An experimental analysis of the tachycardia that follows vagal stimulation.

1. Postvagal tachycardia, the transient increase in heart rate that follows the sinus bradycardia elicited by vagal stimulation, was investigated in thirty chloralosed cats. Maximum postvagal tachycardia was elicited by stimulation at frequencies of 20-60 Hz with train durations of 30-90 sec. A positive correlation was demonstrated between the magnitudes of postvagal tachycardia and of the preceding sinus bradycardia.2. Postvagal tachycardia was not affected by either spinal transection at C7 or by administration of propranolol (1.5 mg/kg I.V.), but was abolished by the administration of atropine (2.0 mg/kg I.V.).3. Postvagal tachycardia was observed to follow the vagal bradycardia induced reflexly either by administration of phenyldiguanide (100-300 mug I.V.) or by stimulation of the aortic depressor nerve.4. In the isolated atria-vagus preparations from six rabbits a positive correlation was demonstrated between the magnitude of postvagal tachycardia and of the preceding bradycardia elicited by vagal stimulation.5. Continuous intracellular recordings were obtained from four sinuatrial node pace-maker cells in the isolated atria-vagus preparation of the rabbit before, during and after vagal stimulation. During postvagal tachycardia the slope of the diastolic prepotential, the maximum diastolic potential, threshold potential and the overshoot were found to be increased; these changes are different from those found in pace-maker cells during adrenergic activation.6. These findings demonstrate that postvagal tachycardia is not mediated by sympathetic adrenergic mechanisms, but suggest that it is dependent upon the preceding vagal bradycardia and may be related to an increase in net sodium influx into pace-maker cells initiated by the hyperpolarization of the pace-maker cell membrane during and immediately after vagal stimulation.