Isoform-specific function and distribution of Na/K pumps in the frog lens epithelium

Epithelial cells from the anterior and equatorial surfaces of the frog lens were isolated and used the same day for studies of the Na/K ATPase. RNase protection assays showed that all cells express α₁- and α₂-isoforms of the Na/K pump but not the α₃-isoform, however the α₂-isoform dominates in anterior cells whereas the α₁-isoform dominates in equatorial cells. The whole cell patch-clamp technique was used to record functional properties of the Na/K pump current (I(P)), defined as the current specifically inhibited by dihydro-ouabain (DHO). DHO-I(P) blockade data indicate the α₁-isoform has a dissociation constant of 100 μM DHO whereas for the α₂-isoform it is 0.75 μM DHO. Both α₁- and α₂-isoforms are half maximally activated at an intracellular Na⁺-concentration of 9 mM. The α₁-isoform is half maximally activated at an extracellular K⁺-concentration of 3.9 mM whereas for the α₂-isoform, half maximal activation occurs at 0.4 mM. Lastly, transport by the α₁-isoform is inhibited by a drop in extracellular pH, which does not affect transport by the α₂-isoform. Under normal physiological conditions, I(P) in equatorial cells is approximately 0.23 μA/μF, and in anterior cells it is about 0.14 μA/μF. These current densities refer to the area of cell membrane assuming a capacitance of around 1 μF/cm². Because cell size and geometry are different at the equatorial vs. anterior surface of the intact lens, we estimate Na/K pump current density per area of lens surface to be around 10 μA/cm² at the equator vs. 0.5 μA/cm² at the anterior pole.