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 of Langerhans. Stimulatory concentrations (11 mm) of glucose caused a transient [Ca2+]i increase followed by an almost flat second phase. Increasing glucose concentrations to 16·7 mm in steps caused a further increase 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 biphasic with a clear second phase. This clearly indicated that in rat islets there is a lack of correlation between [Ca2+]i and insulin release. This was further explored using agents which separately promoted the first (tolbutamide, 200 μm) and second (phorbol-12-myristate-13-acetate; PMA; 5 nm) phases of insulin release. Tolbutamide induced a transient increase in [Ca2+]i paralleled by a transient increase in insulin release, whereas PMA induced a slow increase in insulin release without a clear change in [Ca2+]i. These results suggest that in rat islets the first phase of insulin release is calcium dependent, whereas the second phase is related to the activation of protein kinase C (PKC). However, the glucose-induced second phase of insulin release did not coincide with an increase in membrane-associated PKC activity. Other messengers may contribute to this late phase of insulin release.
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