Calciferols: actions and deficiencies in action

Vitamin D: a review on its effects on muscle strength, the risk of fall, and frailty

Vitamin D is the main hormone of bone metabolism. However, the ubiquitary nature of vitamin D receptor (VDR) suggests potential for widespread effects, which has led to new research exploring the effects of vitamin D on a variety of tissues, especially in the skeletal muscle. In vitro studies have shown that the active form of vitamin D, calcitriol, acts in myocytes through genomic effects involving VDR activation in the cell nucleus to drive cellular differentiation and proliferation. A putative transmembrane receptor may be responsible for nongenomic effects leading to rapid influx of calcium within muscle cells. Hypovitaminosis D is consistently associated with decrease in muscle function and performance and increase in disability. On the contrary, vitamin D supplementation has been shown to improve muscle strength and gait in different settings, especially in elderly patients. Despite some controversies in the interpretation of meta-analysis, a reduced risk of falls has been attributed to vitamin D supplementation due to direct effects on muscle cells. Finally, a low vitamin D status is consistently associated with the frail phenotype. This is why many authorities recommend vitamin D supplementation in the frail patient.

Discovery of alpha-Klotho unveiled new insights into calcium and phosphate homeostasis

alpha-Klotho was first identified as the responsible gene in a mutant mouse line whose disruption results in a variety of premature aging-related phenotypes. alpha-Klotho has been shown to participate in the regulation of parathyroid hormone secretion and trans-epithelial transport of Ca(2+) in the choroid plexus and kidney. alpha-Klotho, acting as a cofactor for FGF23, is also a major regulator of vitamin D biosynthesis and phosphate reabsorption in the kidney. These suggest that alpha-Klotho is a key player that integrates a multi-step regulatory system of calcium and phosphate homeostasis. Collectively, the molecular function of alpha-Klotho reveals a new paradigm that may change current concepts in mineral homeostasis and give rise to new insights in this field.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

[Vitamin D status in the adolescent: seasonal variations and effects of winter supplementation with vitamin D3]

BACKGROUND

Recently, 25 hydroxyvitamin D (25 OHD) blood concentrations measured in adolescents during or at the end of winter were found very low. A concomitant stimulation of parathyroid function was observed. The aim of the present study was to test the biological effects of a treatment with vitamin D3 during winter.

POPULATIONS AND METHODS

The effects of vitamin D3 supplementation (100,000 IU, twice, at the end of November and of January) were assessed in 24 male Caucasian adolescents (mean age +/- SD: 14 y 6 m +/- 9 m). They were pupils in a lad-jockeys training center located in the countryside near Chantilly (49 degrees northern latitude). Blood concentrations of 25 OHD, calcium and intact parathormone (PTH) were measured three times: before each oral intake of vitamin D3 and 2 months after the last intake (March). A group of 32 male adolescents (mean age +/- SD: 14 y 9 m +/- 6 m), pupils in the same center, receiving no vitamin D and sampled in November and in March, served as controls.

RESULTS

In March, mean concentrations of 25 OHD (8.36 +/- 2.73 micrograms/L) were very low in vitamin D-not supplemented adolescents since 34% had levels less than 6 micrograms/L. In March, PTH concentrations (40.5 +/- 12.2 ng/L) were significantly (P = 0.0001) higher than in November (28.8 +/- 9.9 ng/L). In boys receiving vitamin D3 25 OHD serum concentrations measured in January (17.5 +/- 3.2 micrograms/L) and in March (18.7 +/- 4.0 micrograms/L) remained at a level not very different from that measured in November (16.6 +/- 3.8 micrograms/L). During the same period, calcium and PTH concentrations (32.2 +/- 11.7 ng/L in November; 32.4 +/- 14.3 in January and 32.9 +/- 13.5 ng/L in March) remained at their basal level as well.

CONCLUSIONS

The observation that, after winter, a relatively large number of adolescents presented low concentrations of 25 OHD suggests that, during winter, usual dietary intakes and/or vitamin D stores are not sufficient to provide for their needs. Administration of two oral doses of 100,000 IU of vitamin D3 could maintain the vitamin D status at its initial level. The efficiency of such a prophylactic treatment is also assessed by its effect on parathyroid function.

