Transcellular sodium transport and basolateral rubidium uptake in the isolated perfused cortical collecting duct

The relation between transcellular Na⁺ absorption, intracellular Na⁺ concentration and Na⁺/K⁺-ATPase activity (the last estimated by the rubidium uptake across the basolateral cell membrane) was examined in the different cell types of the rabbit cortical collecting duct (CCD). Experiments were performed on isolated perfused CCD in which Na⁺ absorption was varied by perfusing the tubule with solutions containing different Na⁺ concentrations (nominally Na⁺-free, 30 mM and 144 mM). Experiments were terminated by shock-freezing the tubules during perfusion. Precisely 30 s before shock-freezing, the K⁺ in the bathing solution was 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⁺ concentration 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 respectively, when the perfusion solution was Na⁺-free, 10.1±0.7 and 11.6±0.6 mmol/kg wet weight with 30 mM Na⁺ in the perfusion solution, 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 results support the notion that principal, but not intercalated, cells are involved in transepithelial Na⁺ absorption. In addition, the data demonstrate that apical Na⁺ entry and basolateral Na⁺/K⁺-AT-Pase activity are closely coupled in principal cells of the rabbit CCD. A rise in lumen Na⁺ concentration leads to increased Na⁺ entry and augmented intracellular Na⁺ concentration, which then secondarily stimulates active basolateral Na⁺/K⁺(Rb⁺) exchange.