The stoichiometry of the rat and flounder isoforms of the renal type II sodium-phosphate (Na⁺-P_i) cotransporter was determined directly by simultaneous measurements of phosphate (P_i)-induced inward current and uptake of radiolabeled P_i and Na⁺ in Xenopus laevis oocytes expressing the cotransporters. There was a direct correlation between the P_i-induced inward charge and P_i uptake into the oocytes; the slope indicated that one net inward charge was transported per P_i. There was also a direct correlation between the P_i-induced inward charge and Na⁺ influx; the slope indicated that the influx of three Na⁺ ions resulted in one net inward charge. This behavior was similar for both isoforms. We conclude that for both Na⁺-P_i cotransporter isoforms the Na⁺:P_i stoichiometry is 3:1 and that divalent P_i is the transported substrate. Steady-state activation of the currents showed that the Hill coefficients for P_i were unity for both isoforms, whereas for Na⁺, they were 1.8 (flounder) and 2.5 (rat). Therefore, despite significant differences in the apparent Na⁺ binding cooperativity, the estimated Na⁺:P_i stoichiometry was the same for both isoforms.

electrogenic phosphate transport; Xenopus oocyte; voltage clamp; substrate coupling ratio

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