Mechanism of insulin resistance induced by sustained levels of cytosolic free calcium in rat adipocytes.
We have recently provided evidence that elevated levels of cytosolic free Ca2+ ([Ca2+]i) decreased insulin-stimulated glucose uptake in isolated rat adipocytes. To investigate the mechanism of Ca2+ action, we examined the effects of elevated levels of [Ca2+]i on insulin binding, autophosphorylation, and tyrosine kinase activity (TKA) of insulin receptors as well as basal and insulin-stimulated cellular distribution of glucose transporters. The latter was assessed by cytochalasin-B binding to plasma membrane and cytosolic fractions. Elevated concentrations of [Ca2+]i were maintained by incubating adipocytes with a depolarizing concentration of K+ (40 mM). Basal nonstimulated glucose uptake was not altered by increased levels of [Ca2+]i. Adipocytes with higher [Ca2+]i (220 +/- 15 nM) showed 30% reduction in insulin-stimulated 2-deoxyglucose uptake compared with control cells ([Ca2+]i, 140 +/- 18 nM). Moreover, adipocytes with higher levels of [Ca2+]i demonstrated an approximately 10% reduction in autophosphorylation and TKA of insulin receptors without a change in insulin binding. Both basal and insulin-stimulated distributions of glucose transporters were unaffected by sustained levels of [Ca2+]i. The effects of elevated [Ca2+]i were not mimicked by protein kinase-C activation. These observations suggest that 1) elevated or sustained levels of [Ca2+]i impair insulin-stimulated glucose uptake; and 2) Ca2+-induced impairment appears to reside at the postbinding steps of insulin action and probably interferes with the TKA of insulin receptors and the intrinsic activity of glucose transporters.
Draznin
B
B
Research Service, Veterans Administration Medical Center, Denver, Colorado 80220.
Lewis
D
D
Houlder
N
N
Sherman
N
N
Adamo
M
M
Garvey
W T
WT
LeRoith
D
D
Sussman
K
K
D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D003837:Deoxy Sugars / Q000378:metabolism*; D003847:Deoxyglucose / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology; D007333:Insulin Resistance; D015759:Ionomycin / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011972:Receptor, Insulin / Q000302:isolation & purification / Q000378:metabolism*; D013110:Sphingosine / Q000494:pharmacology
D003837:Deoxy Sugars; D007328:Insulin; D009004:Monosaccharide Transport Proteins; D015759:Ionomycin; D003847:Deoxyglucose; D011505:Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D013110:Sphingosine; D011188:Potassium; D002118:Calcium