Gluconeogenesis in the Guinea Pig. Effect of Glucagon on Gluconeogenesis from Lactate by Isolated Perfused Guinea-Pig Liver

Abstract

Gluconeogenesis was stimulated by glucagon in fed but not fasted isolated perfused guinea pig livers. Both the amount and the rate of incorporation of radioactivity into glucose from ~-[U-'~C]lactate were inci-eased in fed livers by the addition of glucagon to the perfusate. The glucagon-stimulated increase in gluconeogenesis was accompanied by an increase in oxygen consumption, an increase in the amount of lactate carbon converted to glucose and a decrease in the amount of lactate carbon converted to COz. The results are interpreted to indicate that glucagon affects gluconeogenesis from lactate in fed livers by redirecting the fate of substrate from other products toward glucose.Much of our knowledge concerning the hormonal control of hepatic gluconeogenesis, especially by glucagon, has been derived from studies with rats, perfused rat livers or isolated hepatocytes prepared from rat livers. The general applicability of such knowledge, however, remains uncertain. For example, Soling et al. [l] have demonstrated that, unlike its positive effect on gluconeogenesis in the rat, glucagon does not stimulate gluconeogenesis in fasted isolated perfused guinea pig liver. In addition, the rat and a few other mammalian species have liver phosphoenolpyruvate carboxykinase activity located almost entirely in the cytosol. Most mammalian species including man and guinea pig, however, have an equal distribution of distinct mitochondria1 and cytosolic phosphoPnolpyruvate carboxykinase activity (cf.[2]). Consequently, the importance of studying the control of hepatic gluconeogenesis in an animal model such as the guinea pig has often been emphasized [I, 3 -51. These and other considerations led to the present investigations on the effects of glucagon on hepatic gluconeogenesis in the guinea pig. MATERIALS AND METHODSMale English short-hair guinea pigs (CAMM Research Institute, Wayne, N.J.) weighing 229 to 349 g Enzyme. Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1. 1.32).were allowed to feed ad lib. or fasted 48 h prior to perfusion. The isolated livers were perfused at 37 "C with a Krebs-Ringer bicarbonate medium containing 3 % fatty-acid-free bovine serum albumin (Miles Laboratories, Elkhart, Ind.) and 20 % washed bovine erythrocytes in a recirculating system as previously described [6]. In this system the livers were perfused for 60 min prior to the addition of substrate. The substrate was 10 mM L-lactate (Schwartz-Mann, Orangeburg, N.J.) containing ~-[U-'~C]lactate (Amersham-Searle, Arlington Heights, Ill.) to give a specific activitity of 1600 dis. min-' pmol-'. Glucagon (a generous gift from Dr William Bromer, Eli Lilly and Co., Indianapolis, Ind.) was prepared in 3 % bovine serum albumin and kept frozen until used. When used, 5 pg of glucacon was added to the perfusate at 30 min and 60 min after the start of the perfusion [6]. Perfusate samples (2 ml) were taken at the indicated times, deproteinized, and assayed for glucose and other metabolities as previously described [6,7]. All of the radioactivity in the neutral fr...

Effect of alloxan-diabetes on gluconeogenesis and ureogenesis in isolated rabbit liver cells

“…On the contrary, in the sheep liver (Filsell et al, 1969;Taylor et al, 1971) both mitochondrial and cytosolic activities are increased under conditions of experimental diabetes. Thirdly, glucagon enhances lactate-induced gluconeogenesis in rabbit liver (Huibregtse et al, 1977), but not in starved guinea-pig liver (Sbling et al, 1970;Cook, 1977). These observations suggest that subtle differences in glucose formation also exist in species with an equal distribution of intracellular phosphoenolpyruvate carboxykinase activity.…”