CONTROL OF PULSATILE 5-MT INSULIN SECRETION FROM SINGLE-MOUSE PANCREATIC-ISLETS BY INTRACELLULAR CALCIUM DYNAMICS/

1. Glucose-induced insulin release from single islets of Langerhans is pulsatile. We have investigated the correlation between changes in cy tosolic free calcium concentration ([Ca2+](i)) and oscillatory insulin secretion from single mouse islets, in particular examining the basis for differences in secretory responses to intermediate and high gluco se concentrations. Insulin release was monitored in real time through the amperometric detection of the surrogate insulin marker 5-hydroxytr yptamine (5-HT) via carbon fibre microelectrodes. The [Ca2+](i) was si multaneously recorded by whole-islet fura-2 microfluorometry. 2. In 82 % of the experiments, exposure to 11 mM glucose evoked regular high-fr equency (average, 3.4 min(-1)) synchronous oscillations in amperometri c current and [Ca2+](i). In the remaining experiments (18%), 11 mM glu cose induced an oscillatory pattern consisting of high-frequency [Ca2](i) oscillations that were superimposed on low-frequency (average, 0. 32 min(-1)) [Ca2+](i) waves. Intermittent high-frequency [Ca2+](i) osc illations gave rise to a similar pattern of pulsatile 5-HT release. 3. Raising the glucose concentration from 11 to 20 mM increased the dura tion of the steady-state [Ca2+](i) oscillations without increasing the ir amplitude. In contrast, both the duration and amplitude of the asso ciated 5-HT transients were increased by glucose stimulation. The amou nt of 5-HT released per secretion cycle was linearly related to the du ration of the underlying [Ca2+](i) oscillations in both 11 and 20 mM g lucose. The slopes of the straight lines were identical, indicating th at there is no significant difference between the ability of calcium o scillations to elicit 5-HT/insulin release in 11 and 20 mM glucose. 4. In situ 5-HT microamperometry has the potential to resolve the high-f requency oscillatory component of the second phase of glucose-induced insulin secretion. This component appears to reflect primarily the dur ation of the underlying [Ca2+](i) oscillations, suggesting that glucos e metabolism and/or access to glucose metabolites is not rate limiting to fast pulsatile insulin release.