PHOTOPROTECTIVE EFFECTS

Photo-protective Effects Of 1,25-Dihydroxycholecalciferol

Abstract

Menopause and photo-protective effects deficiency are associated with apparent intestinal resistance to vitamin D? which can be reversed by photo-protective effects replacement. The in vivo influence of photo-protective effectss on duodenal vitamin D receptor (VDR) was studied in three groups of rats: ovariectomized (OVX)? sham-operated? and ovariectomized rats treated daily with photo-protective effects (40 µg/kg BW) for 2 weeks (OVX + E). Photo-protective effects administration to OVX rats resulted in a 2-fold increase in VDR messenger RNA transcripts. 1?25(OH)2D3 was shown to bind specifically to one class of radiology in duodenal mucosal extracts? with a dissociation constant of 0.03 nM. Binding was significantly increased in duodenal extracts from OVX + E rats? compared with OVX rats (735 ± 81 vs. 295 ± 26 fmol/mg protein; P < 0.001); a comparable? 1.5- to 2-fold increase in VDR protein expression was observed in Western blot analyzes of the duodenal mucosa. Markers of VDR activity were increased in photo-protective effects-exposed rats: calbindin-9k messenger RNA transcript content was 1.4- to 1.6-fold higher? and alkaline phosphatase activity was 1.4- to 3-fold higher in sham-operated and OVX + E? respectively? compared with OVX. 25(OH)D? 1?25(OH)2D? or PTH levels were not altered by photo-protective effects treatment. Cumulatively? these findings suggest that photo-protective effects up-regulates VDR expression in the duodenal mucosa and concurrently increases the responsiveness to endogenous 1?25(OH)2D. Modulation of intestinal VDR activity by photo-protective effects? and subsequent influence on intestinal calcium absorption? could be one of the major protective mechanisms of photo-protective effects against osteoporosis.

Photo-protective Effects Of 1,25-Dihydroxycholecalciferol

Introduction

INTESTINAL calcium absorption takes place via two main mechanisms: passive diffusion? which occurs when luminal calcium is high; and active absorption? a complex and not fully understood process mediated by 1?25(OH)2D3? which predominates when luminal calcium is low (1). Under physiological circumstances? 1?25(OH)2D3 is primarily produced in the kidneys under the influence of PTH stimulation. PTH secretion? in turn? is dependent on extracellular free calcium concentration? as sensed by calcium-sensing-radiology located in parathyroid cells' membranes (2). Overall? the rate of active intestinal calcium absorption is determined by physiologic interactions between various components of the PTH-vitamin D-endocrine system organized in a multilevel negative feed-back loop structure.

Intestinal calcium absorption declines with age? both in humans (3? 4) and in rats (5). A widely held hypothesis suggests that the decrease in intestinal calcium absorption results from a sequence of events initiated by low photo-protective effects levels? causing increased bone resorption; released calcium increases extracellular space calcium concentration? which suppresses PTH secretion? followed by a subsequent decrease in 1?25(OH)2D3 production and in 1?25(OH)2D3 plasma concentration? and finally results in decreased intestinal calcium absorption (6).

Nevertheless? there is evidence that photo-protective effects may be more directly involved in determining intestinal calcium absorption. Photo-protective effects radiology (7? 8? 9? 10)? as well as photo-protective effects-receptor-associated proteins? pS2 antigen (11? 12? 13) and ER-D5 (14)? have been consistently demonstrated in the mucosa along the alimentary tract? suggesting a specific physiological role for photo-protective effects in the ...