Elevated sympathetic nerve activity (SNA) coupled with dysregulated β-adrenoceptor (β-AR) signaling is postulated as a major driving force for cardiac dysfunction in patients with type 2 diabetes; however, cardiac SNA has never been assessed directly in diabetes. Our aim was to measure the sympathetic input to and the β-AR responsiveness of the heart in the type 2 diabetic heart. In vivo recording of SNA of the left efferent cardiac sympathetic branch of the stellate ganglion in Zucker diabetic fatty rats revealed an elevated resting cardiac SNA and doubled firing rate compared with nondiabetic rats. Ex vivo, in isolated denervated hearts, the intrinsic heart rate was markedly reduced. Contractile and relaxation responses to β-AR stimulation with dobutamine were compromised in externally paced diabetic hearts, but not in diabetic hearts allowed to regulate their own heart rate. Protein levels of left ventricular β1-AR and Gs (guanine nucleotide binding protein stimulatory) were reduced, whereas left ventricular and right atrial β2-AR and Gi (guanine nucleotide binding protein inhibitory regulatory) levels were increased. The elevated resting cardiac SNA in type 2 diabetes, combined with the reduced cardiac β-AR responsiveness, suggests that the maintenance of normal cardiovascular function requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the diabetic heart.

Diabetes is a strong independent risk factor for the development of cardiovascular complications and congestive heart failure (13). Cardiac autonomic dysfunction (CAD) is an undervalued, but significant, cause of morbidity and mortality in diabetes, most likely because CAD pathology is poorly understood (4). Among the characteristics of CAD, diabetic patients have an elevated resting heart rate (HR), but a paradoxically lower peak HR, which may result from impaired cardiac sympathetic innervation (46). Uptake of the sympathetic neurotransmitter norepinephrine (NE), using metaiodobenzylguanidine, is reduced in type 2 diabetic versus nondiabetic subjects who have been screened for ischemic heart disease (7). Moreover, metaiodobenzylguanidine uptake is lower in type 2 diabetic patients with CAD than in those without CAD (7). These data suggest that diabetes progressively reduces sympathetic nervous input to the heart, limiting cardiac reserve.

An alternative explanation is that sympathetic nervous input is normal or elevated in diabetes, but that the β-adrenoceptor (AR) responsiveness of the heart is reduced, a mechanism that has been well described in heart failure (8,9). Isolated heart preparations from type 1 diabetic rat models describe a reduced inotropic (contraction), lusitropic (relaxation), and chronotropic (rate) β-AR responsiveness (1013), which is associated with the downregulation of cardiac β1-ARs (11,1416). Increased cardiac NE content and spillover in type 1 diabetic rats also suggest elevated sympathetic nervous activity (SNA) in diabetes (17,18). Moreover, diabetes in humans is associated with augmented activation of SNA in other organs, such as skeletal muscle (19,20). However, central control of SNA to peripheral organs is differentially regulated (2123). Importantly, obese humans with insulin resistance show increased skeletal muscle SNA and renal NE spillover, but reduced cardiac NE spillover (24). Thus, although these studies suggest a key role for the sympathetic nervous system (SNS) in cardiac dysfunction in diabetes, direct measurements of the sympathetic nervous input to the diabetic heart have not yet been reported. Furthermore, it is unclear whether diabetic cardiac dysfunction results from changes in sympathetic nervous input to the heart (cardiac SNA), a reduction in β-AR responsiveness of the heart, or both. Identifying the underlying processes of diabetic autonomic dysfunction is important, as the traditional treatment of patients with type 2 diabetes with β-blockers is of benefit, although to a lesser extent than in the treatment of nondiabetic patients (25), and it is even more critical for the development of potential new therapeutic targets that interrupt myocardial autonomic signaling, such as β-ARKct (26), β3-AR agonists (27), or renal denervation (28). This study uses direct in vivo recordings of efferent cardiac SNA and β-AR responsiveness of isolated denervated hearts from Zucker type 2 diabetic fatty (ZDF) rats to establish whether type 2 diabetes 1) increases cardiac SNA and 2) reduces cardiac β-AR responsiveness. Finally, we tested whether type 2 diabetes reduced the expression of β1- and β2-ARs and their downstream signaling G-proteins.

Animals

All experiments were approved and conducted in accordance with the guidelines of the Animal Ethics Committee of the University of Otago, New Zealand. Experiments were conducted on 20-week-old male type 2 diabetic ZDF (fa/fa, n = 26) rats and their nondiabetic littermates (+/+, n = 26).

Recording of Cardiac SNA and β-AR Responsiveness In Vivo

Recordings of cardiac SNA, arterial blood pressure (ABP), and left ventricular (LV) cardiac function were performed in 12 nondiabetic and 12 type 2 diabetic ZDF rats in vivo, as previously described (29). In brief, animals were anesthetized with urethane (1.5 g/kg i.p.), intubated, and ventilated (tidal volume ∼3.5 mL; breathing rate ∼80 breaths/min). A blood sample was taken to measure blood glucose and insulin levels. A left thoracotomy was performed between the first and second rib, exposing the stellate ganglion. The cardiac sympathetic nerve was identified as a branch from the stellate ganglion and was dissected free of surrounding connective tissue. After cutting the nerve, the proximal section was placed on a pair of platinum recording electrodes to measure nerve activity. Consequently, the nerve activity being measured was from the efferent sympathetic nerves. The recorded signal was filtered (low cutoff 0.1 kHz; high cutoff 1 kHz), amplified, and subsequently passed through an amplitude discriminator to quantify nerve discharge frequency (impulse frequency). The raw signal was rectified and integrated (1-s resetting interval) online, and the integrated nerve signal was displayed in real time. At the end of the experiments, a postmortem nerve activity measurement was performed for background subtraction of noise level, which was not different between groups.

