Short-term insulin infused through the portal vein enhances liver gluconeogenesis and glycogenesis from [3-14C]pyruvate in the starved rat.

Abstract

I. Insulin infusion through the portal vein immediately after a pulse of (3- 14C]pyruvate in 24 hr starved rats enhanced the appearance of (14C]glucose at 2, 5 and IO min and glucose specific activity at I, 2 and 20 min in blood collected from the cava vein at the level of the suprahepatic veins. 2. Insulin infusion for 5 min decreased liver pyruvate concentration and enhanced both liver and plasma lactate/pyruvate ratio, and it decreased the plasma concentration of all amino acids. 3. When insulin was infused together with glucose, [14C]glucose levels and glucose specific activity decreased in blood but there was a marked increase in liver (14C]glycogen, glycogen specific activity and glycogen concentration, and an increase in liver lactate/pyruvate ratio. 4. The effect of insulin plus glucose infusion on plasma amino acids concentration was smaller than that found with insulin alone. 5. It is proposed that insulin effect enhancing liver gluconeogenesis is secondary to its effect either enhancing liver glycolysis which modifies the liver's cytoplasmic oxydoreduction state to its more reduced form, increasing liver amino acids consumption or both. In the presence of glucose, products of gluconeogenesis enhanced by insulin are diverted into glycogen synthesis rather than circulating glucose. 6. This together with results of the preceding paper (Soley et al., 1985), indicates that glucose enhances liver glycogen synthesis from C3 units in the starved rat, the process being further enhanced in the presence of insulin.