Metabolism of 25-hydroxyvitamin D3 during photo-induced reproductive development in female Japanese quail

Effects of estrone and estradiol-17 beta on 25-hydroxycholecalciferol hydroxylase activities in female Japanese quail

Equimolar amounts of estrone and estradiol-17 beta injected into sexually immature female Japanese quail caused comparable increases and decreases in the renal activity of 25-hydroxycholecalciferol (25HCC) 1 alpha-hydroxylase and 25-hydroxycholecalciferol 24-hydroxylase, respectively. Peak activity of the former enzyme was induced by both estrogens within 6 hr and had declined by 24 hr. Plasma concentrations of estrone and estradiol following the injection were maximal within 3-6 hr of injecting the steroid i.m. Following [3H]estradiol injection, 25% of the radioactivity was located with estrone in the plasma following separation by TLC. Conversely, following [3H]estrone injection 30% of the radioactivity in plasma was located with estradiol. Thus, both estrone and estradiol may have physiological effects in quail, which in part may be due to their in vivo interconversion. Evidence that prolactin may be mediating the estrogenic stimulation of the 25HCC 1 alpha-hydroxylase is also presented.

Changes in plasma concentrations of reproductive steroids in female Japanese quail (Coturnix coturnix japonica) raised on long or short photoperiods

Plasma concentrations of progesterone, testosterone, estradiol, and estrone were measured by radioimmunoassay in female Japanese quail 22 to 70 days of age. The birds were raised on long (16 hr light per day) or short (8 hr light per day) photoperiods. Only the birds raised on the long photoperiod layed eggs, starting between 42 and 49 days of age. The concentration of each reproductive steroid increased between 28 and 35 days of age, but the increases were more substantial in the birds raised on long days than in the birds raised on short days. Changes in plasma progesterone and testosterone concentrations showed similar trends: the concentrations increased before the onset of lay and were maintained at a high level in the laying birds. In contrast, plasma concentrations of estradiol and estrone increased to a peak at 35-42 days and declined once egg laying became established. The short-day birds also showed increased plasma estrogen levels, maximal at 35-42 days, with estrone being at a higher concentration than estradiol. The increases in plasma steroid levels occurred during the period of rapid growth of the ovaries and oviducts in the long-day birds, although in the short-day birds the growth of these organs was much less substantial.

Regulation of 25-hydroxyvitamin D3 metabolism in chick embryo

We investigated the time course of the development of renal 25-hydroxyvitamin D-1-hydroxylase and 25-hydroxyvitamin D-24-hydroxylase in chick embryos grown in the presence and absence of the eggshell. In embryos with the eggshell, the specific activity (SA) of 25-hydroxyvitamin D-1-hydroxylase in kidney homogenates increased from 0.68 fmol X min-1 X mg protein-1 at 12 days of gestation to a peak of 2.55 +/- 0.50 fmol X min-1 X mg-1 protein-1 at 17 days. In contrast, the SA of 25-hydroxyvitamin D-24-hydroxylase decreased from 2.5 fmol X min-1 X mg protein-1 to 0.90 +/- 0.25 fmol X min-1 X mg protein-1 during the interval. The total plasma calcium was significantly reduced in embryos without shells at 14 to 15 days of gestation (1.1 +/- 0.1 mM, mean +/- SE) compared with normal embryos of the same gestation (2.3 +/- 0.3 mM, P less than 0.002). In embryos without the eggshell, renal 25-hydroxyvitamin D-1-hydroxylase increased from 6.0 to 8.2 +/- 0.6 fmol X min-1 X mg protein-1 at 17 days of gestation and was from four- to sixfold higher than corresponding enzymatic activities for intact embryos. The increased enzyme activity resulting from loss of the eggshell was due to an increase in Vmax. The findings indicate that renal 25-hydroxyvitamin D-1-hydroxylase and 25-hydroxyvitamin D-24-hydroxylase in the chick embryo exhibit activity and show a large capacity for regulation in response to changes in calcium metabolism.

The binding of estrogen to liver nuclei from Japanese quail

Specific binding sites for estrogen were characterized in liver nuclei from adult Japanese quail. A single class of high-affinity binding sites (KD = 2 X 10(-10) M) was revealed by saturation analysis using an exchange assay. Estradiol (E2), estrone, and estriol competed effectively with [3H]E2 for the nuclear binding sites but progesterone and testosterone were ineffective. The concentration of sites was 0.15 +/- 0.05 pmol/mg DNA (mean +/- 1 SD) in sexually immature females and 0.47 +/- 0.11 pmol/mg DNA in layers. Nuclear estrogen binding sites were not detected in liver nuclei from male Japanese quail. However, after treatment of male birds with E2 there was a dose-dependent accumulation of binding sites. The maximum number of occupied sites was obtained with 50 ng E2/g body wt and equaled that found in laying females. The estrogen binding sites were retained in nuclei of estrogen-treated male quail with a T1/2 of 2.5 hr. Thus, an estrogen binding component was characterized in quail liver nuclei that had features consistent with an estrogen receptor, including high affinity, limited capacity, structural specificity, tissue specificity, nuclear translocation, and nuclear retention.

