Substance P produces sodium and bicarbonate secretion in porcine jejunal mucosa through an action on enteric neurons

The neuropeptide substance P (SP) produces transient elevations in short- circuit current (I(sc)), a measure of active ion transport, across sheets of small intestinal mucosae from several animal species, but the ionic basis of this action remains unknown. The aim of this study was to test the hypothesis that SP promotes electrogenic anion secretion in the porcine proximal jejunum, an intestinal segment analogous to the human upper small intestine. Sheets of jejunal mucosa with attached submucosa responded to serosal (S), but not luminal (L) addition of 0.1 μM SP with a transient increase in I(sc) that was reduced in tissues pretreated with the Na⁺-K⁺-Cl^- cotransport inhibitor bumetanide (10 μM) or bathed in media lacking Cl^- or HCO₃^- ions. SP produced biphasic effects on transepithelial Na⁺ and Cl^- fluxes; it initially stimulated a L-directed Na⁺ secretory flux during a 5-min period in which peptide-induced I(sc) elevations were maximum. The return of the I(sc) to base-line levels was temporally associated with an increase in L-directed Cl^- transport. Both effects of SP were absent in tissues either pretreated with the neuronal conduction blocker tetrodotoxin (0.1 μM) or bathed in HCO₃^--deficient media. Bumetanide abolished the Na⁺ secretory actions of SP, but did not affect peptide-induced Cl^- secretion. pH-Stat titration experiments revealed that mucosal sheets alkalinized the L bathing medium at a rate twice that of the S medium. SP simultaneously increased and suppressed L and S alkalinization, respectively; this effect presumably represents HCO₃^- secretion. Luminal alkalinization subsequently returned to basal levels as the SP-induced I(sc) subsided. In contrast, the cholinomimetic carbachol increased the rate of both L and S alkalinization in the first 30 sec after its S addition; this action was followed by a complete bidirectional suppression of alkalinization. Both tetrodotoxin and bumetanide reduced the L alkalinization rate and abolished SP effects on L alkalinization. These results suggest that SP acts directly on enterocytes and indirectly through submucosal neurons to alter epithelial ion transport in the porcine jejunum. A portion of the mucosal I(sc) response to SP may be attributable to the neurally mediated stimulation of an electrogenic Na⁺- HCO₃^- cotransport mechanism. A tetrodotoxin-resistant action of SP accounts for the remainder of the I(sc) elevation; this does not appear to involve electrogenic Na⁺, Cl^- or HCO₃^- transport pathways.