Identification of enteric hormones with insulin-releasing activity

Hormones secreted from endocrine cells of the gastrointestinal tract appear to be important physiologic stimulators of insulin secretion (incretins). Experiments were performed to assess the magnitude of release of "incretin-like" factors from porcine intestine. The first meter of small intestine from freshly killed adult hogs was excised and cooled in ice-cold transport medium. Groups of 100-130 intestines were rewarmed to 37°C and incubated for 15 min in medium containing 1000 mg/dl glucose. After incubation, the medium and intestines were separated and acidic extracts prepared from them. Samples of medium and intestinal extract were tested for their ability to potentiate glucose-stimulated insulin secretion in the perfused rat pancreas (incretin activity). Both medium and intestinal extracts contained considerable and approximately equal incretin activity. Further purification of the incretin activity indicated that it was due to one or possibly more polypeptides that appeared to be similar in medium and intestinal extract. These data suggest that the endocrine cells of the intestine that secrete incretin peptides can quickly release a relatively large proportion of their total content of active hormone in response to glucose-containing solutions. Large scale incubation techniques such as those employed in this study may be useful in the identification and characterization of other intestinal hormones. The concept that gastrointestinal factors are important in the regulation of glucose metabolism was postulated as early as 1906, when feeding acid extracts of duodenum was shown to decrease the glycosuria of diabetic patients (1). In the 1930's, LaBarre and co-workers partially purified from extracts of small intestine a factor they named "incretin" that was neither insulin nor secretin but improved glucose tolerance in dogs (2-3). Evidence that incretin-like substances released from the gastrointestinal tract might lower the blood glucose by potentiating insulin secretion was obtained by McIntyre and co-workers in 1964, when they demonstrated that intrajejunal administration of glucose caused a greater release of insulin than that released by an equivalent amount of intravenous glucose (4). Several of the polypeptide hormones isolated from intestine have been shown to possess incretin activity (5-6). The best characterized hormone of this group is gastric inhibitory polypeptide (GIP). Ingestion of meals containing either glucose or fat elevates the plasma concentration of immunoreactive GIP to levels (1-2 ng/ml) that can potentiate glucose-stimulated insulin secretion (7). Although GIP may be the major hormone possessing "incretin" activity, there is evidence from in vivo experiments that not all incretin effects can be abolished by infusion of GIP antibodies (8). In addition to GIP, two more peptides purified from porcine intestine have been shown to exert potent incretin activity (9-10). One of them, glicentin, is a 100 amino acid-containing polypeptide that contains the entire sequence of glucagon (11). The other peptide, PHI, is a heptacosapeptide with N-terminal histidine and C-terminal isoleucine amide (10). These experiments were designed to assess the magnitude of release of incretin-like peptides from porcine intestine and to compare the biochemical properties of the released peptides with those of peptides identified in extracts of the same organ.