Vasoconstrictor responses of coronary resistance arteries in exercise- trained pigs

Abstract

Coronary resistance arteries isolated from exercise-trained pigs have been shown to exhibit enhanced myogenic reactivity (J. M. Muller, P. R. Myers, and M. Harold Laughlin. J. Appl. Physiol. 75: 2677-2682, 1993). The purpose of this study was to test the hypothesis that exercise training results in enhanced vasoconstrictor responses of these arteries to all vasoconstrictor stimuli [specifically acetylcholine (ACh), endothelin-1 (ET- 1), KCl, and the Ca2+ channel-agonist Bay K 8644]. Female Yucatan miniature swine were trained (Trn) on a motor-driven treadmill (n = 16) or remained sedentary (Sed, n = 15) for 16-20 wk. Arteries 50-120 μm in diameter were isolated and cannulated with micropipettes, and intraluminal pressure was set at 60 cmH2O throughout experiments. Vasoreactivity was evaluated by examining constrictor responses to increasing concentrations of ACh (10-9 to 10-4 M), ET-1 (10-10 to 10-8 M), KCl (bath replacement with isotonic physiological saline solution containing 30 or 80 mM), and Bay K 8644 (10-9 to 10-6 M). Constricted diameters are expressed relative to the passive diameter observed after 100 μM SNP. All four constrictors produced similar decreases in diameter in arteries from both groups [ACh: 0.52 ± 0.07 (Trn) and 0.54 ± 0,06 (Sed); ET-1: 0.66 ± 0.05 (Trn) and 0.70 ± 0.07 (Sed); KCl: 0.66 ± 0.05 (Trn) and 0.70 ± 0.07 (Sed); Bay K 8644: 0.86 ± 0.05 (Trn) and 0.76 ± 0.05 (Sed)]. Present results combined with previous observations indicate that exercise training does not alter vasoconstrictor responses of porcine coronary resistance arteries but specifically increases myogenic reactivity. Thus the underlying cellular mechanisms for myogenic tone are altered by training but not receptor- mediated mechanisms (ACh and ET-1) nor voltage-gated Ca2+ channels (KCl and Bay K 8644) in coronary resistance arteries.