Identification of a nonendothelial cell thromboxane-like constrictor response and its interaction with the renin-angiotensin system in the aorta of spontaneously hypertensive rats.

Aortic ring preparations from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats were treated with N omega-nitro-L-arginine (NOLA, 10(-4) M). This produced a sustained contraction in preparations from SHR but not WKY rats. A similar contraction in aortic ring preparations from the SHR was produced with methylene blue (10(-5) M) and NG-monomethyl-L-arginine (10(-5) M). The NOLA-induced contraction was reversed with indomethacin (8 x 10(-6) M), ridogrel (10(-5) M) and SQ 29548 (10(-6) M) thus confirming the involvement of thromboxane A2/prostaglandin H2 processes. Subsequent experiments demonstrated that the thromboxane-like contraction was not dependent upon the presence of endothelial cells and occurred in preparations from young, prehypertensive (5 week) and older (17 week) SHRs. The thromboxane-like contraction was markedly suppressed with chronic captopril treatment and reinstated 4 weeks after withdrawal from captopril. The addition of saralasin (10(-6) M) or captopril (10(-6) M) to aortic ring preparations did not suppress the thromboxane-like contractions. The foregoing findings support the presence of a nonendothelial cell thromboxane-like constrictor agent in the aorta of the SHR that is revealed after impairment of nitric oxide production. The activity of the thromboxane-like constrictor process is not tightly linked to prevailing blood pressure, but is reduced with chronic in vivo inhibition of the angiotensin-converting enzyme.