Neurotransmitters involved in the colonic contraction and vasodilatation elicited by activation of the pelvic nerves in the cat.

Stimulation of the pelvic nerves causes a contraction of and an increased blood flow in the feline colon. This study was performed to analyze the possible neurotransmitters involved in this response. The colonic contraction and hyperemia caused by pelvic nerve stimulation (5 Hz, 5 ms, 5 V) were not affected by the substance P antagonist (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-substance P, by substance P tachyphylaxis, or by naloxone (0.2-0.3 mg/kg body wt, i.a.). However, after treatment with atropine (0.5 mg/kg body wt, i.v.), naloxone blocked the contraction evoked by pelvic nerve stimulation (p less than 0.01), whereas the vasodilator response was unchanged. Distension of the rectum with a water-filled balloon or mechanical stimulation of the anal wall with a glass rod elicits a pelvo-pelvic reflex with colonic contraction and hyperemia. The contraction elicited by rectal distension was completely abolished by atropine (0.5 mg/kg body wt, i.v.). However, in atropinized animals, mechanical stimulation of the anal wall caused a contraction of the distal colon that was blocked by naloxone (0.2-0.3 mg/kg body wt, i.a.). The hyperemia was not affected by atropine or by naloxone. The results indicate the existence of at least three motor-neuronal systems in the distal colon: one cholinergic and one enkephalinergic system mediating colonic contractions, and one system mediating vasodilatations with another transmitter.