Insulin releasing effect, gastrointestinal hormone

Glucose, the principal stimulus to insulin secretion, is permissive for the action of many other secretogogues. Glucose provokes insulin secretion more effectively when taken orally than when administered intravenously because the oral route induces the release of GI hormones and stimulates vagal activity. Several GI hormones promote insulin secretion, the most potent of which are glucose-dependent insulinotropic peptide (GIP, also known as gastrointestinal inhibitory peptide) and glucagon-like peptide-1 (GLP-1). 

Somatostatin. Somatostatin, also called growth hormone-inhibiting hormone, is a 14-amino-acid peptide found in the gastrointestinal tract, the hypothalamus, and the 6-cells of the pancreatic islets. Although somatostatin does not appear to have a direct effect on carbohydrate metabolism, it inhibits the release of growth hormone from the pituitary. In addition, somatostatin inhibits secretion of glucagon and insulin by the pancreas, thus modulating the reciprocal relationship between these two hormones. 

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