Electrophysiological analysis of Na+/Pi cotransport mediated by a transporter cloned from rat kidney and expressed in Xenopus oocytes.

Phosphate (Pi) reabsorption in renal proximal tubules involves Na+/Pi cotransport across the brush border membrane; its transport rate is influenced by the Na(+)-coupled transport of other solutes as well as by pH. In the present study, we have expressed a cloned rat renal brush border membrane Na+/Pi cotransporter (NaPi-2) in Xenopus laevis oocytes and have analyzed its electrophysiologic properties in voltage- and current-clamp studies. Addition of Pi to Na(+)-containing superfusates resulted in a depolarization of the membrane potential and, in voltage-clamped oocytes, in an inward current (IP). An analysis of the Na+ and/or Pi concentration dependence of IP suggested a Na+/Pi stoichiometry of 3:1. IP was increased by increasing the pH of the superfusate; this phenomenon seems to be mainly related to a lowering of the affinity for Na+ interaction by increasing H+ concentration. The present data suggest that known properties of Pi handling at the tubular/membrane level are "directly" related to specific characteristics of the transport molecule (NaPi-2) involved.