A constant-flow preparation was used to assess coronary arterial resistance while activating pulmonary C fibres in chloralose-anaesthetized dogs. A Gregg cannula was passed through the left common carotid artery until the tip fitted snugly in the left circumflex coronary artery. The cannula was perfused with blood from the left femoral artery and blood flow through the cannula was maintained constant with a perfusion pump. Perfusion pressure in the coronary artery was measured from a side arm in the cannula, and resistance in that vessel was calculated as the ratio of pressure to blood flow. Capsaicin (10-20 micrograms/kg), injected into the right jugular vein, produced significant (P less than 0.05) decreases in heart rate, systemic arterial pressure, coronary arterial pressure and coronary arterial resistance. These responses were abolished by bilateral vagotomy. Neither left atrial injections of capsaicin nor acutely decreasing the systemic arterial pressure by occluding the inferior vena cava had a significant effect on the coronary arterial pressure or resistance. Additionally, overdrive pacing of the heart had no effect on the reflex decreases in coronary arterial pressure and resistance that were evoked when capsaicin was injected into the jugular vein. alpha-Adrenergic blockade reduced the reflex decrease in coronary arterial pressure and resistance evoked by capsaicin by 31 and 38%, respectively. Combined alpha-adrenergic and cholinergic blockade reduced the reflex decreases in both coronary arterial pressure and resistance by approximately 89%. These results indicate that activation of pulmonary C fibres produces a reflex decrease in coronary arterial resistance, and that this response is due to the combined effects of increased parasympathetic cholinergic activity and withdrawal of alpha-adrenergic tone.
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