The ventricles of the freshwater mollusc Anodonta cygnea were isolated and perfused with serotonin in order to examine its effect on glycolysis. Serotonin induces an increase in the concentration of glycolytic intermediates. Phosphofructokinase (PFK) preparations from ventricles perfused with serotonin exhibited an increased sensitivity to activation by the substrate fructose 6- phosphate (F6P) and to the activators AMP and fructose 2,6-bisphosphate (F2,6P2) and a reduced sensitivity to inhibition by ATP. In vitro phosphorylation/dephosphorylation experiments revealed that incubation of PFK preparations from ventricles perfused with normal saline in the presence of ATP, cyclic AMP and MgCl2 alters the degree of activation by F6P. In contrast, when enzyme preparations from ventricles treated with serotonin were incubated in the presence of alkaline phosphatase and MgCl2, PFK showed a reduced sensitivity to activation by F6P. Serotonin had no significant effect on the kinetic properties of PK, while it increased the proportion of the active a form of glycogen phosphorylase. These results indicate that serotonin induces an increase in the concentration of glycolytic intermediates in the ventricle of A. cygnea and that this is at least partly due to the activation of PFK, possibly via phosphorylation by an endogenous cyclic-AMP-dependent protein kinase.

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