Characterization of the urea transporter in outer medullary descending vasa recta

Partially because of facilitated transport of urea, urea permeability (Pu) of the outer medullary descending vasa recta (OMDVR) frequently exceeds sodium permeability by more than an order of magnitude. This study characterizes the OMDVR urea transporter. Application of the urea analogue thiourea (200 mM) to the abluminal surface of microperfused OMDVR inhibited Pu by 33%. When osmolarity due to thiourea was balanced by addition of mannitol or thiourea, similar results were obtained. Thiourea produced graded inhibition of Pu from 343 +/- 54 (SE) to 191 +/- 43 x 10(-5) cm/s as concentration was increased from 0 to 100 mM. The thiourea concentration needed for half-maximal inhibition was 19 mM. The abilities of urea analogues to reduce Pu were compared by addition of 50 mM concentrations to the bath and perfusate. Thiourea and methylurea produced 32 and 34% inhibition of Pu, respectively, whereas urea and acetamide produced only 3 and 11% inhibition, respectively. The transporter showed negligible saturation as the transmural urea gradient was increased from 0 to 200 mM. Phloretin and p-chloromercuribenzenesulfonate inhibited Pu in a concentration-dependent fashion. It is concluded that a transporter confers high Pu to OMDVR. Pu is equally high when measured by urea influx or efflux. Properties of the transporter are similar to those expressed by the inner medullary collecting duct.