Amino Acid Substitutions in the Pore of the CaV1.2 Calcium Channel Reduce Barium Currents without Affecting Calcium Currents

AbstractBa2+ currents through CaV1.2 Ca2+ channels are typically twice as large as Ca2+ currents. Replacing Phe-1144 in the pore-loop of domain III with glycine and lysine, and Tyr-1152 with lysine, reduces whole-cell GBa/GCa from 2.2 (wild-type) to 0.95, 1.21, and 0.90, respectively. Whole-cell and single-channel measurements indicate that reductions in GBa/GCa result specifically from a decrease in Ba2+ conductance and not changes in Vh or PO. Half-maximal block of ILi is increased by 3.2-, 3.8-, and 1.6-fold in Ca2+, and 3.8-, 4.2-, and 1.8-fold in Ba2+ for F1144G, Y1152K, and F1144K, respectively. High affinity interactions of individual divalent cations to the pore are not important for determining GBa/GCa, because the fold increases in IC50 values for Ba2+ and Ca2+ are similar. On the contrary, conductance-concentration curves indicate that GBa/GCa is reduced because the interactions of multiple Ba2+ ions in the mutant pores are altered. The complexity of these interactions is exemplified by the anomalous mole fraction effect, which is flattened for F1144G and FY/GK but accentuated for F1144K. In summary, the physicochemical properties of the amino acid residues at positions 1144 and 1152 are crucial to the pore’s ability to distinguish between multiple Ba2+ ions and Ca2+ ions