Regional differences of responses to adrenoceptor agonists in isolated and perfused canine coronary arteries.

Regional differences of the responses to adrenoceptor agonists were investigated in isolated canine coronary arteries by use of a cannula inserting method. Acetylcholine induced a dose-dependent vasodilation. Norepinephrine and epinephrine produced a vasoconstriction followed by a strong vasodilation in large coronary arteries and only a weak vasodilation in small coronary arteries. Phenylephrine (a selective alpha-1 agonist) induced a strong vasoconstriction in both arteries. The threshold dose and ED50 value for phenylephrine in small coronary arteries were much larger than those in large coronary arteries, although the vasoconstrictions by KCl and prostaglandin F2 alpha were not different between in large and small coronary arteries. Clonidine and xylazine (selective alpha-2 agonists) produced a slight vasoconstriction but not dose-dependently and a vasodilation with extremely large doses. ED50 value of vasodilation for salbutamol (a selective beta-2 agonist) was approximately 80 times greater than that for isoproterenol (a non-selective beta-agonist) in large coronary arteries, but was approximately 20 times in small coronary arteries. The maximal dilator response to salbutamol was about the same as that to isoproterenol in small coronary arteries, whereas it was much smaller than that to isoproterenol in large coronary arteries. These results suggest that adrenoceptors are heterogeneous according to the distance from the coronary orifice in canine epicardial coronary arteries.