[The action of met-enkephalin on sinus arrhythmia during the burst stimulation of the vagus nerve].

The ranges of the controlled bradycardia were obtained in experimental anaesthetized cats when peripheral cut end of the right vagus nerve was stimulated with bursts of impulses. Within the range each burst of impulses delivered on the vagus induced a single heart contraction and the heart synchronously reproduced any changes of vagal burst frequency. Sinus arrhythmia was observed when vagal burst frequency was beyond the limits of the controlled bradycardia range. Sinus arrhythmia discontinued or considerably decreased after met-enkephalin injection that contributed to the appearance of controlled bradycardia again. It is concluded that met-enkephalin increased the susceptibility of the heart to the series of vagal bursts arriving from the central nervous system.