A comparison of the effects of dietary calcium and phosphorus deficiency on the in vitro and in vivo metabolism of 25-hydroxycholecalciferol in the chick

Acute high environmental temperature and calcium-estrogen relationship in the hen

Much is known about the effects of high environmental temperature (HT) on egg production, but very little is understood about the mechanisms that underlie them. Two experiments were conducted to examine the effects of acute heat stress on circulating estradiol, on calcium uptake by gut tissue, on bone resorption, and on the dynamic relationship between estradiol and calcium in the hen during one ovulatory cycle. In one study, hens were moved individually and randomly into a hot [HT: temperature (T) = 35 C, relative humidity (RH) = 50%; n = 18] or a control, thermoneutral (TN: T = 23 C, RH = 50%; n = 18) environment immediately after a mid-sequence oviposition and brachial vein cannulation. Blood samples (2 mL) were collected every 3 h for 21 h for ionized calcium (Ca2+) and pH determinations and from which aliquots were frozen for 17 beta-estradiol (E2), total calcium (TCa), and inorganic P analysis. Excreta and urine were assayed for TCa and hydroxyproline (OHPr), respectively. A second study was conducted to determine the effects of HT (T = 35, H = 50%, 12 h) vs TN (T = 23 C, RH = 50%, 12 h) on the ability of duodenal cells to take up calcium (CaT). Blood pH and calcium responded to HT as expected (pH increased, Ca2+ decreased, and TCa decreased) and the cyclic pattern of Ca2+ in blood was abolished. The ratio of Ca2+:TCa decreased sharply at approximately the onset of shell calcification in control hens, but in HT hens there was no clear change in the ratio of any point in the cycle. The pattern of E2 typical of hens under normal conditions was significantly depressed in plasma of HT hens. Calcium uptake by duodenal epithelial cells of HT hens was lower than in TN hens. There was a clear inverse correlation between blood Ca2+ and urine OHPr in TN hens (r2 = -73, P = 0.0021) but not in HT hens (r2 = -27, P = 0.32). In addition to alterations in acid-base balance and the status of Ca2+, diminished ability of duodenal cells to transport calcium may be a critical factor in the detrimental effects of heat stress on egg production (numbers), eggshell characteristics, and skeletal integrity often documented in the laying hen.

Determinants of serum 1,25-dihydroxyvitamin D concentration in healthy premenopausal subjects

Concentrations of serum phosphate and parathyroid hormone (PTH) are well known regulators of the production of 1,25-dihydroxyvitamin D (1,25-(OH)2D) and acidosis is known to affect the serum concentration of 1,25-(OH)2D. However, the factors that play a role in the regulation of serum 1,25-(OH)2D concentration in healthy subjects have not been fully evaluated. The associations of ionised calcium, pH, serum concentration of phosphate, PTH, 25-hydroxyvitamin D (25-OHD) and serum 1,25-(OH)2D were examined in 296 healthy premenopausal women (age range 17-40 years). Calculation of partial correlation coefficients showed that serum 1,25-(OH)2D was significantly correlated with phosphate (r = -0.148, P < 0.01), pH (r = 0.221, P < 0.001) and PTH (r = 0.136. P < 0.01). Ionised calcium was not related to serum 1,25-(OH)2D. When the results were stratified according to quartiles based on serum 1,25-(OH)2D concentration, significant trends (by analysis of variance) were seen in phosphate, pH, age, albumin and 25-OHD. Stepwise multiple regression analysis showed that phosphate and pH were the major contributors of serum 1,25-(OH)2D levels. There was a small contribution from PTH and 25-OHD. The results suggest that in young healthy premenopausal women plasma phosphate and pH may be important determinants of serum 1,25-(OH)2D concentration.

Binding properties of duodenal 1,25-dihydroxyvitamin D3 receptors as affected by phosphorus depletion in lactating goats

1. Capacity and affinity of duodenal 1,25(OH)2D3 receptors were measured in P depleted goats and in control animals kept on an adequate P supply. Plasma concentrations of Pi, Ca and vitamin D3 metabolites and activity of plasma alkaline phosphatase were measured to characterize the effects of P depletion. 2. During P depletion plasma Pi concentrations decreased significantly whereas plasma Ca and alkaline phosphatase activity increased. No changes were recorded for plasma vitamin D3, 25OHD3 and 1,25(OH)2D3 concentrations. 3. P depletion resulted in a significant decrease of the equilibrium dissociation constant Kd of duodenal 1,25(OH)2D3 receptors without affecting the maximum binding capacity.