Serpentine receptors for parathyroid hormone, calcitonin and extracellular calcium ions

The cloning of the receptors for PTH, CT and extracellular calcium ions represents a significant advance in the elucidation of the mechanisms through which extracellular calcium ions are regulated. All are members of the superfamily of GPCR, and the inclusion of the Ca2+o-sensing receptor in this superfamily documents that extracellular calcium ions can serve as an extracellular first messenger, in addition to subserving their better known role as a key intracellular second messenger. Furthermore, it has proved possible to identify several human diseases that result from inactivating or activating mutations in the PTH or Ca2+o-sensing receptor. Finally, the availability of these cloned receptors will enable many more studies on structure-function relationships for these receptors as well as clarifying their tissue distribution, regulation and roles in health and disease. It may also be possible to design novel therapeutic agents that permit manipulation of the receptors when their function is abnormal.

Solar ultraviolet B radiation and photoproduction of vitamin D3 in central and southern areas of Argentina

The incidence of nutritional rickets in the southern part of Argentina is 8-12 times higher than in the rest of the country. Winter 25(OH)D serum levels in normal population of southern areas are lower than in central and northern areas. To elucidate these differences, we compared the photoconversion of provitamin D3 (7-DHC) to previtamin D3 in two cities: Ushuaia (latitude 55 degrees S) and Buenos Aires (34 degrees S). Ampules containing 7-DHC were exposed to sunlight one day in the middle of each month either from 10:30 a.m. to 2:30 p.m. or from 8:00 a.m. to 5:00 p.m. The percentages of photoproducts formed were determined by high performance liquid chromatography (HPLC). Previous studies have proved that this is a valid model to assess "in vitro" the photoproduction of vitamin D3 in human skin. Previtamin D3 + vitamin D3 formed in Ushuaia were less (p < 0.02) than those found in Buenos Aires during all seasons: summer, (X +/- SEM) 6.4 +/- 0.8% vs. 13.2 +/- 1.8%; autumn, 1.2 +/- 0.7% vs. 6.3 +/- 1.3%; winter, 0.8 +/- 0.7% vs. 3.6 +/- 0.7%; spring, 3.4 +/- 0.5% vs. 9.1 +/- 1.1%. The photoproducts produced from 10:30 a.m. to 2:30 p.m. were similar for each month and latitude to those formed when the ampules were exposed from 8:00 a.m. to 5:00 p.m. We conclude that in Ushuaia there is a prolonged "vitamin D winter" during which cutaneous synthesis of vitamin D is absent, leading to lower serum values of 25(OH)D and contributing to the higher incidence of rickets.

Activation of the human osteocalcin gene by 24R,25-dihydroxyvitamin D3 occurs through the vitamin D receptor and the vitamin D-responsive element

1 alpha-25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3], together with vitamin D receptor (VDR), directly activates human osteocalcin (hOC) gene expression through a vitamin D-responsive element (VDRE) located in the promoter of the hOC gene. We investigated the effect of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] on the regulation of the hOC gene promoter and compared it with that of 1 alpha,25(OH)2D3. 24R,25(OH)2D3 did not activate the natural promoter in VDR-negative CV-1 cells. 24R,25(OH)2D3, however, induced the activation of this promoter following cotransfection with an hVDR expression vector. In VDR-positive MC3T3-E1 cells, 24R,25(OH)2D3 activated not only the natural hOC promoter but also a chimeric promoter composed of a synthetic hOC VDRE sequence linked to the thymidine kinase promoter. In combination with 1 alpha-25(OH)2D3, 24R,25(OH)2D3 did not exhibit any antagonist activity on the hOC promoter. These results suggest that under conditions of high 24R,25(OH)2D3 levels in vivo, this metabolite of vitamin D3 may activate hOC gene expression through receptor mechanisms identical to that for 1 alpha,25(OH)2D3.

