We examined whether phosphonoformate (PFA) can cause phosphaturia through its direct action on brush-border membrane (BBM) in vivo. Infusion of PFA or of parathyroid hormone (PTH) to thyroparathyroidectomized rats caused a marked increase in fractional excretion of phosphate without changes in excretion of Na+ or of GFR. The PFA-induced phosphaturia was not accompanied by an increase in urinary adenosine-3',5'-cyclic monophosphate (cAMP); moreover, PFA added in vitro did not influence the PTH-sensitive adenylate cyclase and cAMP-phosphodiesterase in proximal convoluted tubules. In BBM vesicles (BBMV) from rats with PFA-elicited phosphaturia, neither the rate of Na+-Pi symport nor Na+-dependent binding of [14C]PFA on BBMV was changed, whereas in BBMV from PTH-infused rats the Vmax of Na+-Pi symport decreased. PFA is almost completely ultrafiltrable; no metabolic transformation of PFA was detected after [14C]PFA exposure to rat renal cortical slices, homogenate, or to blood. We conclude that PFA causes phosphaturia by direct inhibition of Na+-Pi symport across BBM in proximal tubules, acting from the luminal side. Thus PFA (foscarnet) has a unique direct mechanism of phosphaturic effect, via its action on Pi reabsorption in proximal tubules in vivo.

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