Submandibular glands as organs of synthesis and accumulation of insulin-like protein.

Experiments are presented to determine the endocrine function of submandibular salivary glands in the light of a hypothesis about synthesis of insulin-like protein (ILP) and participation of these organs in regulation of the glucose level in the organism. ILP was discovered in submandibular glands of male mice, rats, hamsters, cats, rabbits, pigs, bulls, and men. Electrophoretical and immunological studies of insulin-like protein, extracted from these glands, showed its similarity with pancreatic insulin. Immunofluorescent studies showed that ILP was localized in the granular duct cells of mice and rats. Studies by immunoperoxidase method illustrated that insulin-like protein was distributed along the periphery of the electron-dense secretory granules. In submandibular glands of animals which do not contain granular ducts, ILP was revealed in the striated tubules (by immunofluorescent method). Study of submandibular glands in vitro showed that ILP was synthesized in glands and not simply accumulated there. Under the condition of insulin deficiency (in vivo) and increased glucose concentration in culture media (in vitro), duct cells of submandibular glands released ILP in increasing amounts. Injection of ILP into mice caused a decrease of glucose concentration in the blood and a convulsive reaction. This fact indicates the similarity of ILP with insulin in its physiological action. Our studies permit to draw the conclusion on the participation of submandibular glands in the regulation of glucose homeostasis.