Temporal changes in he sympathetic-parasympathetic interactions that occur in the perfused canine atrium

In the isolated, blood-perfused, canine right atrium, stimulation of the intramural autonomic nerves evoked negative chronotropic and inotropic responses. The responses were not maintained at a constant level during tonic neural stimulation, but they tended to drift back toward their control levels. These time-dependent changes in the cardiac responses were more pronounced the higher the frequency of stimulation. After the beta-adrenergic receptors were blocked in half of the preparations, the negative cardiac responses to autonomic neural stimulation were more pronounced, but the time dependency of those responses was less. After the muscarinic receptors were blocked in the other half of the preparations, only positive responses to neural stimulation were observed. These responses faded significantly at a high stimulation frequency (30 Hz), but not at a lower frequency (5 Hz). The cardiac responses to combined autonomic neural stimulation were substantially more negative than the algebraic sum of the individual responses to sympathetic and parasympathetic stimulation. The extent of this interaction was most pronounced near the beginning of stimulation, but it became less pronounced as the stimulation progressed. Hence, the cardiac sympathetic-parasympathetic interactions change appreciably with time during a continuous train of autonomic neural stimulation.