Cervical efferent vagal stimulation related or unrelated to cardiac cycles: comparison of negative atrial inotropic effects.

Efferent cervical vagal stimulations were performed in 4 anesthetized dogs, either with trains of stimuli of constant frequency (CONT), or with brief trains, triggered by the atrial electrical activity (TRIG). Heart rate was increased by cardiac pacing during vagal stimulations. Negative inotropic effects of vagal stimulations, evaluated as changes in atrial contractility, were partially inhibited by cardiac acceleration during CONT, but were unaffected during TRIG. It is concluded that the number of stimuli per cycle, not the number of stimuli per second, is the critical factor for parasympathetic heart regulation.