Loss of coupling between calcium influx, energy consumption and insulin secretion associated with development of hyperglycaemia in the UCD-T2DM rat model of type 2 diabetes

Aims/hypothesis: Previous studies on isolated islets have demonstrated tight coupling between calcium (Ca²⁺) influx and oxygen consumption rate (OCR) that is correlated with insulin secretion rate (ISR). To explain these observations, we have proposed a mechanism whereby the activation of a highly energetic process (Ca²⁺/metabolic coupling process [CMCP]) by Ca²⁺ mediates the stimulation of ISR. The aim of the study was to test whether impairment of the CMCP could play a role in the development of type 2 diabetes. Methods: Glucose- and Ca²⁺-mediated changes in OCR and ISR in isolated islets were compared with the time course of changes of plasma insulin concentrations observed during the progression to hyperglycaemia in a rat model of type-2 diabetes (the University of California at Davis type 2 diabetes mellitus [UCD-T2DM] rat). Islets were isolated from UCD-T2DM rats before, 1 week, and 3 weeks after the onset of hyperglycaemia. Results: Glucose stimulation of cytosolic Ca²⁺ and OCR was similar for islets harvested before and 1 week after the onset of hyperglycaemia. In contrast, a loss of decrement in islet OCR and ISR in response to Ca²⁺ channel blockade coincided with decreased fasting plasma insulin concentrations observed in rats 3 weeks after the onset of hyperglycaemia. Conclusions/interpretation: These results suggest that phenotypic impairment of diabetic islets in the UCD-T2DM rat is downstream of Ca²⁺ influx and involves unregulated stimulation of the CMCP. The continuously elevated levels of CMCP induced by chronic hyperglycaemia in these islets may mediate the loss of islet function.