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To examine the mechanism of increased Na–H antiport activity in tissues of the spontaneously hypertensive rat (SHR) by measuring the amount of sodium–hydrogen exchanger isoform 1 (NHE-1) in cultured vascular and striated muscle cells, and in ex vivo tissue extracts of membranes from the brain, heart, kidney and skeletal muscle.A polyclonal rabbit antibody was raised against a fusion protein consisting of a section of the carboxyl tail of NHE-1 and β-galactosidase. Cell extracts were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and proteins were transferred to supported nitrocellulose. NHE-1 was detected by Western blotting and quantified by densitometry.Cultured aortic and striated muscle cells from SHR contained similar amounts of NHE-1 on Western blots to those from control Wistar-Kyoto (WKY) rat cells. Ex vivo extracts of crude membranes from SHR tissues also contained quantities of NHE-1 similar to those from WKY rat tissues.The increased Na–H antiport activity observed in SHR cells in vitro and in vivo is not due to an increased amount of NHE-1 protein in SHR cells. This suggests that in this model of hypertension the increased transport activity results from an increased turnover number per NHE-1 molecule.