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 Cl − channel blocker diphenylamine-2-carboxylate, but was insensitive to bumetanide. Elevation of ambient [Ca 2+] from 0.4 to 10 mM resulted in an eminent increase in 125I efflux for up to ∼20 min. Acetylcholine (10 −5 M), which mobilizes cell Ca 2+, also enhanced 125I efflux. Iso although increased intracellular cAMP level ∼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 × 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.