Evidence from incorporation and patch-clamp experiments for a nonselective channel of large conductance at the luminal membrane of rabbit proximal tubule.

The presence of nonselective ionic channels at the apical membrane of rabbit proximal tubule cells was investigated by fusion of proximal tubule brush border membrane vesicles onto a planar bilayer and by means of patch clamp measurements on primary cultured proximal tubule cells. The fusion of brush border membrane vesicles onto a phosphatidylethanolamine-phosphatidylcholine (1:1) planar bilayer led to the identification of a nonselective channel of 115 pS in 200 mM NaCl conditions with an ionic permeability sequence for Na+, K+, and Cl- corresponding to PNa/PK = 0.3 and PNa/PCl = 3, respectively. This channel showed an increased open probability at depolarizing potentials, but its activity was independent of the Ca2+ concentration in both the cis and trans compartments. These findings were subsequently confirmed in inside-out patch clamp experiments carried out on primary cultured proximal tubule cells. This channel was seldom seen in cell-attached recordings on resting cells, but its presence was observed with a higher probability in patch excised experiments performed following superfusion of the cells with a hypotonic solution. It is suggested on the basis of these observations that the apical membrane of rabbit proximal tubule cells contains a voltage-dependent nonselective channel, the activity of which may contribute to the regulation of the membrane potential.