Regulation of splanchnic and renal substrate supply by insulin in humans

Abstract

To determine the effects of peripheral insulin infusion on total, hepatic, and renal glucose production and on the percent contribution to glucose production of gluconeogenesis versus glycogenolysis, 10 healthy subjects had arterialized hand and hepatic vein catheterization after an overnight fast and the results were compared with data from 12 age- and weight-matched subjects with renal vein catheterization during a 180-minute infusion of either insulin (0.25 mU/kg x min) with dextrose, or saline. Endogenous, hepatic, and renal glucose production was measured with [6,6(-2)H2]glucose, regional lactate, alanine, and glycerol balance by arteriovenous difference; hepatic blood flow by indocyanine green clearance; and renal blood flow by p-aminohippurate clearance, before and every 30 minutes during each infusion period. Insulin increased from about 42 to 98 pmol/L and blood glucose remained constant in all studies (3.8 +/- 0.2 v4.4 +/- 0.1 micromol/ml, hepatic vrenal vein). In response to insulin infusion, endogenous, hepatic, and renal glucose production decreased immediately (30 minutes) and reached a lower plateau value (10.8 +/- 0.8 v6.4 +/- 0.7, 10.4 +/- 1.1 v7.8 +/- 1.0, and 2.8 +/- 0.6 v 1.5 +/- 0.6 micromol/kg x min, respectively) between 120 and 180 minutes (all P < .05). Net renal uptake of lactate (2.4 +/- 0.4 v0.9 +/- 0.6) decreased earlier (30 minutes) and returned to baseline between 120 and 180 minutes (2.4 +/- 0.5 micromol/kg x min), whereas net splanchnic uptake of lactate (5.7 +/- 0.7 v 0.7 +/- 0.6) and alanine (1.8 +/- 0.1 v 1.0 +/- 0.5 micromol/kg x min) decreased later (120 to 180 minutes). Net renal (0.3 +/- 0.1 v 0.1 +/- 0.1) and splanchnic (0.7 +/- 0.3 v 0.4 +/- 0.2 micromol/kg x min) glycerol uptake decreased 90 to 180 minutes after insulin and increased (P < .05) with saline infusion (0.4 +/- 0.1 v0.6 +/- 0.3 and 1.0 +/- 0.5 v1.8 +/- 0.4 micromol/kg x min, respectively). These data indicate that the rapid suppression of endogenous glucose production by insulin reflects primarily a decrease in hepatic glucose release, most likely due to inhibition of net glycogenolysis, combined with suppression of renal gluconeogenesis. Inhibition of hepatic gluconeogenesis presumably occurs later during hyperinsulinemia. We conclude that peripheral insulin, in addition to its inhibition of glycogen degradation, regulates endogenous glucose production, in part, by modifying the splanchnic and renal substrate supply.