Calcium- and voltage-dependence of nematocyst discharge in Hydra vulgaris

In Hydra vulgaris, discharge of stenotele nematocysts was induced by contact with prey, electrical stimuli, or increase in the external potassium concentration. In each case 10^-4M calcium was required in the culture medium. The results indicated a voltage- and calcium-dependent mechanism different from mechano- or chemoreception allowing calcium influx from the external medium. A threshold for activation was suggested by the steep increase of the rate of electrically induced discharge in external fields of 3.5 kV/m. Although organic antagonists for vertebrate calcium channels were ineffective in blocking the calcium-induced nematocyst discharge, inorganic divalent and trivalent cations competitively 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 calcium antagonist, decreased nematocyst discharge, while strontium replacing calcium supported the discharge at a lowered rate. It is concluded that in the nematocyte a voltage-activated influx of calcium through apical ion channels initiates the discharge of the nematocyst in an exocytotic process.