The effect of secretin on the glucose-induced insulin release from isolated pancreatic islets in mice

Secretin release after Roux-en-Y gastric bypass reveals a population of glucose-sensitive S cells in distal small intestine

OBJECTIVES

Gastrointestinal hormones contribute to the beneficial effects of Roux-en-Y gastric bypass surgery (RYGB) on glycemic control. Secretin is secreted from duodenal S cells in response to low luminal pH, but it is unknown whether its secretion is altered after RYGB and if secretin contributes to the postoperative improvement in glycemic control. We hypothesized that secretin secretion increases after RYGB as a result of the diversion of nutrients to more distal parts of the small intestine, and thereby affects islet hormone release.

METHODS

A specific secretin radioimmunoassay was developed, evaluated biochemically, and used to quantify plasma concentrations of secretin in 13 obese individuals before, 1 week after, and 3 months after RYGB. Distribution of secretin and its receptor was assessed by RNA sequencing, mass-spectrometry and in situ hybridization in human and rat tissues. Isolated, perfused rat intestine and pancreas were used to explore the molecular mechanism underlying glucose-induced secretin secretion and to study direct effects of secretin on glucagon, insulin, and somatostatin secretion. Secretin was administered alone or in combination with GLP-1 to non-sedated rats to evaluate effects on glucose regulation.

RESULTS

Plasma postprandial secretin was more than doubled in humans after RYGB (P < 0.001). The distal small intestine harbored secretin expressing cells in both rats and humans. Glucose increased the secretion of secretin in a sodium-glucose cotransporter dependent manner when administered to the distal part but not into the proximal part of the rat small intestine. Secretin stimulated somatostatin secretion (fold change: 1.59, P < 0.05) from the perfused rat pancreas but affected neither insulin (P = 0.2) nor glucagon (P = 0.97) secretion. When administered to rats in vivo, insulin secretion was attenuated and glucagon secretion increased (P = 0.04), while blood glucose peak time was delayed (from 15 to 45 min) and gastric emptying time prolonged (P = 0.004).

CONCLUSIONS

Glucose-sensing secretin cells located in the distal part of the small intestine may contribute to increased plasma concentrations observed after RYGB. The metabolic role of the distal S cells warrants further studies.

Differential effects of secretin-fragments imply a dual mechanism of action for secretin

The effects of synthetic peptides, representing different parts of the secretin molecule in isolated mouse pancreatic islets have been investigated in perifusion studies. In the presence of 10 mM D-glucose the C-terminal nonapeptide Leu-Gln-Arg-Leu-Leu-Gln-Gly-Leu-Val-NH2 (S19-27) showed a 2-fold higher activity than that earlier shown for S22-27 and had the same effect on the dynamic pattern of insulin release as secretin, while the elongating sequence Leu-Gln-Arg (S19-21) had no effect on the insulin release. The nonapeptide Leu-Ser-Arg-Leu-Arg-Asp-Ser-Ala-Arg (S10-18) had no influence on the insulin release. Glucagon release seen after intact secretin could not be shown for any of the smaller fragments. Accumulation of cAMP in the islets as seen with secretin, could at 10 mmol/L D-glucose only be demonstrated with S22-27 or S19-27 but not with S10-18 or S1-6. Our results indicate that full size secretin has to be present to stimulate glucagon release while insulin-releasing activity can be confined to the C-terminal part of the hormone.