Mechanism of acid-induced release of secretin in rats. Presence of a secretin-releasing peptide.

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Abstract
In fasting rats, intraduodenal infusion of dilute hydrochloric acid results in significant increases in both pancreatic exocrine secretion and plasma concentration of secretin. To test the hypothesis that acid-induced release of secretin is mediated by a secretin-releasing factor (S-RF), anesthetized rats were prepared with pyloric ligation, duodenal and jejunal cannulas, and pancreatic duct cannulas. Donor rats were infused intraduodenally with 0.01 N HCl, 0.15 M NaCl, or a combination of 0.01 N HCl and 0.05 N NaHCO3 at 0.3 ml/min for 1.5 h, and the perfusates were collected via jejunal cannulas. The perfusates with pH adjusted to 6.0 were concentrated threefold and infused into the duodena of recipient rats. The concentrate of acid perfusate (CAP) significantly increased both pancreatic volume flow and bicarbonate output and plasma concentration of secretin, whereas concentrates of the saline perfusate (CSP) or the perfusate of a combination of 0.01 N HCl and 0.05 N NaHCO3 (CABP) did not influence pancreatic secretion or plasma concentration of secretin. The increased pancreatic secretion by CAP was attributed to increased circulating secretin because when secretin was immunoneutralized by a rabbit antisecretin serum, CAP-stimulated pancreatic secretion was abolished. The bioactivity of CAP was trypsin-sensitive and heat stable. The active substance in CAP had a molecular weight of less than 5,000 and greater than 1,000, as determined by ultrafiltration and bioassay. In conclusion, dilute HCl releases an S-RF into the upper small intestinal lumen to stimulate release of secretin. This substance, with molecular weight of less than 5,000, is heat stable and trypsin sensitive. Thus, the acid-stimulated release of secretin is mediated by a secretin-releasing peptide in the upper small intestinal lumen.