Influence Of Diabetes On Insulin Secretion In The Tear Film Of Rat

Abstract

Abstract: : Purpose: Insulin is a pleiotropic hormone, responsible for metabolic control, tissue growth and differentiation in various tissues. In recent studies we identified insulin in the tear film and its receptor in tissues of the ocular surface. To further understand the secretory mechanisms and physiologic role of insulin in the tear film, the present study examined normal and diabetic rats for: (1) the time–course of insulin secretion in the tear film under glucose intravenous stimulation (2) the glucose, carbachol and K+ induced insulin secretion from isolated lacrimal gland (LG) and (3) the localization of insulin in rat LG tissues. Methods: Diabetes Mellitus was induced by iv streptozotocin at 60mg/kg in 8 week old male Wistar rats. Four weeks later, blood and tears were collected from rats that had fasted for 12 hours and at 5, 15, 30 and 60 minutes after intravenous glucose injection (1.0 g/kg of body weight) (n= 10/group). Isolated LG and pancreatic ß–cells from Wistar male rats were stimulated with glucose from 2.8 mM to 16.7 mM, carbachol 200 µM and KCl 30mM and supernatants were collected after 1h. The samples were analyzed by radioimmunoassay for insulin measurement. Immunohistochemistry was employed to localize insulin in LG. Results: The insulin level in the tear film rose after glucose stimulation. The peak was between 5 and 15 min in both euglycemic and diabetic rats, in contrast to blood insulin level that was higher between 5 and 15 min in control animals but was flat and unchanged in diabetic rats. In vitro assays demonstrated that higher glucose concentration and carbachol 200 µM significantly increased media insulin levels in lacrimal gland samples of both controls and diabetic rats. These effects were reduced in the presence of diazoxide at 20 µg/ml and atropine at 66 µg/ml, respectively. Immunohistochemical assays indicated that insulin is predominantly located at the apex of acinar LG cells. Conclusions: These findings suggest that insulin secretion in the tear film is influenced by systemic conditions and that lacrimal gland response may be modulated by the endocrine and nervous systems. In addition the comparable findings in diabetic and control LG and tear film suggests the capacity for insulin storage and/or local production.

Keywords: diabetes • cornea: tears/tear film/dry eye • lacrimal gland