Important species differences regarding lymph contribution to gut hormone responses

Introduction: GLP-1 is secreted from the gut upon nutrient intake and stimulates insulin secretion. The lymph draining the intestine may transport high levels of GLP-1 to the systemic circulation before it is metabolized by DPP-4. The aims of this study were to investigate to what extent the lymphatic system might contribute to the final level(s) of systemic circulating intact GLP-1 and, in addition, whether secretory profiles in intestinal lymph might reflect lamina propria levels of GLP-1 i.e. before capillary uptake and degradation by endothelial dipeptidyl peptidase-4 (DPP-4).

Method: 7 pigs of the YDL-strain were catheterized in the portal vein, carotid artery and cisterna chyli (lymph). Neuromedin C (NC) was infused through an ear vein catheter, before and after injection of a selective DPP-4 inhibitor (vildagliptin). Total and intact GLP-1 levels were measured throughout the 150min experiments using specific sandwich ELISAs. DPP-4 activity was measured spectrophotometrically.

Results: Concentrations of both total and intact GLP-1 were markedly lower in lymph compared to plasma samples, and did not increase significantly in response to stimulation with NC in the absence/presence of vildagliptin. In contrast, total and intact GLP-1 levels increased significantly in the portal vein and carotid artery. DPP-4 activity was lower in lymph than plasma, and was reduced further by vildagliptin.

Conclusion: Our observations indicate that the lymphatic system does not transport high levels of intact GLP-1 to the systemic circulation, and that GLP-1 levels in cisternal lymph do not reflect the hormone levels in the intestinal lamina propria.