Systemic ABP was measured through a femoral artery cannula, and HR was derived from the arterial systolic peaks. The right carotid artery was cannulated with a 1.5-F Millar pressure-volume catheter (model SPR-869), which was then advanced into the left ventricle for the continuous measurement of LV pressure (LVP) and LV volume, which combined provided LV end-diastolic pressure, LV end-systolic pressure, LV end-diastolic volume (LVEDV), and LV end-systolic volume. From these measurements, stroke volume (SV), cardiac output (CO), maximum rate of contraction (+dP/dtmax), and maximum rate of relaxation (−dP/dtmax) were derived. The Millar pressure-volume catheter was calibrated using a sphygmomanometer (pressure), volumetric cuvettes (volume), and hypertonic saline injections (for volumetric corrections), as previously described in detail (30).

Baseline cardiac SNA was recorded, and administration of dobutamine (β1-agonist; 0.6–10 μg/kg/min) tested the in vivo cardiac response to β-AR stimulation.

Immunohistochemistry

After measuring the cardiac SNA, frozen sections of isolated left sympathetic cardiac nerve of two nondiabetic and three diabetic ZDF rats that had been fixed in 4% paraformaldehyde were cut at 12 μm and processed as described previously (31). Sections were incubated with anti-neurofilament (NF) 160 polyclonal primary antibody (dilution 1:500 in Tris immunodiluent; ab64300; Abcam) for axon staining, or with anti–tyrosine hydroxylase (TH) polyclonal primary antibody (dilution 1:1,000; AB152; Millipore) for the staining of sympathetic axons. Primary antibodies were detected using incubation with anti-species secondary antibody (dilution 1:500 in Tris immunodiluent; Alexa Fluor 488–conjugated goat anti-rabbit IgG; Life Technologies). The total axon number (NF) and the number of sympathetic axons (TH) per nerve were determined for each of the nerves.

Ex Vivo Cardiac β-AR Responsiveness

Intrinsic cardiac function and responsiveness to β-AR agonist stimulation were determined using Langendorff-perfused isolated hearts in 14 nondiabetic and 14 type 2 diabetic ZDF rats ex vivo as previously described (32). In brief, animals were anesthetized with pentobarbital (60 mg/kg), the heart was excised and mounted on a Langendorff apparatus, and the aorta was retrograde perfused with Krebs-Henseleit buffer at 37°C a constant pressure of 80 mmHg. The Krebs-Henseleit buffer contained (in mmol/L) 118.5 NaCl, 4.7 KCl, 1.2 MgSO40.7H2O, 1.2 KH2PO4.H2O, 1.4 CaCl2, 25.0 NaHCO3, and 11.0 glucose and was continuously equilibrated with 95% O2 and 5% CO2 (pH 7.4). A custom-made balloon-tipped catheter connected to a hydrostatic pressure transducer was inserted into the left ventricle to measure isovolumetric LVP. Two stimulating electrodes, one at the apex and the other on the right atrium, were used for pacing of the heart when required. The HR, developed LVP (LVPdev), +dP/dtmax, −dP/dtmax, and time constant of relaxation (Tau) were calculated from the LVP trace.

Diabetic and nondiabetic rats were divided into the following two groups: unpaced and paced. Isolated hearts from the unpaced group were allowed to beat at their own intrinsic rate, enabling the simultaneous determination of inotropic (contraction), lusitropic (relaxation), and chronotropic (rate) properties. The hearts from the paced (to exclude the influence of rate) group were paced at 5 Hz (physiological resting HR ∼300 bpm) to exclude chronotropic properties. Hearts from both the unpaced and paced groups were exposed to dobutamine (β1-agonist; 1 × 10−9 to 1 × 10−6 mol/L, 5 min/dose) to assess responsiveness to β-AR agonist stimulation. Maximal developed pressure was determined with a post–extra-systolic rest potentiation protocol (33).

Determination of G-Protein and β-AR Subtype Protein Expression

Protein lysates from LV and right atrial (RA) tissue of in vivo and ex vivo hearts were separated on 12% SDS-PAGE gels and were transferred onto polyvinylidene fluoride membranes for β1-AR and nitrocellulose for β2-AR Gs (guanine nucleotide binding protein stimulatory) and Gi (guanine nucleotide binding protein inhibitory regulatory), as described previously (34), which were subsequently probed with specific polyclonal antibodies against β1-AR (rabbit, 1:500 dilution in 2% BSA, Novus Biologicals; or 1:1,000 dilution in 2% BSA, GeneTex), β2-AR (rabbit, 1:5,000 dilution; Badrilla), voltage-dependent anion-selective channel protein (VDAC) 1 (1:10,000 dilution in 2% milk; Novus Biologicals), Gs (rabbit, 1:1,000 dilution; Abcam), Gi (rabbit, 1:1,000 dilution; Santa Cruz Biotechnology), or GAPDH (rabbit, 1:25,000 dilution; Badrilla). Proteins were visualized using an enhanced chemiluminescence detection system. The β1- and β2-AR protein expression was normalized to VDAC1 or GAPDH and the Gs and Gi protein expression was normalized to GAPDH to correct for protein loading.

Statistical Analysis

Unpaired t tests were used to test for differences between groups for the in vivo and ex vivo variables and β1-AR and β2-AR protein expression levels. A nonparametric Mann-Whitney test was used to test for differences in nerve cross-sectional area (CSA) and the number of axons per nerve. Two-way ANOVA with repeated measures was used to test for in vivo and ex vivo β-AR responsiveness group comparisons, followed by a Bonferroni post hoc analysis. A P value <0.05 was considered statistically significant. Results are presented as the mean ± SEM.

ZDF (fa/fa) rats had higher nonfasting blood glucose levels, plasma insulin levels, and body mass compared with their ZDF(+/+) littermates, confirming their type 2 diabetes and obese status (Table 1). Type 2 diabetic ZDF rats showed no structural LV hypertrophy (similar to normalized heart mass), but they did show decreased LV filling (e.g., a significant 16 ± 3% decrease in LVEDV; P < 0.05) and decreased SV (21 ± 4% decrease in SV; P < 0.05), suggesting diastolic dysfunction. There was no difference in in vivo resting HR, CO, and mean arterial blood pressure (MABP) between type 2 diabetic and nondiabetic animals (Table 1).

