Alpha-adrenoceptors do not contribute to the chronotropic or inotropic responses of the avian heart to noradrenaline

1. The chronotropic and inotropic responses of the young chick heart to noradrenaline have been investigated in isolated right atria, left atria and ventricular strips from 14-day-old chicks. 2. In the presence of desipramine and metanephrine to inhibit amine uptake, concentration-response curves to noradrenaline in all three tissues were shifted to the right by propranolol (1 microM) but were not altered by the presence of either phentolamine (5 microM) or prazosin (10 microM). 3. Similar results were obtained in the presence of cocaine (10 microM) and corticosterone (10 microM) to inhibit amine uptake. Propranolol (0.3-3.0 microM) produced rightward shifts of noradrenaline concentration-response curves which gave pA2 values of 8.1-8.4. Phentolamine (5 microM), in contrast, did not affect responses to noradrenaline in any tissue, either in the absence or presence of propranolol (1 microM). 4. Isoprenaline produced positive chronotropic responses in right atria and positive inotropic responses in left atria and ventricular strips. Methoxamine elicited positive inotropic responses in left atria but only negative chronotropic responses in right atria and negative inotropic responses in ventricular strips. 5. These results demonstrate that cardiac responses to noradrenaline in the chick heart are mediated via beta-adrenoceptors only, and that cardiac alpha-adrenoceptors are not involved in these responses.