The distal convoluted tubule of rabbit kidney does not express a functional sodium channel

We sought to assess whether the distal convoluted tubule (DCT) segment of t he rabbit nephron expresses a functional epithelial sodium channel. First, the transepithelial voltage (V-te, lumen vs. bath) was measured in isolated perfused DCT segments (assessed separately in the upstream half and the do wnstream half of the DCT). Vte was zero and not affected by amiloride or ba rium in the upstream DCT. Vte was sometimes negative in the downstream DCT and depolarized by amiloride and hyperpolarized by barium, suggesting inclu sion of connecting tubule (CNT) cells. To determine expression of epithelia l sodium channel (ENaC) mRNA subunits by the upstream DCT, rabbit alpha-, b eta-, and gamma -ENaC cDNA fragments were cloned and primers were selected for single-nephron RT-PCR analysis. Although alpha -ENaC was expressed by t he DCT, beta- and gamma -ENaC were not detected in the DCT. In contrast, th e CNT, CCD, and outer medullary collecting duct (OMCD) expressed all three subunits. Nedd4 was also not detected in the DCT but was expressed by the C NT, CCD, and OMCD. When upstream DCT fragments were grown to confluent mono layers in primary culture, the epithelia exhibited negative voltages and hi gh transepithelial resistances and expressed mRNA for all three ENaC subuni ts as well as for Nedd4. The absence of a negative voltage and failure to d etect transcript for beta- and gamma -ENaC and Nedd4 in the native rabbit D CT suggest that the sodium channel is not a significant pathway for sodium absorption by this segment. The phenotype conversion observed when DCT cell s are grown in culture does not rule out the possibility that there may be conditions in which the DCT in the intact kidney expresses sodium channel a ctivity. The results are consistent with the notion that DCT sodium transpo rt is predominantly, if not exclusively, electroneutral.