Effect of GIP on the secretion of insulin and somatostatin and the accumulation of cyclic AMP in vitro in the rat

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Abstract
The effects of gastric inhibitory polypeptide (GIP) on insulin secretion and on the intra-islet accumulation of [3H]cAMP were investigated in isolated pancreatic islets of the rat. In the presence of 6.7 mmol/l of glucose, 3.0 and 30 nmol/l of GIP induced both insulin and [3H]cAMP responses, while lower and higher concentrations of the peptide were ineffective. A coupling of the 2 parameters was also found with regard to the interaction between glucose and GIP. While 30 nmol/l of GIP was stimulatory together with 6.7, 16.7 or 33.3 mmol/l of glucose, the peptide stimulated neither insulin release, nor the accumulation of [3H]cAMP in the presence of a low concentration of glucose (3.3 mmol/l). The concomittant release of insulin and somatostatin was studied in the perfused pancreas to assess a possible influence by somatostatin on the dose-response pattern for GIP-induced insulin release. In this preparation 1.0-10 nmol/l of GIP stimulated insulin and somatostatin secretion; these concentrations were equipotent on insulin release, 10 nmol/l of GIP stimulated somatostatin release more than 1 nmol/l, indicating differences in dose-response curves for the GIP-induced stimulation of the 2 hormones. Modulation of GIP-induced insulin release apparently is coupled to changes in cAMP response in the islet; GIP-induced somatostatin secretion may influence the concomittant insulin response.