Differential beta-adrenergic sensitivity of atrial tissue assessed by chronotropic and inotropic responses to several sympathomimetic amines.

The relative inotropic and chronotropic activity of epinephrine, norepinephrine, isoproterenol, dopamine, dobutamine and ethylephrine was studied on isolated rat atria at 31 C. Dose-response curves were expressed as a percentage of the maximum response. Both atria were used for testing chronotropic effects. Inotropic dose-response curves were performed on left atria driven at 1 Hz, 1.6 Hz, 2.8 Hz and 6.6 Hz. Epinephrine and norepinephrine had a greater chronotropic action than their inotropic one throughout the range of stimulation employed. Their pD2-values changed from 8.12 +/- 0.06 and 8.05 +/- 0.05 to 7.19 +/- 0.03 and 7.14 +/- 0.05, respectively. Isoproterenol, dopamine and ethylephrine produced the same degree of chronotropic and inotropic stimulation. Dobutamine exerted the same effect on heart rate and developed tension at 1 Hz. pD2-values for dobutamine were significantly lower (p less than 0.01, p less than 0.001, p less than 0.001) when the frequency of stimulation was increased to 1.6, 2.8 and 6.6 Hz. These findings may be due to the influence of the negative staircase phenomenon, the temperature and the distribution of beta 1 and beta 2 receptors.