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 t o seawater sensitizes cnidocytes to discharge nematocysts. Extracellul ar calcium ions are required since discharge is inhibited by reducing the Ca2+ concentration in artificial seawater Known inhibitors of L-ty pe Ca2+ channels, nifedipine and verapamil, reduce discharge sensitize d by N-acetylated sugars but not by proline. Conversely, known inhibit ors of certain Ca2+ channels at intracellular storage sites, ryanodine and procaine, reduce discharge sensitized by proline but not by N-ace tylated sugars. Thapsigargin, an agent that inhibits uptake of Ca2+ in to vesicles, sensitizes discharge. Discharge is sensitized upon incuba ting specimens in a caged analog of inositol 1,4,5-trisphosphate (InsP (3)) and subsequently photoactivating it. Furthermore, following prein cubation of specimens in certain low concentrations of caged InsP(3) a nd subsequent photoactivation, lower concentrations of proline are req uired to maximally sensitize discharge. W7, an inhibitor of Ca2+/calmo dulin (CaM), and KT5926, an inhibitor of CaM-kinase II, reduce dischar ge sensitized by both N-acetylated sugars and proline. Apparently, sug ar receptors activate dihydropyridine-sensitive Ca2+ channels, whereas proline receptors stimulate the production of InsP(3), resulting in I nsP(3)-initiated release of Ca2+ from intracellular stores. This proce ss may trigger Ca2+-induced Ca2+ release from InsP(3)-insensitive chan nels, which can be blocked by ryanodine or procaine. With either recep tor, elevated intracellular Ca2+ binds calmodulin to form an active co mplex. CaM activates CaM-kinase II, which, presumably, phosphorylates unidentified substrates, leading to sensitization of discharge. (C) 19 95 Wiley-Liss, Inc.