Free Fatty Acid as a Link in the Regulation of Hepatic Glucose Output by Peripheral Insulin
Overproduction of glucose by the liver in the face of insulin resistance is a primary cause of hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM). However, mechanisms involved in control of hepatic glucose output (HGO) remain less than clear, even in normal individuals. Recent results have supported an indirect extrahepatic effect of insulin as the primary locus of insulin action to restrain HGO. One suggested extrahepatic site is the pancreatic ɑ-cell. To examine whether insulin9s extrahepatic site is independent of the ɑ-cells, HGO suppression was examined independent of changes in glucagon secretion or insulin antagonism of glucagon action. Euglycemic glucose clamps ( n = 40) with somatostatin infusion were performed in conscious dogs ( n = 5). Paired experiments were conducted in which insulin was infused either portally (1.2, 3.0, 6.0 pmol · min −1 · kg −1 ) or peripherally at half the portal infusion rate (0.6, 1.5, 3.0 pmol · min −1 · kg −1 ). Additional zero and saturating portal-dose experiments (100 pmol · min −1 · kg −1 ) were also performed. For the paired experiments, portal insulin infusion resulted in portal insulin concentrations approximately two to three times higher than in the corresponding peripheral insulin infusion experiments, while at the same time peripheral insulin concentrations were approximately matched. Equal peripheral insulin concentration resulted in equivalent HGO suppression irrespective of the portal concentrations. Thus, insulin affects a signal at a peripheral site, other than ɑ-cell, that in turn suppresses hepatic glucose production. To investigate the nature of this signal, we measured alanine, lactate, and free fatty acids (FFAs). There was no clear relationship between alanine or lactate and HGO suppression; however, there was an extremely strong relationship between plasma FFAs and HGO both at steady state and during dynamic changes in insulin. These data suggest, but do not prove, that insulin acts to suppress HGO as follows: Insulin slowly traverses the capillary endothelium in adipose tissue; elevated insulin in adipose tissue interstdtium inhibits lipolysis, thus decreasing FFA levels; and decreased FFAs act as a signal to the liver to suppress endogenous glucose production.