CAFFEINE INHIBITS CYTOPLASMIC CA2-O-(3-THIOTRIPHOSPHATE) IN HYPERPOLARIZED PANCREATIC BETA-CELLS( OSCILLATIONS INDUCED BY CARBACHOL AND GUANOSINE 5')

The effects of caffeine on cytoplasmic Ca2+ oscillations induced by ca rbachol and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) were stu died in individual mouse pancreatic beta-cells clamped at a hyperpolar ized potential. Addition of 10 mM caffeine did not affect the cytoplas mic Ca2+ concentration ([Ca2+](i)) in beta-cells exposed to 20 mM gluc ose and hyperpolarized with diazoxide. Under similar conditions 100 mu M carbachol induced a typical response with a marked [Ca2+](i) peak f ollowed by a lower sustained elevation. Irrespective of whether 10 mM caffeine was present, there were [Ca2+](i) transients with frequencies of 1-5/min superimposed on the sustained phase in 50-60% of the cells . In previously non-exposed cells the introduction of 10 mM caffeine c aused temporary lowering of the sustained phase with disappearance of the transients. Subsequent omission of caffeine in the continued prese nce of carbachol caused a marked [Ca2+](i) peak followed by reappearan ce of the [Ca2+](i) transients. However, in cells oscillating in the p resence of caffeine its omission caused disappearance of the transient s. In this case reintroduction of caffeine restored the transients. In cells kept at -70 mV by a patch pipette containing 100 mu M GTP-gamma -S and 3 mM Mg-ATP there were [Ca2+](i) transients with frequencies of 0.5-2.5/min. These transients were sufficiently pronounced to activat e repetitively a K+ current. Addition of 10 mM caffeine caused disappe arance of the [Ca2+](i) transients or reduction of their amplitudes an d frequencies. The results indicate that caffeine does not activate Ca 2+-induced Ca2+ release in hyperpolarized beta-cells but inhibits the Ca2+-mobilizing effect of inositol 1,4,5-trisphosphate.