Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats

Area of Science:

• Nephrology
• Endocrinology
• Bone Biology

Background:

• Genetic hypercalciuric stone-forming (GHS) rats show significantly higher urine calcium excretion compared to controls.
• Increased intestinal calcium absorption, not elevated serum 1,25(OH)2D3, is a primary driver of hypercalciuria in GHS rats.
• This absorption is linked to an increased number of intestinal vitamin D receptors (VDR).

Purpose of the Study:

• To investigate the role of bone demineralization in GHS rat hypercalciuria.
• To determine if GHS rat bones exhibit increased sensitivity to 1,25(OH)2D3.
• To explore the regulation and distribution of VDR in GHS rat bone and intestine.

Main Methods:

• Cultured calvariae from neonatal GHS and control rats were treated with 1,25(OH)2D3 or parathyroid hormone (PTH).
• Calcium efflux from cultured bone was measured to assess bone resorption.
• Immunoblot and slot blot analyses were used to quantify VDR protein and RNA levels in bone tissue.

Main Results:

• GHS rat calvariae showed significant calcium efflux in response to 1,25(OH)2D3, unlike control calvariae.
• PTH induced similar bone resorption in both GHS and control rats.
• GHS rat calvariae had a fourfold increase in VDR protein levels compared to controls, with no change in VDR RNA, indicating post-transcriptional regulation.

Conclusions:

• GHS rat bones are more sensitive to 1,25(OH)2D3, contributing to hypercalciuria through enhanced bone resorption.
• The increased VDR levels in both bone and intestine suggest a systemic disorder in GHS rats.
• Altered VDR regulation in GHS rats occurs post-transcriptionally.