A, I-Vm relations obtained from a cell after treatment with 0.5 μm of the inactive isomer 4α-PMA. Ligand sensitivity of channels was similar to that of untreated control. B, I-Vm relations obtained from a cell after treatment with 0.5 μm PMA in the presence of 1 μm staurosporin. No modulation of channel activity was observed. C, comparison of channel sensitivity of control cells (dotted line, taken from Fig. 3C), cells treated with 4α-PMA (○, continuous line), cells treated with PMA in the presence of staurosporin (•, dashed line), and cells treated with PMA (dash-dotted line, taken from Fig. 3C).