THE RELEASE OF ACETYLCHOLINE BY SYMPATHETIC NERVE STIMULATION AT DIFFERENT FREQUENCIES.

Supramaximal stimulation of the periarterial nerves in the mesentery to the rabbit isolated ileum causes inhibition which is increased when hyoscine is added to the bath. This increase is, however, usually seen only when the stimulus frequency is low, and the addition of hyoscine usually makes no difference when the frequency is 10 shocks/sec or more. The observations suggest that, at low frequency, stimulation releases both acetylcholine and noradrenaline, but that at higher frequencies only noradrenaline is released. Similar observations have been made for the nictitating membrane of the cat. When the postganglionic fibres from the superior cervical ganglion were stimulated supramaximally, the contractions of the membrane increased in size as the frequency rose. In the presence of hyoscine the contractions were smaller, the greatest difference being for the lowest stimulus frequency, the difference diminishing as the frequency rose. These observations are consistent with the view that sympathetic cholinergic fibres in many situations release acetylcholine to act directly only at low frequencies, and that at higher frequencies the acetylcholine is almost entirely used to release noradrenaline.