Cd²⁺ regulation of the hyperpolarization-activated current I_AB in crayfish muscle

The effects of Cd²⁺ on the hyperpolarization-activated K⁺-mediated current called I_AB (Araque, A., and W, Buño. 1994. Journal of Neuroscience. 14:399-408.) were studied under two-electrode voltage-clamp in opener muscle fibers of the crayfish Procambarus clarkii. I_AB was reversibly reduced by extracellular Cd²⁺ in a concentration-dependent manner, obeying the Hill equation with IC₅₀ = 0.452 ± 0.045 mM and a Hill coefficient of 1 (determined from the maximal chord conductance of I_AB). Cd²⁺ decreased the I_AB conductance (G_AB) and shifted its voltage dependence towards hyperpolarized potentials in a similar degree, without affecting the slope of the voltage dependence. The I_AB activation time constant increased, whereas the I_AB deactivation time constant was not modified by Cd²⁺. The I_AB equilibrium potential (E_AB) was unmodified by Cd²⁺, indicating that the selective permeability of I_AB channels was not altered. I_AB was unaffected by intracellular Cd²⁺. The Cd²⁺-regulation of I_AB did not depend on [K⁺]_o, and the effects of [K⁺]_o on I_AB were unchanged by Cd²⁺, indicating that Cd²⁺ did not compete with K⁺. Therefore, Cd²⁺ probably bound to a different site so that involved in the K⁺ permeability pathway. We conclude that Cd²⁺ affected the gating of I_AB channels, interfering with their opening but not with their closing mechanism. The results can be explained by a kinetic model in which the binding of Cd²⁺ to the I_AB channels would stabilize the gating apparatus at its resting position, increasing the energy barrier for the transition from the closed to the open channel states.