Associated Proteins and Renal Epithelial Na+ Channel Function

The hypothesis that amiloride-sensitive Na+ channel complexes immunopurified from bovine renal papillary collecting tubules contain, as their core conduction component, an ENaC subunit, was tested by functional and immunological criteria. Disulfide bond reduction with dithiothreitol (DTT) of renal Na+ channels incorporated into planar lipid bilayers caused a reduction of single channel conductance from 40 pS to 13 pS, and uncoupled PKA regulation of this channel. The cation permeability sequence, as assessed from bi-ionic reversal potential measurements, and apparent amiloride equilibrium dissociation constant (K amil i ) of the Na+ channels were unaltered by DTT treatment. Like ENaC, the DTT treated renal channel became mechanosensitive, and displayed a substantial decrease in K amil i following stretch (0.44 ± 0.12 μm versus 6.9 ± 1.0 μm). Moreover, stretch activation induced a loss in the channel's ability to discriminate between monovalent cations, and even allowed Ca2+ to permeate. Polyclonal antibodies generated against a fusion protein of αbENaC recognized a 70 kDa polypeptide component of the renal Na+ channel complex. These data suggest that ENaC is present in the immunopurified renal Na+ channel protein complex, and that PKA sensitivity is conferred by other associated proteins.