Possible role of Pi supply in mitochondrial actions of glucagon

Glucagon is able to diminish the net release of inorganic phosphate (Pi) occurring on incubation of isolated hepatocytes from 48-h-starved rats. Concomitantly the hormone increases the cellular Pi content. This is associated with a rise of Pi in the cytosolic fraction. Other hormonal effectors like phenylephrine, vasopressin and angiotensin II exert a smaller and transient effect as compared to glucagon. It is proposed that this increase in Pi availability to the mitochondria, by favouring substrate level phosphorylation at the succinyl-CoA synthetase step plays a role in the development of the metabolite pattern found in the mitochondrial matrix space after exposure of hepatocytes to glucagon or the above agents. With regard to the glutamate level this view is evidenced by the finding that its hormone-dependent decrease was inversely correlated to the respective increase in the cytosolic Pi concentration. Further evidence is provided by experiments with isolated mitochondria incubated under state-3 conditions at medium Pi concentrations corresponding to those metabolically active in the cytosolic compartment of control and glucagon-stimulated hepatocytes, being 2 mM and 3 mM, respectively. Increasing medium phosphate concentration from 2 mM to 3 mM caused a marked decrease in the level of succinyl-CoA and increased the rates of 2-oxoglutarate utilization and of malate and phosphoenolpyruvate production. Citrulline synthesis also was found to be stimulated at 3 mM Pi. Taken together our results suggest a role of Pi supply in mitochondrial actions of glucagon in intact hepatocytes. Moreover, they could contribute to a better interpretation of glucagon effects on isolated mitochondria from hormone-pretreated liver cells.