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High capacity Na(+) /H(+) exchange activity in mineralizing osteoblasts.
MedLine Citation:
PMID:  21413028     Owner:  NLM     Status:  In-Data-Review    
Abstract/OtherAbstract:
Osteoblasts synthesize bone in polarized groups of cells sealed by tight junctions. Large amounts of acid are produced as bone mineral is precipitated. We addressed the mechanism by which cells manage this acid load by measuring intracellular pH (pHi) in non-transformed osteoblasts in response to weak acid or bicarbonate loading. Basal pHi in mineralizing osteoblasts was ∼7.3 and decreased by ∼1.4 units upon replacing extracellular Na(+) with N-methyl-D-glucamine. Loading with 40 mM acetic or propionic acids, in normal extracellular Na(+) , caused only mild cytosolic acidification. In contrast, in Na(+) -free solutions, weak acids reduced pHi dramatically. After Na(+) reintroduction, pHi recovered rapidly, in keeping with Na(+) /H(+) exchanger (NHE) activity. Sodium-dependent pHi recovery from weak acid loading was inhibited by amiloride with the Ki consistent with NHEs. NHE1 and NHE6 were expressed strongly, and expression was upregulated highly, by mineralization, in human osteoblasts. Antibody labeling of mouse bone showed NHE1 on basolateral surfaces of all osteoblasts. NHE6 occurred on basolateral surfaces of osteoblasts mainly in areas of mineralization. Conversely, elevated HCO 3- alkalinized osteoblasts, and pH recovered in medium containing Cl(-) , with or without Na(+) , in keeping with Na(+) -independent Cl(-) /HCO 3- exchange. The exchanger AE2 also occurred on the basolateral surface of osteoblasts, consistent with Cl(-) /HCO 3- exchange for elimination of metabolic carbonate. Overexpression of NHE6 or knockdown of NHE1 in MG63 human osteosarcoma cells confirmed roles of NHE1 and NHE6 in maintaining pHi. We conclude that in mineralizing osteoblasts, slightly basic basal pHi is maintained, and external acid load is dissipated, by high-capacity Na(+) /H(+) exchange via NHE1 and NHE6. J. Cell. Physiol. 226: 1702-1712, 2011. © 2010 Wiley-Liss, Inc.
Authors:
Li Liu; Paul H Schlesinger; Nicole M Slack; Peter A Friedman; Harry C Blair
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  Journal of cellular physiology     Volume:  226     ISSN:  1097-4652     ISO Abbreviation:  J. Cell. Physiol.     Publication Date:  2011 Jun 
Date Detail:
Created Date:  2011-03-17     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  0050222     Medline TA:  J Cell Physiol     Country:  United States    
Other Details:
Languages:  eng     Pagination:  1702-12     Citation Subset:  IM    
Copyright Information:
Copyright © 2010 Wiley-Liss, Inc.
Affiliation:
Departments of Pathology and Physiology & Cell Biology, Pittsburgh Veteran's Affairs Medical Center, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.
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