Bicarbonate Secretion in the Guinea Pig Duodenum: Functional Characterization of Peptide Hormone Receptors in Duodenal Enterocytes

To get information about the peptide hormone receptors involved in duodenal bicarbonate secretion (DBS) and their cellular location, we determined DBS and adenylate cyclase (AC) activity in response to hormones of the vasoactive intestinal polypeptide (VΙP)/secretin family of peptides. DBS was determined in an isolated, perfused (24 mmol/l NaHCO3) loop of the proximal duodenum in urethane- and indometacin-treated guinea pigs. AC stimulation was measured in isolated, homogenized duodenal enterocytes, the histological evaluation of which revealed their villous origin. VIP (10–9 to 10–7 mol × kg–1) dose-dependently increased DBS 3.5-fold (p D-p-Cl-Phe6, Leul 7]VIP (10–6 mol × kg–1). Glucagon (10–8 to 10–6 mol × kg–1) increased DBS 2.1-fold, while secretin (10–9 to 10–6 mol × kg–1) had no effect on DBS, but stimulated pancreatic bicarbonate secretion. VIP concentra-tion-dependently increased AC activity 5.6-fold with an EC50 of 1.3 × 10–9 mol/l. [ D-p-Cl-Phe6, Leul 7] VIP caused a right-ward shift of the VIP concentration-response curve. A Schild plot analysis yielded a slope of 0.85 ± 0.11, indicating competitive inhibition. While secretin also stimulated AC activity, although 1,000-fold less potent than VIP, glucagon was ineffective. These data indicate that specific VIP receptors, which mediate VIP-stimulated bicarbonate secretion, are present on villous enterocytes. Stimulation of AC by secretin seems to be of pharmacological relevance only and is consistent with the lack of effect of this hormone on DBS. Glucagon likely activates a second transmitter of bicarbonate secretion, or works independently of AC.