The mechanism of alpha-adrenergic activation of the dog coronary artery.

Norepinephrine (NE) activates isolated coronary conduit arteries by stimulating Ca2+ uptake into the smooth muscle cells. Blockade of Ca2+ influx by removal of Ca2+ from the bathing medium or addition of 10 mM LaCl3 prevents the NE-induced contraction in the dog coronary artery but still allows NE to induce a rapid transient contraction in the rabbit aorta. Under these conditions, NE stimulates 45Ca2+ efflux from rabbit aorta but fails to do so in the coronary artery. The difference in behaviour between the two arteries is attributed by the presence of an intracellular NE-sensitive Ca2+ fraction in the rabbit aorta and its absence from the dog coronary artery. This difference also explains the much greater sensitivity of the NE-induced contractions in the dog coronary to the relaxant effects of the Ca2+ antagonists, D600 and SKF525A, than that seen in the rabbit aorta. High K+-induced contractions of both the coronary artery and the aorta are equally sensitive to the Ca2+ antagonists.