Phosphate transport by isolated rabbit cortical collecting tubule

Renal handling of phosphate occurs in the proximal convoluted tubule. Absorption of this anion also occurs in the pars recta and distal convoluted tubule, the latter a structurally and functionally diverse segment. The purpose of the present investigation was to examine phosphate transport by the cortical collecting duct of the rabbit. Segments of cortical collecting tubule, beyond the last cortical branch, were isolated and perfused in vitro with an artificial solution simulating plasma as the perfusing and bathing medium. The perfusion solution contained either 3 or 25 mM bicarbonate. Net phosphate transport was measured using 32P as the radionuclide tracer, with identical specific activity in perfusing and bathing solutions. A net absorptive flux for phosphate was demonstrated, amounting to 2-3% of the delivered load. In addition, this absorptive flux was linearly related to perfusion rate and, thus, delivered load, but independent of the lumen bicarbonate concentration or pH.