Increased biological response to 1,25(OH)(2)D(3) in genetic hypercalciuric stone-forming rats

Abstract

Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (U) calcium (Ca) excretion, have increased intestinal Ca absorption and bone Ca resorption and reduced renal Ca reabsorption, leading to increased UCa compared with the Sprague-Dawley (SD) rats. GHS rats have increased vitamin D receptors (VDR) at each of these sites, with normal levels of 1,25(OH)(2)D(3) (1,25D), indicating that their VDR is undersaturated with 1,25D. We tested the hypothesis that 1,25D would induce a greater increase in UCa in GHS rats by feeding both strains ample Ca and injecting 1,25D (25 ng · 100 g body wt(-1) · day(-1)) or vehicle for 16 days. With 1,25D, UCa in SD increased from 1.7 ± 0.3 mg/day to 24.4 ± 1.2 (Δ = 22.4 ± 1.5) and increased more in GHS from 10.5 ± 0.7 to 41.9 ± 0.7 (Δ = 29.8 ± 1.8; P = 0.003). To determine the mechanism of the greater increase in UCa in GHS rats, we measured kidney RNA expression of components of renal Ca transport. Expression of transient receptor potential vanilloid (TRPV)5 and calbindin D(28K) were increased similarly in SD + 1,25D and GHS + 1,25D. The Na(+)/Ca(2+) exchanger (NCX1) was increased in GHS + 1,25D. Klotho was decreased in SD + 1,25D and GHS + 1,25D. TRPV6 was increased in SD + 1,25D and increased further in GHS + 1,25D. Claudin 14, 16, and 19, Na/K/2Cl transporter (NKCC2), and secretory K channel (ROMK) did not differ between SD + 1,25D and GHS + 1,25D. Increased UCa with 1,25D in GHS exceeded that of SD, indicating that the increased VDR in GHS induces a greater biological response. This increase in UCa, which must come from the intestine and/or bone, must exceed any effect of 1,25D on TRPV6 or NCX1-mediated renal Ca reabsorption.

Fig. 1.

Urine (U) calcium (Ca) in Sprague-Dawley (SD) and genetic hypercalciuric stone-forming (GHS) rats, without or with 1,25(OH)₂D₃ (1,25D). UCa was measured in 24-h collected urine and expressed as mg/day. *P < 0.05, compared with SD; ⁺P < 0.05, compared with GHS; ^oP < 0.05, compared with SD + 1,25D.

Fig. 2.

Supersaturation (SS) of calcium oxalate (CaOx; A) and CaHPO₄ (CaP; B) in U from SD and GHS rats, without or with 1,25D. SS for indicated salts in urine was measured as indicated in materials and methods and expressed as a unitless ratio. *P < 0.05 compared with SD; ⁺P < 0.05, compared with GHS; ^oP < 0.05, compared with SD + 1,25D.

Fig. 3.

Expression of transient receptor potential vanilloid (TRPV)5, calbindin D28k, and TRPV6 in kidney from SD and GHS rats, without or with 1,25D. Each sample's RNA content was normalized to the geometric mean of 4 markers (β-actin, rpl13a, YHAWAH, and sdh) and then expressed as a unitless ratio relative to the level of that particular gene product in SD. *P < 0.05, compared with SD; ⁺P < 0.05, compared with GHS; ^oP < 0.05, compared with SD + 1,25D.