Inwardly rectifying potassium current in mammalian lens epithelial cells

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Abstract
Lens potassium conductance is essential for the maintenance of lens volume and transparency. Recent work has identified three major potassium currents in lens: 1) an outwardly rectifying current, 2) an inwardly rectifying current, and 3) a calcium-activated current. This paper presents a study of the lens inward rectifier using whole cell and single-channel patch-clamp techniques. Inwardly rectifying potassium current is present in isolated human, rabbit, rat, and mouse lens epithelia. The voltage about which rectification occurs depends on the external potassium concentration. Internal magnesium is not necessary for rectification. In physiological saline, a time-dependent decrease in current during sustained hyperpolarization is seen. This “droop” is due to voltage-dependent block by external sodium. The inward rectifier is also effectively blocked by external cesium or barium but not by tetraethylammonium or 4-aminopyridine. The mouse lens inward rectifier has a single-channel conductance of 32 pS (measured on-cell with 150 mM potassium in the pipette). The single-channel current-voltage relationship is linear in the inward direction. In contrast to the macroscopic case, no outward current was measurable. The inward rectifier in lens has the necessary properties to be involved in setting resting voltage.