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

J. Gao, X. Sun, V. Yatsula, R. S. Wymore, R. T. Mathias

Research output: Contribution to journalArticle

53 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)89-101
Number of pages13
JournalJournal of Membrane Biology
Volume178
Issue number2
DOIs
StatePublished - 15 Nov 2000

Keywords

  • K-dependence
  • Lens
  • Na-dependence
  • Na/K ATPase
  • Whole cell patch clamp
  • pH
  • α-Isoforms

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