Ca2+ -dependent activation of oxoglutarate dehydrogenase by vasopressin in isolated hepatocytes

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Abstract
Vasopressin stimulated gluconeogenesis from proline in hepatocytes from starved rats; this was attributed to an activation of oxoglutarate dehydrogenase (EC 1.2.4.2). The role of Ca2+ in the activation mechanism was investigated. In the absence of extracellular Ca2+, vasopressin caused a stimulation of gluconeogenesis and a decrease in cell oxoglutarate content that were markedly transient with compared with the effects in the presence of Ca2+. Ca2+ added to cells stimulated for 2 min by vasopressin in the absence of extracellular Ca2+ sustained the initial effects of vasopressin. Ca2+ added 15 min after vasopressin, a time at which both the rate of gluconeogenesis and the cell oxoglutarate content were close to the control values, caused a stimulation of gluconeogenesis and a decrease in cell oxoglutarate content. Under conditions of cell-Ca2+ depletion, vasopressin had no effect on gluconeogenesis or cell oxoglutarate content. Ionophore A23187 [calcimycin] stimulated gluconeogenesis and caused a decreased in cell oxoglutarate content, but the phorbol ester 4.beta.-phorbol 12-myristate 13-acetate had no effects. These data suggest that the initial activation of oxoglutarate dehydrogenase by vasopressin is dependent on an intracellular Ca2+ pool and independent of extracellular Ca2+. For activation of a greater duration, a requirement for extracellular Ca2+ occurs. The activation of oxoglutarate dehydrogenase by A23187 is consistent with a mechanism involving Ca2+, but the lack of effect of 4.beta.-phorbol-12-myristate-13-acetate indicates that protein kinase C is not involved in the mechanism of activation by vasopressin.