Noradrenaline-induced constriction of large and small coronary arteries in the anaesthetized dog.

1. In the anaesthetized dog external diameter of the left circumflex coronary artery and blood flow through that artery were measured to allow the effect of noradrenaline to be compared in large arteries and resistance vessels. 2. The injection of noradrenaline (0.5 micrograms kg-1) into the coronary artery, after bilateral vagotomy and antagonism of beta-adrenoreceptors, decreased large coronary artery diameter and coronary blood flow. Calculation of resistance in the large coronary artery and in the total left circumflex coronary vascular bed revealed that noradrenaline induced increases of 66% and 89% respectively. 3. Intra-coronary (i.c.) injection of the alpha 1-adrenoreceptor agonist phenylephrine (0.5-2 micrograms kg-1) decreased large coronary artery diameter and coronary blood flow. The alpha 2-adrenoreceptor agonist B-HT 920 (0.5-2 micrograms kg-1 i.c.) decreased coronary blood flow but did not significantly affect large coronary artery diameter. 4. Antagonism of alpha 1-adrenoreceptors with prazosin (10 micrograms kg-1 i.c.) abolished the noradrenaline-induced constriction of the large coronary artery but only partially attenuated the decrease in blood flow. The alpha 2-adrenoreceptor antagonist idazoxan (50 micrograms kg-1 i.c.) partially attenuated the noradrenaline-induced decrease in coronary blood flow but did not affect the large artery constriction. 5. It is concluded that noradrenaline constricts both large and small coronary arteries. Noradrenaline-induced constriction of the resistance vessels is more powerful than in the large artery. The constriction of large arteries in response to noradrenaline is mediated by alpha 1-adrenoreceptors. Postjunctional alpha 1- and alpha 2-adrenoreceptors are both involved in the constriction of the resistance vessels.