CHARACTERISTICS OF OSMOLARITY-STIMULATED UREA TRANSPORT IN RAT IMCD

Urea transport across the terminal inner medullary collecting duct (IMCD) is mediated by a urea transporter that is stimulated by vasopressin (AVP) or hyperosmolarity. To determine whether hyperosmolarity stimulates urea transport by an adenylyl cyclase-dependent or -independent mechanism, terminal IMCDs were perfused with 10-mu-M forskolin followed by an increase in osmolality or with increasing osmolality followed by 10 nM AVP. In both protocols, stimulating adenylyl cyclase caused an additive increase in urea permeability (P(urea)) to that stimulated by hyperosmolarity. Next, we investigated whether hyperosmolarity stimulates the same urea transporter as AVP by studying the inhibitor profile and IMCD subsegment response of hyperosmolarity-stimulated urea transport and comparing it to properties already demonstrated for AVP-stimulated urea transport. In terminal IMCDs, luminal phloretin (250-mu-M) reversibly inhibited P(urea) by 63%. Thiourea (100 mM) inhibited P(urea) by 73% at two different levels of osmolality, 690 and 290 mosmol/kgH2O. The half-maximal inhibitory concentration (K1/2) for thiourea at 690 mosmol/kgH2O was not significantly different from the K1/2 value at 290 mosmol/kgH2O, suggesting that stimulation by hyperosmolarity is related to an increase in the V(max) for the urea transporter. Finally, we found that hyperosmolarity did not stimulate P(urea) in the initial IMCD. In summary, the data suggests that hyperosmolarity stimulates urea transport by an adenylyl cyclase-independent mechanism. However, the inhibitor profile and the IMCD subsegment response for hyperosmolarity-stimulated and AVP-stimulated P(urea) are similar, suggesting that both hyperosomolarity and AVP stimulate the same urea transporter.