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 g). After the highest dose (100 g) the maximum variation in heart rate was an increase of 2.2±1.1 bpm at 3 min after injection (mean±SEM;P<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.