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. Chelation of [Ca2+]c with bapta, introduced in rat liver epithelial Clone 9 cells in the acetoxymethyl (AM) form, decreased their basal rate of 2-deoxyglucose uptake in a dose-dependent fashion. Maximal inhibition at 75 μM bapta 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 did not change the abundance of surface GLUT1 and had no significant effect on the affinity of GLUT1 for sugars. In addition, calcium chelation abolished 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 that [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.