Action of noradrenaline on isolated proximal and distal coronary arteries of rat: selective release of endothelium‐derived relaxing factor in proximal arteries

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Abstract
1 The effect of noradrenaline (NA) on the vascular smooth muscle and endothelial cells in isolated ring segments from the proximal and distal part of the left coronary artery (LCA) in rats was examined. 2 NA had a weak concentration-dependent contractile effect on proximal but relaxed distal segments of the LCA. The maximal NA-induced contraction of the proximal segments was doubled while the relaxation of the distal LCA segments was converted to a contraction after blockade of β-adrenoceptors with propranolol 3 × 10−6m, thus indicating the presence of both α- and β-adrenoceptors in the arteries, with dominance of α-adrenoceptors and of β-adrenoceptors in the proximal and distal segments of the LCA, respectively. 3 The contractile effect of NA (β-adrenoceptors blocked) was doubled in the proximal LCA segments after the endothelium was removed. Endothelial denudation had, in contrast, no potentiating effect on the contractile response of the distal arteries to NA. Both proximal and distal segments became more sensitive to the contractile action of NA after removal of the endothelium. 4 The spontaneous myogenic tone increased in both proximal and distal LCAs after endothelial removal, indicating spontaneous release of a relaxing endothelial factor in the vessels. 5 Following contraction with prostaglandin F2α (PGF2α), and in the presence of propranolol, 3 × 10−6 m, and prazosin, 10−6 m, NA induced an endothelium-dependent relaxation of only proximal but not distal segments of the precontracted LCA. The NA-induced relaxation of the proximal segments of the LCA was not altered by indomethacin 10−5 m but was completely abolished after incubation with methylene blue, 3 × 10−6 m, or following endothelium removal. These results are compatible with NA-induced release of EDRF in these arteries. 6 The selective α2-adrenoceptor agonist, B-HT 933, only induced a weak relaxation of PGF2α-precontracted proximal (endothelium intact) LCA segments at a concentration of 10−4m. The NA-induced relaxation of these vessels was unaffected by incubating the vessels with 10−5 m B-HT 933. The NA relaxation response curve was shifted ca 1.1 log unit to the right by rauwolscine, 10−6m, giving an estimated pA2-value of 7.12. The receptor through which NA activates the endothelium appears to be of an atypical α2-subtype.