Guanethidine blocks neuropeptide-Y-like inhibitory action of sympathetic nerves on cardiac vagus.

In the anaesthetised dog there is prolonged attenuation of cardiac vagal action following electrical stimulation of the cardiac sympathetic nerve or intravenous injection of neuropeptide Y (NPY). The effect survives alpha- and beta-adrenoceptor blockade, and is not mimicked by exogenous noradrenaline. Guanethidine blocks the attenuation of cardiac vagal action following sympathetic stimulation but not after injection of NPY. It is suggested that NPY is released from cardiac sympathetic neurons with noradrenaline following activation of the sympathetic nerve and inhibits cardiac vagal action.