Ca 2+ , K + Redistributions and α‐Adrenergic Activation of Glycogenolysis in Perfused Rat Livers

1 The α‐adrenergic activation of glycogenolysis was investigated in isolated rat livers perfused in a non‐recirculating system. Net uptake and/or release of Ca 2+ , K + and H + by the liver (measured by ion‐selective electrodes) were correlated with the glycogenolytic effects of phenylephrine. Uptake and retention of 45 Ca by the mitochondria of perfused livers were studied to obtain information on the role played by exchangeable mitochondrial calcium in α‐adrenergic activation of glycogenolysis. 2 Between 1 and 5 min after starting the addition of phenylephrine a net release of Ca 2+ was observed, this was paralleled by an uptake of K + . Production rates of glucose and lactate from endogenous glycogen started to increase at the same time. During the following minutes K + was released. 2mM EGTA and a high concentration of Mg 2+ strongly diminished the ionic and metabolic responses to phenylephrine, 0.2 mM EGTA was less effective. 3 High concentrations of K + prevented the metabolic response to phenylephrine but had no effect on the release of Ca 2+ into the extracellular medium. Tetracaine activated glycogenolysis and suppressed all the effects of the α‐adrenergic agonist. 4 Experiments with 45 Ca provided no evidence for an α‐adrenergic release of Ca 2+ from the exchangeable mitochondrial pool. Incorporation of 45 Ca into the mitochondria of perfused livers was enhanced by phenylephrine. 5 We propose that the a‐adrenergic release of Ca 2+ from a pool located close to the surface of the cell is capable of triggering the glycogenolytic response.