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

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.