Previous studies have demonstrated that oxidative stress is associated not only with hyperglycemia and hypertension but also with the metabolic syndrome and cadiovascular disease (13).

Adrenomedullin (AM) is a novel vasorelaxant peptide isolated from human pheochromocytoma (4,5). Oxidative stress enhances AM production from endothelium and vascular smooth muscle cells in vitro (6,7). However, whether oxidative stress is associated with circulating levels of AM in vivo is unknown. In the present study, we evaluated the relationship between the plasma levels of 8-epi-prostaglandinF2α (8-epi-PGF2α; currently regarded as the most reliable marker for the assessment of oxidative stress in humans) (8,9) and AM in normal subjects and hypertensive patients with type 2 diabetes.

This study comprised 17 hypertensive patients with type 2 diabetes (15 men and 2 women, age 47.3 ± 3.0 years [mean ± SE], BMI 23.2 ± 0.8 kg/m2, fasting plasma glucose 8.2 ± 0.5 mmol/l, HbA1c 9.9 ± 0.5%, fasting serum insulin 30.6 ± 3.0 pmol/l, systolic blood pressure 150.2 ± 3.4 mmHg, and diastolic blood pressure 76.5 ± 2.2 mmHg) and 18 normal subjects (17 men and 1 women, age 43.0 ± 1.9 years, BMI 23.8 ± 0.4 kg/m2, fasting plasma glucose 5.1 ± 0.2 mmol/l, fasting serum insulin 31.2 ± 3.6 pmol/l, systolic blood pressure 126.8 ± 2.2 mmHg, and diastolic blood pressure 79.8 ± 1.6 mmHg). All subjects were nonsmokers. Hypertensive patients with type 2 diabetes were diagnosed at a local clinic 3.0 ± 0.2 years before the beginning of this study. All patients were treated with diet (1,440–1,720 kcal/day and sodium restriction 304 mmol/day) and exercise (walking 10,000 steps/day); none of them were receiving any kind of drug. Their blood glucose and blood pressure were in good control (HbA1c ≤6.5%, systolic blood pressure <140 mmHg, and diastolic blood pressure <90 mmHg) during the initial several months. However, thereafter the blood glucose and blood pressure of the patients gradually increased (HbA1c ≥8%, systolic blood pressure ≥140 mmHg, and diastolic blood pressure ≥90 mmHg) because of overeating and inactivity. They were referred to our clinical department for control of hyperglycemia and hypertension. None of them had evidence of micro- or macroangiopathy. Informed consent was obtained from all subjects before the beginning of the study. Plasma levels of free 8-epi-PGF2α were measured using a commercially available enzyme immunoassay kit (Cayman Chemical, Ann Arbor, MI). The detection limit of this assay was 1.5 pg/ml, and the intra- and interassay coefficients of variation were 7.5 and 9.2%, respectively. AM in plasma samples was measured using a commercially available immunoradiometric assay kit (Shionogi Pharmaceuticals, Osaka, Japan). The detection limit of this assay was 2 fmol/ml, and the intra- and interassay coefficients of variation were 7.0 and 6.9%. Serum insulin was measured using an immunoradiometric assay kit (Insulin Riabead II kit; Dainabot, Tokyo). The intra- and interassay coefficients of variation of the assay were 1.9 and 2.0%. In addition, we measured blood pressure in supine position after a 5-min rest.

Both plasma levels of 8-epi-PGF2α and AM were significantly increased in hypertensive patients with type 2 diabetes compared with normal subjects (8-epi-PGF2α 48.6 ± 8.6 vs. 11.9 ± 1.3 pg/ml, P < 0.05; AM 14.8 ± 0.7 vs. 12.4 ± 0.2 fmol/ml, P < 0.02). The plasma levels of 8-epi-PGF2α were proportionally correlated with AM (r = 0.696, P < 0.01) in only hypertensive patients with type 2 diabetes. Significant positive correlations were observed between plasma levels of 8-epi-PGF2α (r = 0.540, P < 0.05) or AM (r = 0.875, P < 0.001) and systolic blood pressure in patients with type 2 diabetes.

This is the first report that demonstrated a relationship between oxidative stress and AM in vivo. The mechanism by which plasma levels of 8-epi-PGF2α correlate with AM and the cellular source of AM remain unknown. Previous studies have shown that oxidative stress stimulates secretion of AM from endothelium and vascular smooth muscle cells and that AM mRNA expression is increased by activation of the nuclear factor-κB pathway (6,7). Increased AM secretion from endothelium and vascular smooth muscle may influence the plasma levels of AM. On the other hand, Shimosawa et al. (10) reported that endogenous AM may protect from organ damage by inhibiting oxidative stress production. Increased AM levels may compensate for oxidative stress-induced vasoconstriction and thus may play a protective role against organ injury. Adrenal medulla and pancreatic islets may also be a source of AM (4,11). Further study is needed to clarify the source of plasma AM and the mechanism of correlation between oxidative stress and AM.

In conclusion, there was a significant positive correlation between increased oxidative stress and elevated plasma levels of AM in hypertensive patients with type 2 diabetes. Enhanced oxidative stress may regulate the plasma levels of AM in hypertensive patients with type 2 diabetes.

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Address correspondence to A. Katsuki, MD, Third Department of Internal Medicine, Mie University School of Medicine, 2-174 Edobashi, Tsu, Mie 514-8507, Japan. E-mail: katuki-a@clin.medic.mie-u.ac.jp.