Distribution of target cells for 1,25-dihydroxyvitamin D3 in the brain of the yellow bellied turtle Trachemys scripta

Five h after injection of tritiated 1,25-dihydroxyvitamin D3 into Trachemys scripta, neurons with nuclear concentrations of radioactivity were identified in distinct regions within the central nervous system. Coinjection of a 100-fold excess of non-labeled 1,25-dihydroxyvitamin D3 abolished or reduced the specific nuclear binding of tracer. Target neurons were present in ventral periventricular brain regions including tuberculum olfactorium, nucleus accumbens, cortex piriformis, primordium hippocampi, nucleus striae terminalis, dorsal ventricular ridge, amygdala, nucleus infundibularis and tectum opticum. With the exception of the nucleus infundibularis and the tectum opticum, target neurons can be continuously followed from the ventrolateral nucleus accumbens throughout the nucleus striae terminalis into the amygdala. The general distribution of target neurons is similar to that described for rodents but more restricted to the above regions. The results show that target neurons for vitamin D are located in brain regions including several components of the limbic and thalamic systems in which gonadal steroids as well as aminergic and peptidergic messengers exert their actions. Vitamin D, the heliogenic steroid hormone, may therefore be involved in the orchestration of season-specific processes such as reproduction and related behaviors.

Seasonal variations of 25 hydroxyvitamin D and parathyroid hormone in Ushuaia (Argentina), the southernmost city of the world

Serum levels of calcium, phosphorus, alkaline phosphatase, 250HD, 1.25(OH)2D and PTH were studied in a group of 42 children aged 8.5 +/- 1.8 years (X +/- SD) from the city of Ushuaia (latitude 55 degrees S), at both the end of the winter and the end of summer. Calcium, phosphorus, alkaline phosphatase and 1.25(OH)2D serum levels were not different in summer and winter. The levels of serum 25OHD were significantly higher in summer (18.4 +/- 7.3 ng/ml) than in winter (9.8 +/- 3.8 ng/ml P < 0.001). The levels of 25OHD in children with fair or dark skin were similar in winter but were significantly higher in children with fair skin in summer (20.0 +/- 7.2 ng/l vs 15.3 +/- 5.1 ng/ml (P < 0.05). Serum levels of PTH were higher in winter (58.2 +/- 30.5 pg/ml) than in summer (47.9 +/- 28.3 pg/ml) (P < 0.03). The results demonstrate the existence of a population with low serum levels of 25OHD in winter. The higher levels of PTH in winter when serum 25OHD levels are lower could be the cause of the lack of seasonal variation in serum calcium and 1.25(OH)2D levels. Further studies are needed to establish whether these changes besides increasing the incidence of rickets, could also affect the mineral density of the skeleton in the population of this vitamin-D-deficient area.

Heterogeneity of pseudohypoparathyroidism type I from the aspect of urinary excretion of calcium and serum levels of parathyroid hormone

Urinary excretion of calcium (Ca) was measured in 9 patients with pseudohypoparathyroidism (PHP) type I--3 with Albright's hereditary osteodystrophy (AHO): AHO(+) and 6 without AHO: AHO(-)--and in 13 with idiopathic hypoparathyroidism (IHP), treated with active vitamin D3 (1,25(OH)2D3 or 1 alpha OHD3) to maintain serum Ca levels at 8.4-9.5 mg/dl. Fasting urinary excretion of Ca in PHP was significantly lower than that in IHP. Moreover, fasting urinary excretion of Ca in PHP AHO(+) was lower than that in PHP AHO(-). This difference was also seen in the urine after oral loading of Ca. Urinary excretion of sodium (Na) was not different between PHP AHO(+) and PHP AHO(-). Serum levels of immunoreactive PTH in PHP AHO(+) were higher than those in PHP AHO(-). The difference in urinary excretion of Ca between PHP AHO(+) and PHP AHO(-) may come from the difference in the circulating levels of PTH.

Altered hepatic function in vitamin D-deprived rats

Rats were rendered vitamin D-deficient by housing them in a room free of ultraviolet light and maintaining them for 20 weeks on a diet devoid of only vitamin D. The vitamin D-deficiency state was confirmed by the undetectable levels of circulating vitamin D metabolites, severe hypocalcaemia and significantly reduced intestinal calcium transport. Liver function and protein metabolism in these rats were assessed by bromosulphthalein (BSP) clearance, liver histology, plasma transaminases and alkaline phosphatase, and 14C-labelled amino acid incorporation into liver and plasma proteins. Subtle alterations in hepatic function, as manifested by delayed BSP clearance, elevated levels of plasma transaminases and alkaline phosphatase, were noticed. Liver histology revealed changes consistent with periportal necrosis. Synthesis of liver and plasma proteins were reduced by 26-34% (P less than 0.01), without affecting the circulating levels of plasma proteins, suggesting reduced protein turnover in vitamin D-deprived rats. The results strongly suggest the direct/indirect involvement of vitamin D in mediating the altered liver function.

