A novel calcium-dependent chloride current in Xenopus oocytes injected with brain messenger RNA

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.