The effect of noradrenaline and isoprenaline were studied in the 15-day old chick atria. Noradrenaline increased both the rate and force of atrial contractions whilst isoprenaline only slightly increased force at very high concentrations. The increase in force after noradrenaline and isoprenaline was reduced by phentolamine, an alpha-adrenoceptor antagonist. The beta-antagonist, propranolol and the cardioselective beta 1-adrenoceptor antagonist, atenolol, did not reduce the positive inotropy. Noradrenaline-induced positive chronotropy was, however, resistant to both alpha- and beta-adrenoceptor blockade. It is concluded that the force of contraction of the chick atria may be alpha-adrenoceptor-mediated whilst the rate seems to be mediated by a mechanism sensitive to noradrenaline but insensitive to alpha- and beta-adrenoceptor blockade.
Adrenergic beta-Antagonists Age Factors