Cardiac effects produced by long-term stimulation of thoracic autonomic ganglia or nerves: implications for interneuronal interactions within the thoracic autonomic nervous system.

Electrical stimulation of an acutely decentralized stellate or middle cervical ganglion or cardiopulmonary nerve augments cardiac chronotropism or inotropism; as the stimulation continues there is a gradual reduction of this augmentation following the peak response, i.e., an inhibition of augmentation. The amount of this inhibition was found to be dependent upon the region of the heart investigated and the neural structure stimulated. The cardiac parameters which were augmented the most displayed the greatest inhibition. Maximum augmentation or inhibition occurred, in most instances, when 5-20 Hz stimuli were used. Inhibition of augmentation was overcome when the stimulation frequency was subsequently increased or following the administration of nicotine or tyramine, indicating that the inhibition was not primarily due to the lack of availability of noradrenaline in the nerve terminals of the efferent postganglionic sympathetic neurons. Furthermore, as infusions of isoproterenol or noradrenaline during the period of inhibition could still augment cardiac responses, whereas during the early peak responses they did not, the inhibition of augmentation does not appear to be due primarily to down regulation of cardiac myocyte beta-adrenergic receptors. The inhibition was modified by hexamethonium but not by phentolamine or atropine. Inhibition occurred when all ipsilateral cardiopulmonary nerves connected with acutely decentralized middle cervical and stellate ganglia were stimulated, whereas significant inhibition did not occur when these nerves were stimulated after they had been disconnected from the ipsilateral decentralized ganglia. Taken together these data indicate that the inhibition of cardiac augmentation which occurs during relatively long-term stimulation of intrathoracic sympathetic neural elements is due in large part to nicotinic cholinergic synaptic mechanisms that lie primarily in the major thoracic autonomic ganglia. They also indicate that long-term stimulation in intrathoracic sympathetic neural elements with frequencies as low as 2 Hz may augment the heart as much as higher stimulation frequencies, depending upon the structure stimulated and the cardiovascular parameter monitored.