-Identification and characterization of Ca2+-phospholipid-dependent protein kinase in rat islets and hamster p-cells

Changes in the intracellular concentrations of Ca2 + and cyclic AMP (for reviews see Hedeskov, 1980; Wollheim & Sharp, 1981), as well as alterations in phosphatidylinositol turnover (Freinkel et al., 1975; Clements & Rhoten, 1976; Clements et al., 1977), have been proposed to be key events in stimulus-secretion coupling in the pancreatic fl-cell. The principal mediator of the action of cyclic AMP is cyclic AMP-dependent protein kinase, which has been characterized in rat islets of Langerhans (Sugden et al., 1979). One important intracellular target for Ca2 + is calmodulin, and a Ca2 +-calmodulin-dependent protein kinase has been identified in rat islets (Gagliardino et al., 1980; Harrison & Ashcroft, 1982) and in hamster insulinoma cells (Schubart et al., 1980). Various workers have established the existence in many tissues (Takai et al., 1977; Kuo et al., 1980) of another protein kinase, which is sensitive to Ca2 + but is quite independent of calmodulin (Wrenn et al., 1980). This kinase has the unique property of being dependent on phospholipid for its activity, and its sensitivity to Ca2 + is in addition modulated by diacylglycerol. Since diacylglycerol is liberated as a consequence of the breakdown of phosphoinositides by phospholipase C (Michell, 1975), an important role for the kinase has been suggested in stimulus-response systems showing a phosphatidylinositol effect (Nishizuka & Takai, 1981); the latter include islets of Langerhans (Clements & Rhoten, 1976; Best & Malaisse, 1983). Although the presence of a Ca2 +-phospholipid-dependent protein kinase in rat islets has been shown (Tanigawa et al., 1982), the properties of the enzyme and the existence of endogenous protein substrates have not been reported. The aims of this present study were to confirm the presence of the enzyme in islets of Langerhans, to show that the