Vasodilator response of coronary smooth muscle to the sympathetic co‐transmitters noradrenaline and adenosine 5′‐triphosphate

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Abstract
1 Vasodilator and vasoconstrictor responses to noradrenaline (NA), adenosine and adenosine 5′-triphosphate (ATP) were examined in isolated ring segments of the left anterior descending coronary artery of the rabbit in the absence of endothelium. 2 NA caused dose-dependent relaxation of potassium-constricted vessels in the absence of β-adrenoceptor blockade, with a pD2 of 5.96 ±0.21. This was inhibited by 1 μm propranolol. Constrictor responses of vessels at baseline tension were only seen at concentrations greater than 1 mm, and reached a maximum of 6% of the contraction to 30 mm KCl. 3 ATP caused relaxation of the potassium-constricted ring segments in a dose-dependent manner, although a transient constriction often preceded the relaxation. Adenosine was equipotent with ATP in producing relaxation; this was significantly inhibited by the P1-purinoceptor antagonist, 8-phenyltheophylline (8-PT). The responses to ATP were little affected by 8-PT, indicating that ATP was not acting through breakdown to adenosine. At basal tone, ATP produced transient vasoconstriction only at concentrations greater than 0.1 mm, reaching a maximum of 38% of the contraction to 30 mm KCl. 4 We conclude that in the rabbit coronary artery both NA and ATP produce vasodilatation by a direct action on the smooth muscle.