Measurement of steady currents around the frog lens

Steady electrical currents were measured and mapped around the frog lens using a vibrating probe. Ion substitutions and ouabain were used to help identify the ionic basis of the observed currents. In physiological frog Ringer solution, current densities at the equator averaged 26 microA cm-2 and were directed outward. At both optical poles, inward currents were measured, with those at the posterior pole about 30% greater than those at the anterior pole. Currents at all loci decreased when external potassium concentration was increased at the expense of sodium, and when Na and K concentrations approached those of lens cytosol, all currents approached zero. Current direction reversed in a solution containing 105 mM K and 2.5 mM Na. In an Na-free medium, where sodium was replaced with choline, currents were reduced significantly at the poles and equator, but they changed direction only at the anterior pole. This effect was fully reversible on return to solution containing sodium. Ouabain (0.1 mM) caused a significant decrease in currents only at the equator, and this effect had two components. A rapid 10% decline in current was complete in 2 min, followed by a steady, slower decrease which stabilized 8-12 min later at a new level approximately 30% lower than the initial current.