FIGURE 4. Mut-C channels are insensitive to by calcicludine.

(A, D) Peak amplitudes of Ba²⁺ currents evoked by 300 msec step depolarizations from −100 to −10 mV every 20 seconds from cells expressing wild-type (A) and Mut-C (D) Ca²⁺ channels in the presence of 0, 100, 500 and 5000 nM calcicludine. Note that Mut-C currents are insensitive to block by calcicludine. (B, E) Sample traces resulting from voltage steps from −100 to −10 mV in the absence and presence of 500 nM calcicludine. (C, F) Current-voltage relations in the absence and presence of calcicludine were generated by depolarizing cells from a holding potential of −100 mV to 300 msec step depolarizations to potentials ranging from −60 to +80 mV. Peak currents were plotted against corresponding test voltages to give the current-voltage relationship (I-V). These data were fit using the equation, I=G(V_m−V_rev)/(1+exp[(V_h−V_m)/k]), where G is the maximal slope conductance, V_rev is the reversal potential, V_m is the membrane potential, V_h is the half activation potential and k is the slope factor. I-V measurements were made in each cell in the absence and presence of calcicludine, and were normalized by dividing the peak current at each step potential by the peak of the fit through I-V data acquired in the absence of calcicludine. Current voltage relations indicate that the gross gating properties of Mut-C (F) are similar to that of wild-type (D).