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The ileal proglucagon-derived peptides (PGDPs) play important roles in the regulation of gastric function and insulin secretion. We have previously demonstrated the existence of a proximal-distal enteroendocrine loop mediated by glucose-dependent insulinotropic peptide (GIP), through which intestinal PGDP secretion is stimulated after placement of nutrients in the duodenum. To study the possible involvement of neuropeptides in this loop, we infused gastrin-releasing peptide (GRP) into anesthetized rats at 47, 235, or 470 ng/h. GRP stimulated secretion of the intestinal PGDPs only at the highest dose, from 250 +/- 18 to 321 +/- 9 pg/ml (P < 0.05). Gastrin secretion was also increased by GRP infusion (from 20 +/- 7 to 75 +/- 19 pg/ml; P < 0.05); however, GIP release was not altered from basal levels. Placement of fat into the duodenum stimulated the secretion of both the intestinal PGDPs and GIP (to 618 +/- 221 and 417 +/- 154 pg/ml above basal, respectively; P < 0.05). However, infusion of the GRP antagonist, BW10 (3 ng/h), completely abrogated the PGDP response to duodenal fat (P < 0.05). In contrast, the GIP response was reduced by 56 +/- 15% (P < 0.05) with BW10 administration, but remained elevated compared to the basal level (P < 0.05). Removal of the entire intestine distal to the fat-infused duodenal segment prevented the rise in intestinal PGDPs, but not that of GIP, demonstrating the distal and proximal origins of these peptides, respectively. Thus, placement of fat into the duodenum stimulated the release of the PGDPs from the distal intestine through a GRP-dependent mechanism. Although GIP plays a role in regulating intestinal PGDP secretion in the rat, GRP appears to exert its effects independent of changes in GIP.

Incretins are intestinal factors that stimulate postprandial insulin secretion in preparation for subsequent rises in plasma levels of ingested nutrients. Glucose-dependent insulinotropic peptide (GIP), a duodenal endocrine peptide, is ideally located for such a function. In contrast, the intestinal proglucagon-derived peptide (PGDP), truncated GLP-1 [GLP-1(7-37) or tGLP-1] is equipotent to GIP in insulinotropic activity, but due to its localization in the distal ileum, appears to be poorly situated to fulfill an incretin role in response to direct nutrient stimulation. Despite its distribution, rapid increments in plasma levels of tGLP-1 have been noted in response to nutrient ingestion. We have recently reported that GIP (but not other nutrient-stimulated duodenal endocrine peptides) can stimulate intestinal PGDP secretion in vitro, and therefore hypothesized that GIP might regulate secretion of the intestinal PGDPs, including tGLP-1, in response to nutrient ingestion in vivo in the rat. Placement of either fat or glucose directly into the ileal lumen was demonstrated to significantly stimulate secretion of the intestinal PGDPs (P < 0.05), whereas fat or glucose in the duodenal lumen significantly increased plasma levels of GIP (P < 0.05). In addition, however, duodenal fat treatment also increased the secretion of intestinal PGDPs into the circulation (P < 0.05), with levels rising to same extent as observed after direct administration of fat into the ileum. The rise in plasma GIP levels in response to duodenal fat treatment occurred slightly before the increments in intestinal PGDP levels, suggesting a relationship between the two peptides. Intravenous infusion of GIP to give concentrations similar to those observed after duodenal fat administration induced a 2-fold increase in plasma levels of intestinal PGDPs that was independent of glycemic levels (P < 0.05). No increment in intestinal PGDPs was found in response to infusion of another duodenal endocrine peptide, cholecystokinin. Thus, these data demonstrate a specific effect of GIP to stimulate secretion of the intestinal PGDPs in vivo in the rat. This enteroendocrine loop between the duodenal peptide GIP and the ileal PGDPs may account for some of the early rises in secretion of tGLP-1 observed in response to nutrient ingestion.

To establish whether secretion of proglucagon-derived peptides (PGDPs) by the intestinal L cell is nutrient- and/or location-dependent, 0.9% saline, 200 mM glucose, or emulsified fats were administered into the ileal or duodenal lumen of normal rats. Fat administration, but not saline or glucose treatment, significantly increased circulating levels of the intestinal PGDPs in a time-dependent fashion, indicating a selectivity of the L cell in its response to nutrients. Interestingly, the response to duodenal fats was quantitatively and qualitatively identical to the response to ileal fats, despite 50-fold lower concentrations of PGDPs in the duodenum. These results suggest the existence of a duodenal factor that stimulates ileal PGDP secretion in response to fat ingestion. Ileal and plasma levels of gut PGDPs have been reported to be elevated in poorly controlled streptozotocin-diabetic rats. Whether the sensitivity of the L cell to luminal nutrients is altered in diabetes was, therefore, also examined. The L cell responses to luminal nutrients in diabetic rats were not significantly different from those of the normal rat, indicating a normal responsiveness of the L cell. However, independent of changes in glycemia, luminal glucose perfusion significantly decreased circulating glucagon levels in normal rats, but not in diabetics. Furthermore, luminal fat administration increased plasma glucagon levels in normal rats only. These results indicate that moderately controlled diabetes is associated with alterations in the pancreatic A cell, but not the intestinal L cell response to ingested nutrients. The results of the present study indicate that the response of the intestinal L cell to ingested food is nutrient-specific and that this specificity is not altered in diabetes. A duodenal-ileal axis is proposed to contribute to increments in circulating intestinal PGDPs in response to nutrient ingestion.

Using several novel in vitro culture systems, we have examined the tissue-specific regulation of the proglucagon-derived peptides, at the levels of proglucagon gene expression and pGdp synthesis and secretion. Our studies indicate that proglucagon gene expression in intenstine, hypothalamus and pancreas is under the regulatory control of protein kinase A- but not a protein kinase C-dependent pathway. PKA and PKC stimulate secretion of the intestinal pGdp's, whereas only PKA stimulates secretion of the hypothalamic peptides. Pancreatic glucagon secretion in response to PKA is subject to further modulation by prevailing glucose concentrations. This diversity in intracellular regulation of the pGdp's may account for some of the tissue-specific differences in synthesis and secretion of the pGdp's that we have observed in diabetes and during development.