IONIC CHANNELS IN CORNEAL ENDOTHELIUM

Single-channel patch-clamp techniques as well as standard and perforat ed-patch whole cell voltage-clamp techniques have been applied to the study of ionic channels in the corneal endothelium of several species. These studies have revealed two major K+ currents. One is due to an a nion- and temperature-stimulated channel that is blocked by Cs+ but no t by most other K+ channel blockers, and the other is similar to the f amily of A-currents found in excitable cells. The A-current is transie nt after a depolarizing voltage step and is blocked by both 4-aminopyr idine and quinidine. These two currents are probably responsible for s etting the -50 to -60 mV resting voltage reported for these cells. A C a2+-activated ATP-inhibited nonselective cation channel and a tetrodot oxin-blocked Na+ channel are possible Na+ inflow pathways, but, given their gating properties, it is not certain that either channel works u nder physiological conditions. A large-conductance anion channel has a lso been identified by single-channel patch-clamp techniques. Single c orneal endothelial cells have input resistances of 5-10 G Ohm and have steady-state K+ currents that are similar to 10 pA at the resting vol tage. Pairs or monolayers of cells are electrically coupled and dye co upled through gap junctions.