Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium.
The extracellular Ca2+ (Ca(0)2+)-sensing receptor (CaR) recently cloned from mammalian parathyroid, kidney, brain, and thyroid plays a central role in maintaining near constancy of Ca(0)2+. We previously showed that the hypercalcemia normally present in New Zealand white rabbits is associated with an elevated set point for Ca(02+)-regulated PTH release (the level of Ca(0)2+ half-maximally inhibiting hormonal secretion). This observation suggested an alteration in the Ca(02+)-sensing mechanism in the rabbit parathyroid, a possibility we have now pursued by isolating and characterizing the rabbit homolog of the CaR. The cloned rabbit kidney CaR (RabCaR) shares a high degree of overall homology (> 90% amino acid identity) with the bovine, human, and rat CaRs, although it differs slightly in several regions of the extracellular domain potentially involved in binding ligands. By Northern analysis and/or immunohistochemistry, a similar or identical receptor is also expressed in parathyroid, thyroid C cells, small and large intestine, and in the thick ascending limb and collecting ducts of the kidney. When expressed transiently in HEK293 cells and assayed functionally through CaR agonist-evoked increases in Ca(i)2+, the rabbit CaR shows apparent affinities for Ca(0)2+, Mg(0)2+, and Gd(0)3+ that are indistinguishable from those observed in studies carried out concomitantly using the human CaR. Therefore, at least as assessed by its ability to increase Ca(i)2+ when expressed in HEK293 cells, the intrinsic functional properties of the rabbit CaR cannot explain the hypercalcemia observed in vivo in the New Zealand white rabbit.
Butters
R R
RR
Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts, USA.
Chattopadhyay
N
N
Nielsen
P
P
Smith
C P
CP
Mithal
A
A
Kifor
O
O
Bai
M
M
Quinn
S
S
Goldsmith
P
P
Hurwitz
S
S
Krapcho
K
K
Busby
J
J
Brown
E M
EM
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D015152:Blotting, Northern; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D002417:Cattle; D002460:Cell Line; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000737:chemistry / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism*; D005682:Gadolinium / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D006934:Hypercalcemia / Q000235:genetics* / Q000378:metabolism; D007668:Kidney / Q000737:chemistry; D008274:Magnesium / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010280:Parathyroid Glands / Q000737:chemistry; D017433:Protein Structure, Secondary; D011817:Rabbits; D051381:Rats; D016415:Sequence Alignment; D014018:Tissue Distribution; D014162:Transfection
D002135:Calcium-Binding Proteins; D018076:DNA, Complementary; D005682:Gadolinium; D008274:Magnesium; D002118:Calcium