Effect of estrogen on calcium homeostasis and pituitary hormones in the growing chick

An experiment was carried out to investigate the effect of a range of estradiol (E2) doses (0.1-6.5 micrograms/g body wt/day) on vitamin D metabolism and the plasma levels of growth hormone (GH) and prolactin (PRL) in the growing chick. Doses of 0.5-0.7 microgram/g E2, which are insufficient to raise the plasma calcium level, did induce an increase in growth rate, an increase in 25-hydroxyvitamin D 1 alpha-hydroxylase (1-hydroxylase) and 24-hydroxylase activities, and an increase in plasma GH level. These parameters leveled off or fell over the dose range 1-2 micrograms/g E2 but there was evidence of a second peak in 1-hydroxylase activity at 6 micrograms/g E2. At this high dose rate, the plasma Ca level rose to 8 mM, as it does in the laying hen; 24-hydroxylase activity, growth rate, and plasma GH and plasma PRL levels all decreased. It was concluded that the dose response to estrogen in the growing chick is not linear and, in the case of 1-hydroxylase activity, may even be biphasic.

The effect of low levels of dietary protein and calcium on growth rate, growth hormone, and vitamin D metabolism in the chick

Two experiments were carried out on 2-week-old chicks in which they were fed protein-deficient diets for varying lengths of time with or without calcium (Ca). All deficient diets depressed the rate of growth; this depression was more marked in chicks fed the protein-deficient diet for 21/2 days than in those fed the diet for 4 days. The circulating growth hormone level was high in all protein-deficient diets but low in the diet only deficient in calcium. The low Ca-induced-rise in renal vitamin D, 1-hydroxylase activity was reduced, and the plasma Ca level was particularly low, in chicks on the low protein/low Ca diet. The 24-hydroxylase activity was high on the low protein diets. It was concluded that adaptation to a low protein diet occurs over the first 4 days and that this may be associated with the high growth hormone level and also that adequate dietary protein is essential for maximal adaptation to dietary Ca deficiency.

Thyroid function, growth hormone, and organ growth in broilers deficient in phosphorus

The effects of a dietary P deficiency on thyroid function, serum growth hormone, and growth parameters in 10 to 29-day-old broiler cockerels was determined. Chicks fed severely P-deficient diets (.05% or .10% available P) grew more slowly and ate less feed than controls fed .65% P. The deficiency was also accompanied by hypercalcemia, hypophosphatemia, and decreases in percent bone ash, fat-free tibial weight, and tibial length and width. Increases in the relative weights of kidneys, hearts, and pituitary glands (.05% P only) occurred as well. Most of these changes occurred to a lesser extent or not at all in pair-fed controls, showing that they resulted specifically from the P deficiency and were not simply a result of reductions in feed intake. Phosphorus deficiency also was accompanied by peripheral edema and hydropericardium. Relative thyroid weight was unaffected. Serum triiodothyronine was consistently lower in the P-deficient chicks, although effects were significant only in one experiment. Thyroxine levels tended to be low also, but not consistently so. Serum growth hormone in P-deficient chicks in both studies was consistently lower than that in pair-fed controls, but this was significant only when .10% but not .05% available P was fed. The findings suggest that serum levels of both thyroid hormone and growth hormone are altered by P deficiency, but the results were not clearly definitive.

Impact of prepartum dietary phosphorus intake on calcium homeostasis at parturition

A study was conducted to determine the impact of dietary P intake on vitamin D metabolism and incidence of parturient paresis in aged dairy cows. Thirty dairy cows (10/group) were fed one of three experimental diets for approximately 28 d precalving. Phosphorus intake was .7, 1, or 3 times daily maintenance requirement and Ca intake was three times daily maintenance requirement for all cows. There was a 20% incidence of parturient paresis in each group. Prepartum dietary P intake had no effect on precalving or calving plasma Ca concentrations. Cows fed the low P (.7 times) diet had higher plasma Ca at 3 and 5 d postcalving than did cows fed P at 1 or 3 times maintenance. Plasma phosphorus concentrations reflected dietary P intake. Dietary P intake had no effect on plasma Mg, free hydroxyproline, 1,25-dihydroxyvitamin D, or 24,25-dihydroxyvitamin D concentrations. The range in dietary P from .7 to 3 times maintenance requirement had no effect on the incidence of parturient paresis. However, it did appear to influence Ca homeostasis during the postpartum period as cows fed the low P diet had higher plasma calcium concentrations postcalving. This may be a result of the low P diet enhancing intestinal C absorption by a vitamin D-mediated transport mechanism.

Effect of phosphorus deficiency on thyroid function and growth hormone in the white Leghorn male

Leghorn males, 66 to 96 days of age, were fed a diet marginally deficient in phosphorus (.14% nonphytate). Growth and feed intake were significantly reduced, but plasma calcium and phosphorus were unchanged. Bone ash was significantly reduced compared to pair-fed controls but not ad libitum-fed controls. Thyroid size increased and plasma growth hormone (GH) levels decreased with the deficiency, apart from any effect attributable to the lower food intake and weight gain. Plasma 3,5,3'triiodothyronine (T3) and thyroxine (T4) were not significantly changed, although there was a tendency for T4 to be lower in the deficient chickens. As both thyroid size and plasma GH are regulated partly by pituitary function, dietary phosphorus may alter endocrine function through changes in pituitary metabolism.

