Figure 9.

Block of αS580D, βG522D, γG534E mutants by external Ca²⁺. Mutant subunits were coexpressed with α, γ wild-type subunits. (A) Concentration dependence of inhibition of Li⁺current by external Ca²⁺. The ratio I/I₀ represents the measured I_Liin the presence/absence of external Ca²⁺ions. Ca²⁺ inhibitory constants (K _i) obtained from fit to inhibition Langmuir isotherm (dotted lines) were 2.1 mM for the αS580D (n = 8), 0.4 mM for βG522D (n = 10), and 2.2 mM for γG534E (n = 8). (B) Voltage dependence of βG522D mutant block by external Ca²⁺ at concentrations of 0.5, 1, and 2 mM (n = 4). Dotted lines represent fit of the data to Eq. 2 (see materials and methods) gave K _i(0) values of 0.67, 0.97, and 0.49, z′ values of 0.26, 0.28, and 0.35 for the experiments done with respectively 0.5, 1, and 2mM Ca²⁺.