Letter to the editors

Three types of channel activity in frog lens epithelial cells

Using the perforated patch version of whole-cell recording, we have measurd currents from isolated frog lens epithelial cells. Three types of currents were seen. A time-independent outwardly rectifying potassium current was identified that sets the resting voltage. This potassium current differs significantly from any of the potassium currents recorded with the whole-cell technique in mammalian lens epithelial cells. In addition to the potassium current, the two other currents present were both outwardly rectifying: one was time-independent while the other showed distinct activation.

Microelectrode studies of fresh human and rabbit lens epithelia revealed stable membrane potentials [V_R (human) = −36 mV; V_R (rabbit) = −45 mV] and low input resistances [$\text{R}{}_{\mathrm{i}}\text{(human) = 10 MΩ}\text{; R}{}_{\mathrm{i}}\text{(rabbit) = 20 MΩ}$]. Coupling studies, using two voltage microelectrodes, demonstrated that the low input resistance of the fresh epithelial tissue was due to electrotonic coupling, which was found to be extremely labile and sensitive to perfusion of the apical (fibre-facing) surface of the epithelium. The intercellular coupling could be stabilized by raising the calcium concentration of the perfusate.

Studies performed on confluent monolayers of cultured human lens epithelial (HLE) cells demonstrated a membrane potential (V_R = −33 mV) and input resistance ($\text{R}{}_{\mathrm{i}}\text{= 29 M}\text{Ω}$) similar to their fresh counterparts. The intercellular coupling of these cells was found to be much more robust. Ultrastructural studies revealed that the apical junction of cultured HLE cells was less complex than that found in fresh tissue, the latter exhibiting multiple interdigitations and folds. The cultured monolayer was dissociated into single cells by a variety of methods and the membrane properties of individual cells were studied. Single cells were found to have a lower membrane potential (−20 to −25 mV) and an input resistance in the range 110–170 MΩ, depending on the method of dissociation. Channel blocking and ion replacement studies revealed significant conductance pathways for potassium, sodium and chloride and a cell-attached patch clamp investigation revealed three distinct channel types. Of the two channels with inward currents at the resting potential, one, with a conductance of 25 pS, is identified as a non-selective cation channel, and the other, with a conductance of 14 pS and reversal potential of −14 mV, is a possible candidate for a chloride channel but has yet to be characterized. A third channel with an outward current at the resting potential is didentified as a potassium channel with a conductance of 49 pS.

A link between epithelial uncoupling and certain types of cataract is proposed.