DEFECTIVE GLUCOSE-DEPENDENT ENDOPLASMIC-RETICULUM CA2+ SEQUESTRATION IN DIABETIC MOUSE ISLETS OF LANGERHANS

Non-insulin-dependent diabetes mellitus (NIDDM) is a metabolic disease associated with abnormal insulin secretion, the underlying mechanisms of which are unknown. Glucose-dependent signal transduction pathways were investigated in pancreatic islets derived from the db/db mouse, a n animal model of NIDDM. After stimulation with glucose (4-12 mM), the changes in intracellular Ca2+ concentration ([Ca2+](i)) were differen t; unlike control islets, db/db islets lacked an initial reduction of [Ca2+](i) and the subsequent [Ca2+](i) oscillations following stimulat ion with 12 mM glucose. The severity of these defects in Ca2+ signalin g correlated with the age-dependent development of hyperglycemia. Simi larly defective glucose-induced Ca2+ signaling were reproduced in cont rol islets by pre-exposure to thapsigargin, a selective inhibitor of e ndoplasmic reticulum (ER) Ca2+-ATPase. Estimation of ATPase activities from rates of ATP hydrolysis and by immunoblot hybridization with an antiserum directed against the sarco/endoplasmic reticulum Ca2+-ATPase both demonstrated that the ER Ca2+-ATPase was almost entirely absent from db/db islets. The effects of inhibition of ER Ca2+-ATPase on insu lin secretion were also examined; a 4-day exposure of control islets t o 1 mu M thapsigargin resulted in basal and glucose-stimulated insulin secretion levels similar to those found in db/db islets. These result s suggest that aberrant ER Ca2+ sequestration underlies the impaired g lucose responses in the db/db mouse and may play a role in defective i nsulin secretion associated with NIDDM.