The role of 25-hydroxycholecalciferol-1-hydroxylase in the responses of calcium absorption to the reproductive activity in birds

Calcium transport in strongly calcifying laying birds: mechanisms and regulation

Birds that lay long clutches (series of eggs laid sequentially before a "pause day"), among them the high-producing, strongly-calcifying Gallus gallus domesticus (domestic hen) and Coturnix coturnix japonica (Japanese quail), transfer about 10% of their total body calcium daily. They appear, therefore, to be the most efficient calcium-transporters among vertebrates. Such intensive transport imposes severe demands on ionic calcium (Ca2+) homeostasis, and activates at least two extremely effective mechanisms for Ca2+ transfer from food and bone to the eggshell. This review focuses on the development, action and regulation of the mechanisms associated with paracellular and transcellular Ca2+ transport in the intestine and the eggshell gland (ESG); it also considers some of the proteins (calbindin, Ca2+ATPase, Na+/Ca2+ exchange, epithelial calcium channels (TRPVs), osteopontin and carbonic anhydrase (CA) associated with this phenomenon. Calbindins are discussed in some detail, as they appear to be a major component of the transcellular transport system, and as only they have been studied extensively in birds. The review aims to gather old and new knowledge, which could form a conceptual basis, albeit not a completely accepted one, for our understanding of the mechanisms associated with this phenomenon. In the intestine, the transcellular pathway appears to compensate for low Ca2+ intake, but in birds fed adequate calcium the major drive for calcium absorption remains the electrochemical potential difference (ECPD) that facilitates paracellular transport. However, the mechanisms involved in Ca2+ transport into the ESG lumen are not yet established. In the ESG, the presence of Ca2+-ATPase and calbindin--two components of the transcellular transport pathway--and the apparently uphill transport of Ca2+ support the idea that Ca2+ is transported via the transcellular pathway. However, the positive (plasma with respect to mucosa) electrical potential difference (EPD) in the ESG, among other findings, indicates that there may be major alternative or complementary paracellular passive transport pathways. The available evidence hints that the flow from the gut to the ESG, which occurs during a relatively short period (11 to 14 h out the 24- to 25.5-h egg cycle), is primarily driven by carbonic anhydrase (CA) activity in the ESG, which results in high HCO3(-) content that, in turn, "sucks out" Ca2+ from the intestinal lumen via the blood and ESG cells, and deposits it in the shell crystals. The increased CA activity appears to be dependent on energy input, whereas it seems most likely that the Ca2+ movement is secondary, that it utilizes passive paracellular routes that fluctuate in accordance with the appearance of the energy-dependent CA activity, and that the level of Ca2+ movement mimics that of the CA activity. The on-off signals for the overall phenomenon have not yet been identified. They appear to be associated with the circadian cycle of gonadal hormones, coupled with the egg cycle: it is most likely that progesterone acts as the "off" signal, and that the "on" signal is provided by the combined effect of an as-yet undefined endocrine factor associated with ovulation and with the mechanical strain that results from "egg white" formation and "plumping". This strain may initially trigger the formation of the mammillae and the seeding of shell calcium crystals in the isthmus, and thereafter initiate the formation of the shell in the ESG.

Relationships among age, eggshell thickness and vitamin D metabolism and its expression in the laying hen

Hens forming uncalcified shells synthesized less 1,25-hydroxycholecalciferol (1,25(OH)2D3) and less duodenal and eggshell gland (ESG) calbindin than normal laying hens. Hens forming thin shells had lower intestinal and ESG calbindin and its mRNA. Reducing ESG calcium (Ca2+) transport by the carbonic anhydrase inhibitor acetazolamide, but not by dietary Ca2+ restriction, reduced ESG calbindin and its mRNA. Two sub-populations of hens characterized by shell thickness (ST) maintained this characteristic throughout the whole production period. The differences between the two sub-populations increased with age. In old laying hens, the two sub-populations responded differently to dietary Ca2+ restriction and to exogenous 1,25(OH)2D3. Those forming a thin shell responded to 1,25(OH)2D3 by a significant improvement in ST. The results suggest that: (a) the mechanism responsible for Ca2+ transport to the egg shell consists of a vitamin D-dependent absorption of Ca2+ and a multi-factor-dependent transfer of Ca2+ to the shell; (b) both steps are, most likely, calbindin-mediated; however, the induction of calbindin gene expression in the ESG is predominantly calcium-dependent; and (c) the apparent defect in vitamin D metabolism or its expression in old hens is typical of, or even exclusive, to thin-shell-forming hens.

