Single inward rectifier channels in horizontal cells.

The ion channels responsible for inward rectification in horizontal cells were studied using the patch clamp technique applied to isolated cells from goldfish retina. Inward currents recorded from these cells were identified as due to the opening of inward rectifier channels based on their ion selectivity, channel gating behavior, and the effects of external blocking ions. The single channel conductance was 20 pS in 125 mM external K+. The null current potential shifted with changes in the K+ concentration as expected for a channel permeable to K+, and the channel appeared to have little permeability to Na+. The probability of a channel being in an open state increased as the membrane was hyperpolarized from the K+ equilibrium potential (0 to -10 mV) over potentials ranging to -80 mV, in the presence of external Na+. The closing rate was insensitive to membrane potential in the presence of external Na+. The opening rate of the channel increased as the membrane was hyperpolarized. The increase in the probability of a channel being open at negative potentials was therefore caused by the voltage sensitivity of the rate of channel opening.