Glucose (20 mM) and carbachol (1 mM) produced a rapid increase in [3H]inositol trisphosphate (InsP3) formation in isolated rat islets of Langerhans prelabelled with myo-[3H]inositol. The magnitude of the increase in InsP3 formation was similar when either agent was used alone and was additive when they were used together. In islets prelabelled with 45Ca2+ and treated with carbachol (1 mM), the rise in InsP3 correlated with a rapid, transient, release of 45Ca2+ from the cells, consistent with mobilization of 45Ca2+ from an intracellular pool. Under these conditions, however, insulin secretion was not increased. In contrast, islets prelabelled with 45Ca2+ and exposed to 20mM-glucose exhibited a delayed and decreased 45Ca2+ efflux, but released 7-8-fold more insulin than did those exposed to carbachol. Depletion of extracellular Ca2+ failed to modify the increase in InsP3 elicited by either glucose or carbachol, whereas it selectively inhibited the efflux of 45Ca2+ induced by glucose in preloaded islets. Under these conditions, however, glucose was still able to induce a small stimulation of the first phase of insulin secretion. These results demonstrate that polyphosphoinositide metabolism, Ca2+ mobilization and insulin release can all be dissociated in islet cells, and suggest that glucose and carbachol regulate these parameters by different mechanisms.
Studies on the role of inositol trisphosphate in the regulation of insulin secretion from isolated rat islets of Langerhans