TRANSCELLULAR SODIUM-TRANSPORT AND BASOLATERAL RUBIDIUM UPTAKE IN THEISOLATED-PERFUSED CORTICAL COLLECTING DUCT

The relation between transcellular Na+ absorption, intracellular Na+ c oncentration and Na+/K+-ATPase activity (the last estimated by the rub idium uptake across the basolateral cell membrane) was examined in the different cell types of the rabbit cortical collecting duct (CCD). Ex periments were performed on isolated perfused CCD in which Na+ absorpt ion was varied by perfusing the tubule with solutions containing diffe rent Na+ concentrations (nominally Na+-free, 30 mM and 144 mM). Experi ments were terminated by shock-freezing the tubules during perfusion. Precisely 30 s before shock-freezing, the K+ in the bathing solution w as exchanged for Rb+. Intracellular element concentrations, including Rb+, were determined in freeze-dried cryosections of the tubules using energy-dispersive X-ray analysis. Increasing Na+ concentration in the perfusion solution caused significant rises in intracellular Na+ conc entration and Rb+ uptake of principal cells. Principal cell Na+ and Rb + concentrations were 7.8 +/- 0.9 and 7.0 +/- 0.8 mmol/kg wet weight r espectively, when the perfusion solution was Na+-free, 10.1 +/- 0.7 an d 11.6 +/- 0.6 mmol/kg wet weight with 30 mM Na+ in the perfusion solu tion, and 14.5 +/- 1.5 and 14.9 +/- 0.9 mmol/kg wet weight with 144 mM Na+ in the perfusion solution. In contrast, a comparable relationship between lumen Na+ concentration, intracellular Na+ concentration and basolateral Rb+ uptake was not seen in intercalated cells. These resul ts support the notion that principal, but not intercalated, cells are involved in transepithelial Na+ absorption. In addition, the data demo nstrate that apical Na+ entry and basolateral Na+/K+-ATPase activity a re closely coupled in principal cells of the rabbit CCD. A rise in lum en Na+ concentration leads to increased Na+ entry and augmented intrac ellular Na+ concentration, which then secondarily stimulates active ba solateral Na+/K+(Rb+) exchange.