Recent developments in the roles of vitamins and minerals in reproduction

Vitamins and minerals affect reproductive function. Vitamin A deficiency has long been known to affect reproductive function in cattle. More recently, a role has been proposed for the vitamin A percursor, beta-carotene, in reproductive efficiency. Dietary supplementation with vitamin E and selenium may reduce the incidence of retained placenta, but these nutrients may also affect reproductive function in other ways. Calcium and phosphorus deficiencies affect reproduction in cattle, and vitamin D may directly affect reproductive function in addition to its role in calcium and phosphorus metabolism. Dietary manipulation of a number of other vitamins and minerals also influences reproductive function. However, the specific roles of nutrients in reproductive tissues are not well-defined in dairy cattle, and nutrient requirements for optimal reproductive efficiency in modern dairy cattle deserve careful reevaluation. This review provides a background of the effect of vitamins and minerals on reproduction and it attempts to provide a basis for further investigation of specific mechanisms by which reproductive function is affected. The interface between nutritional science and reproductive physiology provides considerable potential for optimizing reproductive efficiency in dairy cattle.

Multiple sites of action of the vitamin D endocrine system: FSH stimulation of testis 1,25-dihydroxyvitamin D3 receptors

1,25-Dihydroxyvitamin D [1,25(OH)2D] receptors exist in numerous unexpected tissues. These include, for example, rat lung, heart, testis, and uterus, but not prostate and bladder. The issues of 1,25(OH)2D effects on and receptor location in the testis were addressed by (a) physiological and pharmacological manipulations of tubule cell types and (b) histological examination of testes of vitamin D-deficient rats. FSH treatment in hypophysectomized adult rats increased 1,25(OH)2D receptor levels by 135% (P less than 0.01). Busulfan treatment reduced testis receptor levels by 35% (P less than 0.05) after 35 days (maximum effect), and the effect was reversed after recovery (85 d). Cryptorchidism for 5 or 50 days resulted in modest (33%, P less than 0.05) or substantial (79%, P less than 0.001) reductions in receptor levels. Only the FSH treatment and 50 days cryptorchidism reduced receptor levels in the residual tissue. The testes of vit. D-deficient rats showed incomplete spermatogenesis and degenerative changes. Although interpretation is complicated by the intricate communication among testis cell types, these data suggest that the Sertoli cell is a primary site of action of 1,25(OH)2D in the testis. Moreover, these data indicate that 1,25(OH)2D receptor function in the testis relates to germ cell division/maturation, although this may be an indirect effect via the Sertoli cells.

Vitamin D metabolism in pregnant and pseudopregnant rats: identification of 25-hydroxycholecalciferol-1-hydroxylase in decidual tissue

Elevated levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are found in late pregnancy but the factors responsible for this are not known. To determine if the maternal-fetal calcium flux or the presence of a previously described extrarenal 25-hydroxycholecalciferol-1-hydroxylase (25(OH)-D3-1-hydroxylase) play a role, serum calcium and 1,25(OH)2D3 were measured in pregnant, nonpregnant, and decidua-bearing pseudopregnant rats. Serum calcium was 8.74 +/- 0.26 mg/dl (mean +/- SEM) in nonpregnant rats. In pregnant rats, serum calcium was not significantly different from nonpregnant controls on day 12 and only slightly higher on day 15. Pseudopregnant rats were significantly hypercalcemic on days 12 (11.93 +/- 0.19 mg/dl) and 15 (11.45 +/- 0.23 mg/dl) compared with nonpregnant rats (P less than 0.001). In nonpregnant controls the serum level of 1,25(OH)2D3 was 44.6 +/- 6.3 pg/ml. Levels in pregnant rats were not significantly different on days 12 or 15 but tended to be higher by day 15 (75.2 +/- 19.7 pg/ml). Pseudopregnant rats had levels of 72.6 +/- 13.5 pg/ml on day 12 and 102.8 +/- 10.9 pg/ml on day 15, the latter of which was significantly higher than nonpregnant values (P less than 0.05). 25(OH)D3-1-hydroxylase activity was determined in whole tissue homogenates of placenta and decidua. Placenta from pregnant rats and decidua from pregnant and pseudopregnant rats both formed putative 1,25(OH)2D3 in short-term incubation with 25(OH)D3 as identified by comigration with authentic 1,25(OH)2D3 on high pressure liquid chromatography (HPLC).(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine control and disturbances of calcium and phosphate metabolism in children

