Regulation of renal calcium receptor gene expression by 1,25-dihydroxyvitamin D3 in genetic hypercalciuric stone-forming rats

Hypercalciuria in inbred genetic hypercalciuric stone-forming (GHS) rats is due, in part, to a decrease in renal tubule Ca reabsorption. Activation of the renal Ca receptor (CaR) may decrease renal tubule Ca reabsorption and cause hypercalciuria through suppression of Ca-sensitive potassium channel activity. Because the. rat renal CaR gene is regulated by extracellular calcium and 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] and GHS rats have increased renal vitamin D receptor content, the current study was undertaken to determine the level of CaR gene expression in GHS rat kidney and whether CaR gene expression is regulated by 1,25(OH)(2)D-3. Male GHS and normal control (NC) rats were fed a Ca-sufficient diet (0.6% Ca). Western blotting revealed a four-fold increase in CaR protein in GHS rat renal tissue, and 1,25(OH)(2)D-3 administration increased renal CaR in both GHS and NC rats. Northern blot analysis of extracts of renal cortical tissue from GHS and NC rats revealed a major 7-kb transcript of CaR and a more modest 4-kb transcript, both of which were readily detectable. Both Northern blotting and real-time reverse transcription-PCR revealed increased basal CaR mRNA expression levels in GHS rat kidney. 1,25(OH)(2)D-3 administration increased renal CaR mRNA levels 2.0- and 3.3-fold in GHS and NC rats, respectively. Despite the greater incremental increase by 1,25(OH)(2)D-3 in NC rats, CaR mRNA levels remained higher in GHS rat kidney, and the elevation was more sustained. 1,25(OH)(2)D-3 increased CaR mRNA through both elevated CaR gene expression and prolonged tissue half-life. These results demonstrate that GHS rats have high levels of CaR gene expression and CaR protein that may contribute to the hypercalciuria and calcium nephrolithiasis.