Regulation of endogenous chloride conductance in Xenopus oocytes

Radiotracer (86Rb, 125I) efflux measurements and intracellular microelectrode recording were performed to study the cellular mechanisms that regulate the endogenous ionic conductances in Xenopus oocytes. Addition of isoproterenol (Iso, 10(-5) M) caused a marked increase in 86Rb efflux, with a time course that is in good agreement to Iso-elicited membrane hyperpolarization. Thus, radiotracer efflux measurement appears to be a sensitive assay method to study stimulus-secretion coupling in oocytes. 125I efflux was suppressed by the C1- channel blocker diphenylamine-2-carboxylate, but was insensitive to bumetanide. Elevation of ambient [Ca2+] from 0.4 to 10 mM resulted in an eminent increase in 125I efflux for up to approximately 20 min. Acetylcholine (10(-5) M), which mobilizes cell Ca2+, also enhanced 125I efflux. Iso although increased intracellular cAMP level approximately 2-fold, but showed no stimulatory effect on 125I efflux. Addition of 8-(-4-chlorophenylthio)-cAMP (1 mM), or of forskolin (10(-5) M) plus the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (2 x 10(-4) M), also failed to enhance 125I efflux. These results suggest that, in sharp contrast to the mechanisms for Cl-conductance regulation in mammalian Cl-secreting epithelia, the endogenous Cl- conductance in Xenopus oocytes is, under normal physiological conditions, primarily regulated by intracellular Ca(2+)- rather than a cAMP-mediated signaling mechanism.