The stoichiometry of the electrogenic sodium bicarbonate cotransporter NBC1 is cell-type dependent

1. The pancreatic variant of the sodium bicarbonate cotransporter, pNBC1, m ediates basolateral bicarbonate influx in the exocrine pancreas by coupling the transport of bicarbonate to that of sodium, with a 2 HCO3-:1 Na+ stoic hiometry. The kidney variant, kNBC1, mediates basolateral bicarbonate efflu x in the proximal tubule by coupling the transport of 3 HCO3- to 1 Na+. The molecular basis underlying the different stoichiometries is not known. 2. pNBC1 and kNBC1 are 93% identical to each other with 41 N-terminal amino acids of kNBC1 replaced by 85 distinct amino acids in pNBC1. In this study we tested the hypothesis that the differences in stoichiometry are related to the difference between the N-termini of the two proteins. 3. Mouse renal proximal tubule and collecting duct cells, deficient in both pNBC1- and kNBC1-mediated electrogenic sodium bicarbonate cotransport func tion were transfected with either pNBC1 of kNBC1. Cells were grown on a per meable support to confluence, mounted in an Ussing chamber and permeabilize d apically with amphotericin B. Current through the cotransporter was isola ted as the difference current due to the reversible inhibitor dinitrostilbe ne disulfonate. The stoichiometry was calculated from the reversal potentia l by measuring the current-voltage relationships of the cotransporter at di fferent Na+ concentration gradients. 4. Our data indicate that both kNBC1 and pNBC1 can exhibit either a 2:1 or 3:1 stoichiometry depending on the cell type in which each is expressed. In proximal tubule cells, both pNBC1 and kNBC1 exhibit a 3 HCO3-:1 Na+ stoich iometry, whereas in collecting duct cells, they have a 2:1 stoichiometry. T hese data argue against the hypothesis that the stoichiometric differences are related to the difference between the N-termini of the two proteins, Mo reover, the results suggest that as yet unidentified cellular factors(s) ma y modify the stoichiometry of these cotransporters.