Ageing suppresses endothelium-dependent relaxation and generates contraction mediated by the muscarinic receptors in vascular smooth muscle of normotensive Wistar-Kyoto and spontaneously hypertensive rats.

We evaluated the effects of ageing and hypertension on endothelium-dependent relaxation and contraction of vascular smooth muscles. Aortic rings with and without endothelium from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR), divided into young, adult and old groups, were examined for relaxation in response to acetylcholine, adenosine triphosphate (ATP) and sodium nitroprusside. Relaxation responses to acetylcholine and ATP in SHR were either equal to or greater than those in WKY. Relaxations induced by ATP were not altered by ageing, while the degree of acetylcholine-induced endothelium-dependent relaxations was in an inverse order of age in both SHR and WKY. In old SHR and WKY and adult SHR, lower doses of acetylcholine caused relaxations, but increases in the dose of acetylcholine induced contraction. Acetylcholine-induced contractions were abolished by indomethacin or endothelium rubbing. We conclude that endothelium-dependent relaxations are not diminished by spontaneous hypertension but the relaxations mediated by the muscarinic receptors are reduced with ageing. Further, acetylcholine causes endothelium-dependent contractions by releasing cyclo-oxygenase products, not only in SHR but also in aged normotensive WKY.