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AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, Cambridge.

1. The role of inositol 1,4,5-trisphosphate (Ins1,4,5P3) in controlling the membrane potential oscillations induced by acetylcholine in Xenopus oocytes was investigated by studying the effect of injecting Ins1,4,5P3. 2. Perfusing Xenopus oocytes with low concentrations of acetylcholine (less than or equal to 1 x 10(-7) M) induced regular oscillations in membrane potential. The frequency of these oscillations accelerated as the concentration of acetylcholine was increased. 3. Ionophoretic application of low doses of Ins1,4,5P3 stimulated membrane depolarization in the form of an initial brief spike which was followed by a burst of oscillations when the amount of Ins1,4,5P3 injected was increased. 4. When low doses of Ins1,4,5P3 were injected at 30 s intervals, there was rapid desensitization of the early response which recovered if the interval between injections was extended to 2 min or longer. 5. In comparison to the vegetal pole, the animal pole was much more sensitive to Ins1,4,5P3. This localization of Ins1,4,5P3 sensitivity in the animal pole may contribute to the electrical field which surrounds Xenopus oocytes. 6. A model is presented to explain these oscillations based on the phenomenon of calcium-induced calcium release. It is proposed that Ins1,4,5P3 releases calcium from an Ins1,4,5P3-sensitive pool which is then periodically taken up and released by an Ins1,4,5P3-insensitive pool. It is the overloading of this Ins1,4,5P3-insensitive pool which may provide the trigger to spontaneously release calcium back into the cytoplasm.

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