Responsiveness of sheep isolated coronary artery rings under simulated myocardial ischaemia.

The effects of simulated myocardial ischaemia on tone and on the responsiveness to contractile and relaxant agents were examined on sheep circumflex coronary artery rings. Introduction of ischaemia caused a transient relaxation followed by sustained contraction which was inhibited by adenosine or by the removal of the endothelium. Under ischaemic conditions, the contractile effects of 5-hydroxytryptamine and acetylcholine were markedly depressed while those of potassium and U46619 (a stable thromboxane analogue) were not modified. In contrast, the vasodilating effects of adenosine, noradrenaline and iloprost (a prostacyclin mimetic) were significantly potentiated. Addition of adenosine to the ischaemic Krebs solution abolished the contractile effects of 5-hydroxytryptamine and U46619 but did not modify the vasorelaxant effects of noradrenaline and iloprost. These results indicate that the ischaemic-induced contraction is endothelium-dependent and the responsiveness of the coronary artery to both constrictors and vasodilators changed under conditions of simulated myocardial ischaemia.