SECRETAGOGUE-INDUCED [CA2-RAT PANCREATIC-ISLETS AND CORRELATION WITH SIMULTANEOUSLY MEASURED INSULIN RELEASE(](I) CHANGES IN SINGLE)

The effects of secretagogues (glucose, tolbutamide and phorbol esters) on simultaneously measured intracellular free calcium concentration ( [Ca2+](i)) and insulin release were studied in rat pancreatic islets o f Langerhans. Stimulatory concentrations (11 mM) of glucose caused a t ransient [Ca2+]i increase followed by an almost flat second phase. Inc reasing glucose concentrations to 167 mM in steps caused a further inc rease in [Ca2+](i). In contrast with mouse islets, rat islets scarcely showed glucose-induced [Ca2+](i) oscillations. Digital image analysis showed that [Ca2+](i) changes occurred synchronously across the whole islet. As expected, simultaneously measured insulin release was bipha sic with a clear second phase. This clearly indicated that in rat isle ts there is a lack of correlation between [Ca2+](i) and insulin releas e. This was further explored using agents which separately promoted th e first (tolbutamide, 200 mu M) and second (phorbol-12-myristate-13-ac etate; PMA; 5 nM) phases of insulin release. Tolbutamide induced a tra nsient increase in [Ca2+](i) paralleled by a transient increase in ins ulin release, whereas PMA induced a slow increase in insulin release w ithout a clear change in [Ca2+](i). These results suggest that in rat islets the first phase of insulin release is calcium dependent, wherea s the second phase is related to the activation of protein kinase C (P KC). However, the glucose-induced second phase of insulin release did not coincide with an increase in membrane-associated PKC activity. Oth er messengers may contribute to this late phase of insulin release.