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2011 ; 226
(6
): 1702-12
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High capacity Na+/H+ exchange activity in mineralizing osteoblasts
#MMPMID21413028
Liu L
; Schlesinger PH
; Slack NM
; Friedman PA
; Blair HC
J Cell Physiol
2011[Jun]; 226
(6
): 1702-12
PMID21413028
show ga
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.
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