EVIDENCE FOR THE SHORT-TERM REGULATION OF GLYCOLYTIC FLUX IN THE ISOLATED-PERFUSED VENTRICLE OF THE LAND SNAIL HELIX-LUCORUM (L.) AFTER TREATMENT WITH SEROTONIN (5-HYDROXYTRYPTAMINE)

The glycolytic flux and the regulation of phosphofructokinase (PFK) ac tivity by fructose 2,6-bisphosphate and covalent modification was inve stigated in isolated ventricles of land snail Helix lucorum perfused w ith or without serotonin. Serotonin evoked a significant increase in t he level of glycolytic intermediates and a threefold increase of glyco lytic flux. Studies of saturation curves of PFK for the substrate fruc tose 6-phosphate at pH similar to intracellular pH of heart muscle sho wed that serotonin increases enzyme sensitivity to activation by fruct ose 6-phosphate. Moreover, PFK preparations from ventricles perfused w ith serotonin exhibited lower K-a values for the activators AMP and fr uctose 2,6-bisphosphate, compared with the enzyme preparations from se rotonin-untreated ventricles. The results suggest that PFK was convert ed to a more active form when exposed to serotonin. In vitro experimen ts of PFK phosphorylation showed that the conversion of the enzyme to a more active form was possibly due to its phosphorylation by an endog enous cyclic-AMP-dependent protein kinase. The concentration of fructo se 2,6-bisphosphate increased in serotonin-treated ventricles and it e xerted a synergistic effect with AMP on the activation of PFK. The bou nd fraction of glycolytic enzymes increased in the serotonin-treated v entricles only after the 4th min of perfusion. The results suggest tha t the stimulation of glycolytic flux in the ventricles of H. lucorum i n the first minutes of perfusion with serotonin was partly due to the activation of PFK via enzyme molecule covalent modification and to inc rease of fructose 2,6-bisphosphate.