THE EFFECTS OF CALCIUM ON SEROTONIN-STIMULATED ADENYLATE-CYCLASE IN FRESH-WATER MUSSELS

When serotonin (5-hydroxytryptamine, (5-HT) is added to a pondwater bath (final concentration, 10-4 mM/l) containing freshwater mussels, Na influx is rapidly stimulated, as are behavioral responses. Serotonin also stimulates adenylate cyclase (AC) in gill homogenate pellets from 2 different freshwater bivalve families, suggesting that a serotonin-cAMP system is associated with control of Na transport and that this system may be common in many ion regulating mussel species. Ac activity was detected in several tissues of Ligumia subrostrata; the highest activity being observed in the foot. Endogenous Ca depressed the AC activity measured under basal, 5-HT- and dopamine-stimulated conditions; the reduced enzyme activity was most pronounced in the monoamine-stimulated preparations. AC activity of the crude homogenate pellet was significantly lower than a purified pellet formed by an additional 100 .times. g centrifugation prior to a 3000 .times. g centrifugation. Exogenous Ca (2.5 mM/l) inhibited monoamine-stimulated AC activity .apprx. 50%. Ca concretions exist in gill tissue and may influence the observed AC activity by increasing the protein measure or increasing the Ca concentration. Prostaglandin E2 had no effect on basal or 5-HT-stimulated AC activities in the purified pellet. Although phosphodiesterase and nonspecific phosphatase activities were high in the supernatant, their activities in the homogenate pellet were low and had little effect on the AC activity measurements.