Mechanism and function of high vitamin D receptor levels in genetic hypercalciuric stone-forming rats.
The functional status and mechanism of increased VDR in GHS rats were investigated. Basal VDR and calbindins were increased in GHS rats. 1,25(OH)(2)D(3) increased VDR and calbindins in controls but not GHS rats. VDR half-life was prolonged in GHS rats. This study supports the mechanism and functional status of elevated VDR in GHS rats.


BACKGROUND
Genetic hypercalciuric stone-forming (GHS) rats form calcium kidney stones from hypercalciuria arising from increased intestinal calcium absorption and bone resorption and decreased renal calcium reabsorption. Normal serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels and increased vitamin D receptor (VDR) protein suggest that high rates of expression of vitamin D-responsive genes may mediate the hypercalciuria. The mechanism of elevated VDR and state of receptor function are not known.


METHODS
GHS and non-stone-forming control (NC) male rats (mean, 249 g), fed a normal calcium diet, were injected intraperitoneally with 1,25(OH)2D3 (30 ng/100 g BW) or vehicle 24 h before cycloheximide (6 mg/100 g, IP) and were killed 0-8 h afterward. Duodenal VDR was measured by ELISA and Western blot, and duodenal and kidney calbindins (9 and 28 kDa) were measured by Western blots.


CONCLUSIONS
Duodenal VDR protein by Western blot was increased 2-fold in GHS versus NC rats (633 +/- 62 versus 388 +/- 48 fmol/mg protein, n = 4, p < 0.02), and 1,25(OH)2D3 increased VDR and calbindins (9 and 28 kDa) further in NC but not GHS rats. Duodenal VDR half-life was prolonged in GHS rats (2.59 +/- 0.2 versus 1.81 +/- 0.2 h, p < 0.001). 1,25(OH)2D3 prolonged duodenal VDR half-life in NC rats to that of untreated GHS rats (2.59 +/- 0.2 versus 2.83 +/- 0.3 h, not significant). This study supports the hypothesis that prolongation of VDR half-life increases VDR tissue levels and mediates increased VDR-regulated genes that result in hypercalciuria through actions on vitamin D-regulated calcium transport in intestine, bone, and kidney.
Karnauskas
Alexander J
AJ
Department of Medicine, University of Chicago Pritzker School of Medicine, Chicago, Illinois 60637, USA.
van Leeuwen
Johannes P T M
JP
van den Bemd
Gert-Jan C M
GJ
Kathpalia
Paru P
PP
DeLuca
Hector F
HF
Bushinsky
David A
DA
Favus
Murray J
MJ
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D064026:Calbindins; D002117:Calcitriol / Q000008:administration & dosage*; D002120:Calcium Channel Agonists / Q000008:administration & dosage*; D004386:Duodenum / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D006934:Hypercalcemia / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D008297:Male; D051381:Rats; D018167:Receptors, Calcitriol / Q000096:biosynthesis*; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D014545:Urinary Calculi / Q000378:metabolism
D064026:Calbindins; D002120:Calcium Channel Agonists; D018167:Receptors, Calcitriol; D064030:S100 Calcium Binding Protein G; D002117:Calcitriol