Effect of high extracellular Ca(2+) levels in spontaneously hypertensive rat aorta

The release of endothelial relaxing factors has been suggested to be important in modulating the inhibition of the contractile activity caused by the increase in extracellular Ca(2+) concentration in arterial tissue. Since the hypertensive process in spontaneously hypertensive rats (SHR) could be associated with the release of endothelial vasoconstrictor factors (mainly cyclooxygenase-dependent endoperoxides and endothelin-1), we studied the contractile responses to KCl, methoxamine and phenylephrine in different aorta ring preparations (intact, de-endothelized, 10(-5) M indomethacin-treated, 10(-6) M CGS-27830 [meso-1,4-dihydro-5-methoxycarbonyl-2, 6-dimethyl-4-(3-nitrophenyl)-3-pyridine carboxylic acid anhydride]-treated, and treated simultaneously with 10(-5) M indomethacin and 10(-6) M CGS-27830) from SHR and normotensive Wistar Kyoto rats (WKY), at various Ca(2+) concentrations (1.25, 2.5, 5 and 10 mM) in the organ bath. In endothelium-intact preparations from WKY rats we observed a decrease in KCl, methoxamine and phenylephrine contractions with high Ca(2+) concentrations (5 and 10 mM), but in the endothelium-intact preparations from SHR, the increase in extracellular Ca(2+) concentration potentiated methoxamine contractions and caused no change in KCl and phenylephrine contractions. When the endothelium was disrupted in preparations from both WKY rats and SHR, we observed a decrease in KCl and methoxamine contractions with high Ca(2+) concentrations. The decrease in phenylephrine contractions caused by high Ca(2+) concentrations was clear in de-endothelized preparations from WKY rats but slight in de-endothelized preparations from SHR. In all indomethacin- and CGS-27830-treated preparations, and also in the preparations from WKY rats and SHR treated with both drugs, we observed a decrease in all the contractile responses with increased Ca(2+) concentration. Besides, there was a clear reduction in the responses of the alpha(1)-adrenoceptor agonists in the WKY and SHR preparations treated with both drugs. The results indicate that, in the hypertensive arteries, endothelium-derived contractile factors can counteract the relaxing effect of high extracellular Ca(2+) concentrations.