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 alpha (1)- and alpha (2)-isoforms of the Na/K pump but not the alpha (3)-isoform, however the alpha (2)-isoform dominates in anterior cells whereas the alpha (1)-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 alpha (1)-isoform has a dissociation constant of 100 muM DHO whereas for the alpha (2)-isoform it is 0.75 muM DHO, Both alpha (1)- and alpha (2)-isoforms are half maximally activated at an intracellular Na+-concentration of 9 mM. The alpha (1)-isoform is half maximally activated at an extracellular K+-concentration of 3.9 mM whereas for the alpha (2)-isoform, half maximal activation occurs at 0.4 mM. Lastly, transport by the or,isoform is inhibited by a drop in extracellular pH, which does not affect transport by the alpha (2)-isoform. Under normal physiological conditions, I-P in equatorial cells is approximately 0.23 muA/muF, and in anterior cells it is about 0.14 muA/muF. These current densities refer to the area of cell membrane assuming a capacitance of around 1 muF/cm(2). 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 muA/cm(2) at the equator vs. 0.5 muA/cm(2) at the anterior pole.