Improvement of insulin sensitivity by chelation of intracellular Ca(2+) in high-fat-fed rats

It has been postulated that sustained high levels of intracellular calcium concentration ([Ca(2+)](i)) in the insulin target cells may cause insulin resistance. We evaluated this hypothesis by examining the effect of an intracellular Ca(2+) chelator, 5,5'-dimethyl derivative of bis (o-aminophenoxy) ethane-N,N,N',N' tetraacetic acetoxymethyl ester (dimethyl-BAPTA/AM), on insulin resistance. Insulin resistance was induced in rats by feeding a high-fat diet for 3 to 4 weeks. The whole body insulin sensitivity was determined by the steady state glucose infusion rate (GIR) under euglycemic hyperinsulinemic (6 mU x kg(-1) x min(-1)) clamps. Compared with control rats, the high-fat diet (HFD) fed rats showed significantly lower GIR (12.2 +/- 0.7 v 20.2 +/- 0.9 mg x kg(-1) x min(-1); P <.01). In the HFD rats, an intravenous injection of dimethyl-BAPTA/AM (6 mg/kg) 90 minutes before the clamps significantly increased GIR to 16.3 +/- 0.9 mg x kg(-1) x min(-1) (P <.02), reversing insulin resistance by about 50%; but this intervention had no effect in the controls. This increase in GIR by dimethyl-BAPTA/AM was observed without an increase in femoral artery blood flow, indicating that the chelator increased GIR directly through improving cellular responsiveness to insulin. The stimulatory effect of insulin on 2-deoxy glucose (2-DG) uptake by the isolated epididymal adipocytes was reduced by 35% in the HFD rats compared with the control rats (P <.01). Pretreatment of the HFD rats with dimethyl-BAPTA/AM restored 2-DG uptake to the level in the control rats. The direct measurement of [Ca(2+)](i) using fura-2/AM in isolated adipocytes showed that basal [Ca(2+)](i) was significantly higher in the HFD rats than in the control rats (145 +/- 11 v 112 +/- 9 nmol/L; P <.05). An injection of dimethyl-BAPTA/AM in the HFD rats lowered [Ca(2+)](i) to 127 +/- 11 nmol/L, which did not differ from the level in the control rats (P >.2). The present study clearly demonstrates that an injection of intracellular Ca(2+) chelator in the HFD rats reverses insulin resistance, as well as normalizes elevated [Ca(2+)](i) in the insulin target cells. The results strongly support that sustained high levels of [Ca(2+)](i) in the insulin target cells may play an important role in insulin resistance, at least in the HFD rats.