A rat kidney-specific calcium transporter in the distal nephron

Active absorption of calcium from the intestine and reabsorption of calcium from the kidney are major determinants of whole body calcium homeostasis. Two recently cloned proteins, CaT1 and ECaC, have been postulated to mediat e apical calcium uptake by rat intestine and rabbit kidney, respectively. B y screening a rat kidney cortex library with a CaT1 probe, we isolated a cD NA encoding a protein (CaT2) with 84.2 and 73.4% amino acid identities to E CaC and CaT1, respectively. Unlike ECaC, CaT2 is kidney-specific in the rat and was not detected in intestine, brain, adrenal gland, heart, skeletal m uscle, liver, lung, spleen, thymus, and testis by Northern analysis or reve rse transcription polymerase chain reaction. The expression pattern of CaT2 in kidney was similar to that of calbindin D-28K and the sodium calcium ex changer 1, NCX1, by in situ hybridization of adjacent sections. Furthermore , the mRNAs for CaT2 and calbindin D-28K were colocalized in the same cells . CaT2 mediated saturable calcium uptake with a Michaelis constant (K-m) of 0.66 mM when expressed in Xenopus laevis oocytes, Under voltage clamp cond ition, CaT2 promoted inward currents in X. laevis oocytes upon external app lication of Ca2+. Sr2+ and Ba2+ but not Mg2+ also evoked inward currents in CaT2-expressing oocytes. Similar to the alkaline earth metal ions, applica tion of Cd2+ elicited inward current in CaT2-expressing oocytes with a K-m of 1.3 mM. Cd2+, however, also potently inhibited CaT2-mediated Ca2+ uptake with an IC50 of 5.4 mu M. Ca2+ evoked currents were reduced at low pH and increased at high pH and were only slightly affected by the L-type voltage- dependent calcium channel antagonists, nifedipine, verapamil, diltiazem, an d the agonist, Bay a 8644, even at relatively high concentrations, In concl usion, CaT2 may participate in calcium entry into the cells of the distal c onvoluted tubule and connecting segment of the nephron, where active reabso rption of calcium takes place via the transcellular route. The high sensiti vity of CaT2 to Cd2+ also provides a potential explanation for Cd2+-induced hypercalciuria and resultant renal stone formation.