Table 1

Animal characteristics and in vivo hemodynamics in 20-week-old male nondiabetic ZDF+/+ and type 2 diabetic ZDF (fa/fa) rats

Nondiabetic ZDF+/+ rats (n = 12)Type 2 diabetic ZDF (fa/fa) rats (n = 12)
Body weight (g) 338 ± 6 412 ± 16** 
Blood glucose level (mmol/L) 8.0 ± 1.2 23.8 ± 2.9*** 
Plasma insulin level (ng/mL) 3.3 ± 0.5 20.5 ± 4.5** 
Heart weight (g) 1.83 ± 0.14 1.75 ± 0.05 
Heart weight/tibia length ratio 0.064 ± 0.003 0.070 ± 0.003 
Tibia length (mm) 27.2 ± 1.3 26.0 ± 0.6 
EF (%) 63 ± 2 60 ± 3 
SV (μL) 109 ± 7 84 ± 6* 
HR (bpm) 379 ± 10 378 ± 8 
CO (mL/min) 41.0 ± 1.8 35.6 ± 4.2 
LVEDV (μL) 121 ± 7 101 ± 4* 
LVESV (μL) 48 ± 4 44 ± 4 
LVESP (mmHg) 136 ± 6 130 ± 6 
LVEDP (mmHg) 16 ± 4 12 ± 4 
LV +dP/dtmax (mmHg/s) 9,812 ± 549 11,228 ± 1,009 
LV −dP/dtmax (mmHg/s) −5,513 ± 217 −5,263 ± 357 
Tau (ms) 12.3 ± 1.2 13.1 ± 2.1 
MABP (mmHg) 80 ± 8 85 ± 7 
Nondiabetic ZDF+/+ rats (n = 12)Type 2 diabetic ZDF (fa/fa) rats (n = 12)
Body weight (g) 338 ± 6 412 ± 16** 
Blood glucose level (mmol/L) 8.0 ± 1.2 23.8 ± 2.9*** 
Plasma insulin level (ng/mL) 3.3 ± 0.5 20.5 ± 4.5** 
Heart weight (g) 1.83 ± 0.14 1.75 ± 0.05 
Heart weight/tibia length ratio 0.064 ± 0.003 0.070 ± 0.003 
Tibia length (mm) 27.2 ± 1.3 26.0 ± 0.6 
EF (%) 63 ± 2 60 ± 3 
SV (μL) 109 ± 7 84 ± 6* 
HR (bpm) 379 ± 10 378 ± 8 
CO (mL/min) 41.0 ± 1.8 35.6 ± 4.2 
LVEDV (μL) 121 ± 7 101 ± 4* 
LVESV (μL) 48 ± 4 44 ± 4 
LVESP (mmHg) 136 ± 6 130 ± 6 
LVEDP (mmHg) 16 ± 4 12 ± 4 
LV +dP/dtmax (mmHg/s) 9,812 ± 549 11,228 ± 1,009 
LV −dP/dtmax (mmHg/s) −5,513 ± 217 −5,263 ± 357 
Tau (ms) 12.3 ± 1.2 13.1 ± 2.1 
MABP (mmHg) 80 ± 8 85 ± 7 

Data are presented as the mean ± SEM. EF, ejection fraction; LVEDP, left ventricular end-diastolic pressure; LVESP, left ventricular end-systolic pressure; LVESV, left ventricular end-systolic volume.

*P < 0.05;

**P < 0.01;

***P < 0.001, significantly different from nondiabetic rat, unpaired t test.

In Vivo Cardiac SNA

A representative direct recording of cardiac SNA in vivo in nondiabetic and type 2 diabetic ZDF rats is shown in Fig. 1A. The direct cardiac SNA recordings revealed a 45% increase in resting cardiac SNA in the type 2 diabetic group, as shown from the averaged integrated cardiac SNA values (1.25 ± 0.17 and 1.87 ± 0.18 μV · s−1, nondiabetes vs. type 2 diabetes, respectively; P < 0.05; Fig. 1B). Moreover, the mean nerve firing rate was twice as high in diabetic versus nondiabetic rats (21.1 ± 4.1 and 45.0 ± 8.7 impulses/s, nondiabetes vs. type 2 diabetes, respectively; P < 0.05; Fig. 1C). This elevated SNA, recorded in vivo from the left efferent cardiac sympathetic nerve, shows that the sympathetic input to the heart is increased in a type 2 diabetic rat model.

Figure 1

Direct in vivo recordings of cardiac SNA (cSNA): increased sympathetic input to the heart in type 2 diabetic ZDF rats. A: Representative LabChart recordings showing ABP, raw efferent cardiac SNA, and the derived integrated cardiac SNA in nondiabetic and type 2 diabetic ZDF rats. B: Increased integrated resting cardiac SNA. C: A trend toward an increased nerve firing rate in type 2 diabetic ZDF rats. *P < 0.05, significantly different from nondiabetic rats, unpaired t test, n = 11 per group. Data are presented as the mean ± SEM.

Figure 1

Direct in vivo recordings of cardiac SNA (cSNA): increased sympathetic input to the heart in type 2 diabetic ZDF rats. A: Representative LabChart recordings showing ABP, raw efferent cardiac SNA, and the derived integrated cardiac SNA in nondiabetic and type 2 diabetic ZDF rats. B: Increased integrated resting cardiac SNA. C: A trend toward an increased nerve firing rate in type 2 diabetic ZDF rats. *P < 0.05, significantly different from nondiabetic rats, unpaired t test, n = 11 per group. Data are presented as the mean ± SEM.

