Calcium dependence of settlement and nematocyst discharge in actinulae of the hydroid Tubularia mesembryanthemum

The influence of Ca2+ and Mg2+ ions on both atrichous isorhiza (AI) dischar ge and settlement of actinular larvae of the hydroid Tubularia mesembryanth emum was investigated. Mg2+-supplemented artificial seawater (ASW) complete ly inhibited both events at a concentration of 206 mM, whereas lowered Mg2 concentrations enhanced them. Ca2+ ions in the bathing solution highly reg ulated Al discharge and settlement, and Mg2+ ions may down-regulate these e vents. The effect of inorganic Ca2+-channel blockers, including Gd3+ and La 3+, was also examined. Larval settlement was inhibited by Co2+, Ni2+, Cd2+, La3+, and Gd3+, with half inhibitory concentrations (IC50) of 5800, 260, 5 3, 45, and 7 mu M, respectively; AI discharge was also inhibited by these i ons, with IC50 values of 6600, 500, 78, 41, and 5 mu M respectively. These results suggest possible involvement of stretch-activated Ca2+ channels in the signal transmission of both AI discharge and larval settlement.