Gastric inhibitory polypeptide release into the portal vein in response to intraduodenal amino acid loads in anesthetized rats

The hepatic vagal nerve is receptive to incretin hormone glucagon-like peptide-1, but not to glucose-dependent insulinotropic polypeptide, in the portal vein

To examine whether incretin hormones, truncated glucagon-like peptide-1 (tGLP-1) and glucose-dependent insulinotropic polypeptide (GIP), are recognized by the hepatic vagal nerve, changes of the impulse discharge rate in the afferent vagus upon their intraportal administrations were measured in situ in rats anesthetized with urethan and chloralose. One-min injection of tGLP-1 at a periphysiological dose of 0.2 pmol or a pharmacological dose of 4.0 pmol, but not of the vehicle, significantly facilitated the hepatic vagal afferents. However, the injection of GIP at either a physiological dose of 0.2 pmol, a periphysiological dose of 4.0 pmol, or an even much larger dose of 40.0 pmol did not change the afferents at all. The present results indicate that the hepatic vagus specifically recognizes an intraportal appearance of tGLP-1 in the hepatoportal area, suggesting that the vagal monitoring system for intraportal levels of the incretin hormone operates on ingestion of a mixed meal.