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1 Department of Medicine, University of Sydney, Sydney, NSW, Australia
2 Department of Physiology, University of Wurzburg, Wurzburg, Germany
3 Department of Anatomy, University of Sydney, Sydney, NSW, Australia
4 Department of Medicine, University of Sydney, Sydney, NSW, Australia; School of Biomedical Sciences, University of Queensland, St Lucia, QLD, Australia
* To whom correspondence should be addressed. E-mail: p.poronnik{at}uq.edu.au.
The progression of renal disease correlates strongly with both hypertension and the degree of proteinuria suggesting a link between excessive Na+ reabsorption and exposure of the proximal tubule to protein. The current study investigated the effects of albumin on cell growth and Na+ uptake in primary cultures of human proximal tubule cells (PTC). Albumin (1.0 mg/ml) increased cell proliferation to 134.1 ± 11.8% (P < 0.001) of control levels with no change in levels of apoptosis. Total 22Na+ uptake was increased following exposure to 0.1 and 1.0 mg/ml albumin to 119.1 ± 6.3% (P = 0.005) and 115.6 ± 5.3% (P < 0.006) of controls levels respectively that was due to an increase in NHE3 activity. This was associated with an increase in NHE3 mRNA level to 161.1 ±15.1% (P < 0.005) of control in response to 0.1 mg/ml albumin. Using confocal microscopy with a novel antibody raised against the predicted extracellular N-terminus of human NHE3, we observed in non-permeabilsed cells that exposure of PTC to albumin increased the levels of NHE3 at the cell surface to 115.4 ± 2.7% (P < 0.0005) and 122.4 ± 3.7% (P < 0.0001) of control levels (0.1 and 1.0 mg/ml albumin respectively). This was paralleled by significant increases in NHE3 in the subplasmalemmal region as measured in permeabilised cells. These albumin-induced increases in expression and activity of NHE3 in PTC suggest a possible mechanism for Na+ retention in response to proteinuria.
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