The Na⁺-HCO ⁻₃ cotransporter operates with a coupling ratio of 2 HCO ⁻₃ to 1 Na⁺ in isolated rabbit renal proximal tubule

Abstract

All the relevant literature reports indicate that net rates of salt and water absorption and cell membrane potentials (V _b) are lower, but intracellular Na⁺ concentration is higher in rabbit renal proximal tubule in vitro than in rat proximal tubule in vivo. Since the different driving forces should influence basolateral Na⁺-HCO₃ ⁻ cotransport we have studied the operation of the cotransporter in isolated rabbit renal proximal tubule in vitro with special emphasis on the stoichiometry of flux coupling (q). Using conventional and ion-selective intracellular microelectrodes three series of experiments were performed: (a) we determined the V _b response to a 2∶1 reduction of bath HCO₃ ⁻ or Na⁺ concentration, (b) we determined initial efflux rates of HCO₃ ⁻ or Na⁺ ions in response to a sudden 10∶1 reduction of bath HCO₃ ⁻ concentration, and (c) we collapsed the tubules and determined electrochemical driving forces of Na⁺ and HCO₃ ⁻ across the basolateral cell membrane under conditions approaching zero net flux in the control state in the presence of Ba²⁺- and in Cl⁻-free solutions. All measurements concurrently yielded a coupling ratio of approximately two HCO₃ ⁻ ions to one Na⁺ ion (q=2). This result contrasts with the ratio q=3, which we have previously observed in similar experiments on rat renal proximal tubule in vivo [Yoshitomi et al. (1985) Pflügers Arch 405:360] and which was also observed on rabbit renal basolateral cell membrane vesicles in vitro [Soleimani et al. (1987) J Clin Invest 79:1276]. This indicates that — depending on the functional state of the cell — the Na⁺-HCO₃ ⁻ cotransporter can operate with variable stoichiometry and suggests that it transports either 1 Na⁺ + 1 HCO₃ ⁻ + 1 CO₃ ²⁻ ion [Soleimani and Aronson (1989) J Biol Chem 264:18 302] or 1 Na⁺ + 2 HCO₃ ⁻ ions.