Circular muscle contraction, messenger signalling and localization of binding sites for neurokinin A in human sigmoid colon

1. Neurokinin (NK)A is the endogenous ligand for the tachykinin NK2 recepto r. In the present study, tachykinins and selective receptor agonists were t ested as contractile agonists in human colon circular muscle and [I-125]-NK A was used to localize binding sites in human colon. 2. In strips of circular muscle, removal of mucosa and submucosa significan tly (P < 0.05) increased the potency and the maximum response achieved by N KA. 3. The rank order of potency of tachykinin and selective receptor agonists in contracting circular muscle strips was NKA greater than or equal to [Lys (5),MeLeu(9),Nle(10)]NKA(4-10) greater than or equal to neuropeptide (NP)ga mma greater than or equal to [beta Ala(8)]NKA(4-10) >> NKB > substance P (S P) >> senktide approximate to [Pro(9)]SP. 4. Specific binding sites for [I-125]-NKA were densely localized over circu lar muscle and muscularis mucosae. Weak specific binding was seen on longit udinal muscle and taenia coli, whereas no binding sites were seen on mucosa , ganglia or blood vessels. 5. In circular muscle, the selective NK2 receptor agonist [Lys(5),MeLeu(9), Nle(10)]NKA(4-10) produced weak increases (maximum 37%) in inositol monopho sphate formation with a pD(2) of 6.8 +/- 0.51 (n = 3). Carbachol (100 mu mo l/L) was also a weak stimulant (maximum 45%). These agonists were over 10-f old more efficacious in stimulation of inositol monophosphate in rat urinar y bladder. 6. In conclusion, [I-125]-NKA binding sites localized on human colon circul ar muscle were characterized as NK2 receptors. Functionally, the tachykinin NK2 receptor is mediating circular smooth muscle contraction. Although the human NK2 receptor is coupled to the phosphatidylinositol pathway, other s econd messenger mechanisms may also operate in this tissue.