AFLUID March 47/3

Wong, Norman L. M., Bonita P. H. Wong, and Joseph K. C. Tsui. Vasopressin regulates endothelin-B receptor in rat inner medullary collecting duct. Am. J. Physiol. Renal Physiol. 278: F369–F374, 2000.—Previous studies have shown that endothelin (ET) antagonizes the actions of arginine vasopressin (AVP) in the renal collecting ducts. On the other hand, the effects of AVP on ET function within the collecting ducts of the kidney have not been investigated extensively. Using isolated inner medullary collecting ducts (IMCD), we examined the possibility that a decrease in ETB receptor mRNA accompanied AVP-induced downregulation of ETB receptors. Binding studies revealed that overnight incubation of rat IMCD cells with AVP significantly reduced the maximal binding capacity (Bmax) of ET. Activation of adenylate cyclase by forskolin decreased the total ETB receptor density by ,42% but did not affect the density of ETA receptors. The Rp diastereoisomer of adenosine 38,58-cyclic monophosphothionate, Rp-cAMPS (a specific inhibitor of protein kinase A), blocked the AVP-induced reduction in ET receptor density. Using competitive PCR method, we also observed downregulation of ETB receptor mRNA in IMCD treated with AVP. Additional studies were done with IMCD to determine whether AVP inhibited the ET-induced accumulation of cGMP. We saw a reduction in ET-induced cGMP accumulation when IMCD was incubated overnight with AVP. This inhibition of ETinduced accumulation of cGMP was blocked by Rp-cAMPS. These results suggest thatAVP regulates ETB receptor expression in IMCD.