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 alpha-cell. To examine whether insulin's extrahepatic site is independent of the alpha-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 alpha-cell, that in turn suppresses hepatic glucose production. To investigate the nature of this signal, we measured alanine, lactate, and free fatty acids (FFAs).(ABSTRACT TRUNCATED AT 250 WORDS)