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 (10 microM) [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 (delta = -2 + 3 nM, n = 13). The observed parallel changes in [Ca2+]i and [Na+]i 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 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.