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Contractile responsiveness of coronary arteries from exercise-trained rats.

The purpose of this study was to determine whether exercise training alters vasomotor reactivity of rat coronary arteries. In vitro isometric microvessel techniques were used to evaluate vasomotor properties of proximal left anterior artery rings (1 ring per animal) from exercise-trained rats (ET; n = 10) subjected to a 12-wk treadmill training protocol (32 m/min, 15% incline, 1 h/day, 5 days/wk) and control rats (C; n = 6) restricted to cage activity. No differences in passive length-tension characteristics or internal diameter (158 +/- 9 and 166 +/- 9 micron) were observed between vessels of C and ET rats. Concentration-response curves to K+ (5-100 mM), prostaglandin F2alpha (10(-8)-10(-4) M), and norepinephrine (10(-8)-10(-4)) were unaltered (P > 0.05) in coronary rings from ET rats compared with C rats; however, lower values of the concentration producing 50% of the maximal contractile response in rings from ET rats (P = 0.05) suggest that contractile sensitivity to norepinephrine was enhanced. Vasorelaxation responses to sodium nitroprusside (10(-9)-10(-4) M) and adenosine (10(-9)-10(-4) M) were not different (P > 0.05) between vessels of C and ET rats. However, relaxation responses to the endothelium-dependent vasodilator acetylcholine (ACh; 10(-10)-10(-4) M) were significantly blunted (P < 0.001) in coronary rings from ET animals; maximal ACh relaxation averaged 90 +/- 5 and 46 +/- 12%, respectively, in vessels of C and ET groups. In additional experiments, two coronary rings (proximal and distal) were isolated from each C (n = 7) and ET (n = 7) animal. Proximal coronary artery rings from ET animals demonstrated decreased relaxation responses to ACh; however, ACh-mediated relaxation of distal coronary rings was not different between C and ET groups. NG-monomethyl-L-arginine (inhibitor of nitric oxide synthase) blocked ACh relaxation of all rings. L-Arginine (substrate for nitric oxide synthase) did not improve the blunted ACh relaxation in proximal coronary artery rings from ET rats. These studies suggest that exercise-training selectively decreases endothelium-dependent (ACh) but not endothelium-independent (sodium nitroprusside) relaxation responses of rat proximal coronary arteries; endothelium-dependent relaxation of distal coronary arteries is unaltered by training.