Vagal chemoreflex coronary vasodilation evoked by stimulating pulmonary C-fibers in dogs

We performed experiments on anesthetized, open-chest dogs to determine whether the pulmonary chemoreflex (bradycardia and systemic hypotension) evoked by stimulating pulmonary C-fibers also involves reflex changes in coronary vascular resistance. We perfused the circumflex coronary artery at constant pressure (usually 100 mm Hg) and recorded mean circumflex blood flow. Stimulation of pulmonary C-fibers by right atrial injection of capsaicin (10 micrograms/kg) decreased arterial blood pressure and heart rate and increased circumflex blood flow by 32-109% (P less than 0.001). Circumflex blood flow also increased, by 26-100% (P less than 0.001), when heart rate was kept constant by pacing. Coronary vasodilation was not secondary to the reflex decrease in arterial blood pressure. Injecting capsaicin (10 micrograms/kg) into the left atrium did not increase circumflex blood flow. Reflex coronary vasodilation could still be evoked when myelinated nerve fibers were blocked selectively by cooling the cervical vagus nerves to 7-8 degrees C but was abolished by cooling to 0 degrees C, by cutting the pulmonary vagal branches, or by giving atropine. Reducing coronary perfusion pressure shifted the stimulus (dose of capsaicin)-response (increase in coronary blood flow) curve to the right, but, even at low perfusion pressures, significant reflex vasodilation still occurred. Regional (transmural) distribution of myocardial blood flow was measured by the microsphere technique at various perfusion pressures. The endocardial:epicardial blood flow ratio decreased significantly as perfusion pressure was reduced, but was not altered by right atrial injection of capsaicin at any perfusion pressure. Our results indicate that stimulation of pulmonary C-fibers triggers reflex cholinergic vasodilation in all layers of the myocardium.