Ascending enteric reflex contraction: roles of acetylcholine and tachykinins in relation to distension and propagation of excitation.

The ascending reflex contraction of intestinal circular muscle involves both cholinergic and tachykininergic transmission, which are thought to be activated by different degrees of distension. Substance P, however, is colocalized with acetylcholine in myenteric neurons, and the present study examined the role of these transmitter substances in relation to low- and high-degree distension and with regard to ascending propagation of excitation. Reflex contractions in segments of the guinea pig isolated small intestine were evoked by inflation of an intraluminal balloon and recorded orally to the site of distension. Atropine (12.5 nM), hexamethonium (3 microM) and the neurokinin (NK)2-selective tachykinin antagonist MEN 10,376 (10 microM) inhibited contractions induced by low-degree distension to a larger extent than contractions induced by high-degree distension, whereas the receptor-nonselective tachykinin antagonist spantide (30 microM) did not differentiate in this way. Atropine, hexamethonium and spantide also depressed the propagation of excitation, i.e., the response recorded 2 cm away from the distension site was inhibited to a larger degree than the response recorded 1 cm away from the distension site. In contrast, MEN 10,376 did not interfere with the ascending propagation of excitation, and the NK1-selective tachykinin antagonist GR 82,334 (10 microM) was without effect on the ascending reflex contraction. These observations show that tachykinins and acetylcholine comediate ascending reflex contractions triggered by both low- and high-degree distension. When seen in context with the known projection of myenteric neurons, the findings relating to the ascending propagation of excitation indicate that NK2 receptors participate in neuromuscular transmission only, whereas neuroneuronal transmission involves both nicotinic and muscarinic acetylcholine receptors.