We have recently shown that the intrinsic rate response to an increase in right atrial pressure is augmented when cardiac muscarinic receptors are activated. This present study examines the cardiac pacemaker response to vagal stimulation at different values of right atrial pressure in isolated rat right atrium and in the rabbit heart in situ. In the rat atrium, when pressure was raised in steps from 2 to 10 mmHg, there was a progressive reduction in the response to vagal stimulation [40.5 ± 7.2% reduction (mean ± SE) at 8 mmHg, P < 0.01], which was independent of the level of vagal bradycardia, that persisted in the presence of the -adrenergic agonist isoproterenol. In barbiturate-anesthetized rabbits with cervical vagi cut and -adrenergic blockade, raising right atrial pressure ~2.5 mmHg by blood volume expansion reduced the bradycardia elicited by electrical stimulation of the peripheral end of the right vagus nerve (9.1 ± 1.1% reduction, P < 0.0001). These results demonstrate that vagal bradycardia is modulated by the level of right atrial pressure and suggest that normally right atrial pressure may interact with cardiac vagal activity in the control of heart rate.

heart rate; intrinsic regulation

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