Voltage-dependent interaction of Barium and Cesium with the Potassium Conductance of the Cortical Collecting Duct Apical cell membrane

Summary

The influence of Ba⁺⁺ and Cs⁺ on the K⁺ conductive properties of the rabbit isolated perfused cortical collecting tubule were assessed using electrophysiological methods. As before, elevation of K⁺ from 5 to 25 or 50mm (choline⁺ for K⁺ substitution) in the luminal perfusate, caused a marked hyperpolarization (lumen negative) of the transepithelial voltage,V _te, and increase in the transepithelial conductance,G _te-indicative of a high luminal (apical) K⁺ conductance-where-as a similar elevation of K⁺ in the bath caused only minor changes inV _te andG _te. In the presence of 5mm K⁺ in perfusate and bath, addition of 5mm Ba⁺⁺ to the perfusate causedG _te to decrease from 16.1 to 7.4 mScm⁻² (10mm Cs⁺ had qualitatively similar effects) and greatly diminished the response ofV _te andG _te to K⁺ elevation in the lumen, reflecting a decrease in the apical membrane K⁺ conducatance. In contrast, a similar addition of 5mm Ba⁺⁺ to the bath caused only a modest reduction inG _te of 0.4 mScm⁻², consistent with a relatively low K⁺ conductance of the basolateral membrane and tight junction. In other studies the effects of luminal addition of Ba⁺⁺ and amiloride were found to be relatively independent, with the magnitude of the Ba⁺⁺-sensitiveG _te (8.7 mScm⁻²) being several-fold greater than the amiloride-sensitiveG _te (1.4 mScm⁻¹), indicative of a dominat K⁺ conductance at that border which is spatially distinct from the Na⁺ conductance. Furthermore, from the input current-voltage relation of the tubule, the effects of Ba⁺⁺ (0.1–5mm) and Cs⁺ (10mm) at the luminal border on tissue conductance were found to be highly voltage-dependent — the effects on conductance being diminished with lumennegative voltages and more pronounced, approaching a maximum, with lumen-positive voltages. It is concluded that the apical (luminal) cell membrane contains a dominant K⁺ conductive pathway that is blocked by luminal addition of Ba⁺⁺ and Cs⁺. The voltage-dependent nature of the block is consistent with a K⁺ conductive pathway which has a binding site for Ba⁺⁺ and Cs⁺, and presumably K⁺, located deep within the channel.