Defective glycolysis and calcium signaling underlie impaired insulin secretion in a transgenic mouse

Pancreatic beta cells from mice that overexpress the Ca(2+)-binding protein calmodulin have a unique secretory defect that leads to chronic hyperglycemia. To further understand the molecular basis underlying this defect, we have studied signaling pathways in these beta cells. Measurements of cytosolic free Ca2+ concentration ([Ca2+]i) using fura-2 or indo-1 revealed a markedly reduced response when glucose was the stimulant. However, eliciting membrane depolarization with 50 mM K+ or the addition of the ATP-sensitive K+ (K+ ATP) channel antagonist tolbutamide restored [Ca2+]i transients to near normal levels. Electrophysiological analysis of the beta cell ion channels revealed that Ca2+ currents, delayed rectifier K+ currents, and K+ATP channel currents were similar in transgenic and nontransgenic cells, suggesting that these ion channels were able to function normally. However, whereas K+ATP channel currents in control cells were reduced by 50% by the presence of high glucose, those in transgenic cells were unaltered. Addition of tolbutamide inhibited this channel and enhanced the secretion of insulin in response to glucose for both control and transgenic cells. As these observations implicated a metabolic defect, glucose utilization, which is an indicator of glucose metabolism and ATP production in beta cells, was measured and found to be reduced by 40% in the transgenic cells. These data support the contention that excessive levels of calmodulin may compromise the ability of the beta cell to metabolize glucose and to modulate the state of the K+ATP channel, resulting in an inadequate control of the membrane potential, which collectively impair [Ca2+]i and thus insulin secretion in response to glucose.