Cardiac Sympathetic Nerve

The CSA of the left cardiac sympathetic nerve was not different between the nondiabetic and type 2 diabetic groups (4,399 ± 397 and 4,268 ± 620 μm2, nondiabetes [n = 2] vs. type 2 diabetes [n = 3], P > 0.05; Fig. 2B). Moreover, the immunohistochemical NF and TH staining of the nerves revealed that neither the total number of axons (Fig. 2C) nor the number of sympathetic axons (Fig. 2D) per nerve was different between the nondiabetic and diabetic groups (1,087 ± 196 and 989 ± 213 NF axons per nerve and 984 ± 93 and 928 ± 181 TH axons per nerve; nondiabetes [n = 2] vs. type 2 diabetes [n = 3], P > 0.05).

Figure 2

A: Examples of left cardiac sympathetic nerves from nondiabetic (control) and type 2 diabetic ZDF rats immunohistochemically stained for NF and TH. Scale bars are 50 μm. B: The CSA of the cardiac sympathetic nerves. The total number of axons per nerve (stained with NF) (C) and the total number of sympathetic axons per nerve (stained with TH) (D) were not different between nondiabetic and type 2 diabetic ZDF rats. Nondiabetic (n = 2) vs. type 2 diabetic (n = 3) rats, P > 0.05, nonparametric Mann-Whitney test. Data are presented as the mean ± SEM.

Figure 2

A: Examples of left cardiac sympathetic nerves from nondiabetic (control) and type 2 diabetic ZDF rats immunohistochemically stained for NF and TH. Scale bars are 50 μm. B: The CSA of the cardiac sympathetic nerves. The total number of axons per nerve (stained with NF) (C) and the total number of sympathetic axons per nerve (stained with TH) (D) were not different between nondiabetic and type 2 diabetic ZDF rats. Nondiabetic (n = 2) vs. type 2 diabetic (n = 3) rats, P > 0.05, nonparametric Mann-Whitney test. Data are presented as the mean ± SEM.

In Vivo Hemodynamic Responses to β-AR Stimulation

Both the nondiabetic and type 2 diabetic ZDF animals responded to the β-AR agonist dobutamine with increases in HR, SV, and CO (Fig. 3A–C). No significant acceleration of the cardiac contractility (+dP/dtmax) or relaxation (−dP/dtmax and Tau) parameters were observed (Fig. 3D–F), and the MABP and total peripheral resistance (TPR) both decreased (Fig. 3G and H) compared with saline injection. During incremental dobutamine infusion, the increase in SV was attenuated in type 2 diabetic vs. nondiabetic rats (∆21.7 ± 3.3 vs. ∆6.7 ± 3.9 μL at 10 μg/kg/min dobutamine; respectively; P < 0.05; Fig. 3B), indicating a reduced inotropic response to β-AR stimulation in rats with type 2 diabetes in vivo. There were no differences in the response of any other hemodynamic variables between the two groups.

Figure 3

Preserved hemodynamic responses to β-AR stimulation in vivo in type 2 diabetic ZDF rats. In both the nondiabetic (closed circles) and type 2 diabetic (open circles) ZDF rats, incremental doses of β-AR agonist (dobutamine) resulted in an increase in HR (A), SV (B), and CO (C) compared with saline injection. No changes in the cardiac contractile parameter speed of contraction (+dP/dtmax) (D), the relaxation parameters speed of relaxation (−dP/dtmax) (E), and Tau (F) were observed compared with saline injection, whereas the MABP (G) and the calculated total peripheral resistance (TPR) (H) both decreased with β-AR agonist stimulation compared with saline injection. The type 2 diabetic rats only had a lower SV compared with the nondiabetic animals. *Significant vs. saline (baseline), #significant vs. nondiabetes, both P < 0.05, two-way repeated-measures ANOVA; n = 7 per group. Data are presented as the mean ± SEM.

Figure 3

Preserved hemodynamic responses to β-AR stimulation in vivo in type 2 diabetic ZDF rats. In both the nondiabetic (closed circles) and type 2 diabetic (open circles) ZDF rats, incremental doses of β-AR agonist (dobutamine) resulted in an increase in HR (A), SV (B), and CO (C) compared with saline injection. No changes in the cardiac contractile parameter speed of contraction (+dP/dtmax) (D), the relaxation parameters speed of relaxation (−dP/dtmax) (E), and Tau (F) were observed compared with saline injection, whereas the MABP (G) and the calculated total peripheral resistance (TPR) (H) both decreased with β-AR agonist stimulation compared with saline injection. The type 2 diabetic rats only had a lower SV compared with the nondiabetic animals. *Significant vs. saline (baseline), #significant vs. nondiabetes, both P < 0.05, two-way repeated-measures ANOVA; n = 7 per group. Data are presented as the mean ± SEM.

Ex Vivo Animal and Cardiac Characteristics

The type 2 diabetic rats had elevated nonfasting blood glucose levels (Table 2), whereas their heart weight and maximum LV volume were not different, which is indicative of type 2 diabetes without cardiac structural remodeling. Interestingly, resting intrinsic HR was 32% lower in the unpaced isolated hearts of type 2 diabetic rats. Consistent with the in vivo data, the LVPdev, the +dP/dtmax, and the −dP/dtmax were unchanged in type 2 diabetic animals, whereas late Tau was prolonged in the paced type 2 diabetic hearts. The maximal LVPdev was not different between the two groups. No differences in animal characteristics and resting cardiac function were observed between the unpaced and paced groups (Table 2).

