An experiment was carried out to investigate the effects of age of laying hens (young = 22 wk vs old = 120 wk) in maintaining Ca homeostasis during periods of Ca depletion then repletion with Ca. Plasma Ca and P, tibia breaking strength and percentage ash, renal 25-hydroxycholecalciferol-1-hydroxylase (1 alpha-hydroxylase), and parathyroid hormone (PTH)-stimulated adenylate cyclase activities were studied during 28 d of Ca depletion on a .08% Ca diet (LC) and 28 d of Ca repletion on a 3.75% Ca diet (HC). When laying hens on a HC diet were placed on a LC diet, plasma Ca and P, tibia breaking strength and ash percentage, and renal PTH-dependent adenylate cyclase activity were significantly depressed, but renal 1 alpha-hydroxylase activity was significantly stimulated. These changes were greater in the young hens than in the older hens; therefore an interaction between age and dietary Ca was found. These changes were of a lesser magnitude at 28 d of Ca depletion, probably due to the cessation of egg laying and to the desensitization of hormone-mediated function. 1 alpha-Hydroxylase activity was significantly less during the repletion period. The age effect was most pronounced for 1 alpha-hydroxylase, with the younger birds expressing significantly higher activity and ability to respond to hypocalcemia. There was a significant increase in kidney weights in Ca-deficient groups at 14 and 28 d of Ca depletion. It is concluded that younger hens have greater adaptive responses to Ca restriction than do older hens.

ABSTRACTcDNA cones ening Japanese quidl chorioallantoic membrane and chicken kidney 1,25-dihydroyvitamin D3 receptors were isolated and the total 448-amino acid (aa) sequence was deduced. (7). Another report from the same group (9), based on in vitro translation, suggested that the VDR protein is not synthesized as a large (>>60 kDa) parent molecule that is subsequently processed into a smaller protein. They concluded that the presence of the lower molecular mass protein could be a translation product of multiple transcripts, as is the case for glucocorticoid receptors or represents post-translationally modified species (9). Pike et al. (7) showed that both A and B forms ofcVDR bind similarly to DNA and, thus, are not analogous in properties to the A and B polypeptides of the avian progesterone receptor (10). The considerable debate over the origin of the B form of the avian VDR and the suggestion that this 58-kDa form may arise by proteolysis of the larger A form receptor protein led us to isolate the full-length cDNA encoding the avian VDRs.*We present evidence to demonstrate that the A and B forms of the avian VDR are derived by alternate initiation of translation from two in-phase ATG initiation signals located on a single mRNA transcript.MATERIALS AND METHODS Identification Of Japanese Quail (q) VDR cDNAs. An oligo(dT)-primed and random-primed Agtll cDNA library made from 5 pug of jqCAM poly(A)+ mRNA was screened using 358-and 1783-bp fragments from the coding region of the rVDR (3). For the first round of screening, recombinant phages were plated at a density of 15,000 plaque-forming units per 150-mm plate on a lawn of Escherichia coli y1090. Lifted Hybond-N nylon filters were prehybridized for 12 h at 420C in 50%o (vol/vol) formamide/Sx SSPE/5x Denhardt's solution/0.1% SDS containing denatured salmon sperm DNA (100 pg/ml) (lx SSPE = 0.18 M NaCl/10 mM sodium phosphate, pH 7.4/1 mM EDTA). Nick-translated probe was added to the-filters (1 x 106 cpm) (11). After hybridization for 30 h at 420C, the filters were washed successively in 2x SSC/0.1% SDS at room temperature for 15 min, lx SSC/ 0.1% SDS at 650C for 1 h, and finally, if necessary, in 0.1x SSC/0.1% SDS at 650C for 1 h. Filters were then exposed to

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The chorioallantoic membrane (CAM) of birds is an epithelial tissue that actively transports large amounts of Ca during embryonic development. In this study the effect of vitamins D and K on Ca uptake by the CAM was studied. Four dietary treatments were used to produce eggs that are the following: deficient in vitamins D and K (-D/-K), sufficient in both (+D/+K), or deficient in one and sufficient in the other (-D/+K or +D/-K). Vitamin D-deficient (-D) Japanese quail embryos (from hens fed 1,25-dihydroxyvitamin D3) do not hatch because of severe Ca deficiency resulting from their inability to obtain Ca from shell, whereas vitamin K deficiency results in only 14% reduction in hatchability. The results demonstrate that Ca uptake by CAM is vitamin D dependent and only slightly vitamin K dependent. Ca-binding activity of CAM extracts was unchanged by vitamin K deficiency, and only a small increase was provided by vitamin D treatment. Vitamin D stimulated both Ca entry and exist from the chorion cells as indicated by the increased accumulated 45Ca in +D embryos. We conclude that vitamin D is essential for the utilization of eggshell Ca by the developing embryo and hence its survival, suggesting that Ca transport across the CAM is largely a vitamin D-dependent process.

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