PROPERTIES AND REGULATION OF PH-DEPENDENT CATION CHANNELS IN THE APICAL MEMBRANE OF CULTURED PROXIMAL TUBULE CELLS

The established opossum kidney (OK) cell line serves as a model system for ion and substrate transport in the renal proximal tubule. Previou s experiments on OK cells revealed a channel-mediated Na+ conductance which is regulated by intracellular pH (pH(i)). In this study we repor t on patch clamp experiments determining the properties and pH(i) depe ndence of a cation channel located in the apical membrane. This channe l is selective for sodium over chloride but discriminates poorly betwe en the monovalent cations Na+, K+, Li+ and Cs+. Its open probability ( P-o) rises at depolarising membrane potentials. Under normal condition s the channel is inactive in the cell-attached patch mode and is activ ated upon excision. However, after excision the channel usually runs d own within 30-90 s which cannot be overcome by either altering the Ca2 +-concentration (l0(-3)mol/l, 10(-6)mol/l, Ca2+-free) or adding 1 mmol /l Mg-ATP to the bath solution. In the cell-attached patch mode the ch annel could be activated by decreasing pH(i) from pH 7.4 to pH 6.5, by either the ammonium prepulse technique or the nigericin K+ method, in more than 50% of the experiments performed. In the renal proximal tub ule such a non-selective cation channel would constitute a functional Na+ channel and might therefore support Naf reabsorption especially du ring the intracellular acidification due to hormonal inhibition of the Na+/H+ exchanger.