Direct assessment of splanchnic uptake and metabolic effects of human and porcine insulin.

The hepatic vein catheterization technique was used to quantitate the splanchnic uptake and the metabolic effects of biosynthetic human insulin (BHI) and porcine insulin (PI) in normal man. BHI and PI were infused into a peripheral vein (0.9-1.3 mU kg-1 min-1) for 60 min together with SRIH (0.6 mg/h) to inhibit endogenous insulin secretion and glucose to induce moderate hyperglycemia (9-10 mmol/liter). During the infusion period, arterial-hepatic venous difference of plasma C-peptide as well as splanchnic C-peptide output fell by more than 98% indicating virtually complete cessation of endogenous insulin release. Under these conditions, the arterial-hepatic venous differences in plasma insulin concentrations represent a valid and direct measurement of splanchnic insulin uptake. During BHI infusion, arterial insulin levels rose to 82 +/- 11 (SE) microU/ml (range: 33-105 microU/ml). Splanchnic insulin uptake paralleled the rise of arterial insulin, reaching 430 +/- 72 microU kg-1 min-1 at 60 min. No appreciable difference between BHI and PI was demonstrable. A highly significant correlation between arterial insulin concentrations and splanchnic insulin uptake was found (r = 0.816; P less than 0.001). Accordingly, both fractional splanchnic insulin extraction and splanchnic insulin clearance remained unchanged throughout insulin infusion and averaged 70 +/- 4% and 5.3 +/- 2 ml kg-1 min-1, respectively. With BHI infusion, splanchnic glucose balance (-8.5 +/- 0.9 mumol kg-1 min-1, basal) became positive (7.3 +/- 1 mumol kg-1 min-1). In contrast, basal splanchnic lactate uptake was inhibited by BHI and there was lactate production (from 3.4 +/- 0.9 to -1.7 +/- 1.4 mumol kg-1 min-1). Similar changes in splanchnic glucose and lactate metabolism occurred during PI infusion. These studies indicate that: 1) A considerable amount of insulin (70 +/- 4%) is extracted by the splanchnic bed on a single passage, after exogenous administration of either human insulin or PI; 2) over a physiological range of insulin concentrations (33-105 microU/ml) a linear relationship exists between arterial insulin concentrations and splanchnic insulin removal; and 3) BHI and PI do not differ appreciably with respect to their uptake and metabolic effects at the splanchnic level.