Abstract
Physiological and pharmacological analyses were made on the relaxing responses of intestinal smooth muscle elicited by the electrical stimulation and by ganglion stimulants. The relaxation produced by the perivascular nerve stimulation and the transmural stimulation at high frequencies were prevented by adrenergic blocking agents, whereas those produced by the transmural stimulation at low frequencies and by the ganglion stimulant (nicotine) were resistant to them. Furthermore, this resistant type was abolished in the presence of tetrodotoxin. These results imply that the relaxing responses elicited by the transmural stimulation of low frequencies may involve the non-adrenergic inhibitory nerve system. Some substances were tested to mimic the relaxing response to the transmural stimulation. Dopamine produced a relaxation which was abolished by an adrenergic blocking agent (phentolamine). 5-Hydroxytryptamine (5-HT) caused a contraction which was reversed to a relaxation by atropine. This relaxing effect of 5-HT is mediated by the nerve, since the relaxation by 5-HT is completely prevented by tetrodotoxin. GABA produced only contractions and the other amino acids tested failed to produce a relaxation. These trials to mimic the relaxing response due to the transmural stimulation by some naturally occuring substances were unsuccessful, and at the present time, the nature of the transmitter of the non-adrenergic inhibitory nerves remains unknown.
