Inhibition of urea transport in inner medullary collecting duct by phloretin and urea analogues

Abstract

Arginine vasopressin (AVP) increases the urea permeability of the rat terminal inner medullary collecting duct (IMCD) to levels much greater than can be explained by lipid-phase permeation or paracellular diffusion, suggesting the presence of an AVP-stimulated facilitated transport pathway. We tested whether inhibitors of facilitated urea transport in erythrocytes and toad bladder also inhibit urea transport in the isolated perfused IMCD. Apparent urea permeability (Purea) was determined by measuring the flux due to an imposed 5 mM concentration gradient. Phloretin (0.25 mM in lumen or bath) reversibly inhibited Purea. Phloretin, however, did not alter the osmotic water permeability. Urea analogues (200 mM) in the bath inhibited Purea (thiourea, 74% inhibition; methylurea 65%; acetamide 35%). Urea analogues in the lumen decreased Purea with the same order of potency. The inhibitory K1/2 for thiourea in the lumen was 27 +/- 2 mM and did not change with 10(-10) M AVP (28 +/- 3), despite a fourfold increase in Purea. We conclude the following. 1) Inhibitor actions on urea transport in the IMCD are similar to those in red blood cells and toad bladder, suggesting that the urea transporter could be a membrane protein similar to that in the other tissues. 2) Inhibition of Purea by phloretin without an effect on vasopressin-stimulated water permeability supports the view that the urea pathway is not the vasopressin-stimulated water channel. 3) The ability of AVP to increase Purea without an effect on the inhibitory K1/2 for thiourea indicates that AVP probably does not act by altering the binding affinity of individual transporters for urea.