Proton-sensitive transitions of renal type IINa+-coupled phosphate cotransporter kinetics

In the kidney proximal tubule, acidification of the glomerular filtrate lea ds to an inhibition of inorganic phosphate (P-i) reabsorption by type II Na +-coupled cotransporters (NaPi-II). As external pH also alters the divalent /monovalent P-i ratio, it has been difficult to separate putative proton in teractions with the cotransporter from direct titration of divalent P-i, th e preferred species transported. To distinguish between these possibilities and identify pH-sensitive transitions in the cotransport cycle, the pH-dep endent kinetics of two NaPi-II isoforms, expressed in Xenopus laevis oocyte s, were investigated electrophysiologically. At -50 mV, both isoforms showe d >70% suppression of electrogenic response for an external pH change from 8.0 to 6.2, not attributable to titration of divalent P-i. This was accompa nied by a progressive removal of steady-state voltage dependence. The NaPi- II-related uncoupled slippage current was unaffected by a pH change from 7. 4 to 6.2, with no shift in the reversal potential, which suggested that pro tons do not function as substrate. The voltage-dependence of pre-steady-sta te relaxations was shifted to depolarizing potentials in 100 mM and 0 mM Na -ext(+) and two kinetic components were resolved, the slower of which was p H-dependent The changes in kinetics are predicted by a model in which proto ns interact with the empty carrier and final Na+ binding step.