Evidence for intracellular stores of calcium ions involved in regulating nematocyst discharge

In sea anemones, nematocyst discharge is regulated in part by chemical substances derived from prey. Adding N‐acetylated sugars or proline to seawater sensitizes cnidocytes to discharge nematocysts. Extracellular calcium ions are required since discharge is inhibited by reducing the Ca2+ concentration in artificial seawater. Known inhibitors of L‐type Ca2+ channels, nifedipine and verapamil, reduce discharge sensitized by N‐acetylated sugars but not by proline. Conversely, known inhibitors of certain Ca2+ channels at intracellular storage sites, ryanodine and procaine, reduce discharge sensitized by proline but not by N‐acetylated sugars. Thapsigargin, an agent that inhibits uptake of Ca2+ into vesicles, sensitizes discharge. Discharge is sensitized upon incubating specimens in a caged analog of inositol 1,4,5‐trisphosphate (InsP3) and subsequently photoactivating it. Furthermore, following preincubation of specimens in certain low concentrations of caged InsP3 and subsequent photoactivation, lower concentrations of proline are required to maximally sensitize discharge. W7, an inhibitor of Ca2+/calmodulin (CaM), and KT5926, an inhibitor of CaM‐kinase II, reduce discharge sensitized by both N‐acetylated sugars and proline. Apparently, sugar receptors activate dihydropyridine‐sensitive Ca2+ channels, whereas proline receptors stimulate the production of InsP3, resulting in InsP3‐initiated release of Ca2+ from intracellular stores. This process may trigger Ca2+‐induced Ca2+ release from InsP3‐insensitive channels, which can be blocked by ryanodine or procaine. With either receptor, elevated intracellular Ca2+ binds calmodulin to form an active complex. CaM activates CaM‐kinase II, which, presumably, phosphorylates unidentified substrates, leading to sensitization of discharge.