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 greater than [beta-Ala8]NKA4-10 greater than or equal to [Sar9,Met(O2)11]SP greater than NKA = Arg-NKB greater than 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 greater than SP greater than Arg-NKB = [beta-Ala8]NKA4-10 greater than or equal to 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, omega-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 Kd = 0.3 and 0.1 nM and Bmax = 18 and 31 fmol/mg protein, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)