Cytosolic [Ca2+] modulates basal GLUT1 activity and plays a permissive role in its activation by metabolic stress and insulin in rat epithelial cells

The aim of this work was to investigate the role of cytosolic free calcium ([Ca2+]c) in the stimulation of GLUT1 by metabolic stress and insulin. Chel ation of [Ca2+]c with bapta, introduced in rat liver epithelial Clone 9 cel ls in the acetoxymethyl (AM) form, decreased their basal rate of 2-deoxyglu cose uptake in a dose-dependent fashion. Maximal inhibition at 75 mu M bapt a was by 38 +/- 8% (n = 8). The effect was partially reversed by ionomycin. Basal sugar uptake was also decreased by lowering extracellular [Ca2+] in ionomycin-permeabilized cells. Increasing [Ca2+]c over its resting level of 168 +/- 32 (n = 27) had no affect on sugar uptake. Chelation of [Ca2+]c di d not change the abundance of surface GLUT1 and had no significant effect o n the affinity of GLUT1 for sugars. In addition, calcium chelation abolishe d the activation of GLUT1 by azide, arsenate, 2,4-dinitrophenol and insulin . However, [Ca2+]c did not increase in the presence of azide. We conclude t hat [Ca2+]c, near or below its resting level, modulates GLUT1 activity over a considerable range and plays a permissive role in the activation of the carrier by metabolic stress and insulin. (C) 2000 Harcourt Publishers Ltd.