Non-selective cation conductance in a Xenopus renal epithelial line

The currents activated in single cells of an epithelial cell line from renal tubules of Xenopus laevis, were studied, using a whole-cell voltage-clamp technique. After the patched membrane was broken for the whole-cell recording, the currents developed transiently, reaching a peak in 20 min, in cells held under voltage-clamp. The currents did not show voltage- and time-dependent gating behavior at membrane potentials between -100 and +100 mV. The current-voltage relationships showed strong outward rectification. The ratio of the absolute amplitude of the current at a potential of +60 mV to that at -60 mV was 24.4 +/- 3.6. The reversal potentials of the currents under various ionic conditions imply that the membrane conductance was preferentially permeable to cations but that it had a finite permeability to anions. The cation conductance was non-selective for monovalent cations and also permeable to Ca2+, with a permeability sequence (relative to Na+); Na+: K+: Cs+: Li+: Ca2+ = 1.0: 1.3: 1.4: 1.0: 0.7. These results suggest that in A6 cells, the non-selective cation conductance activated after the rupture of the patch membrane, may play an important role in epithelial transport of fluid and electrolyte and that its Ca2+ permeability could be involved in some physiological signal transduction.