Stoichiometry and Na+ binding cooperativity of rat and flounder renal type II Na+-Pi cotransporters

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.