Little or no change in Ca²⁺ permeation EC₅₀ at V < 0 mV. (A) The tail currents in 100 µM roscovitine were normalized to the current in 10 mM Ca²⁺ and plotted versus [Ca²⁺]_o. The points of the dose–response curves are mean ± SD measured from 6–13 neurons. For a given [Ca²⁺]_o, the points at −20 and −40 are not significantly different from each other but are different from those at 20 mV (P < 0.05). The data from each tail voltage were fit using the mass-action equation to yield the indicated EC₅₀ values for Ca²⁺ permeation. (B) To better visualize the data, the dose–response curves in roscovitine (from A) were normalized to the maximum current values calculated from the mass-action fits to the experimental data. The symbols have the same meaning as in A.