1. Membrane currents were studied in voltage‐clamped Xenopus laevis oocytes which had been injected with total rat brain RNA. 2. When the membrane potential was stepped from ‐100 to +10 mV, two components of outward current were observed which were named Tout1 and Tout2. 3. Both Tout1 and Tout2 were eliminated in chloride‐free or calcium‐free media and blocked by 9‐anthroic acid, indicating that they represented calcium‐dependent chloride currents. 4. Both currents were dependent on extracellular calcium (1.8‐10 mM), with Tout1 showing a greater sensitivity to changes in calcium concentration. 5. Tout2 but not Tout1 was blocked by intracellular injection of 300‐600 pmol, BaCl2 (final concentration in the oocyte: 0.3‐0.6 mM). Injection of KCl had no effect on either Tout1 or Tout2. 6. Tout2 but not Tout1 was enhanced by low concentrations of serotonin (0.5‐2 nM). This effect was blocked by 0.1 microM‐mianserin. Higher concentrations (above 10 nM) of serotonin decreased the amplitude of Tout2. The effect of serotonin was blocked by the protein kinase inhibitor, H‐7 (25 microM). 7. Tout2 but not Tout1 was enhanced by 10 nM‐phorbol myristate acetate. Higher concentrations of the phorbol ester decreased the amplitude of Tout2. 8. It is concluded that in oocytes injected with RNA there is an induction of a novel component of the calcium‐induced chloride current (Tout2). This current reflects a second process of chloride channel opening which can be enhanced by serotonin via activation of protein kinase C.
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