Embryonic intestinal receptor

In addition, in chick embryonic intestine, 1,25-(OH)2D2 and 1,25-(OH)2D3 are equally effective in stimulating production of calcium binding protein (O. Parkes and H. F. DeLuca, unpublished results). These results provide evidence that the discrimination does not take place at the receptor level at least. 

The gastrointestinal tract is the only system outside the central nervous system (CNS) with significant concentrations of opioid receptors. This reflects their common embryonic origins. Opioids increase intestinal tone and decrease propulsive peristalsis, resulting in delayed gastric emptying and constipation or ileus. Opioids increase common bile duct pressure and decrease bile production and flow, primarily because of spasm of the sphincter of Oddi. The tone of the bile duct itself is also increased. 

Vitamin A (retinol) is a fat-soluble vitamin important for the maintenance of skin, bone, and blood vessels, as well as for the promotion of vision (Theodosiou et al. 2010). It is obtained from the diet either as all-trans-retinol, retinyl esters, or P-carotene (Blomhoff and Blomhoff 2006) and is stored in the liver (Moise et al. 2007). Vitamin A is converted to retinoic acid (RA), which is formed mainly through intracellular oxidative metabolism by retinal dehydrogenases (RALDHs) (Lampen et al. 2000). RA plays important roles in embryonic development, organogenesis, tissue homeostasis, cell proliferation, differentiation, and apoptosis (Theodosiou et al. 2010). In adult mammals, RALDH is found in intestinal epithelial cells (lECs) and gut associated-dendritic cells (DCs) from Peyer s patches and mesenteric lymph nodes (Iwata 2004, Coombes et al. 2007). Gut-associated DCs and lECs can metabolize vitamin A to RA in vitro (Lampen 2000), which indicates they may be a source of RA in gut mucosa. RA binds to two families of nuclear receptors, RA receptor (RAR) isotypes (a, p, and y) and retinoic X receptor (RXR) isotypes (a, p, and y). RAR and RXR form heterodimers and interact with retinoic acid response elements (RAREs) within the promoters of retinoic acid responsive genes (Blomhoff and Blomhoff 2006). RAR is ubiquitously expressed and up-regulated by RA. RXR also... 

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