Defective glucose-dependent cytosolic Ca 2+ handling in islets of GK and nSTZ rat models of Type 2 diabetes J-C Marie 1 , D Bailbé 1 , E Gylfe 2 and B Portha 1 1 Laboratoire de Physiopathologie de la Nutrition, CNRS-ESA 7059, Université Paris VII/D. Diderot, 2 Place Jussieu, F-75251 Paris, France 2 Uppsala University, Department of Medical Cell Biology, Biomedical Centre, SE-75123 Uppsala, Sweden (Requests for offprints should be addressed to J-C Marie, INSERM U410, Faculté de Médecine Xavier Bichat, 16, rue Henri Huchard, BP 416, F-75870 Paris Cedex 18, France; Email: ugh@bichat.inserm.fr) Abstract We examined to what extent the abnormal glucose- dependent insulin secretion observed in NIDDM (non- insulin-dependent diabetes mellitus) is related to alterations in the handling of cytosolic Ca 2+ of islets of Langerhans. Using two recognized rat models of NIDDM, the GK (Goto–Kakizaki) spontaneous model and the nSTZ (neonatal streptozotocin) induced model, we could detect several common alterations in the glucose-induced [Ca 2+ ] i cytosolic responses. First, the initial reduction of [Ca 2+ ] i following high glucose (16·7 mM) observed routinely in islets obtained from non-diabetic Wistar rats could not be detected in GK and nSTZ islets. Second, a delayed response for glucose to induce a subsequent 3% increase of [Ca 2+ ] i over basal level was observed in both GK (321 40 s, n =11) and nSTZ (326 38 s, n =13) islets as compared with Wistar islets (198 20 s, n =11), values representing means ... Third, the rate of increase in [Ca 2+ ] i in response to a high glucose challenge was 25% and 40% lower in GK and nSTZ respectively, as compared with Wistar islets. Fourth, the maximal [Ca 2+ ] i level reached after 10 min of perifusion with 16·7 mM glucose was lower with GK and nSTZ islets and repre- sented respectively 60% and 90% of that of Wistar islets. Further, thapsigargin, a blocker of Ca 2+ -ATPases (SERCA), abolished the initial reduction in [Ca 2+ ] i ob- served in response to high glucose and induced fast [Ca 2+ ] i oscillations with high amplitude in Wistar islets. The latter effect was not seen in GK and nSTZ islets. In these two NIDDM models, several common alterations in glucose- induced Ca 2+ handling were revealed which may contrib- ute to their poor glucose-induced insulin secretion. Journal of Endocrinology (2001) 169, 169–176 Introduction Prior to the triggering of insulin exocytosis from beta cells exposed to high glucose, a number of metabolic and ionic changes occurs leading to an increase in the cytoplasmic concentration of Ca 2+ or [Ca 2+ ] i . The main pathway involves the closure of ATP-sensitive K + channels in the plasma membrane due to a generation of ATP by the metabolism of glucose. As a consequence of the membrane depolarization, there is an opening of voltage-dependent Ca 2+ channels. The increasing [Ca 2+ ] i which follows is known to be a major trigger of insulin secretion. However, there are some recent indications that Ca 2+ -independent pathways can mediate some of the effects of glucose in stimulating insulin secretion (Komatsu et al. 1995). Several alterations specific for the glucose signalling pathway in beta cells have been described in different diabetic rats such as a reduced ATP/ADP ratio, a low production of inositol 1,4,5-triphosphate (IP 3 ) and cAMP (Giroix et al. 1993, Dachicourt et al. 1996, 1997a,b, Morin et al. 1996, 1997). Since ATP is one of the major metabolic signals associated with glucose, the reduced ATP/ADP ratio in beta cells would have a severe impact on the function of ATP-sensitive K + channels in the plasma membrane and on the subsequent ionic events. Further- more, it has been postulated that a diminished glucose- stimulated insulin secretion can be ascribed to abnormal [Ca 2+ ] i handling of pancreatic islets. In this respect, the disappearance of glucose-induced [Ca 2+ ] i oscillations in mouse beta cells treated with streptozotocin suggests that disturbances in [Ca 2+ ] i may contribute to insulin secretory impairments in diabetes (Hellman et al. 1990). Moreover, rat islets cultured at high glucose had a weaker increase in [Ca 2+ ] i with a concomitant reduced insulin secretion in response to glucose as compared with freshly isolated pancreatic islets (Boschero et al. 1990). Another observed defect in [Ca 2+ ] i handling that has been reported is the absence of the initial sequestration of Ca 2+ by beta-cell sarco-endoplasmic reticulum Ca 2+ - ATPase (SERCA) in diabetic db/db mouse islets (Roe et al. 1994a). The lowering of [Ca 2+ ] i represents an early action of glucose observed in normal mouse islets and depends on functional SERCA which is thought to play an important part in glucose regulation of insulin secretion 169 Journal of Endocrinology (2001) 169, 169–176 0022–0795/01/0169–169  2001 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology.org