Figure 5.

Novel mechanisms of action, Na⁺/K⁺-ATPase inhibitors (cardiotonic steroids) and GSPE. Cells were treated as described in Figs. 2–4. A, concentration response. The increase in cell-surface Glut4 was dose-dependent and saturable for both ouabain and GSPE. Unlike other activators, GSPE had its largest effect in quiescent cells (filled symbols), increasing cell-surface Glut4 to the same extent as 1 nm insulin (open symbols). Ouabain had no effect in quiescent cells. B, time course. The effect of GSPE was slow relative to insulin (t_½ = 20 min) but much faster than ouabain (t_½ = 60 min). Ouabain had a complex kinetics profile, with an initial rapid decrease in cell-surface Glut4, followed by a slow increase (black filled symbols). C, insulin concentration response. GSPE increased cell-surface Glut4 at sub-maximal insulin (<3 nm) but inhibited translocation at saturating insulin (black filled symbols).