In vitro synthesis of 1 alpha,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol by isolated calvarial cells

The question of whether the skeleton metabolizes 25-hydroxycholecalciferol [25(OH)D3] to more-polar products was studied. Calvarial cells were dispersed from 16-day old chicken embryos by using collagenase and then grown in culture in serum-free medium. Confluent cell cultures were incubated with 7 nM 25(OH)[3H]D3 for 2 hr, and the vitamin D metabolites were then extracted. At least four polar metabolites were produced. Based on separation by Sephadex LH-20 chromatography followed by high-pressure liquid chromatography, two of these metabolites were identified as 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3]. These metabolites were also produced by cultured kidney cells but not by liver, heart muscle, or skin cells isolated from the same embryos. The specific activities of the calvarial 1- and 24-hydroxylases were similar in magnitude to those in isolated kidney cells. The specific activity of the calvarial 25(OH)D3:1-hydroxylase was inhibited by an 8-hr preincubation with 1,25(OH)2D3, whereas the 24-hydroxylase was enhanced. It is concluded that (i) vitamin D metabolism by isolated cells is organ-specific, (ii) calvarial cells produce active metabolites of vitamin D in significant amounts, (iii) vitamin D metabolism by calvarial cells is regulated by 1,25(OH)2D3, and (iv) locally produced, active metabolites could act locally, thereby adding a new dimension to the regulation of mineral metabolism by vitamin D metabolites.

In vitro metabolism of 25-hydroxyvitamin D3 by isolated rat kidney cells

Cells were dispersed from rat kidney after enzymatic digestion of the extracellular matrix. When the cells were suspended in a serum-free medium and incubated with (3)H-labeled 25-hydroxyvitamin D(3) (25-OH-D(3)) several polar metabolites, including 1,25-(OH)(2)[(3)H]D(3) and 24,25-(OH)(2)[(3)H]D(3) were produced. The specific activities of the 25-OH-D(3):1- and 24-hydroxylases in isolated rat kidney cells were 10-100 times greater than in avian kidney homogenates. The rates of production of 1,25-(OH)(2)D(3) and 24,25-(OH)(2)D(3) were linear over a wide range in cell densities (0.65-5.0 x 10(6) cells per ml) and substrate concentrations (3.5-70 nM). The rate of production of 24,25-(OH)(2)[(3)H]D(3) from 25-OH-[(3)H]D(3) by cells isolated from rats fed control diet was linear with time for up to 30 min, while the synthesis of 1,25-(OH)(2)[(3)H]D(3) was linear for over 90 min. The specific activity of the 25-OH-D(3):1-hydroxylase was increased in kidney cells from vitamin D-deficient rats (11.5 fmol/min per 10(6) cells) as well as calcium-deficient rats (8.1 fmol/min per 10(6) cells) when compared to cells from rats fed the control diet (2.0 fmol/min per 10(6) cells). Also, the specific activity of the 25-OH-D(3):24-hydroxylase was reduced in cells from the vitamin D-deficient rats (<0.2 fmol/min per 10(6) cells) and calcium-deficient rats (5.1 fmol/min per 10(6) cells) compared to the controls (15.2 fmol/min per 10(6) cells). On the basis of these results, as well as previous in vivo studies, we conclude that the metabolism of 25-OH-D(3) by freshly isolated rat kidney cells reflects the in vivo activities of the renal vitamin D-metabolizing enzymes and may prove useful as an assay.

Serum-free culture of Japanese quail kidney cells. Regulation of vitamin D metabolism

Cells obtained from male quail kidneys by digestion with collagenase and hyaluronidase were plated and maintained in a chemically defined, serum-free medium. Culture dishes (35 mm) were inoculated with 1.5 . 10(6) cells which became confluent in 5 days. The cells maintained an epithelial-like morphology over the entire culture period. During a 2 h incubation the cells metabolized 25--30% of the 10 nM 25-hydroxyvitamin D-3 (25-OH-D-3) provided. Seven metabolites were chromatographically separated on Sephadex LH-20. Three have been identified as 1 alpha, 25-dihydroxyvitamin D-3 (1,25(OH)2D-3), 24,25-dihydroxyvitamin D-3 (24,25(OH)2D-3) and 1 alpha, 24,25-trihhydroxyvitamin D-3 (1,24,25(OH)3D-3). The activities of the 25-OH-D-3:1 alpha- and 24-hydroxylases increased eight times faster than the cell number in 5 days. Preincubation of the cells with 10 nM 25-OH-D-3 or 1,25(OH)2D-3 decreased 1,25(OH)2D-3 synthesis, and increased both 24,25(OH)2D-3 and metabolite IV synthesis. The decrease in 25-OH-D-3:1 alpha-hydroxylase activity required a 2 h preincubation with 25-OH-D-3, while stimulation of 25-OH-D-3:24-hydroxylase activity and metabolite IV production required a 6 h preincubation. Incubations of cells for 1 h with parathyroid hormone resulted in a 30-fold increase in cyclic AMP in the medium. A 6 h preincubation with parathyroid hormone decreased 24,25(OH)2D-3) synthesis 50% relative to control cells. These results demonstrate the amenability of this system for studying the regulation of 25-OH-D-3 metabolism, as well as its use for other in vitro studies on renal cell function in a chemically defined culture system.