Characterization of apical cell membrane Na⁺ and K⁺ conductances of cortical collecting duct using microelectrode techniques

The apical cell membrane ionic conductive properties of the isolated perfused rabbit cortical collecting duct (tubule) were assessed at 37°C using microelectrode techniques. In the initial evaluation of the methodology, it was observed that stable cell membrane voltage recordings could be obtained by impaling cells either from the luminal side across the apical cell membrane or from the bath side across the basolateral cell membrane, providing initial evidence supporting the application of these techniques to this tissue. With the latter method of impalement, it was observed that addition of amiloride (50 μM) to the luminal perfusate caused a hyperpolarization of the apical cell membrane voltage, a decrease in the transepithelial conductance, and an increase in the fractional resistance (estimated as the ratio of the resistance of the apical cell membrane to the sum of apical and basolateral cell membrane resistances). These results are consistent with an amiloride-sensitive Na⁺ conductance at the apical cell border. In a similar manner it was deduced from the effects of elevating K⁺ in the luminal perfusate from 5 to either 25 or 50 mM that there was a high K⁺ conductance at the apical border. This conductive pathway was blocked by the luminal addition of 5 mM Ba²⁺ or reduction of the luminal pH to 4.0. Furthermore, since addition of both amiloride and Ba²⁺ to the perfusate caused the fractional resistance to increase from 0.52 ± 0.04 to 0.91 ± 0.03, the Na⁺ and K⁺ conductances are the apparent dominant conductive pathways at that border. It is concluded that microelectrode techniques can be applied successfully to the cortical collecting duct and that the apical cell membrane possesses an amiloride-sensitive Na⁺ conductance and a Ba²⁺- and H⁺-sensitive K⁺ conductance.