Substance P enteric neurons mediate non-colinergic transmission to the circular muscle of the guinea-pig intestine

The sites of action and possible roles of substance P in contracting the circular muscle of the guinea-pig ileum were studied using two analogues of substance P that act as antagonists of some of its actions. These are d-Arg¹, d-Pro², d-Trp^7,9, Leu¹¹-substance P and d-Pro², d-Trp^7,9-substance P, referred to by the single letter amino acid codes for the substituting amino acids as (RPWWL)-SP and (PWW)-SP, respectively. Records of circular muscle activity were taken from strips of intestine free of mucosa and submucosa and from rings with all layers of intestine intact. Substance P was equally effective in contracting the circular muscle strips as it was in contracting the longitudinal muscle. The contractions of strips were not blocked by hyoscine (2×10^-6 M) or tetrodotoxin (6×10^-7 M), but were substantially reduced by (RPWWL)-SP (6.7×10^-6 M) or (PWW)-SP (2×10^-5 M). In contrast, contractions of the circular muscle of whole rings of intestine elicited by low concentrations of substance P (4×10^-7M) were blocked by hyoscine or tetrodotoxin but notreduced by the substance P antagonists in the concentrations referred to above. These observations indicate that the antagonists are effective at receptors for substance P on the muscle, but not at substance P receptors on enteric cholinergic nerves. Transmural stimulation of strips of circular muscle or of intestinal rings in the presence of hyoscine evoked contractions that were blocked by tetrodotoxin. These hyoscineresistant, nerve-mediated contractions could be elicited by single pulses in the strips. The contractions were reduced to less than 20% of original amplitude by (RPWWL)-SP (6.7×10^-6M). Reflex contractions of the circular muscle recorded on the oral side of a distension stimulus had a low-threshold, hyoscine-sensitive and a high-threshold, hyoscine-insensitive, component. The low threshold component was unaffected by the substance P antagonists whereas the high threshold component was depressed. It is concluded that substance P nerves are effective in transmitting to the circular muscle, that they are final nerves in non-cholinergic excitatory reflexes, and that the substance P antagonist analogues can be used to distinguish actions of substance P at neural and muscle receptors.