Responses of the rabbit epicardial coronary artery to acetylcholine and adrenoceptor agonists.

STUDY OBJECTIVE

The aim was to identify the role of the endothelium in mediating the responses to acetylcholine in the rabbit coronary artery, and to determine whether alpha or beta adrenergic stimulation may cause relaxation via endothelial receptors in the coronary arteries of this species.

DESIGN

Responses to acetylcholine and adrenoceptor agonists were compared in isolated ring preparations with and without endothelium. The adrenoceptor agonists were examined in the presence of phentolamine or propranolol to block alpha and beta adrenoceptors, respectively.

EXPERIMENTAL MATERIAL

30 New Zealand white rabbits (2.3-3.4 kg) were killed by an overdose of barbiturate and exsanguination, and the left epicardial coronary artery was dissected free. Ring preparations were suspended in organ baths under isometric tension and, where required, the tone of the preparations was raised by KC1.

MEASUREMENTS AND MAIN RESULTS

Concentrations of acetylcholine up to 10(-6) mol.litre-1 produced dose dependent relaxation of the preparations with endothelium intact, but no relaxation in preparations denuded of endothelium. At higher concentrations, a marked vasoconstrictor response was seen in all preparations regardless of the presence of endothelium. At basal tone, acetylcholine produced vasoconstriction which reached a maximum of 1.0 (SEM 0.14)g tension in preparations with endothelium and 1.74(0.27) g tension in those without endothelium (p less than 0.05). In coronary arteries pretreated with 50 mumol.litre-1 phenoxybenzamine to block alpha adrenoceptors, noradrenaline, isoprenaline, and salbutamol produced dose dependent relaxation of the preparations; this was unaffected by the absence of endothelium. In vessels not pretreated with phenoxybenzamine, propranolol inhibited the relaxation to noradrenaline and isoprenaline but again there was no difference between vessels with and without endothelium.

CONCLUSIONS

In the rabbit isolated epicardial coronary artery, acetylcholine produces an endothelium dependent relaxant response over a limited concentration range; a vasoconstrictor response via smooth muscle dominates at higher concentrations. beta Adrenoceptors mediating relaxation are present on the smooth muscle, but there was no evidence for either alpha or beta adrenoceptor mediated responses via the endothelium. Important differences with coronary arteries from other species are discussed.