Divalent Cation Interactions with Light-Dependent K Channels: Kinetics of Voltage-Dependent Block and Requirement for an Open Pore

The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca²⁺ and Mg²⁺ that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca²⁺ than Mg²⁺ (K_1/2 ≈ 16 and 61 mM, respectively, at V_m = −30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca²⁺ block is consistent with a single binding site located at an electrical distance of δ ≈ 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (τ = 5–20 ms) that disappears on removal of Ca²⁺ and Mg²⁺ and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca²⁺ than with Mg²⁺, suggesting that the process is dominated by the “on” rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside.