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The ability of endothelium to alter contractile events in phenylephrine (PE)-triggered contraction has been tested on ring segments of the thoracic aorta removed from obese Zucker rats (plasma cholesterol 3·63 mM; n = 8) and from age matched lean rats (plasma cholesterol 2·38 mM; n = 8). In normal medium, PE (1 μM) elicited similar contractions in endothelium-denuded arteries of both strains. However, the presence of endothelium reduced these contractile events and the endothelium-dependent relaxation induced by carbachol (10 μM) was higher in obese rats. In rings incubated in Ca2+free medium containing EGTA (1 mM), PE (1 μM) induced a phasic contraction and a sustained contraction following addition of Ca2+(2·5 mM) to the medium. The phasic contraction was due to intracellular Ca + release, whereas the sustained response was dependent on extracellular Ca2+influx. In endothelium-free preparations, the size of both the phasic and sustained contraction was similar for the two strains. The Ca2+antagonist gallopamil (1 μM) reduced the sustained contraction of lean (24%) and obese (34%) rats without affecting the phasic contraction. In preparations possessing endothelium, the sustained, but not the phasic contraction, of both strains was inhibited. This inhibitory effect of endothelium on the sustained contraction was significantly higher in obese than in lean rats. Thus, it can be concluded that phenylephrine elicited quantitatively and qualitatively similar contractions in obese and lean rats. In both strains, the endothelium diminished the contraction induced by PE, however, this effect was more pronounced in obese rats than in lean ones. These results may explain, in part, the described absence of atherosclerotic lesions in the obese strain.

We investigated the constrictor effects of Angiotensin I (Ang I) and Angiotensin II (Ang II) on rabbit peripheral (aorta, carotid artery, mesenteric artery, saphenous artery) and cerebral (basilar artery) vessels and in rat aorta in functional organ bath studies. The effect of angiotensin converting enzyme (ACE) inhibition by captopril was also assessed in these preparations. Ang II elicited concentration-dependent contractions with comparable potency in rabbit and rat endothelium-free vascular rings (pD2 about 8.5) which indicates a lack of species and regional variation in the contractile responses to Ang II. The responses to Ang II were reduced by the presence of a functional endothelium in rabbit mesenteric artery and in rat aorta. Since ACE determines the plasma and tissue conversion of Ang I to active Ang II, we calculated the ratio R (EC50 Ang I-induced contraction: EC50 Ang II-induced contraction) as an indicator of the tissue ACE effectiveness. In the aorta without endothelium, Ang I was found to be much less potent than Ang II in the rabbit (R = 44) compared with the rat (R = 3.5). This species difference in the aortic conversion of Ang I to Ang II was confirmed by the use of captopril. Captopril (10-6M) shifted the Ang I concentration/response curve by 2- and 14-fold to the right in rabbit and rat respectively. In other rabbit blood vessels, the rank order of potency to Ang I in endothelium denuded rings was basilar artery » carotid artery = aorta = saphenous artery. In addition, the R value was the lowest for the basilar artery (R = 2.5). This is in agreement with the highest rightward shift (78-fold) of the Ang I concentration/response curve by captopril for basilar artery in comparison with only 3-, 8- and 3-fold shifts observed in carotid artery, saphenous artery and aorta respectively. In conclusion, our data provide evidence for a greater influence of ACE in rabbit basilar artery than in peripheral vessels.