Neurokinin receptors and mucosal ion transport in porcine jejunum

Neurokinin (NK) peptides such as substance P (SP) may modulate epithelial ion transport in the small intestine. The present study was undertaken to examine the pharmacological mechanisms by which SP and its endogenous homologs NKA and NKB affect active electrolyte transport in the mucosa of porcine jejunum. Neurokinins and NK agonist analogs increased short circuit current, a measure of active ion transport, across sheets of jejunal mucosa- submucosa with the order of potency: SP > [β-Ala⁸]NKA_4-10 ≥ [Sar⁹,Met(O₂)¹¹]SP > NKA = Arg-NKB > NKB after their addition to the serosal aspect of tissues (SP EC₅₀ = 11 nM). Epithelial responses to SP or NKA underwent rapid autotachyphylaxis and unidirectional cross-tachyphylaxis after repeated peptide administration. The neuronal conduction blocker tetrodotoxin significantly reduced NK efficacy. SP activity was significantly reduced in tissues pretreated with the muscarinic cholinoceptor blocker atropine or the eicosanoid synthesis inhibitor eicosa-5,8,11,14-tetraynoic acid. NK peptides increased contractility in longitudinally oriented strips of jejunal smooth muscle with an order of potency: [Sar⁹,Met(O₂)¹¹]SP > SP > Arg-NKB = [β-Ala⁸]NKA_4-10 ≥ NKB = NKA (SP EC₅₀ = 11 nM). SP- induced contractions were reduced by 70 to 80% in tissues pretreated with atropine or the neuronal Ca⁺⁺ channel blocker, ω-conotoxin. [¹²⁵I]Bolton-Hunter-substance P (BHSP) bound specifically to a single population of sites in slide-mounted sections of mucosa-submucosa and smooth muscle with K(d) = 0.3 and 0.1 nM and B(max) = 18 and 31 fmol/mg protein, respectively. Unlabeled SP and [Sar⁹,Met(O₂)¹¹]SP competed with highest affinities for muscular and mucosal BHSP binding sites. Specific BHSP binding sites visualized autoradiographically were at their highest densities over the jejunal circular muscle and external submucosal plexus and diffusely localized over the epithelium, myenteric and internal submucosal plexuses and longitudinal muscle. These results indicate that NK peptides are capable of modulating ion transport and propulsion in the porcine jejunum. A portion of their action is mediated through SP-preferring receptors which are linked to cholinergic neural pathways in the gut wall. In addition, their transport- related effects may involve the formation of intestinal eicosanoids.