GLUT-4 phosphorylation and its intrinsic activity. Mechanism of Ca(2+)-induced inhibition of insulin-stimulated glucose transport

In this study, we examined the influence of high levels of cytosolic calcium on phosphorylation status and function of GLUT-4 in isolated rat adipocytes. Intracellular calcium was elevated by exposing adipocytes to either extracellular ATP (1.6 mM) or thapsigargin (100 nM). Both agents increased cytosolic calcium 2-3 fold. While basal glucose uptake was unaffected, both ATP and thapsigargin reduced insulin-stimulated glucose transport by 40-70% (p < 0.05). Neither ATP nor thapsigargin affected GLUT-4 content or its translocation from the low density microsomes to the plasma membrane (PM). In contrast, GLUT-4 immunoprecipitated from the PM of adipocytes exposed to either ATP or thapsigargin was phosphorylated to a greater extent than the GLUT-4 isolated from control cells. ATP and thapsigargin also abolished insulin-stimulated dephosphorylation of GLUT-4. At the same time, GLUT-4 intrinsic activity was significantly reduced in adipocytes with high levels of cytosolic calcium (p < 0.05). Preincubation of adipocytes with cAMP antagonist, RpcAMP (10(-4) M), and calcium channel blocker, nitrendipine (30 microM), improved the ability of insulin to dephosphorylate GLUT-4 and restored insulin-stimulated GLUT-4 intrinsic activity. We conclude that elevated levels of cytosolic calcium interfere with insulin's ability to dephosphorylate GLUT-4, thus reducing its intrinsic activity.