Cl^- channels in basolateral renal medullary membranes VI. Cl^- conductance expression in Xenopus oocytes

Cl^- channels from basolateral membranes of the mammalian thick ascending limb of Henle differ significantly, in certain of their functional characteristics, from Cl^- channels in apical membranes of secretory epithelia such as trachea or small intestine and from certain Cl^- channels in the central nervous system. Yet there is no sequence information available about basolateral thick ascending limb Cl^- channels. As an initial step in the isolation of a Cl^- channel from the thick ascending limb of Henle's loop, we attempted a functional expression of a Cl^- conductance in oocytes from Xenopus laevis. Oocytes injected with mRNA isolated from the outer medulla of rabbit kidney had a membrane conductance, measured using a two- electrode voltage clamp, which was sixfold greater than in water-injected oocytes (9.05 ± 1.56 vs. 1.43 ± 0.15 μS, respectively). Ion substitution experiments showed the conductance in the RNA-injected oocytes to be Cl^- selective. In addition, the Cl^- channel blocker diphenylamine-2-carboxylate (1 mM) almost completely inhibited (79 ± 6%) the increased conductance seen in the RNA-injected oocytes. Fractionation of the mRNA by sucrose gradient centrifugation revealed that peak Cl^- channel activity was expressed using an mRNA fraction of 1.8-3.2 kb in size. These results demonstrate that a membrane Cl^- conductance can be expressed in X. laevis oocytes injected with size-selected fractions of mRNA from rabbit outer renal medulla.