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Gluconeogenesis in Isolated Lamb Hepatocytes

University of California^2,3, Davis 95616

Abstract

Isolated sheep hepatocytes were used to obtain estimates of kinetic parameters, identify substrate preference and interactions and study regulation of gluconeogenesis. Respective V_max estimates for propionate, pyruvate and alanine conversion to glucose were 59.5, 12.8 and 21.5 mol glucose formed·(h·g dry weight)^–1. Respective K_s estimates for propionate and pyruvate were 1 mM and 18 to 40 µM. Rates of lactate utilization varied among cell preparations, possibly because of loss of lactate dehydrogenase during isolation. Dihydroxyacetone and glycerol were utilized for glucose synthesis at similar rates of 8.6 and 8.7 µmol glucose formed·(h·g dry weight)^–1, respectively. Respective rates of glucose synthesis from 5 mM fructose and 10 mM galactose were 63.2 and 31.4 µmol·(h·g dry weight)^–1. Maximum rates of pyruvate carboxylase and phosphoenolpyruvate carboxykinase were estimated to be 101.6 and 160.4 µmol substrate converted·(h·g dry weight)^–1, respectively. Neither butyrate nor acetate accelerated gluconeogenesis from propionate while acetate increased glucose synthesis from pyruvate, presumably through activation of pyruvate carboxylase. Glucagon stimulated gluconeogenesis from propionate. Dibutyrylcyclic AMP mimicked the effect of glucagon, implying that the glucagon effect is translated via the adenyl cyclase system as in rats. The kinetic parameters established in these experiments should be useful in future experiments and in computer modeling analyses of ruminant liver and whole animal metabolism where Michaelis-Menten type equations are widely used.

¹ Send all correspondence to R. L. Baldwin.

² Dept. of Anim. Sci.

³ Submitted in partial satisfaction for requirements of the Ph.D. degree by M. C. Looney.