The effect of the carotid sinus reflex on large coronary artery diameter in anaesthetized dogs

Reflex epicardial coronary vasoconstriction elicited by nicotine in anaesthetized dogs

The effects of arterial chemoreceptor activation by nicotine on coronary artery diameter was studied in anaesthetized, artificially ventilated dogs. Left circumflex coronary artery diameter, coronary blood flow, calculated mean coronary resistance, systemic arterial blood pressure and heart rate were measured. In control dogs (n = 10) the injection of nicotine (100 micrograms) into the carotid artery evoked an increase of arterial pressure (+22 +/- 9 mm Hg) and a decrease in heart rate (-36 +/- 13 beats/min), and tended to increase coronary blood flow (+7 +/- 4 ml/min). Intracarotid nicotine had no effect on large coronary artery diameter (+0.02 +/- 0.03 mm) or total coronary resistance (+0.04 +/- 0.09 mm Hg min/ml) under these conditions. When heart rate was controlled by (1) beta-adrenoceptor blockade (propranolol, 1 mg/kg i.v.) plus pacing of the right ventricle (n = 4) or (2) beta-adrenoceptor blockade plus bilateral vagotomy (n = 7), the chemoreflex-induced constriction of the large coronary artery (-0.07 +/- 0.02 mm and -0.12 +/- 0.03 mm, respectively; p less than 0.05). In contrast, there was no chemoreflex-induced change in total coronary resistance after beta-adrenoceptor blockade plus pacing (+0.01 +/- 0.09 mm Hg min/ml, but after beta-adrenoceptor blockade plus vagotomy coronary resistance was increased (+0.75 +/- 0.31 mm Hg min/ml; p less than 0.05). The constriction of both large and small coronary arteries was abolished by phentolamine (0.5 mg/kg i.v.). These results suggest that carotid body chemoreceptor stimulation by nicotine can produce reflex alpha-adrenoceptor-mediated constriction of both large and small coronary arteries, and that the constriction of the small vessels is balanced by vagally-mediated dilatation.

Noradrenaline-induced constriction of large and small coronary arteries in the anaesthetized dog

1. In the anaesthetized dog external diameter of the left circumflex coronary artery and blood flow through that artery were measured to allow the effect of noradrenaline to be compared in large arteries and resistance vessels. 2. The injection of noradrenaline (0.5 micrograms kg-1) into the coronary artery, after bilateral vagotomy and antagonism of beta-adrenoreceptors, decreased large coronary artery diameter and coronary blood flow. Calculation of resistance in the large coronary artery and in the total left circumflex coronary vascular bed revealed that noradrenaline induced increases of 66% and 89% respectively. 3. Intra-coronary (i.c.) injection of the alpha 1-adrenoreceptor agonist phenylephrine (0.5-2 micrograms kg-1) decreased large coronary artery diameter and coronary blood flow. The alpha 2-adrenoreceptor agonist B-HT 920 (0.5-2 micrograms kg-1 i.c.) decreased coronary blood flow but did not significantly affect large coronary artery diameter. 4. Antagonism of alpha 1-adrenoreceptors with prazosin (10 micrograms kg-1 i.c.) abolished the noradrenaline-induced constriction of the large coronary artery but only partially attenuated the decrease in blood flow. The alpha 2-adrenoreceptor antagonist idazoxan (50 micrograms kg-1 i.c.) partially attenuated the noradrenaline-induced decrease in coronary blood flow but did not affect the large artery constriction. 5. It is concluded that noradrenaline constricts both large and small coronary arteries. Noradrenaline-induced constriction of the resistance vessels is more powerful than in the large artery. The constriction of large arteries in response to noradrenaline is mediated by alpha 1-adrenoreceptors. Postjunctional alpha 1- and alpha 2-adrenoreceptors are both involved in the constriction of the resistance vessels.

Adrenoceptor subtypes involved in the baroreceptor reflex constriction of large coronary arteries in the anaesthetized dog

The baroreceptor reflex was invoked by bilateral occlusion of the carotid arteries in anaesthetized dogs. The effect of bilateral carotid occlusion on the diameter of the left circumflex coronary artery and late diastolic coronary resistance was examined after bilateral vagotomy and antagonism of beta-adrenoceptors and then compared to responses after selective antagonism of alpha 1- and alpha 2-adrenoceptors. In the absence of any change in heart rate, bilateral carotid occlusion decreased coronary artery diameter (-8 +/- 1%) and increased coronary resistance (59 +/- 14%). Prazosin (0.01 mg/kg i.a.), a selective antagonist of alpha 1-adrenoceptors, significantly attenuated the bilateral carotid occlusion-induced changes in coronary artery diameter (-2 +/- 2%) and late diastolic coronary resistance (20 +/- 11%). Selective antagonism of alpha 2-adrenoceptors by the intra-coronary injection of idazoxan (0.05 mg/kg) significantly reduced the bilateral carotid occlusion-induced increase in coronary resistance (14 +/- 12%) but did not affect the large artery constriction (-8 +/- 4%). When injected into the coronary circulation the alpha 1-adrenoceptor agonist phenylephrine constricted both the large artery and the resistance vessels. In contrast B-HT 920, a selective alpha 2-adrenoceptor agonist, constricted the resistance vessels but did not affect large coronary artery tone. The responses to phenylephrine and B-HT 920 were selectively antagonised by prazosin and idazoxan respectively. Reflex activation of the sympathetic nervous system results in constriction of both large coronary arteries and coronary resistance vessels when there is no change in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)