Hemocytes of Pomacea canaliculata: I. Reversible aggregation induced by Ca2+.

Using a platelet aggregometer, factors involved in hemocyte aggregation of a prosobranch snail Pomacea canaliculata were investigated. Aggregation-inducing tests revealed that extracellular Ca2+ is required to provoke the cellular response which is reversible. The aggregation-dispersion response can be induced by adding Ca2+ and EDTA alternately. Aggregation was inhibited by a calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) in a dose-dependent manner, and by a protein kinase-C inhibitor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine dihydrochloride (H-7). Thus, calmodulin and protein kinase-C may contribute to the calcium-mediated aggregation of hemocytes, which resembles a phenomenon that occurs in vertebrate leucocytes. Unlike the aggregation of mammalian leucocytes, no stimulus other than Ca2+ is required to induce the response of Pomacea hemocytes. Thus, this system may serve as a simple model for analysing cellular aggregation responses.

Possible roles of cAMP and Ca2+ in the regulation of miracidial transformation in Schistosoma mansoni.

The triggering action of physiological saline in the miracidial transformation of Schistosoma mansoni was analyzed using various agents affecting cAMP- and Ca2+-dependent pathways. Potent activators of adenylate cyclase, such as forskolin and serotonin, strongly inhibited the transformation provoked by saline in RPMI-1640. These inhibitory actions were diminished by the combined administration of phosphodiesterase activators such as ammonium salts or imidazole. Furthermore, the exposure of miracidia to ammonium salts or imidazole in dechlorinated tap water "mimicked" the transformation, i.e., the cessation, of swimming and then shedding of epithelial plates. This mimic transformation was also inhibited by serotonin or forskolin. In contrast, treatment of miracidia with Ca2+ antagonists such as TMB-8 (an inhibitor of Ca2+ release), nicardipine (a Ca2+ channel blocker), or W-7 (a calmodulin inhibitor) in tap water produced severe vesiculation on their body surfaces and resulted in death. However, these toxic effects were abolished by a combined administration of these Ca2+ antagonists with saline or NH4Cl, and the transformation was reestablished except with W-7 treatment. W-7 strongly inhibited the triggering action of saline and NH4Cl and the worms swam slowly, whereas W-5, an inactive analogue of W-7, had no inhibitory effect on the transformation. These results suggest that the initiation of micracidial transformation to young sporocysts may be synergistically regulated by cAMP and Ca2+ and that a decrease in cAMP levels and an increase in Ca2+ mobilization may be provoked in worms transformed by saline, ammonium salts, or imidazole.