| Abstrakt: |
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 50 mm (choline for K substitution) in the luminal perfusate, caused a marked hyperpolarization (lumen negative) of the transepithelial voltage, V, and increase in the transepithelial conductance, G-indicative of a high luminal (apical) K conductance-where-as a similar elevation of K in the bath caused only minor changes in V and G. In the presence of 5 mm K in perfusate and bath, addition of 5 mm Ba to the perfusate caused G to decrease from 16.1 to 7.4 mScm (10 mm Cs had qualitatively similar effects) and greatly diminished the response of V and G to K elevation in the lumen, reflecting a decrease in the apical membrane K conducatance. In contrast, a similar addition of 5 mm Ba to the bath caused only a modest reduction in G of 0.4 mScm, 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-sensitive G (8.7 mScm) being several-fold greater than the amiloride-sensitive G (1.4 mScm), 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 (0.1-5 mm) and Cs (10 mm) 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. [ABSTRACT FROM AUTHOR] |
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