The Rat Parotid Binds the COOH-terminal Tetrapeptide of Gastrin/Cholecystokinin

Objectives: Because the mouth is the beginning of the alimentary tract, the possibility exists that salivary glands might likewise contain enteric peptides. This study sought to determine whether the parotid gland might contain gastrin which is primarily produced in the gastric antrum. The most antigenic part of gastrin is its COOH-terminal tetrapeptide. Unfortunately, this is also present in cholecystokinin (CCK), a small intestinal peptide. Methods: In each of six adult rats one parotid was excised, fixed in formalin, and immunohistochemistry performed using a rabbit polyclonal antibody marketed by Cell Marque (Hot Springs, AR) as an anti-human Gastrin I which cross-reacts with CCK. Our findings indicate that it also cross-reacts with rat tissues. The procedure was performed according to the manufacturer's directions. Briefly, deparaffinized 4 micra tissue sections were boiled in citrate buffer. Endogenous peroxidase was blocked and either the antibody or a rabbit immunoglobulin fraction as the negative control solution was applied, followed by a horseradish-peroxidase anti-rabbit link, DAB chromogen and a Mayer's hematoxylin counterstain. Results: The primary antibody immunostained the intralobular intercalated ducts and caused appreciable immunostaining of the striated ducts and interlobular collecting ducts and luminal fluid. The acini were unreactive. Thus, the duct system contains either gastrin, CCK or both. To differentiate these possibilities aliquots of the primary antibody were preabsorbed with either enteric peptide. Preabsorption with gastrin leaves the antibody reactive for CCK, and doing so with CCK leaves it reactive for gastrin. To our surprise because the parotid is nearer the stomach than the intestine, most cells of the duct system stained heavily for CCK and not at all, or perhaps minimally, for gastrin. Conclusion: The results suggest but do not prove incontrovertibly that the rat parotid duct system contains CCK which might enter the saliva. Its function in the parotid is unknown.