Oscillatory chloride current evoked by temperature jumps during muscarinic and serotonergic activation in Xenopus oocyte.

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Abstract
1. Membrane currents were recorded from voltage‐clamped oocytes of Xenopus laevis, during temperature jumps imposed by a heating light. Resting oocytes usually showed little response, but large oscillatory membrane currents developed in response to cooling steps applied during activation of 'native' muscarinic receptors. 2. Similar temperature jump (Tjump) currents were seen during activation of oscillatory chloride currents mediated by muscarinic acetylcholine (ACh), serotonin, glutamate and noradrenaline receptors, expressed in the oocyte following injection with messenger ribonucleic acid (mRNA) from rat brain. The Tjump response during muscarinic activation was selectively blocked by atropine, and that during serotonergic activation by methysergide. In contrast, the 'smooth' membrane currents elicited by nicotinic ACh, kainate and gamma‐aminobutyric acid (GABA) were not accompanied by Tjump responses. 3. Rapid cooling of the oocyte gave larger Tjump currents than a gradual cooling over a few seconds. The size of the Tjump current elicited by a fixed cooling step increased linearly with the preceding time of warming, becoming maximal at intervals greater than about 100 s. 4. The Tjump current was inward at a clamp potential of ‐60 mV and reversed direction at about ‐22 mV, which corresponds to the chloride equilibrium potential in the oocyte. In low‐chloride solution the reversal potential was shifted to more positive potentials, but it was almost unchanged by changes in potassium and sodium concentration. The size of the Tjump current decreased as the membrane potential was made more negative than about ‐40 mV. 5. The period of oscillation of the Tjump current increased with decreasing temperature, following a Q10 of 3.15. Depolarization also caused a small increase in period. 6. The Tjump current was not abolished in calcium‐free solution, or by addition of manganese or lanthanum to the bathing solution. However, it was abolished by intracellular injection of the calcium‐chelating agent EGTA. 7. Intracellular injection of inositol 1,4,5‐trisphosphate evoked an oscillatory membrane current, during which Tjump responses developed similar to those after muscarinic activation. Intracellular injection of calcium evoked a chloride current, but this was not accompanied by Tjump responses. 8. We conclude that the oscillatory currents evoked by temperature jumps arise from chloride channels activated by intracellular calcium. This calcium is probably mobilized from intracellular stores by inositol trisphosphate which is liberated as a result of activation of muscarinic receptors, and also receptors for serotonin and glutamate.(ABSTRACT TRUNCATED AT 400 WORDS)

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