Short-term exercise training enhances reflex cholinergic nitric oxide-dependent coronary vasodilation in conscious dogs.

The effects of exercise training on the coronary vasodilation following activation of the Bezold-Jarisch reflex were examined in conscious dogs. Mongrel dogs were chronically instrumented using sterile techniques for measurements of systemic hemodynamics and left circumflex coronary blood flow (CBF). With the heart rate controlled (150 bpm), veratrine (0.5 to 20 micrograms/kg) caused dose-dependent increases in CBF; eg, 5 micrograms/kg of veratrine increased CBF by 61 +/- 6% from 31 +/- 1.3 mL/min (P < .05). After exercise training, the dose-response curve of CBF in response to veratrine was shifted to the left; eg, 5 micrograms/kg of veratrine increased CBF by 101 +/- 12% (P < .05 compared with control) from 34 +/- 2.3 mL/min. The enhanced coronary vasodilation was blunted by nitro-L-arginine (NLA, 35 mg/kg). In anesthetized dogs after exercise training, electrical stimulation of the left vagus nerve caused greater increases in CBF, and NLA inhibited increases in CBF. Acetylcholine, norepinephrine, angiotensin II, and bradykinin caused greater increases in NO2- production in coronary microvessels from exercise-trained dogs compared with those from normal dogs. Our results indicate that the coronary vasodilation following activation of the Bezold-Jarisch reflex is enhanced in conscious dogs after exercise training. Since electrical stimulation of the vagus nerve caused greater coronary vasodilation and since the agonists resulted in greater increases in NO production in coronary microvessels from exercise-trained dogs, the mechanism responsible for the enhanced coronary vasodilation following activation of the Bezold-Jarisch reflex is most likely due to the increased release of NO from the endothelial cells.