Na(+)-H+ Exchanger Expression in Vascular Smooth Muscle of Spontaneously Hypertensive and Wistar-Kyoto Rats

The Na(+)-H+ exchanger has important modulatory effects on vascular smooth muscle cell proliferation and contractility. Increased Na(+)-H+ exchange activity is a general property of many tissues, including mesenteric artery and cultured vascular smooth muscle cells, in the spontaneously hypertensive rat (SHR). In the present work, we investigated whether alterations in the steady-state levels of specific Na(+)-H+ exchanger mRNA isoforms (NHE-1 through NHE-4) are associated with the observed increases in exchanger activity. Poly(A+) mRNA prepared from 12-week-old hypertensive SHR and normotensive Wistar-Kyoto (WKY) aorta, kidney, and intestine was hybridized to cDNAs specific for each NHE isoform. By Northern blot analysis, NHE-1 was detected in all tissues as well as cultured vascular smooth muscle cells and was not regulated differently in SHR compared with WKY tissues. There was no expression of NHE-2, NHE-3, or NHE-4 in SHR and WKY aortas or in cultured vascular smooth muscle cells from SHR and WKY aortas. Stimulation of NHE-1 mRNA expression by growth factors was similar in cultured SHR and WKY vascular smooth muscle cells. We conclude that the previously observed increase in exchanger activity in blood vessels and cultured vascular smooth muscle cells of the SHR is not caused by induction of the NHE-2, NHE-3, and NHE-4 isoforms or by alterations in steady-state NHE-1 mRNA expression. These findings suggest that posttranslational regulation of the Na(+)-H+ exchanger is responsible for increased activity in the SHR.