Correlation between Intracellular Activities of Ca2+ and Na+ in Rat Cortical Collecting Duct – A Possible Coupling Mechanism between Na+-K+-ATPase and Basolateral K+ Conductance

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Abstract
In principal cells of rat cortical collecting ducts (CCD) the K+ conductance of the basolateral membrane is functionally coupled to the Na+-K+-ATPase. Inhibition of the Na+-K+-ATPase by ouabain resulted in a decrease of this conductance. This inhibition was absent in the presence of amiloride. In the present study we attempted to measure the activities of intracellular Na+ ([Na+]i) and intracellular Ca2+ ([Ca2+]i) fluorimetrically, and discuss their role for the functional coupling of the activity of the Na+-K+ pump and the recently identified K+ channels in the basolateral membrane. [Na+]i and [Ca2+]i were measured in isolated CCD segments using the Na+-sensitive dye sodium-binding benzofuran isophthalate (SBFi) and the Ca2+-sensitive dye fura-2 as fluorescence indicators. Basal [Na+]i and [Ca2+]i were 22 ± 4 mM(n = 23) and 84 ± 13 nM(n = 28), respectively. With amiloride (10µM) [Na+]i and [Ca2+]i decreased by 5 ± 1 mM(n= 18) and by 19+9 nM(n = 21), respectively. With ouabain (0.5 mM), [Na+]i and [Ca2+]i increased by 30+7 mM (n = 7) and by 25+10 nM (n = 13), respectively. In the presence of amiloride, ouabain increased [Na+]i by only 8 ± 3 mM(n = 7), while [Ca2+]i did not change significantly (Δ = -2 ± 3 nM, n = 13). The observed parallel changes in [Ca2+]i and [Na+]j are compatible with the function of the Na+/Ca2+ exchanger present in the basolateral membrane of the CCD. The K+ channels in the basolateral membrane of rat CCD are inhibited by increases in [Ca2+]i. These data suggest the changes in [Na+]i and consecutive changes in [Ca2+]i as possible functional link between the K+ conductance of the basolateral membrane and the Na+-K+-ATPase of rat CCD.

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