Figure 4. RA effect on Ca²⁺ gating of HGJ channel activity

A: Outward current was elicited by external Ca²⁺ concentration switch from 2 mM to 0 mM at a constant holding potential of 30 mV in an H-2 type horizontal cell. When the current was stable, the cell was exposed to 100 mM Ca²⁺ and the current dropped from 484 pA to 269 pA (-215 pA). After washout, 0.3 mM at-RA produced a 114 pA reduction. In the presence of RA, 100 mM Ca²⁺ produced an additional 95 pA inhibition for a total during coapplication of 209 pA. Each point represents the mean current from a 3 s interval of recording. B: Normalized dose inhibition relationships for Ca²⁺ inhibition of HGJ channel currents in the absence and presence of 0.3 mM at-RA. Each point is an average from 4 to 6 cells and smooth curves were fit with the Hill equation. RA shifted the dose response to the right with an increase of IC₅₀ from 192 mM to 384 mM, but failed to affect maximal inhibition.