Hyperpolarization-activated chloride currents in Xenopus oocytes.

During hyperpolarizing pulses, defolliculated Xenopus oocytes have time- and voltage-dependent inward chloride currents. The currents vary greatly in amplitude from batch to batch; activate slowly and, in general, do not decay; have a selectivity sequence of I- > NO3- > Br- > Cl- > propionate > acetate; are insensitive to Ca2+ and pH; are blocked by Ba2+ and some chloride channel blockers; and have a gating valence of approximately 1.3 charges. In contrast to hyperpolarization-activated chloride currents induced after expression of phospholemman (Palmer, C. J., B. T. Scott, and L. R. Jones. 1991. Journal of Biological Chemistry. 266:11126; Moorman, J. R., C. J. Palmer, J. E. John, J. E. Durieux, and L. R. Jones. 1992. 267:14551), these endogenous currents are smaller; have a different pharmacologic profile; have a lower threshold for activation and lower voltage-sensitivity of activation; have different activation kinetics; and are insensitive to pH. Nonetheless, the endogenous and expressed current share striking similarities. Recordings of macroscopic oocyte currents may be inadequate to determine whether phospholemman is itself an ion channel and not a channel-modulating molecule.