Pharmacologic inhomogeneity between the reactivity of intramural coronary arteries and arterioles

We hypothesized that because of their size, anatomic location, and hemodyna mic function, coronary arteries and arterioles would respond differently to vasoactive substances. Intramural arteries (281.7 +/- 23.1 mum) and arteri oles (77.3 +/- 6.6 mum) of the left anterior descending coronary of rats we re isolated and cannulated. Spontaneous tone was lower in arteries than in arterioles (81.1 +/- 5.7 vs. 53.0 +/- 3.9% of passive diameter, p < 0.05 at 60 mm Hg intraluminal pressure). Arterial tone was adjusted by the thrombo xane receptor agonist U46619 (5 x 10(-8) M) to reach an active tone close t o that of arterioles. Bradykinin elicited dilations in both types of vessel s. Acetylcholine (10(-6)-10(-5) M) dilated arteries (by 42.6 +/- 11.5 <mu>m ) but constricted arterioles (by 16.4 +/- 9.3 mum). Sodium nitroprusside an d adenosine elicited significantly greater dilations in arterioles than in arteries (by 7.9 and 11.9%, respectively, p < 0.05), whereas dilations to n orepinephrine were similar. Inhibition of nitric oxide synthesis caused a s ignificantly smaller constriction in arteries (10.2 +/- 3.31%) than in arte rioles (31.6 +/- 6.9%) and completely blocked bradykinin- and acetylcholine -induced dilations. whereas it did not affect dilations to sodium nitroprus side, adenosine, and norepinephrine. Compared with arteries, arterioles hav e a greater spontaneous tone and enhanced nitric oxide modulation of basal tone and exhibit greater responsiveness to nitric oxide and adenosine. In a ddition, nitric oxide synthase is activated differently by pharmacologic st imuli in these segments. The qualitative and quantitative differences among vasoactive responses of coronary arteries and arterioles demonstrated in t his study suggest segment-specific roles for endothelial and metabolic fact ors in regulation of coronary vascular resistance.