Light-dependent ion channels in Octopus visual cell membranes: The permeability of large organic ions and effects of cyclic GMP

Abstract

Electroretinograms (ERG) of Octopus vulgaris were obtained in various physiological solutions containing large organic cations in the place of Na + and large organic anions in the place of Cl − in the normal physiological solution. The permeabilities of the large organic ions to the light-activated membranes were plotted against the maximum spans of the smallest molecular silhouettes of these organic ions. Results suggested that the light-dependent ion channels in the octopus visual cell membranes have a higher permeability to these large organic ions than the Na +-channel and the acetylcholine acceptor. In the planar bilayer, lipid membranes, modified with membrane fractions of microvillar outer segments of octopus visual cells, ion channels of various conductance, 5–400 pS, were observed in 200 mM NACl, but those of the conductance, less than 50 pS, were detected in 200 mM large organic ions (Tris +-Hepes −). Cyclic GMP activated 10 pS-channels and inactivated 20 pS-channels in the lipid membranes. That DBcGMP abolished the fast component of the ERG suggests that the ion channels of 20 pS which can be blocked by cGMP are the most promising candidates for the light-dependent ion channels in the octopus visual cells.