The dynamics of the chronotropic effect of a single vagal stimulus in cats under the action of met-enkephalin and neurotensin]

Following a burst of pulses applied to the vagus nerve with progressively incremental delay after the P wave of the ECG, the narrow zone of the cardiac cycle was identified where even a small shift of the vagal burst position evoked an abrupt alteration of the chronotropic effect magnitude. Met-enkephalin potentiated the phase-dependent vagal chronotropic effect, whereas neurotensin moved its limits toward the initial part of the P-P interval.