Cardiac alpha-1 adrenoceptors are not involved in heart rate control of the anaesthetized dog.
To study the possible role of cardiac postsynaptic alpha-1 adrenoceptors in heart rate control of the anaesthetized open-chest dog we injected a specific alpha-1 agonist (amidephrine) into the right coronary artery or stimulated electrically the right stellate ganglion. Reflex influences were minimized by bilateral cervical vagotomy and de-afferentiation of both stellate ganglia. Activation of alpha-2, beta- and muscarinic receptors was prevented by intravenous administration of yohimbine, propranolol and atropine, respectively. Since alpha-1 receptor stimulation could affect heart rate indirectly via coronary constriction, a continuous intracoronary infusion of adenosine (0.25 mg/kg/h) was given. Amidephrine did not affect heart rate at the lower dose (1-10 microgram). After the highest dose (100 micrograms) the maximum variation in heart rate was an increase of 2.2 +/- 1.1 bpm at 3 min after injection (mean +/- SEM; P less than 0.05). This slight cardioacceleration was simultaneous with an aortic pressure rise of 13.8 +/- 3.4 mm Hg and it was abolished by alpha-1 blockade with prazosin (1 mg/kg i.v.). After propranolol (1 mg/kg +0.5 mg/kg/h) the residual positive chronotropic effect of sympathetic stimulation (12.2 +/- 4.0 bpm) was not significantly altered (13.8 +/- 5.7 bpm) by prazosin administration. Similar results were recorded without adenosine infusion. We conclude that in the anaesthetized dog chronotropic effects directly mediated by alpha-1 adrenoceptors either do not exist or lack physiological significance.
D000241:Adenosine / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000758:Anesthesia; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004285:Dogs; D004562:Electrocardiography; D004983:Ethanolamines / Q000494:pharmacology; D005260:Female; D006321:Heart / Q000502:physiology*; D006339:Heart Rate; D008297:Male; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013233:Stellate Ganglion / Q000502:physiology; D014628:Vagotomy
D000316:Adrenergic alpha-Agonists; D004983:Ethanolamines; D011942:Receptors, Adrenergic, alpha; C016980:amidephrine; D000241:Adenosine