CHARACTERIZATION OF A CATION CHANNEL ON THE APICAL SURFACE OF THE FROG LENS EPITHELIUM

The properties of a single conductance pathway of the apical (fibre-facing) surface of the frog lens epithelium are reported. Using the patch-clamp technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981), the most common single-channel currents had an amplitude of 1·9 pA, the mean open time 2·1 ms and a conductance of 25 pS. One open-state time constant (t o = 3·3 ms) and two closed-state time constants (τ c 1 = 0·9 ms, τ c 2 = 23·1 ms) were resolved. The channel current and the mean open time were both increased when Ca 2+ was removed from the external solution and the open time distribution was no longer fitted by a single exponential. Multiple-channel events in cell-attached patches containing two or more identical channels were distributed in a binomial fashion and the probability that an individual channel was open, obtained by fitting the binomial distribution, was 0·039. The channel was found to have a Na + :K + selectivity ratio of 3:1. When Ca 2+ was removed from the pipette solution the probability that an individual channel was open increased to 0·137 and the Na + :K + selectivity ratio increased to 4:1. Channel activity was observed in the presence of tetrodotoxin (10 -6 M) in the bathing medium and the pipette solution but was abolished by internal perfusion of the patch pipette with 0·5 x 10 -4 M amiloride. This apical conductance pathway is identified as an amiloride-sensitive cation channel. These channels are clustered in groups on the apical membrane, spontaneously active at the resting potential and with the possibility of altering their Na + :K + selectivity. They represent a distinct type of channel, that differ from nerve and muscle Na + channels in their manner of activation, but do share some common features with both Na + and Ca 2+ channels in excitable cells.