Increased expression of Ca2+-sensitive K+ channels in aorta of hypertensive rats

Potassium efflux through Ca2+-sensitive K+ channels (K-Ca channels) is increased in arterial smooth muscle cells from hypertensive rats, but the molecular mechanism is un known. The goal of this study was to compare the levels of K-Ca channel current between aortic smooth muscle cells from adult Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) and then use Western blot methods and ribonuclease protection assays to examine the expression and mRNA levels for the K-Ca channel in these same vascular tissues. Whole-cell patch-clamp methods indicated a larger component of K-Ca channel current, sensitive to block by iberiotoxin (100 nmol/L), in single aortic smooth muscle cells from SHR compared with WKY. Subsequent Western blot analysis using a site-specific antibody (anti-alpha(913-926)) directed against the S9/S10 linker of the alpha-subunit of the K-Ca channel revealed a 125-kD immunoreactive band in lanes loaded with either WKY or SHR aortic muscle membranes. The immunoreactive density of this band, which corresponded to the known molecular size of the alpha-subunit, was 2.2-fold greater in lanes loaded with aortic smooth muscle membranes from the hypertensive animals. However, despite this evidence for an increased expression and functional enhancement of K-Ca channels in aortic smooth muscle membranes of SHR, ribonuclease protection assays with a P-32-labeled riboprobe targeted against the S9/S10 linker of the K-Ca channel alpha-subunit revealed no difference in mRNA levels for the alpha-subunit between WKY and SHR aortic tissue. These findings provide initial evidence that (1) an increased expression of K-Ca channels may be a mechanism for the enhanced K-Ca current in aortic smooth muscle membranes of SHR, and (2) the upregulation of K-Ca channels in arterial muscle membranes during hypertension, which is regarded as a homeostatic mechanism for buffering vascular excitability, may rely on posttranscriptional events.