THE EFFECT OF SEROTONIN (5-HYDROXYTRYPTAMINE) ON GLYCOLYSIS IN THE PERFUSED VENTRICLE OF THE FRESH-WATER BIVALVE ANODONTA-CYGNEA - EVIDENCEFOR PHOSPHORYLATION DEPHOSPHORYLATION CONTROL OF PHOSPHOFRUCTOKINASE

The ventricles of the freshwater mollusc Anodonta cygnea were isolated and perfused with serotonin in order to examine its effect on glycoly sis. Serotonin induces an increase in the concentration of glycolytic intermediates. Phosphofructokinase (PFK) preparations from ventricles perfused with serotonin exhibited an increased sensitivity to activati on by the substrate fructose 6-phosphate (F6P) and to the activators A MP and fructose 2,6-bisphosphate (F2,6P2) and a reduced sensitivity to inhibition by ATP. In vitro phosphorylation/dephosphorylation experim ents revealed that incubation of PFK preparations from ventricles perf used with normal saline in the presence of ATP, cyclic AMP and MgCl2 a lters the degree of activation by F6P. In contrast, when enzyme prepar ations from ventricles treated with serotonin were incubated in the pr esence of alkaline phosphatase and MgCl2, PFK showed a reduced sensiti vity to activation by F6P. Serotonin had no significant effect on the kinetic properties of PK, while it increased the proportion of the act ive a form of glycogen phosphorylase. These results indicate that sero tonin induces an increase in the concentration of glycolytic intermedi ates in the ventricle of A. cygnea and that this is at least partly du e to the activation of PFK, possibly via phosphorylation by an endogen ous cyclic-AMP-dependent protein kinase.