Early changes in the adaptation to a low calcium diet in the chick

Twelve hours after the diet of 3-week-old chicks was changed from a 1% to a 0.1% concentration of calcium (Ca), the growth rate and circulating levels of growth hormone had fallen while renal 25-hydroxecholecalciferol-24-hydroxylase had risen. The amount of 47Ca incorporated into bone from an injection given 18 h previously was lower than in the control birds. Over the following 2 1/2 days on the low Ca diet, the renal 1-hydroxylase activity rose and the plasma prolactin level fell, but the other parameters moved back toward the control level. It was concluded that early adjustments in hormonal and mineral metabolism counteract the acute effects of a dietary Ca shortage until longer-term adaptive changes begin to compensate for a continuing Ca deficiency. The renal hydroxylase activities were not directly influenced by the level of circulating growth hormone or prolactin.

Inhibition of 25-hydroxyvitamin D3-1-hydroxylase by chronic metabolic acidosis

Chronic metabolic acidosis had been shown to influence the renal metabolism of 25-hydroxyvitamin D3. Using the isolated perfused rat kidney model, we evaluated the rates of synthesis of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in vitamin D-depleted [D(-)] and 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] in vitamin D-replete [D(+)] rats. Metabolic acidosis was induced in both groups of rats by feeding aqueous ammonium chloride for 9 days. Kidneys isolated from D(-) acidotic rats (mean pH, 7.11) exhibited a decreased rate of 1,25(OH)2D3 synthesis (0.79 +/- 0.17 pmol produce . h-1 . g kidney-1) when compared with that (1.27 +/- 0.09) of D(-) nonacidotic (mean pH, 7.33) rats. There was a significant negative linear correlation between the rate of synthesis of 1,25(OH)2D3 and the hydrogen ion concentration of the animal (r = 0.79, P less than 0.005). The rate of synthesis of 24,25(OH)2D3 by the kidneys from D(+) acidotic (mean pH, 7.06) and nonacidotic (mean pH, 7.39) rats did not differ (0.81 +/- 0.21 vs. 0.60 +/- 0.12 pmol product . h-1 . g kidney-1). It is concluded that chronic acidosis suppressed 1-hydroxylase activity, but does not suppress 24-hydroxylase activity.

Calcium accumulation by chick intestinal mitochondria. Regulation by vitamin D-3 and 1,25-dihydroxyvitamin D-3

We have the evaluated the effect of vitamin D-3 and its metabolite 1,25-dihydroxyvitamin D-3 on Ca2+ accumulation by chick intestinal mitochondria. Ca2+ accumulation appears to occur in two phases: an early, transient accumulation into an Na+-labile pool followed by an ATP-dependent accumulation into an Na+-resistant pool. Ca2+ accumulation is extensive at free Ca2+ concentrations greater than 3 . 10(-6) M in the presence of ATP. Ruthenium red and dinitrophenol block Ca2+ accumulation, but atractyloside does not. Oligomycin blocks ATP-supported accumulation completely with a partial inhibition of ATP and malate-supported accumulation. Little difference could be found in mitochondrial preparations from vitamin D-deficient chicks compared to those from vitamin D-3 (or 1,25(OH)2D-3)-supplemented chicks with respect to respiratory control, oxygen consumption, efficiency of oxidative phosphorylation, affinity for Ca2+, or the rate and extent of ATP-supported Ca2+ accumulation. Intestinal cytosol stimulated Ca2+ accumulation, but this was not specific with respect to vitamin D status or tissue of origin, nor was it duplicated by chick intestinal Ca2+-binding protein. 30 ng/ml 1,25(OH)2D-3 stimulated Ca2+ accumulation directly, regardless of the presence of intestinal cytosol. Other vitamin D metabolites were less potent: 25-hydroxyvitamin D-3 greater than 24,25-dihydroxyvitamin D-3 = vitamin D-3. Since increasing the free Ca2+ concentration from 3 . 10(-6) to 1 . 10(-5) M increased Ca2+ accumulation approx. 50-fold, whereas direct stimulation by 1,25(OH)2D-3 in vitro increased Ca2+ accumulation less than 2-fold, we conclude that 1,25(OH)2D-3 influences mitochondrial accumulation of Ca2+ in vivo primarily by altering cytosol concentrations of free Ca2+.

The in vitro metabolism of 25-hydroxycholecalciferol by pig kidney: effect of low dietary levels of calcium and phosphorus

1. Homogenates of pig kidney incubated in vitro convert 25-hydroxycholecalciferol to the dihydroxymetabolites almost as efficiently as homogenates of chick kidney under similar conditions. 2. The cortex of the kidney carries out this conversion more efficiently than does the medulla. 3. Kidneys from pigs given low calcium or low phosphorus diets produce about 40% more 1,25-dihydroxycholecalciferol than those from pigs given diets containing normal levels of Ca and P.