Phosphate transport in isolated rat inner medullary collecting duct

Abstract

Phosphate transport by the inner medullary collecting duct of normal rats was studied using an in vitro microperfusion technique. Net (J_net), lumen-to-bath (J_lb) and bath-to-lumen (J_bl) phosphate fluxes were measured using ³²PO₄ as tracer, in the absence of net water absorption. A net absorption of phosphate (22.3 ±3.3 pmol cm⁻² s⁻¹) was observed by direct determination, and was similar to the difference between the J_lb and J_bl (57.7±8.2 and 32.2±1.5 pmol cm⁻² s⁻¹ respectively). The addition of amiloride (10 μM) to the perfusate did not change the J_lb of phosphate but blocked the efflux of sodium. Also, the withdrawal of sodium from the bath and perfusion solution did not change the J_lb of phosphate. In parallel, the addition of ouabain (10 mM) to the bath fluid decreased the J_lb of sodium more (37%) than the J_lb of phosphate (12%) and did not change the J_bl of phosphate. The addition of arsenate (10 μM) to the perfusate both in the presence and in the absence of sodium caused a decrease in J_lb, but J_bl remained unchanged, and parathyroid hormone (10 U) added to the bath did not change the J_lb. The increase in pH of the bath and perfusion fluid was associated with an increase in the J_lb of phosphate, and the decrease in pH was similarly followed by a decrease in phosphate efflux. The J_bl did not change with the pH alterations. These data demonstrate that a net phosphate absorption takes place in rat inner medullary collecting duct perfused in vitro and that this transport appears to be independent of sodium absorption and the action of parathyroid hormone. Moreover, a decrease in luminal and bath pH induces a decrease in phosphate efflux.