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Hepatic Na(+)-K(+)-ATPase enzyme activity correlates with polarized beta-subunit expression
F R Simon1, H L Leffert, M Ellisman
1Department of Medicine, Denver Veterans Affairs Hospital, Colorado, USA.
The American Journal of Physiology
|July 1, 1995
Summary
Hepatocytes contain Na(+)-K(+)-ATPase alpha-subunits in both membrane domains, but functional activity and beta-subunits are primarily at the sinusoidal domain. This suggests functional complexes form mainly on the sinusoidal side of hepatocytes.
Area of Science:
- Cell Biology
- Biochemistry
- Membrane Protein Localization
Background:
- Hepatocytes exhibit Na(+)-K(+)-ATPase in both apical and basolateral membrane domains.
- Functional activity of Na(+)-K(+)-ATPase is preferentially associated with sinusoidal membrane sites.
Purpose of the Study:
- To investigate the underlying causes for the differential localization and function of Na(+)-K(+)-ATPase subunits in hepatocytes.
- To determine the specific isoforms of Na(+)-K(+)-ATPase alpha and beta subunits present in hepatocytes and their membrane distribution.
Main Methods:
- Light and electron microscopy for subunit localization.
- Purified liver plasma membrane subfractions for biochemical analysis.
- Ouabain inhibition curves, immunoblotting, and Northern blot hybridization.
Main Results:
- Na(+)-K(+)-ATPase alpha-subunits (predominantly alpha 1-isoform) were found in both apical and sinusoidal membrane domains.
- The beta-subunit (beta 1-isoform) was predominantly localized to the sinusoidal domain.
- Other proteins like fodrin, ankyrin, actin, and E-cadherin showed symmetrical distribution in both domains.
Conclusions:
- Functionally competent Na(+)-K(+)-ATPase alpha/beta-complexes appear to form preferentially at the sinusoidal domain of hepatocytes.
- Only alpha 1-subunits are present at the apical pole, while the complete functional complex is found at the basolateral (sinusoidal) pole.