Activity and expression of Na(+)-H+ exchanger isoforms 1 and 3 in kidney proximal tubules of hypertensive rats.

Increased activity of the cellular Na(+)-H+ exchangers (NHEs), especially isoform 1 (NHE-1), is a recognized intermediate phenotype of hypertension. NHE activity has been demonstrated to be increased in proximal tubules of the spontaneously hypertensive rat (SHR). However, with the recent cloning of other members of this family of transporters, it is unclear which isoforms may contribute to this increased activity. We have used specific antibodies raised against glutathione-S-transferase fusion proteins of rat NHE-1 and NHE-3 to determine the relative contributions of these isoforms to the NHE activity in freshly isolated and cultured proximal tubule cells from SHR and Wistar-Kyoto (WKY) normotensive control rats. In freshly isolated proximal tubule cells, NHE activity was elevated almost 3-fold in SHR cells (P < .001), and in both rat strains, the contribution from NHE-1 and NHE-3 was approximately equal. Western blots of membranes from these cells showed equal amounts of NHE-1 protein in SHR and WKY cells. However, NHE-3 protein expression was increased 50% in SHR cells (P < .001), and this may account for the elevated activity of this isoform in SHR. The effect of culturing these cells in vitro was then examined. Although total NHE activity in both cell types was decreased during culture, this was mainly due to loss of expression of NHE-3 protein. NHE-1 activity was persistently elevated in the SHR cells in culture. These findings suggest that elevated NHE activity in SHR proximal tubules could be mediated by two mechanisms: (1) increased NHE-1 activity without any increased NHE-1 protein content that persists despite culture and may resemble those changes described for extrarenal tissues and (2) increased NHE-3 activity due to increased expression of NHE-3 protein. Disappearance of NHE-3 during culture implies that our culture conditions did not replicate the in vivo environment and may have removed the factors contributing to the increased NHE-3 expression in SHR cells.