ARTERIAL CONTRACTIONS INDUCED BY CUMULATIVE ADDITION OF CALCIUM IN HYPERTENSIVE AND NORMOTENSIVE RATS - INFLUENCE OF ENDOTHELIUM

Responses to cumulative addition of Ca2+ (0.2-2.5 mM) after precontraction with potassium chloride (KCl) and noradrenaline in Ca2+-free medium were studied in isolated mesenteric arterial rings from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The Ca2+ contractions in 125 mM KCl-stimulated endothelium-denuded rings in the presence of atenolol (10 mu M) and phentolamine (10 mu M) were less marked in SHR than WKY, although the contractions to high concentrations of KCI in normal organ bath Ca2+ (1.6 mM) were similar in these strains. The difference in Ca2+ contractions between SHR and WKY during KCl stimulation was also present after 10-min pretreatment with 1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in Ca2+-free medium. However, when noradrenaline (1 mu M) was used as the agonist the Ca2+ contractions of endothelium-denuded rings in the two strains were comparable, while exposure to EGTA reduced these responses more effectively in SHR than WKY. Nifedipine (0.5 nM and 10 nM in KCl- and noradrenaline-stimulated rings, respectively) more efficiently inhibited the Ca2+ contractions in hypertensive than in normotensive rats. The presence of intact vascular endothelium attenuated the contractions to Ca2+ addition comparably (during KCl stimulation) or even more (during noradrenaline) in SHR when compared with WKY. N-G-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) counteracted this attenuation correspondingly in WKY and SHR, and L arginine (1 mM) restored it in both strains, whereas indomethacin (10 mM) was without effect on the response. However, mesenteric arterial relaxations induced by the endothelium-dependent agonists acetylcholine and ADP in noradrenaline-precontracted (1 mu M) rings were clearly impaired in SHR, and also L-NAME (0.1 mM) reduced the responses to acetylcholine more efficiently in SHR. In contrast, the relaxations to acetylcholine and ADP in KCl-precontracted (60 mM) rings in the absence and presence of L-NAME were comparable between the two strains. In conclusion, attenuated contractile response to cumulative Ca2+ addition during stimulation with KCl clearly differentiated arterial smooth muscle of hypertensive and normotensive rats, suggesting altered function of cell membrane in SHR. The more pronounced effect of nifedipine on the response indicates abnormal function of voltage-dependent Ca2+ channels, and higher diminishing effect of EGTA on the contraction during noradrenaline suggests exaggerated action of the chelator on membrane-bound Ca2+ in SHR. Interestingly, the depressant effect of intact endothelium on the Ca2+ con traction response, mediated largely via nitric oxide, was not attenuated in SHR. Furthermore, impaired endothelium-dependent agonist-induced relaxations can be attributed to reduced release of endothelium-derived hyperpolarizing factor in this type of genetic hypertension.