Table 2

Animal characteristics and ex vivo cardiac function in unpaced and paced isolated hearts of 20-week-old male nondiabetic and diabetic ZDF rats

Unpaced
Paced
Nondiabetic ZDF+/+ rats (n = 7)Type 2 diabetic ZDF (fa/fa) rats (n = 7)Nondiabetic ZDF+/+ rats (n = 7)Type 2 diabetic ZDF (fa/fa) rats (n = 7)
Body weight (g) 357 ± 7 401 ± 13* 347 ± 7 384 ± 15* 
Blood glucose level (mmol/L) 10.5 ± 0.5 25.8 ± 2.7** 9.4 ± 0.6 28.7 ± 1.8** 
Heart weight (g) 1.33 ± 0.14 1.5 ± 0.02 1.37 ± 0.06 1.43 ± 0.02 
LVVmax (μL) 343 ± 16 357 ± 30 328 ± 13 350 ± 11 
LVPdev (mmHg) 142 ± 6 135 ± 9 125 ± 8 111 ± 7 
+dP/dtmax (mmHg/s) 3,577 ± 194 4,099 ± 177 3,235 ± 141 3,335 ± 228 
−dP/dtmax (mmHg/s) −2,353 ± 127 −2,230 ± 106 −2,344 ± 143 −2,080 ± 65 
Tau (ms) 74 ± 12 73 ± 18 67 ± 13 82 ± 5* 
Pmax (mmHg) 338 ± 16 330 ± 23 382 ± 11 350 ± 22 
HR (bpm) 245 ± 10 165 ± 9** 300 ± 1 300 ± 1 
Unpaced
Paced
Nondiabetic ZDF+/+ rats (n = 7)Type 2 diabetic ZDF (fa/fa) rats (n = 7)Nondiabetic ZDF+/+ rats (n = 7)Type 2 diabetic ZDF (fa/fa) rats (n = 7)
Body weight (g) 357 ± 7 401 ± 13* 347 ± 7 384 ± 15* 
Blood glucose level (mmol/L) 10.5 ± 0.5 25.8 ± 2.7** 9.4 ± 0.6 28.7 ± 1.8** 
Heart weight (g) 1.33 ± 0.14 1.5 ± 0.02 1.37 ± 0.06 1.43 ± 0.02 
LVVmax (μL) 343 ± 16 357 ± 30 328 ± 13 350 ± 11 
LVPdev (mmHg) 142 ± 6 135 ± 9 125 ± 8 111 ± 7 
+dP/dtmax (mmHg/s) 3,577 ± 194 4,099 ± 177 3,235 ± 141 3,335 ± 228 
−dP/dtmax (mmHg/s) −2,353 ± 127 −2,230 ± 106 −2,344 ± 143 −2,080 ± 65 
Tau (ms) 74 ± 12 73 ± 18 67 ± 13 82 ± 5* 
Pmax (mmHg) 338 ± 16 330 ± 23 382 ± 11 350 ± 22 
HR (bpm) 245 ± 10 165 ± 9** 300 ± 1 300 ± 1 

Data are presented as the mean ± SEM. LVVmax, maximum LV volume; Pmax, maximal developed pressure.

*P < 0.05,

**P < 0.01, significantly different from nondiabetic rats, unpaired t test.

Ex Vivo Cardiac Responses to β-AR Stimulation—Paced Versus Unpaced

A greater β-AR agonist concentration was required to achieve a target HR in unpaced type 2 diabetic versus nondiabetic rat hearts (Fig. 4A). In addition, diabetic hearts reached a lower peak HR than nondiabetic hearts at the highest dobutamine dose used. In contrast, the LVP for a given β-AR agonist concentration was not different between the groups (Fig. 4C), suggesting altered chronotropic, but preserved inotropic, β-AR responsiveness.

Figure 4

Altered chronotropic but preserved inotropic and lusitropic β-AR responsiveness in unpaced isolated type 2 diabetic ZDF hearts. A: Unpaced isolated type 2 diabetic hearts (open circles) required a higher β-AR agonist (dobutamine) concentration to achieve a given absolute HR compared with nondiabetic hearts (closed circles). B: However, after normalization, the HR response was not different between the groups. LVPdev increased with β-AR agonist stimulation in both groups, but showed no difference between nondiabetic and type 2 diabetic hearts (C), even after normalization (D). Both nondiabetic and type 2 diabetic hearts showed increased speed of early relaxation (−dP/dtmax) (E) and a faster late Tau (F) due to β-adrenergic stimulation. Thus, there was altered chronotropic but preserved inotropic and lusitropic β-AR responsiveness in unpaced isolated hearts of 20-week-old male type 2 diabetic ZDF rats. *P < 0.05 vs. baseline, #P < 0.05 vs. nondiabetes, two-way repeated-measures ANOVA; n = 7 per group. For normalization, data were fitted with a four-parameter sigmoidal dose-response curve with a variable slope. Data are presented as the mean ± SEM.

Figure 4

Altered chronotropic but preserved inotropic and lusitropic β-AR responsiveness in unpaced isolated type 2 diabetic ZDF hearts. A: Unpaced isolated type 2 diabetic hearts (open circles) required a higher β-AR agonist (dobutamine) concentration to achieve a given absolute HR compared with nondiabetic hearts (closed circles). B: However, after normalization, the HR response was not different between the groups. LVPdev increased with β-AR agonist stimulation in both groups, but showed no difference between nondiabetic and type 2 diabetic hearts (C), even after normalization (D). Both nondiabetic and type 2 diabetic hearts showed increased speed of early relaxation (−dP/dtmax) (E) and a faster late Tau (F) due to β-adrenergic stimulation. Thus, there was altered chronotropic but preserved inotropic and lusitropic β-AR responsiveness in unpaced isolated hearts of 20-week-old male type 2 diabetic ZDF rats. *P < 0.05 vs. baseline, #P < 0.05 vs. nondiabetes, two-way repeated-measures ANOVA; n = 7 per group. For normalization, data were fitted with a four-parameter sigmoidal dose-response curve with a variable slope. Data are presented as the mean ± SEM.

When HR and LVP values were normalized (as a percent of maximum), the relative chronotropic and inotropic β-AR responsiveness was not different between the groups (Fig. 4B and D). Moreover, the normal physiological lusitropic (relaxation) response to β-adrenergic stimulation was observed in both groups by an increased −dP/dtmax (Fig. 4E) and an accelerated late Tau (Fig. 4F).

When the dobutamine protocol was performed at a paced HR of 300 bpm (to exclude the influence of rate), type 2 diabetic rat hearts developed less LVP at higher concentrations of the β-AR agonist (Fig. 5A), and the normalized LVP curve shifted to the right (Fig. 5B), confirming reduced inotropic responsiveness to β-AR stimulation. When paced, the changes in −dP/dtmax (Fig. 5C) were greater during β-AR stimulation in the nondiabetic hearts compared with the unpaced hearts, whereas the change in late Tau was absent in the diabetic heart (Fig. 5D).

