Caffeine inhibits inositol-trisphosphate-induced membrane potential oscillations inXenopusoocytes

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Abstract
Immature Xenopus oocytes injected with inositol 1,4,5-trisphosphate (Ins(1,4,5) P$_{3}$) give a complex electrophysiological response comprising an early depolarizing spike followed by a burst of oscillations. These two components have been interpreted on the basis of an interaction between two internal calcium stores: an Ins(1,4,5) P$_{3}$-sensitive pool responsible for the early spike which then primes an Ins(1,4,5) P$_{3}$-insensitive pool to begin to oscillate through a process of calcium-induced calcium release (Berridge, M. J., J. Physiol., Lond. 403, 589-599 (1988)). The role of the latter was investigated in Xenopus oocytes by using the drug caffeine which can trigger calcium-induced calcium release in muscle cells. Caffeine had no effect on the early Ins(1,4,5) P$_{3}$-induced spike but it suppressed the subsequent oscillations. The spontaneous oscillations observed in some oocytes were also abolished by caffeine. Oscillation amplitude and duration was slightly reduced following incubation of oocytes with adenosine or isobutyl-methylxanthine. Because these two agents gave large membrane hyperpolarizations indicative of an increase in cyclic AMP, it can be concluded that this second messenger is not responsible for the inhibitory action of caffeine. The ability of caffeine to abolish oscillations while not affecting the early Ins(1,4,5) P$_{3}$ response is discussed with regard to the two-pool model for generating calcium oscillations.