4-aminopyridine and l-cis-diltiazem block the cGMP-activated K+ channels closed by light in the molluscan extra-ocular photoreceptors.

The cGMP-activated K+ channels closed by light lead to the depolarizing photocurrent of photoreceptors in the Onchidium ganglion. Whole-cell current records showed that external application of 100-200 microM 4-aminopyridine or 200-400 microM l-cis-diltiazem completely blocked the macroscopic photocurrent at any depolarizing and hyperpolarizing potentials. Single-channel current recordings suggested that both 4-aminopyridine and l-cis-diltiazem act to block the cGMP-activated K+ channels in their open state from inside the cell.