Caffeine inhibits cytoplasmic Ca²⁺ oscillations induced by carbachol and guanosine 5′-O-(3-thiotriphosphate) in hyperpolarized pancreatic ß-cells

Abstract

The effects of caffeine on cytoplasmic Ca²⁺ oscillations induced by carbachol and guanosine 5′-O-(3-thiotriphosphate) (GTP-γ-S) were studied in individual mouse pancreatic ß-cells clamped at a hyperpolarized potential. Addition of 10 mM caffeine did not affect the cytoplasmic Ca²⁺ concentration ([Ca²⁺]₁) in ß-cells exposed to 20 mM glucose and hyperpolarized with diazoxide. Under similar conditions 100 μM carbachol induced a typical response with a marked [Ca²⁺]_i peak followed 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 caused temporary lowering of the sustained phase with disappearance of the transients. Subsequent omission of caffeine in the continued presence of carbachol caused a marked [Ca²⁺]_i peak followed by reappearance of the [Ca²⁺]_i, transients. However, in cells oscillating in the presence of caffeine its omission caused disappearance of the transients. In this case reintroduction of caffeine restored the transients.

In cells kept at −70 mV by a patch pipette containing 100 μM GTP-γ-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 activate repetitively a K⁺ current. Addition of 10 mM caffeine caused disappearance of the [Ca²⁺]_i transients or reduction of their amplitudes and frequencies.

The results indicate that caffeine does not activate Ca²⁺-induced Ca²⁺ release in hyperpolarized ß-cells but inhibits the Ca²⁺-mobilizing effect of inositol 1,4,5-trisphosphate.