Figure 1.

Incretin hormone secretion from K and L cells.

Glucose dependent insulinotropic polypeptide (GIP) is secreted from K cells, which are predominantly found in the duodenum, whereas glucagon-like peptide-1 (GLP-1) is secreted from L cells, which increase in numbers in the distal intestine. Both cell types are so-called open enteroendocrine cells with direct contact to the intestinal lumen, allowing sampling of the chyme. Nutrient transporters expressed in K and L cells play an important part in this chemosensation (see text for details). The difference in location, however, is likely to impact on the observed hormone responses to nutrient ingestion. It is now thought that sufficient L cells are present in the proximal intestine to account for early rises of GLP-1 under normal conditions (left hand side). However, when absorption is blocked (right hand side), the increased delivery to the distal intestine brings more L cells in contact with nutrients and/or their fermentation/putrification products, resulting in elevated GLP-1 levels. This in turn slows gastric emptying, potentially thereby prolonging exposure of proximal cells to chyme. While glucose stimulated GIP secretion is thus abolished in Sglt-1 knockout mice, emphasizing the importance of this transporter in K cells, GIP (as well as GLP-1) levels in B⁰at1 knockout animals are elevated in comparison with wildtype litter mates 60 min after food consumption.