Attenuation of vagal action following sympathetic stimulation is modulated by prejunctional alpha 2-adrenoceptors in the dog.

We examined the effect of the alpha 2-adrenoceptor agonist clonidine and the alpha-adrenoceptor antagonist phentolamine on the attenuation of cardiac vagal action seen following stimulation of the cardiac sympathetic nerve. In the presence of phentolamine both the maximum percentage of inhibition of cardiac vagal action and the half-time to recovery were significantly increased. In the presence of clonidine both the maximum percentage of inhibition of vagal action and the half-time to recovery were significantly reduced. The results demonstrate that the inhibition of cardiac vagal action seen following sympathetic stimulation--an effect which is attributed to actions of the co-transmitter neuropeptide Y released during sympathetic stimulation--can be modulated by drugs acting on alpha-adrenoceptors.