KINETICS OF INSULIN ACTION IN-VIVO - IDENTIFICATION OF RATE-LIMITING STEPS

To examine the kinetic steps in insulin's in vivo action, we have asse ssed 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 produ ction (HGP), and IRK activity in liver. Interstitial insulin, as measu red by lymph-insulin concentration (muscle only), and IRK activity wer e used as independent methods to determine the arrival of insulin at i ts tissue site of action. Euglycemic clamps were conducted in seven mo ngrel dogs and consisted of an activation phase with a venous insulin infusion (7.2 nmol . kg(-1). min(-1), 100 min) and a deactivation phas e. Liver and muscle biopsies were taken to assess IRK activity. Arteri al, portal, and lymph insulin rose to 636 +/- 12, 558 +/- 18, and 402 +/- 24 pmol/l, 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 mu mol . kg( -1). min(-1). Muscle and liver IRK activity increased significantly fr om 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 res ponse (t(1/2)a) for stimulation of GDR (19.8 +/- 4.8 min) and suppress ion of HGP (21.5 +/- 3.7 min) were similar. The t(1/2)a for stimulatio n of GDR, muscle IRK, and rise in lymph insulin were not significantly different from one another and were all markedly greater than that fo r the approach to steady state of arterial insulin (2.3 +/- 1.2 min, P < 0.01). The t(1/2)a for portal. insulin (1.8 +/- 0.8 min) was less t han that for activation of Liver IRK (11.3 +/- 4.3, P < 0.05), which i n 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 occur s before IR binding and is due to the time required for plasma insulin to gain access to the interstitial compartment. In liver, however, id entification 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. Lf,however, insulin's effect on suppressing HGP is indirect, then a single extrahepatic site of de lay is likely, which represents the time-limiting step of insulin's ab ility to stimulate GDR and suppress HGP. Then, the locus of the site i nvolves transendothelial passage of insulin to the interstitial space.