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Caffeine inhibits inositol-trisphosphate-induced membrane potential oscillations in Xenopus oocytes
1University of Cambridge, Department of Zoology, U.K.
Abstract:
Immature Xenopus oocytes injected with inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) give a complex electrophysiological response comprising an a 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) P3-sensitive pool responsible for the early spike which then primes an Ins(1,4,5) P3-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)P3-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 isobutylmethylxanthine. 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) P3 response is discussed with regard to the two-pool model for generating calcium oscillations.
Insights
Caffeine inhibits calcium oscillations in Xenopus oocytes by affecting an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-insensitive store, without impacting the initial Ins(1,4,5)P3-induced spike. This supports a two-pool model for calcium signaling.
Area of Science:
- Cellular Biology
- Biochemistry
- Physiology
Background:
- Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) triggers complex electrophysiological responses in Xenopus oocytes, including an early spike and subsequent calcium oscillations.
- These responses are hypothesized to involve two internal calcium stores: an Ins(1,4,5)P3-sensitive pool and an Ins(1,4,5)P3-insensitive pool.
- Calcium-induced calcium release (CICR) is proposed to mediate oscillations from the second pool.
Purpose of the Study:
- To investigate the role of the Ins(1,4,5)P3-insensitive calcium store in Xenopus oocyte oscillations.
- To determine the effect of caffeine, a known CICR activator, on Ins(1,4,5)P3-induced responses.
Main Methods:
- Xenopus oocytes were injected with Ins(1,4,5)P3.
- The effects of caffeine on Ins(1,4,5)P3-induced electrophysiological responses were recorded.
- Spontaneous oscillations and responses to adenosine/isobutylmethylxanthine were also assessed.
Main Results:
- Caffeine abolished Ins(1,4,5)P3-induced oscillations but did not affect the initial Ins(1,4,5)P3-evoked spike.
- Caffeine also suppressed spontaneous oscillations in some oocytes.
- Adenosine and isobutylmethylxanthine reduced oscillation amplitude and duration, but their effects were not mediated by cyclic AMP.
Conclusions:
- Caffeine's selective inhibition of oscillations suggests it targets the Ins(1,4,5)P3-insensitive calcium pool.
- These findings support the two-pool model for generating calcium oscillations in Xenopus oocytes.
- The results differentiate the mechanisms underlying the initial Ins(1,4,5)P3 response and subsequent CICR-mediated oscillations.