Effect of Ca2+ on MAO activity in the mouse hippocampal HT-22 cell line. HT-22 cell lysates (100 μg/100 μL; n = 4) were incubated with increasing concentrations of Ca2+ and assayed radioenzymatically for (A) MAO-A and (B) MAO-B activities. (C) HT-22 lysates (n = 3) were incubated with either Ca2+ or the Ca2+ antagonist Mg2+ (1 mM) or co-incubated with Ca2+ (1 mM) and increasing concentrations of Mg2+ to test for the potential of Mg2+ to block Ca2+ (1 mM)-sensitive MAO-A activity. **: P < 0.01, ***: P < 0.001 vs. control. Data represent mean ± SD.