Ge Y, Strait KA, Stricklett PK, Yang T, Kohan DE. Role of prostaglandins in collecting duct-derived endothelin-1 regulation of blood pressure and water excretion. Collecting duct (CD)-derived endothelin-1 (ET-1) exerts natriuretic, diuretic, and hypotensive effects. In vitro studies have implicated cyclooxygenase (COX) metabolites, and particularly PGE2, as important mediators of CD ET-1 effects. However, it is unknown whether PGE2 mediates CD-derived ET-1 actions in vivo. To test this, CD ET-1

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... knockout (KO) and control mice were studied. During normal salt and water intake, urinary PGE2 excretion was unexpectedly increased in CD ET-1 KO mice compared with controls. Salt loading markedly increased urinary PGE2 excretion in both groups of mice; however, the levels remained relatively higher in KO animals. Acutely isolated inner medullary collecting duct (IMCD) from KO mice also had increased PGE2 production. The increased IMCD PGE2 was COX-2 dependent, since NS-398 blocked all PGE2 production. However, increased CD ET-1 KO COX-2 protein or mRNA could not be detected in inner medulla or IMCD, respectively. Inner medullary COX-1 mRNA and protein levels and IMCD COX-1 mRNA levels were unaffected by Na intake or CD ET-1 KO. KO mice on a normal or high-Na diet had elevated blood pressure compared with controls; this difference was not altered by indomethacin or NS-398 treatment. However, indomethacin or NS-398 did increase urine osmolality and reduce urine volume in KO, but not control, animals. In summary, IMCD COX-2-dependent PGE2 production is increased in CD ET-1 KO mice, indicating that CDderived ET-1 is not a primary regulator of IMCD PGE2. Furthermore, the increased PGE2 in CD ET-1 KO mice partly compensates for loss of ET-1 with respect to maintaining urinary water excretion, but not in blood pressure control. MATERIALS AND METHODS Transgenic mouse lines. All experiments were performed with approval from the Institutional Animal Care and Use Committees at the University of Utah. CD ET-1 knockout (KO) and littermate control mice were generated utilizing the Cre/loxP system, as previously described (1, 5). Briefly, mice with exon 2 of the ET-1 gene flanked by loxP-sites (floxed) (provided by Dr. Masashi Yanagisawa, Howard Hughes Institute, University of Texas Southwestern Medical Center) were mated with AQP2-Cre mice, containing 11 kb of the mouse aquaporin-2 (AQP2) gene driving expression of Cre recombinase. Female AQP2-Cre mice were mated with male floxed ET-1 mice; female offspring heterozygous for both AQP2-Cre and floxed ET-1 were bred with males homozygous for floxed ET-1. Animals homozygous for floxed ET-1 and heterozygous for AQP2-Cre (CD ET-1 KO) were used in this study. Littermates that were homozygous for the floxed ET-1 gene, but without Cre, were used as control mice. Genotyping was performed as previously described (1). All mice were