Inactivation of the Na-Cl co-transporter (NCC) gene is associated with high BMD through both renal and bone mechanisms: analysis of patients with Gitelman syndrome and Ncc null mice.
Chronic thiazide treatment is associated with high BMD. We report that patients and mice with null mutations in the thiazide-sensitive NaCl cotransporter (NCC) have higher renal tubular Ca reabsorption, higher BMD, and lower bone remodeling than controls, as well as abnormalities in Ca metabolism, mainly caused by Mg depletion.


BACKGROUND
Chronic thiazide treatment decreases urinary Ca excretion (UVCa) and increases BMD. To understand the underlying mechanisms, Ca and bone metabolism were studied in two models of genetic inactivation of the thiazide-sensitive NaCl cotransporter (NCC): patients with Gitelman syndrome (GS) and Ncc knockout (Ncc(-/-)) mice.


METHODS
Ca metabolism was analyzed in GS patients and Ncc(-/-) mice under conditions of low dietary Ca. BMD was measured by DXA in patients and mice, and bone histomorphometry was analyzed in mice.


RESULTS
GS patients had low plasma Mg. They exhibited reduced UVCa, but similar serum Ca and GFR as control subjects, suggesting increased renal Ca reabsorption. Blood PTH was lower despite lower serum ionized Ca, and Mg repletion almost corrected both relative hypoparathyroidism and low UVCa. BMD was significantly increased in GS patients at both lumbar (+7%) and femoral (+16%) sites, and osteocalcin was reduced. In Ncc(-/-) mice, serum Ca and GFR were unchanged, but UVCa was reduced and PTH was elevated; Mg repletion largely corrected both abnormalities. Trabecular and cortical BMD were higher than in Ncc(+/+) mice (+4% and +5%, respectively), and despite elevated PTH, were associated with higher cortical thickness and lower endosteal osteoclastic surface.


CONCLUSIONS
Higher BMD is observed in GS patients and Ncc(-/-) mice. Relative hypoparathyroidism (human) and bone resistance to PTH (mice), mainly caused by Mg depletion, can explain the low bone remodeling and normal/low serum Ca despite increased renal Ca reabsorption.
Nicolet-Barousse
Laurence
L
Département de Physiologie, Hôpital Européen George Pompidou, Institut National de la Santé et de la Recherche Médicale INSERM U.652, IFR 58, Université Paris V, Paris, France.
Blanchard
Anne
A
Roux
Christian
C
Pietri
Laurence
L
Bloch-Faure
May
M
Kolta
Sami
S
Chappard
Christine
C
Geoffroy
Valérie
V
Morieux
Caroline
C
Jeunemaitre
Xavier
X
Shull
Gary E
GE
Meneton
Pierre
P
Paillard
Michel
M
Houillier
Pascal
P
De Vernejoul
Marie-Christine
MC
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000818:Animals; D001581:Benzothiadiazines; D001835:Body Weight; D015519:Bone Density; D001842:Bone and Bones / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D016022:Case-Control Studies; D004232:Diuretics; D005260:Female; D006801:Humans; D007011:Hypoparathyroidism; D007668:Kidney / Q000378:metabolism*; D007674:Kidney Diseases / Q000235:genetics*; D008274:Magnesium / Q000097:blood / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D008875:Middle Aged; D009154:Mutation; D010641:Phenotype; D012965:Sodium Chloride / Q000494:pharmacology; D049993:Sodium Chloride Symporter Inhibitors / Q000494:pharmacology*; D049992:Sodium Chloride Symporters; D027981:Symporters / Q000737:chemistry* / Q000378:metabolism; D013577:Syndrome; D013829:Thiadiazines / Q000494:pharmacology*; D013977:Tibia / Q000378:metabolism; D013997:Time Factors; D019076:Transgenes
D001581:Benzothiadiazines; D004232:Diuretics; D049993:Sodium Chloride Symporter Inhibitors; D049992:Sodium Chloride Symporters; D027981:Symporters; D013829:Thiadiazines; D012965:Sodium Chloride; D008274:Magnesium; D002118:Calcium