Effects of age at onset of production, light regime and dietary calcium on performance, eggshell traits, duodenal calbindin and cholecalciferol metabolism

1. Rate of production and shell thickness (ST) decreased, while body weight (BW), egg weight (EW) and percentage breakage increased progressively with age. Shell weight (SW) increased until 8 to 13 months of age and then decreased. 2. Early onset of production resulted in lower BW and EW at the onset of production, and lower pooled averages of BW, EW, SW and ST, as compared with late or medial onset of production. In 4 out of 5 trials, early onset did not result in the production of more eggs during the laying period. 3. Early onset of production is associated with physiological Ca deficiency as indicated by increases in kidney-1-hydroxylase and duodenal calbindin in early layers as compared with late layers. Early layers exhibited a more severe reduction in shell quality as the result of Ca deficiency as compared with late layers. 4. Feeding pullets with a prelaying diet containing 3.9% Ca did not affect unequivocally the performance or shell quality during the whole productive period, whether the birds started to lay early or late. The dietary treatment did not cause renal damage, as indicated by morphological examination and by plasma calcium and uric acid concentration.

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.

Modulation of quail intestinal and egg shell gland calbindin (Mr 28,000) gene expression by vitamin D3, 1,25-dihydroxyvitamin D3 and egg laying

The effects of vitamin D3 sources, egg production and egg cycle on the genomic expression of calbindin (Mr 28,000) in the intestine and egg shell gland (ESG) of quail were characterized by Northern blot and solution hybridization, using synthetic oligonucleotide probe. In vitamin D3- or 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-fed quail, onset of egg production induced duodenal and ESG calbindin mRNA and calbindin synthesis. Duodenal calbindin mRNA was slightly higher during the period of shell calcification as compared with the period during which shells were not formed (ESG inactivity). ESG calbindin mRNA was markedly higher during the period of shell calcification than of ESG inactivity. Increasing dietary intake of [3H]1 alpha-hydroxyvitamin D3 increased the duodenal, but not ESG, content of 1,25-(OH)2D3 and calbindin. Duodenal calbindin and its mRNA were absent in vitamin D-deficient quail and were not affected by egg laying. ESG calbindin in the vitamin D-deficient quail was not affected by egg laying, but calbindin mRNA increased in the vitamin D-deficient birds during shell calcification. The results suggest that: (a) intestinal calbindin mRNA and calbindin are induced and/or regulated, either directly or indirectly, by 1,25-(OH)2D3; (b) intestinal calbindin and its mRNA are further induced at the onset of egg laying by an additional stimulator besides 1,25-(OH)2D3; (c) 1,25-(OH)2D3 is required for the expression of the latter stimulator; (d) ESG calbindin mRNA and calbindin are induced in egg-laying birds by a stimulator associated with the egg cycle; and (e) the induction of ESG calbindin mRNA does not need vitamin D metabolites, but 1,25-(OH)2D3 is required for the translation of the mRNA.

Regulation of net intestinal calcium uptake in hens laying obligatory soft-shelled eggs

1. Individually caged laying hens had a loop of thread inserted into the shell gland. This resulted in the laying of soft shelled eggs. 2. A balance study was performed for a one week period before and after the operation. After the operation birds with threads consumed less calcium than before. Their requirements for calcium for eggshells decreased, resulting in increases in both calcium excreted and calcium retained. 3. Net calcium extraction in the digestive tract was measured in groups of birds with threads and intact controls, when shelling or not, by examining ratios of Ca to TiO2 in different gut segments. Observations were made during the period following premature oviposition in birds with threads, but within the normal shelling period of control birds. The period of study was at least two weeks after the operation. 4. Birds with threads absorbed less calcium than control birds up to the upper jejunum. 5. Control birds secreted calcium between the upper jejunum and colon, but birds with threads showed little change in absorption in this part of the digestive tract. 6. The increase in calcium absorption in intact birds was a response to the stimulus of shelling an egg or replacing calcium in medullary bone during a pause day, rather than of ovulation.

Renal cAMP and 1,25(OH)2D3 synthesis in estrogen-treated chick embryos and hens

Onset of sexual maturity in female chickens or administration of estrogen to mature males or to juveniles of either sex results in increased parathyroid hormone (PTH)-dependent adenylate cyclase activity and increased 25-hydroxyvitamin D3-1-hydroxylase activity in kidney. The relationship between estrogen-mediated alterations of these two enzyme systems was investigated in embryonic and mature, egg-laying chickens treated in vivo with 17 beta-estradiol (E2). Basal and PTH- and forskolin-stimulated adenylate cyclase activity in kidney plasma membrane preparations was not affected by E2 treatment of 19-day-old chick embryos or of 41-wk-old egg-laying females. High, possibly maximal, levels of catalytic activity in control embryos and hens may have precluded further stimulation by E2. In contrast, E2 significantly enhanced 25-hydroxyvitamin D3-1-hydroxylase activity of embryonic kidney up to 10-fold (P less than 0.005). In mature females, E2 caused cessation of egg laying accompanied by a significant reduction (P less than 0.005) of 25-hydroxyvitamin D3-1-hydroxylase activity. These results indicate that the PTH-dependent adenylate cyclase and the 25-hydroxyvitamin D3-1-hydroxylase systems of avian kidney can be regulated independently and suggest that factors in addition to estrogen are involved in their regulation.

