SODIUM-DEPENDENT PHOSPHATE-TRANSPORT IN A RAT-KIDNEY ENDOSOMAL FRACTION

An endosome-enriched fraction was prepared froin rat kidney cortex by a standard procedure employing centrifugation on a Percoll gradient. This fraction showed time-dependent accumulation of inorganic phosphate (Pi) which was stimulated two- to threefold during the initial phase by an inwardly directed Na+ gradient. Na+ gradient-dependent Pi accumulation decreased with increasing medium osmolality and Pi binding accounted for only 16% of the total accumulation at two minutes. Like the Pi transporter in the brush border membrane (BBM), the Na+ gradient-dependent Pi uptake (but not the Na+-independent component) by the endosomal fraction was stimulated by intravesicular Pi and by an outwardly directed proton gradient, and was inhibited by extravesicular arsenate. Unlike the Pi transporter in BBM, the endosomal Pi transporter was not changed by acidic pH under non-gradient conditions. Activation of the endosomal proton pump by extravesicular ATP, leading to acidification of the vesicle interior, was accompanied by stimulation of endosomal Na+ gradient-dependent Pi transport. Inhibition of the proton pump by deletion of chloride or addition of N-ethylmaleimide abolished the stimulation of Pi uptake by ATP. The data indicate that the Na+-dependent Pi transporter in renal endosomal fractions is an intrinsic endosomal component. It remains to be determined if the endosomal Pi transporter plays a role in regulation of renal Pi transport.