[Ionic mechanisms of the nonlinearity of retinal horizontal cell membranes]

Changes in ionic conductivity underlying the nonlinearity a voltage-current curve of the nonsynaptic membrane of horizontal cells were investigated in experiment on goldfish and turtle retina. The measurements were made under conditions when conductivity of a subsynaptic membrane was minimal, i.e. when the retina was illuminated with bright light or synaptic transmission from photoreceptors was blocked with Co2+. An increase in [K+] led to depolarization and diminution os steepness i a hyperpolarizing part of the voltage-current curve; a decrease in K+ evoked an opposite effect. A decrease in [C1-] did not change both the membrane potential and the shape of the voltage-current curve. Substitution of Na+ by tris or choline did not evoke a reliable change in the membrane potential, and produce only a slight change in the shape of he current-voltage curve. It is concluded that the membrane is permeable for K+; permeability for C1- is absent or small; permeability for Na + is small too. Ba2+ (2-5 mM) increased the steepness of the hyperpolarizing part of the voltage current curve, and the curve became more linear. It is concluded that the nonlinearity is determined mainly by potential-dependent k+-channels whose conductivity increases with hyperpolarization, and Ba2+ blocks this increase in permeability. An increase in Ca2+ from 1 to 20 mM enhanced the steepness of the depolarizing part of the voltage-current curved without altering the hyperpolarizing part. It is supposed that the horizontal cell membrane has potential-dependent Ca2+- channels whose conductivity increases with depolarization.