Figure 5

Altered inotropic and lusitropic β-AR responsiveness in paced isolated type 2 diabetic ZDF hearts. A: In isolated nondiabetic (closed circles) and type 2 diabetic (open circles) hearts paced at a fixed HR of 300 bpm, LVPdev increased with β-AR agonist stimulation, but less so in the type 2 diabetic group. B: Consequently, the normalized dose-response curve shifted to the right in the type 2 diabetic hearts, which is indicative of reduced inotropic β-AR responsiveness. The arrow indicates significant rightward shift in the β-AR responsiveness. β-AR agonist stimulation increased the speed of early relaxation (−dP/dtmax) (C) and accelerated late Tau (D) in the nondiabetic hearts. In the type 2 diabetic hearts, the change in early relaxation was impaired, and the change in late relaxation was absent. *P < 0.05 vs. baseline, #P < 0.05 vs. nondiabetes, two-way repeated-measures ANOVA; n = 7 per group. For normalization, data were fitted with a four-parameter sigmoidal dose-response curve with a variable slope. Data are presented as the mean ± SEM.

Figure 5

Altered inotropic and lusitropic β-AR responsiveness in paced isolated type 2 diabetic ZDF hearts. A: In isolated nondiabetic (closed circles) and type 2 diabetic (open circles) hearts paced at a fixed HR of 300 bpm, LVPdev increased with β-AR agonist stimulation, but less so in the type 2 diabetic group. B: Consequently, the normalized dose-response curve shifted to the right in the type 2 diabetic hearts, which is indicative of reduced inotropic β-AR responsiveness. The arrow indicates significant rightward shift in the β-AR responsiveness. β-AR agonist stimulation increased the speed of early relaxation (−dP/dtmax) (C) and accelerated late Tau (D) in the nondiabetic hearts. In the type 2 diabetic hearts, the change in early relaxation was impaired, and the change in late relaxation was absent. *P < 0.05 vs. baseline, #P < 0.05 vs. nondiabetes, two-way repeated-measures ANOVA; n = 7 per group. For normalization, data were fitted with a four-parameter sigmoidal dose-response curve with a variable slope. Data are presented as the mean ± SEM.

β-AR Subtype and G-Protein Expression Levels

β1-AR protein expression was 23% lower in the left ventricle (Fig. 6A and B) and 17% higher in the right atrium (Fig. 6E and F) of type 2 diabetic rat hearts compared with the nondiabetic rat hearts (LV 1.41 ± 0.11 vs. 1.09 ± 0.07 arbitrary units [A.U.]; RA 0.88 ± 0.02 vs. 1.02 ± 0.04 A.U.; nondiabetes vs. type 2 diabetes, P < 0.05 for both). The β2-AR protein expression level was 41% higher in the left ventricle (Fig. 6C and D) and 20% higher in the right atrium (Fig. 6G and H) of the type 2 diabetic hearts compared with the nondiabetic hearts (LV expression 1.26 ± 0.10 vs. 1.78 ± 0.19 A.U.; RA expression 0.82 ± 0.03 vs. 0.99 ± 0.2 A.U.; nondiabetes vs. type 2 diabetes, P < 0.05 for both).

Figure 6

LV and RA β1-AR and β2-AR protein expression levels in type 2 diabetic ZDF rats. A, C, E, and G: Representative images of β1-AR and β2-AR Western blots in the left ventricle and right atrium. B, D, F, and H: Quantitative analysis of the protein expression (mean ± SEM; 3 replications). Shown are reduced β1-AR protein expression levels in the left ventricle (A and B) and increased β1-AR levels in the right atrium (E and F) in the type 2 diabetic group relative to the nondiabetic group, normalized to VDAC or GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. LV: nondiabetes n = 17; type 2 diabetes n = 20; RA: nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications). The β2-AR protein expression levels were elevated in the left ventricle (C and D) and in the right atrium (G and H) in the type 2 diabetic group relative to the nondiabetic group, normalized to VDAC or GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. LV: nondiabetes n = 17; type 2 diabetes n = 17; RA: nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications).

Figure 6

LV and RA β1-AR and β2-AR protein expression levels in type 2 diabetic ZDF rats. A, C, E, and G: Representative images of β1-AR and β2-AR Western blots in the left ventricle and right atrium. B, D, F, and H: Quantitative analysis of the protein expression (mean ± SEM; 3 replications). Shown are reduced β1-AR protein expression levels in the left ventricle (A and B) and increased β1-AR levels in the right atrium (E and F) in the type 2 diabetic group relative to the nondiabetic group, normalized to VDAC or GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. LV: nondiabetes n = 17; type 2 diabetes n = 20; RA: nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications). The β2-AR protein expression levels were elevated in the left ventricle (C and D) and in the right atrium (G and H) in the type 2 diabetic group relative to the nondiabetic group, normalized to VDAC or GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. LV: nondiabetes n = 17; type 2 diabetes n = 17; RA: nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications).

The Gs expression level was 28% lower in the left ventricle (Fig. 7A and B) and 39% lower in the right atrium (Fig. 7E and F) of type 2 diabetic rat hearts compared with the nondiabetic rat hearts (LV expression 0.89 ± 0.04 vs. 0.64 ± 0.07 A.U.; RA expression 0.90 ± 0.07 vs. 0.54 ± 0.03 A.U.; nondiabetes vs. type 2 diabetes, P < 0.05 for both). The Gi expression level was 30% higher in the left ventricle (Fig. 7C and D) and 27% higher in the right atrium (Fig. 7G and H) of the type 2 diabetic rat hearts compared with the nondiabetic rat hearts (LV expression 0.52 ± 0.03 vs. 0.68 ± 0.06 A.U.; RA expression 0.90 ± 0.04 vs. 1.14 ± 0.05 A.U.; nondiabetes vs. type 2 diabetes, P < 0.05 for both).

