STIMULATION OF CILIARY BEAT FREQUENCY BY SEROTONIN IS MEDIATED BY A CA2-TRIVOLVIS EMBRYOS( INFLUX IN CILIATED CELLS OF HELISOMA)

Serotonin (5-HT) has been established as a regulator of ciliary beatin g in numerous systems. In early embryos of Helisoma trivolvis, a cilia -driven rotational movement is modulated by the release of endogenous serotonin from a pair of embryonic neurons, ENC1s, directly onto the c iliated epithelium. The present study was undertaken to examine the si gnal transduction mechanisms underlying serotonin-mediated cilio-excit ation in Helisoma trivolvis embryos. Using time-lapse videomicroscopy, the ciliary beat frequency (CBF) of cultured embryonic ciliated cells was measured in response to various pharmacological manipulations. Se rotonin increased CBF in a dose-dependent manner. Addition of 8-bromo- cyclic AMP, isobutylmethylxanthine (IBMX) or a combination of forskoli n and IBMX, treatments that elevate the concentration of intracellular cyclic AMP, did not mimic the serotonin-induced increase in CBF. Thus , cyclic AMP does not appear to be involved in the regulation of CBF i n this system. In contrast, depolarizing the cells with KCl or veratri dine, and artificially raising the intracellular Ca2+ concentration wi th thapsigargin or A23187, caused a serotonin-like increase in CBF. Fu rthermore, the serotonin response was abolished in a Ca2+-depleted med ium or in a medium containing the L-type Ca2+ channel blockers verapam il or nifedipine. These results suggest that serotonin-stimulated cili o-excitation in cultured Helisoma trivolvis cells involves an influx o f Ca2+ to increase intracellular Ca2+ concentration. The link between serotonin-receptor binding and Ca2+ influx in these cells has yet to b e determined.