Stimulation of pulmonary C fibres decreases coronary arterial resistance in dogs.

Abstract

SUMMARY1. A constant-flow preparation was used to assess coronary arterial resistance while activating pulmonary C fibres in chloralose-anaesthetized dogs.2. 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.3. Capsaicin (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) ,ug/kg), injected into the right jugular vein, produced significant (P<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.4. a-Adrenergic blockade reduced the reflex decrease in coronary arterial pressure and resistance evoked by capsaicin by 31 and 38 %, respectively. Combined z-adrenergic and cholinergic blockade reduced the reflex decreases in both coronary arterial pressure and resistance by approximately 89 %.5. 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 oz-adrenergic tone.

Regulation of Coronary Blood Flow

“…Although data from Duncker et al indicate that the withdrawal of parasympathetic activity may facilitate β-adrenoceptor dilation in swine during exercise (280), the effects of the parasympathetic nervous system on the control of coronary blood flow are quite modest under most physiologic conditions (198, 280, 332, 335). However, there is evidence to support a role for parasympathetic coronary vasodilation in the carotid chemoreceptor reflex (430,442,700,703,950), Bezold-Jarisch reflex (186,330,1027), and during activation of afferent vagal C fibers in the lung (187, 734). …”

Reflex coronary vasodilation evoked by chemical stimulation of cardiac afferent vagal C fibres in dogs.

“…Even in studies using generally similar methods, the efferent pathways mediating the coronary vasomotor response to a given afferent input may vary. Thus, in chloralose-anaesthetized dogs, Clozel et al (1985) found that the coronary vasodilatation evoked by stimulating pulmonary C fibres was blocked by atropine, whereas Ordway & Pitetti (1986) found that the response was due to the combined effects of increased parasympathetic cholinergic activity and withdrawal of a-adrenergic tone.…”

Reflex effects on the coronary circulation

“…Lung inflation or activation of pulmonary receptors by foreign chemical agents has been shown to cause increases in coronary blood flow (Vatner & McRitchie, 1975;Clozel, Roberts, Hoffman, Coleridge & Coleridge, 1985;Ordway & Pitetti, 1986). In the studies which used chemical stimuli, myocardial blood flow was shown to increase in both subendocardial and subepicardial layers of the ventricle.…”