Involvement of Calcium-Independent Phospholipase A_2 in Regulation of Ca~(2+)Signals Induced by a Calmodulin Inhibitor in Rat Thymocytes

摘要

Previous studies have shown that micromolar concentrations of calmodulin inhibitor calmidazo-lium induce fast activation of nonselective Ca~(2+)channels in plasma membranes of Ehrlich ascites carcinomacells (Zinchenko, V.P., Kasymov, V.A., Li, V.V., and Kaimachnikov, N.P., Biofizika (Rus.), 2005, vol. 50 (6),pp. 1055-1069). In order to detect this type of Ca~(2+)channels in other cells and to establish common regulatorymechanisms, we studied calmidazolium effects on rat thymocytes. It was found that calmidazolium inducesbiphasic increases in Ca~(2+)content in cytosol of rat thymocytes due to Ca~(entry from external medium andreflects the activity of nonselective Ca~(2+)channels permeable for Mn2+and Ni~(ions. The rate the amplitudeof the fast phase are decreased, while those of the slow phase are increased in the presence of specific inhibitorsof Ca2+-independent phospholipase A2 (bromoenol lactone and palmitoyl trifluoromethyl ketone). The rate andthe amplitude of the fast phase are also inhibited by arachidonic acid and the lipoxygenase inhibitor nordihy-droguaiaretic acid, while the Ca2+-dependent phospholipase A2 inhibitor bromophenacyl bromide, the cycloox-ygenase inhibitor indomethacin, the specific store-operated Ca~(2+)channel inhibitor gadolinium and the phos-pholipase C inhibitor U73122 have no such effect. The rate of the fast phase only slightly depends on temper-ature, while that of the slow phase shows a strong temperature dependence and increases with a rise intemperatures (Q10 = 2). The amplitude of the fast phase of the Ca2+ signal increases with a decrease of temper-atures due to prolongation of the maximum activity of the Ca~(2+)channel. The data obtained suggest that iPLA_2is an intermediate link in the activation of calmidazolium-induced nonselective Ca~(2+)channels. The iPLA_2prod-ucts lysophospholipids and arachidonic acid activate and inhibit Ca~(2+)channels, respectively. The fact that thesecompounds manifest different affinities for Ca~(2+)channels shed additional light on the mechanisms of biphasicCa~(2+)elevation in thymus cell cytosol and prolongation of the active state of Ca~(2+) channels at low temperatures.