Action of serotonin antagonists on cytoplasmic calcium levels in early embryos of sea urchin Lytechinus pictus

Possible interaction of the serotonergic system with intracellular calcium mechanisms was investigated using techniques of ratio imaging measurement o f intracellular Ca2+ and confocal microscopy in cleaving embryos of sea urc hin Lytechinus pictus. Some serotonin antagonists specifically increase fre e intracellular Ca2+ and evoke transient regression of the first cleavage f urrow, suggesting possible linkage of serotonergic and calcium mechanisms i n the regulation of cellular events during cleavage divisions. These effect s were more pronounced in the experiments with hydrophilic 5-HT-antagonists , quarternary ammonium salts that do not penetrate the cell membrane. Thus, it appears that 5-HT-receptors which mediate these effects are localised o n the cell membrane, whereas previously studied receptors mediating the cyt ostatic action of lipophilic 5-HT-antagonists are localised intracellularly .