Abstract
We investigated the β 2-adrenoceptors-mediated effects of atrial and ventricular effective refractory period (ERP), SA node pacemaker activity, and AV conductivity induced by sympathetic nerve stimulation or epinephrine infusion in anesthetized dogs. A β 2-adrenoceptors antagonist, ICI 118,551 up to 100 μg/kg, i.v., inhibited the positive chronotropic and dromotropic responses to sympathetic stimulation but did not shorten the atrial or ventricular ERP. ICI 118,551 also attenuated the positive chronotropic and dromotropic responses and the shortening of atrial ERP in response to epinephrine but not the shortening of ventricular ERP. A selective β 1-adrenoceptor antagonist, atenolol, inhibited each electrical cardiac response to sympathetic stimulation and epinephrine infusion in a similar manner. These results suggest that β 2-adrenoceptors-mediated electrical cardiac responses to endogenous catecholamines also exist in addition to the predominant β 1-adrenoceptors-mediated responses, and that the order of the proportion of β 2-adrenoceptors-mediated cardiac effects was SA node pacemaker activity ⪢AV conductivity = atrial ERP ⪢ ventricular ERP in the dog heart.
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