GLUCOSE, POTASSIUM, AND CCK-8 INDUCE INCREASES IN MEMBRANE-ASSOCIATEDPKC ACTIVITY THAT CORRESPOND TO INCREASES IN [CA2+](I) IN ISLET CELLSFROM NEONATAL RATS

The effects of glucose, K+, and cholecystokinin octapeptide (CCK-8) on intracellular free Ca2+ concentration ([Ca2+](i)) and membrane-associ ated protein kinase C (PKC) activity were examined in cultured islet c ells from neonatal rats. Raising the glucose concentration from 2.8 to 22.2 mM or external K+ (from 5 to 45 mM), or adding CCK-8 (200 nM) al l triggered a [Ca2+](i) surge that peaked between 3 and 10 min afterwa rd, depending on the stimulus, and then declined, either to a suprabas al plateau (glucose and K+) or to basal levels (CCK-8). These same man ipulations triggered a burst of membrane-associated PKC activity that peaked between 5 and 10 min and then variously declined. Incubation in Ca2+-free medium abolished both the effects of glucose and K+ on [Ca2 +](i) and the stimulation of membrane-associated PKC activity. The K+- triggered stimulation of PKC activity was also inhibited by pretreatin g the cells with the general Ca2+ entry blocker lanthanum (1 mM). Howe ver, incubation in Ca2+-free medium did not affect the CCK-8-induced r elease Ca2+ from internal stores, although it abolished the burst of m embrane-associated PKC activity, which showed the importance of Ca2+ i nflux as opposed to internal release for PKC activation. Thus, glucose , the principal stimulator of insulin secretion, rapidly stimulates Ca 2+ influx into islet cells from neonatal rats, and it is probably this influx that stimulates membrane-associated PKC activity.