Sodium-dependent transport of phosphate by rat liver plasma membrane vesicles

Plasma membrane fractions were prepared from homogenates of rat liver by density gradient centrifugation and then used for the formation of right-side-out vesicles. Uptake of Pi into the vesicles is rapid when an inwardly directed sodium gradient is present and an overshoot of uptake occurs indicative of accumulation against a Pi concentration gradient. Initial Pi uptake rate in the presence of a K+ gradient is approx half that seen with Na+, but uptake in the presence of a choline chloride gradient is very slow. An overshoot does not occur with either K+ or choline gradients. The Km(Pi) for the Na-dependent component of Pi uptake is approx. 1 mM and Vmax at 20 degrees C is 0.8 nmol/min per mg protein. The relationship between initial uptake rate and Na+ concentration is sigmoid, with a Hill coefficient of 2.6. It is concluded that the cotransporter resembles that of kidney and intestine in possessing at least two interacting sites for Na+ and that in intact cells the Na+ gradient maintained by the sodium pump ATPase provides the energy for accumulation of Pi against the unfavourable membrane potential.