Abstract
Radioimmunoassays were developed for measurement of insulin, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) in rat salivary gland extracts and human saliva. Insulin and GIP were found in rat salivary submandibular glands (SSG), and human saliva. Insulin was also found in rat salivary parotid glands (SPG), but GLP-1 was not detected in rat SSG or SPG or human saliva. Chronic whole black tea consumption in rats increased GIP levels in the duodenum and reduced plasma glucose levels, these effects were primarily attributable to caffeine. However red wine polyphenols appeared to be responsible for increased duodenal GLP-1 concentrations in rats. Insulin levels in human saliva rose following a swallowed meal but not following a sham fed meal, GIP levels in saliva decreased following both swallowed and sham fed meals. Acute red wine consumption increased salivary α-amylase activity in volunteers, whole black tea consumption modulated circulating GIP and GLP-1 levels. This is the first known report of GIP in human saliva. On the basis of time-course studies and integration of data from rats and volunteers it is concluded that insulin in saliva is derived from circulating insulin, whereas GIP in saliva is probably secreted by the SSG. It is suggested that GIP is involved in the inhibition of gastric acid secretion via the somatostatin-gastrin pathway. Further work is necessary to verify this hypothesis. The modulation of circulating GIP and GLP-1 can be explained by the consumption of black tea and red wine increasing gut transit time. Consumption of black tea and red wine does not modulate the endocrine activity of the salivary glands. Relative dietary physiological doses of black tea and red wine were used in all experiments to determine the effects that would be seen in the consumer. Therefore, although this research found relatively little effects of black tea or red wine consumption on the hormonal and metabolic response to a meal or the endocrine activity of the salivary glands, it is representative of the effects seen in the consumer.