Characterization of glucagon and catecholamine effects on isolated sheep hepatocytes

Abstract

The purpose of this study was to characterize the glycogenolytic response to catecholamines and glucagon in isolated sheep hepatocytes. In this species, epinephrine appeared to exert its action on hepatic glycogenolysis by altering the cytosolic concentrations of both adenosine 3',5'-cyclic monophosphate (cAMP) and Ca2+. In contrast to results obtained in rat hepatocytes, glucagon failed to induce a rise in free cytosolic Ca2+ in sheep liver. Experiments on isolated hepatocytes or on liver plasma membranes showed that in sheep, glucagon was more efficient than epinephrine in promoting the production of cAMP. In the presence of glucagon or epinephrine, the activation of the glycogen phosphorylase a always appeared greater in sheep than in rat liver cells, whereas the variations in cellular cAMP were quite limited in sheep. The alpha 1- and beta-agonists (phenylephrine and isoproterenol) were alone as efficient as epinephrine in promoting phosphorylase a activation in sheep hepatocytes. All these results indicate the existence in sheep liver of a glycogen phosphorylase highly responsive to hormones.

Hormonal Control of Glucose Metabolism

“…Recent studies have demonstrated that the degree of stimulation of glycogen Phosphorylase activity in sheep hepatocytes by glucagon or epi nephrine is greater than that in rat liver (Morand et al 1988). The response to glucagon was mediated purely by the ß-adrenergic pathway, whereas both «i-and /3-receptors mediated the effect of epinephrine.…”

Chronic and transitional regulation of gluconeogenesis and glyconeogenesis by insulin and glucagon in neonatal calf hepatocytes.

“…The proportion of glucose diverted into glycogen following preincubation with glucagon increased from 7 to 21% and from 11 to 29% for propionate and lactate, respectively (Table 1). These effects are surprising in light of the acute inhibitory effects of glucagon on net de novo glycogen synthesis and stimulatory effects on glycogenolysis in hepatocytes from ruminants (Morand et al, 1988;Donkin and Armentano, 1995) and other animals (Larner, 1990). The effects of long-term exposure to glucagon on net de novo glycogenesis in the present experiments may represent a dimunition of cell glycogen that is readily replenished when glucagon is decreased or removed (Figure 1).…”

Metabolic responses of lactating dairy cows to exogenous glucagon

“…1984;Robson et al, 1984). Atwal and Sauer, 1973;Cardorniga-Valino et al, 1997;Zhu et al, 1997 Bovine +++ Morand et al, 1988Morand et al, , 1990 Bovine - Bovine 0 Cardorniga-Valino et al, 1997Rat + Bjornsson et al, 1992Zupke et al, 1997 Bovine - Atwal and Sauer, 1973 Bovine - Atwal andSauer, 1973 Bovine Cardorniga-Valino et al, 1997 Bovine +++ Zhu et al, 1997 Bovine ++ Strang et al, 1998a;Zhu et al 1997Zhu et al , 2000 Rats - Beynen et al, 1981;Bjornsson et al, 1994;Duerden and Gibbons, 1993 Rat - Foufelle et al, 1994 Bovine + Metz and van den Bergh, 1977…”