Kinetics of Insulin Action In Vivo: Identification of Rate-Limiting Steps

To examine the kinetic steps in insulin's in vivo action, we have assessed the temporal relationship between arterial insulin, interstitial insulin, glucose disposal rate (GDR), and insulin receptor kinase (IRK) activity in muscle and between portal insulin, hepatic glucose production (HGP), and IRK activity in liver. Interstitial insulin, as measured by lymph-insulin concentration (muscle only), and IRK activity were used as independent methods to determine the arrival of insulin at its tissue site of action. Euglycemic clamps were conducted in seven mongrel dogs and consisted of an activation phase with a venous insulin infusion (7.2 nmol · kg⁻¹ · min⁻¹,100 min) and a deactivation phase. Liver and muscle biopsies were taken to assess IRK activity. Arterial, portal, and lymph insulin rose to 636 ± 12, 558 ± 18, and 402 ± 24 pmol/1, respectively. GDR increased from 13.9 ± 0.6 to 41.7 ± 2.8, and HGP declined from 14.4 ± 0.6 to 1.1 ± 0.6 μmol · kg⁻¹ · min⁻¹. Muscle and liver IRK activity increased significantly from 5.9 ± 0.9 to 14.6 ± 0.6 and 5.5 ± 0.7 to 23.7 ± 1.9 fmol P/fmol insulin receptor (IR), respectively. The time to half-maximum response (t_½a) for stimulation of GDR (19.8 ± 4.8 min) and suppression of HGP (21.5 ± 3.7 min) were similar. The t_½a for stimulation of GDR, muscle IRK, and rise in lymph insulin were not significantly different from one another and were all markedly greater than that for the approach to steady state of arterial insulin (2.3 ± 1.2 min, P < 0.01). The t_½a for portal insulin (1.8 ± 0.8 min) was less than that for activation of liver IRK (11.3 ± 4.3, P < 0.05), which in turn was less than that for suppression of HGP (21.5 ± 3.7 min, P < 0.05). In skeletal muscle, the delay in insulin-stimulated GDR occurs before IR binding and is due to the time required for plasma insulin to gain access to the interstitial compartment. In liver, however, identification of the site(s) of delay in insulin's effects to suppress HGP is dependent on whether insulin acts directly or indirectly on the liver. If its action is direct, there are two separate sites of delay: a prereceptor and a postreceptor delay. If, however, insulin's effect on suppressing HGP is indirect, then a single extrahepatic site of delay is likely, which represents the time-limiting step of insulin's ability to stimulate GDR and suppress HGP. Then, the locus of the site involves transendothelial passage of insulin to the interstitial space.