Figure 7

LV and RA Gs and Gi expression levels in type 2 diabetic ZDF rats. A, C, E, and G: Representative images of Gs and Gi Western blots in the left ventricle and right atrium. B, D, F, and H: Quantitative analysis of the protein expression. Shown are reduced Gs expression levels in the left ventricle (A and B) and right atrium (E and F) in the type 2 diabetic group relative to the nondiabetic group, normalized to GAPDH protein expression levels. Gi expression levels are elevated in the left ventricle (C and D) and right atrium (G and H) in the type 2 diabetic group relative to the nondiabetic group, normalized to GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. Nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications, except for LV Gs duplicates).

Figure 7

LV and RA Gs and Gi expression levels in type 2 diabetic ZDF rats. A, C, E, and G: Representative images of Gs and Gi Western blots in the left ventricle and right atrium. B, D, F, and H: Quantitative analysis of the protein expression. Shown are reduced Gs expression levels in the left ventricle (A and B) and right atrium (E and F) in the type 2 diabetic group relative to the nondiabetic group, normalized to GAPDH protein expression levels. Gi expression levels are elevated in the left ventricle (C and D) and right atrium (G and H) in the type 2 diabetic group relative to the nondiabetic group, normalized to GAPDH protein expression levels. *P < 0.05 vs. nondiabetes, unpaired t test. Nondiabetes n = 7; type 2 diabetes n = 6 (mean ± SEM; 3 replications, except for LV Gs duplicates).

This study showed that resting and β-AR–stimulated hemodynamics of type 2 diabetic and nondiabetic rats were similar in vivo, despite the diabetic heart receiving a greater efferent SNA and having reduced β1-AR expression. Ex vivo experiments (e.g., denervated hearts) revealed that the diabetic heart had a lower intrinsic HR and required a greater β-AR stimulation to achieve a given HR or contractile state (reduced β-AR responsiveness). These findings suggest that the maintenance of normal cardiovascular hemodynamics requires elevated cardiac sympathetic input to compensate for changes in the intrinsic properties of the type 2 diabetic heart.

Previous studies (1720) have suggested a key role for increased sympathetic input to the diabetic heart in modulating cardiac dysfunction; however, findings were all based on indirect measurements of cardiac SNA. This study, using direct recording of the left efferent cardiac sympathetic nerve, is the first to provide direct evidence that SNA is elevated in the diabetic heart. Ganguly and colleagues (17,18) demonstrated a nearly twofold increase in cardiac NE concentrations, enhanced catecholamine turnover, and increased initial rate of NE uptake after 8 weeks of type 1 diabetes in rats. On the other hand, the reduced inotropic response of electrically stimulated left atria of type 1 diabetic rats has been related to an impairment of NE release from sympathetic nerve terminals (35). Cardiac NE spillover provides an estimation of cardiac SNA; however, the relationship between actual SNA and the NE spillover is not linear, and high rates of nerve discharge produce a plateau in the neurotransmitter release (36). In addition, only a variable fraction (estimated at 20%) of the NE released from the nerve terminals actually enters the plasma, while the majority of the NE returned to the nerve varicosity via the NE transporter (37). Elevated “low-frequency” HR variability (HRV) is reported in diabetic patients and has been associated with increased cardiac sympathetic activity and cardiovascular morbidity and mortality (38). However, others have shown (39,40) that atropine abolishes most of the low-frequency HRV, calling into question the association of HRV with cardiac SNA. Our data, therefore, are the first to directly quantify the increase in sympathetic input to the type 2 diabetic heart, revealing a 45% increase in integrated efferent cardiac nerve activity and a doubling of its firing rate in diabetes. Chidsey and Braunwald (41) noted a loss of cardiac sympathetic innervation in failing hearts, whereas Levin et al. (42) showed a marked lowering of cardiac NE levels in obese Zucker rats, both suggesting reduced cardiac sympathetic innervation. We were unable to demonstrate a difference in the nerve CSA or in the number of NF-positive or TH-positive axons within the left cardiac sympathetic nerve between nondiabetic and type 2 diabetic ZDF rats. This suggests that the increase in cardiac SNA in diabetes may not be due to atrophy or partial denervation or to hypertrophy of the cardiac nerve. Overall, these data suggest that type 2 diabetes does not affect the numbers of sympathetic axons (structural input) within the cardiac sympathetic nerve, but increases its sympathetic nerve activity (throughput).

The increase in cardiac SNA may have been a consequence of cellular changes in the diabetic heart. The intrinsic HR was 32% lower in the isolated type 2 diabetic hearts at rest, whereas the chronotropic (rate), inotropic (contraction), and lusitropic (relaxation) responses to β-AR stimulation were the same as those in nondiabetic hearts. This lower intrinsic HR at rest is observed in many experimental rodent models of diabetes (12,43,44). Increased sympathetic input to the diabetic heart, which accelerates spontaneous depolarization and increases conduction velocity through the heart, could explain the observed changes in function and β-AR and G-protein expression in the diabetic heart and why, under in vivo conditions, the HR was not different between nondiabetic and diabetic animals.

However, changes in the intrinsic HR are primarily determined by intracellular mechanisms within the sinoatrial node cells (vs. autonomic nervous system). Recently, it was shown (45) in rodents that the training-induced decrease in HR was not a consequence of changes in the activity of the autonomic function, but was caused by training-induced intrinsic electrophysiological changes within the sinoatrial node. Metabolic changes occurring during diabetes are also known to affect the modulation of the intrinsic HR and, therefore, might provide an alternative explanation for our finding that the intrinsic HR of the diabetic heart was reduced. For example, carnitine supplementation corrected the reduced free carnitine levels and normalized intrinsic HR in type 1 diabetic rats, suggesting that myocardial substrate availability plays an important role in HR regulation beyond autonomic tone (46).

