Glucose-dependent insulinotropic peptide (GIP) gene expression in the rat salivary gland

Previous studies have indicated that following nutrient ingestion, GIP is released principally from the upper small intestine. In addition to its presence in the rat small intestine, GIP transcripts have also been localized to the submandibular salivary gland (SSG). The present studies were directed to further characterize expression of the GIP gene in the SSG. Pregnant rats were sacrificed at gestational days 18 and 20, followed by the removal of rat fetuses. The duodenum pancreas, and SSG were then excised from the fetuses, as well as from neonatal pups at ages 1, 3, 7, 10, 14, and 21 days. RNA was extracted and measured by Northern blot analysis using specific rat GIP probes. GIP transcripts were first detected in the duodenum in the 18-day fetus and reached maximum levels at birth. In contrast, GIP mRNA was not observed in the SSG until 10 days postnatally and was not detected at all in either the fetal or neonatal pancreas. In situ hybridization of the SSG using an 35S-labelled antisense GIP RNA probe demonstrated expression of the GIP gene to be limited to ductal cells, with no transcripts present in acini. In separate experiments, rats fasted overnight were given water or 10% glucose. While no changes were detected in water-fed rats following oral glucose ingestion, small, but significant increases in SSG GIP gene expression were detected at 60 and 240 min. The results of these initial studies suggest the possibility of a functional role for GIP in the rat salivary gland by the demonstration of GIP mRNA in the SSG by Northern analysis and in situ hybridization, as well as by an increase in SSG GIP gene expression following a glucose meal.