CALCIUM-DEPENDENCE AND VOLTAGE-DEPENDENCE OF NEMATOCYST DISCHARGE IN HYDRA-VULGARIS

In Hydra vulgaris, discharge of stenotele nematocysts was induced by c ontact with prey, electrical stimuli, or increase in the external pota ssium concentration. In each case 10(-4) M calcium was required in the culture medium. The results indicated a voltage- and calcium-dependen t mechanism different from mechano- or chemoreception allowing calcium influx from the external medium. A threshold for activation was sugge sted by the steep increase of the rate of electrically induced dischar ge in external fields of 3.5 kV/m. Although organic antagonists for ve rtebrate calcium channels were ineffective in blocking the calcium-ind uced nematocyst discharge, inorganic divalent and trivalent cations co mpetitively inhibited the process, with a sequence (Co2+ < Ni2+ < Cd2 < La3+ < Gd3+) similar to that seen for antagonism of calcium influx through voltage-dependent channels. Magnesium, an intracellular calciu m antagonist, decreased nematocyst discharge, while strontium replacin g calcium supported the discharge at a lowered rate. It is concluded t hat in the nematocyte a voltage-activated influx of calcium through ap ical inn channels initiates the discharge of the nematocyst in an exoc ytotic process.