When isolated hearts were paced to remove the confounding effects of rate, we found that the diabetic hearts had reduced inotropic and lusitropic responsiveness to β-AR stimulation. Interestingly, in the diabetic rats under in vivo conditions (no difference in HR at rest) and in their unpaced isolated hearts (HR reduced at rest), both the inotropic and lusitropic responses to β-AR stimulation were preserved. This suggests that changes in the intrinsic functions of the diabetic heart, by means of a reduction in absolute intrinsic HR, would ensure normal inotropic β-AR responsiveness (potentially by improving its lusitropic window). These findings, therefore, may indicate that metabolically altered chronotropic incompetence, rather than inotropic impediment, is an important, and undervalued, feature of the diabetic heart (47).

The reduced inotropic and lusitropic responses to β-AR stimulation in the paced diabetic hearts have been observed in other experimental diabetes models (1013,15) and have been attributed to reductions in mRNA and protein levels of β1-ARs (11,1416), the most predominant β-AR subtype in the healthy heart. β1-AR stimulation increases cAMP levels via Gs-coupled proteins, which augment contraction by increasing intracellular calcium flux during systole. Therefore, the reductions in β1-AR (23%) and Gs (28%) in the diabetic left ventricle are consistent with a blunted stimulatory response from the increased efferent SNA and likely contributed to the observed changes in cardiac function of our diabetic hearts. Surprisingly, the β1-AR expression in the right atrium was increased (17%), with an associated reduction of Gs expression (39%). This could indicate specific uncoupling of the β1-AR from its downstream signaling proteins (48). Alternatively, β2-ARs have a higher relative expression level in the sinoatrial node region (49) and, as a consequence of the colocalization of β2-AR with the funny channels in the caveolae, have been shown to contribute more to chronotropic changes compared with β1-ARs (50). We found that β2-AR protein expression was increased in the left ventricle (40%) and in the right atrium (20%) of the diabetic hearts, with concomitant increases of Gi expression (30% and 27%, respectively), which, to the best of our knowledge, is the first report in a type 2 diabetic rodent model. It is unclear what the functional relevance of this shift from β1-AR to β2-AR in type 2 diabetes is, although similar relative results have been observed during heart failure (9). In the healthy heart, it is believed that β2-ARs do not directly contribute to changes in contraction because their colocalization with Gi-activated phosphodiesterases inhibits the cAMP pathway and prevents the resultant increase in calcium fluxes. Interestingly, in heart failure redistribution of β2-ARs changes this compartmentation of cAMP and might contribute to the failing myocardial phenotype (51). Moreover, recent evidence indicates that β2-ARs modulate intracellular oxygen availability (52) and are linked to increases in metabolic kinases, such as AMPK (52,53). AMPK is considered a critical sensor of cellular energy, activated by biguanide drugs (metformin and phenformin), and an attractive metabolic target for novel therapies in the treatment of type 2 diabetes (54). Therefore, the relation of β2-ARs, their coupling (and uncoupling) to Gs and Gi, and their links to functional and metabolic challenges in type 2 diabetes, especially related to chronotropic modulation of cardiac function, warrants further research.

Limitations

Surgery and anesthetic agents modulate the SNS and therefore caution is needed when interpreting data from anesthetized animals during surgery. While direct assessment of cardiac SNA in conscious sheep is possible (55), no ovine diabetic model exists. Unfortunately in rodents, because of the limited accessibility of the cardiac nerves, telemetric recordings are not feasible. We used urethane because of its minimal effects on autonomic, cardiovascular, and respiratory systems (56).

SNA measurements in other cardiac diseases, such as myocardial infarction (29,57) or more severe heart failure (58,59), have found larger changes in cardiac SNA. Nevertheless, our relatively small, but significant, change in cardiac SNA in type 2 diabetes, if maintained over long periods during the progression of diabetes, may still have very detrimental effects on cardiac function.

Short-term central administration of leptin activates the SNS (60), increases lumbar and renal SNS activity (61), and increases ABP (62), all of which are consistent with the increased cardiac sympathetic activity. However, this suggests that a leptin-deficient animal would, if anything, have reduced cardiac sympathetic activity. Therefore, we believe that the development of type 2 diabetes independently explains the changes in cardiac SNA and resultant changes in myocardial β-adrenergic responsiveness in our study.

In conclusion, this is the first study to demonstrate, with direct measurement of the efferent cardiac sympathetic nerves, elevated resting cardiac SNA in type 2 diabetes in vivo. In contrast, the intrinsic properties of the diabetic heart showed lower HR and rate-independent contractility. In this context, the elevated cardiac SNA may have been a compensatory response to a diabetes-induced lower intrinsic HR, ensuring normal hemodynamics in vivo. The proportion of β1-/β2-AR and associated presence of Gs and Gi in the left ventricle was reduced by diabetes, which is consistent with other models of high cardiac SNA, such as congestive heart failure. Thereby, in our type 2 diabetic animal model without any other cardiovascular disease present, we demonstrated that sympathetic hyperactivity and a defective intrinsic HR regulation are associated with cardiac dysfunction and β-AR dysregulation in type 2 diabetes.

See accompanying article, p. 2708.

Funding. This work was supported by grants from Healthcare Otago Charitable Trust, New Zealand National Heart Foundation grant no. 1491–NH Taylor Charitable Trust, and the Department of Physiology of the University of Otago.

Duality of Interest. No potential conflicts of interest relevant to this article were reported.

Author Contributions. H.P.A.T. performed the experimental work, the primary analysis, and the secondary analysis and drafted the manuscript. J.C.B. designed the study and drafted the manuscript. H.-Y.W. performed the experimental work, the primary analysis, and the secondary analysis. G.H., R.F.C., C.T.B., and P.W.S. performed the experimental work and the primary analysis. A.B., P.P.J., and D.O.S. managed the experimental work and edited the manuscript. R.R.L. designed the study, performed the secondary analysis, managed the experimental work, and drafted the manuscript. All authors read and approved the final manuscript. R.R.L. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Prior Presentation. Parts of this study were presented in abstract form at 2012 Sydney: Joint Australian Physiological Society/Physiological Society of New Zealand/Australian Society for Biophysics Meeting, Sydney, New South Wales, Australia, 2–5 December 2012.

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