Survey of vitamin D metabolite levels during growth and development in Japanese quail

Plasma calcium and vitamin D metabolite levels were monitored during growth and development in male and female Japanese quail. In male Japanese quail, plasma calcium levels were constant (range 9.0 to 10.9 mg/dl); the 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels remained undetectable in all age groups. Plasma calcium levels in the females rose at 6 weeks of age and remained elevated. The female 1,25-(OH)2D levels increased from undetectable levels at 3 to 5 weeks to a peak of 297 pg/ml at 8 weeks and remained elevated at 12 weeks of age in ovulating birds. Twelve-week old female without an egg in their oviduct had plasma 1,25-(OH)2D levels similar to males and immature females. No marked changed occurred in plasma 25-(OH)D levels due to sex or age. Plasma 24, 25-(OH)2D levels stayed relatively constant at about 2 ng/ml for both male and females at all ages studied.

Effects of suppression of eggshell calcification and of 1,25(OH)2D3 on Mg2+, Ca2+ and Mg2+HCO-3 ATPase, alkaline phosphatase, carbonic anhydrase and CaBP levels--II. The laying hen intestine

Activity of a HCO-3 stimulated Mg2+ dependent ATPase is demonstrated in mitochondrial fractions of the avian duodenum. Suppression of eggshell calcification resulted in a slight reduction in Mg2+, Ca2+ and Mg2+HCO-3 ATPase activities. Duodenal carbonic anhydrase activity was lower in birds laying soft-shelled eggs than in birds laying normal eggs. Alkaline phosphatase and calcium binding protein levels both decreased along the length of the small intestine, but the effect was more pronounced for alkaline phosphatase. Suppression of eggshell calcification and treatment of shell-less laying hens with 1,25(OH)2D3 influenced alkaline phosphatase activity only in the duodenal mucosa. Suppression of eggshell calcification reduced CaBP levels in all sections of the intestine. Treatment with 1,25(OH)2D3 restored CaBP levels. Regulation of intestinal CaBP levels by 1,25(OH)2D3 would therefore, seem to be controlled more directly by calcium requirements associated with eggshell calcification than by gonadal hormones.

25(OH)D3 metabolism in kidney cell cultures: lack of a direct effect of estradiol

There are several reports of increased production of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] by the kidney of birds in response to estrogen treatment. To determine whether estradiol influences the renal cell directly, primary cultures of chick kidney cells were exposed to the steroid under a variety of conditions. In the absence of 1,25(OH)2D3, treatment of cultures for 20-24 h with 10(-5) and 10(-6) M estradiol led to inhibition of 25-hydroxyvitamin D3 [25(OH)D3]-1-hydroxylase activity. When the 1-hydroxylase was suppressed and 25(OH)2D3-24-hydroxylase was induced by treatment with 1,25(OH)2D3, estradiol in concentrations of 10(-9) and 10(-5) M either had no effect or slightly inhibited 1,25(OH)2D3 production. Similarly, 24R,25-dihydroxyvitamin D3[24,25(OH)2D3] production was not affected consistently by estradiol. These results were unaltered when either testosterone (10(-6) M) or insulin (5 micrograms/ml) was present in the medium. Shorter treatments (0.5, 2, 4, and 8 h) with estradiol resulted in a transient decrease in both 1,25(OH)2D3 and 24,25(OH)2D3 production, but at no time was stimulation observed. These results suggest that the effects of estrogens on 25(OH)D3 metabolism observed in vivo are exerted elsewhere than directly at the renal cell.

Involvement of cholecalciferol metabolism in birds in the adaptation of calcium absorption to the needs during reproduction

1. The metabolism of calcium and cholecalciferol in quail (Coturnix coturnix japonica) and chicken (Gallus domesticus) during maturation was correlated to gonadal activity and plasma oestrogen levels. 2. Birds with undeveloped ovaries (immature), developed ovaries but not laying (mature), and after laying 3-8 eggs (laying), were used in the first series. 3. Birds in which egg production had been arrested by Nicarbazin, were used in the second series. 4. Plasma 17 beta-oestradiol and calcium were elevated in the mature bird, with no further change in the laying bird. Kidney 25-hydroxycholecalciferol-1-hydroxylase and intestinal calcium-binding protein increased slightly in the mature bird, whereas they were grossly elevated in the laying bird. 5. Calcium and phosphorus absorption were markedly elevated in the laying bird. 6. No changes were noted in plasma 25-hydroxycholecalciferol, at any stage of maturation. 7. During the arrest of egg production by Nicarbazin, 17 beta-oestradiol level, calcium concentration of plasma, and medullary bone were maintained. Kidney 25-hydroxy-cholecalciferol-1-hydroxylase, intestinal calcium-binding protein and absorption of calcium were strikingly reduced. 8. The results suggest that changes in calcium absorption and cholecalciferol metabolism during maturation in birds are not directly affected by gonadal hormones; they appear to represent an adaptation to the increased calcium needs due to medullary bone formation and, more importantly, to the large losses of calcium imposed by shell formation.