Most disorders of extracellular calcium and phosphate metabolism in childhood can be attributed to primary increased or decreased secretion/action of 1,25-dihydroxyvitamin D3 and parathyroid hormone or primary increased or decreased urinary excretion of phosphate and calcium. Based on this pathogenetic classification the most important diseases related to calcium and phosphate metabolism will be discussed.

Neoplastic hypercalcemia: physiologic response to intravenous etidronate disodium

Following a four-day control period during which an elevated serum calcium level either stabilized or continued to rise despite maximally tolerated saline diuresis, 12 patients with neoplastic hypercalcemia were treated with intravenous etidronate disodium (etidronate) 7.5 mg/kg/day for up to seven days. Serum calcium reverted to normal levels in all patients, with the mean pretreatment serum calcium level of 12.5 +/- 0.4 mg/dl dropping to 9.2 +/- 0.2 mg/dl (p less than 0.01) by Day 7. Elevated urinary calcium (1,107 +/- 134 mg/g creatinine) and hydroxyproline levels (154 +/- 16 mg/g creatinine) declined to 245 +/- 52 mg/g creatinine and 75 +/- 14 mg/g creatinine, respectively, suggesting a marked reduction in bone resorption following treatment. Serum phosphorus levels were unchanged, but urinary phosphorus levels dropped rapidly from 1,181 +/- 125 mg/g creatinine before treatment to 723 +/- 94 mg/g creatinine after two days. Serum parathyroid hormone levels (mid-molecule assay) were suppressed before treatment (64 +/- 16 pg/ml), but rose rapidly to 223 +/- 68 pg/ml by Day 7 of treatment. The value of serum 1,25-dihydroxyvitamin D was initially below normal (16 +/- 3 pg/ml), but rose rapidly with treatment to 42 +/- 12 pg/ml by Day 7. Symptoms of hypercalcemia and bone pain improved with treatment, and no serious adverse reactions to treatment were encountered. Intravenous etidronate is apparently an effective and safe treatment for neoplastic hypercalcemia.

Homologous up-regulation of the 1,25 (OH)2 vitamin D3 receptor in rats

This study investigates the ability of vitamin D-metabolites to regulate 1,25(OH)2D3 receptors in vivo. Rats made vitamin D-deficient were treated with 1,25(OH)2D3 or vehicle for 1-5 days. In treated animals, receptors for 1,25(OH)2D3 in kidney increased dramatically compared with control levels. An increase in specific binding to 220% of control was seen after 2 doses of hormone, which reached to 336% after 5 days of treatment. Intestinal receptors increased to only 130% of control levels after 5 days of treatment. In vitamin D-replete animals, the difference between control and treated groups was slightly greater when endogenously occupied sites were measured by exchange (TPCK). However, significant changes were observed only after 4 days of hormone treatment. The data indicate that homologous up-regulation of the 1,25(OH)2D3 receptor occurs in vivo. The difference in response in kidney and in intestine suggests differential importance of up-regulation in various organs.

Pancreatic secretion in man with subclinical vitamin D deficiency

The effects of subclinical vitamin D deficiency and vitamin D supplementation on oral glucose tolerance and secretion of pancreatic hormones were studied in 10 diphenylhydantoin-treated epileptic patients and 15 geriatric patients. Their mean serum concentrations of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 decreased markedly, but returned to normal within 2 weeks of oral supplementation with 25-hydroxyvitamin D3. The serum concentration of ionized calcium was within the normal range before treatment, and remained unchanged. Serum parathyroid hormone was increased during vitamin D deficiency, but decreased significantly (p less than 0.05) afterwards. In vitamin D-deficient epileptic and geriatric patients, the 2- and 3-h insulin levels after glucose ingestion were increased when compared with control values, and glucagon secretion was not suppressed by glucose. Oral glucose tolerance of both groups of patients did not change after 25-hydroxyvitamin D3 supplementation. Insulin secretion remained unchanged in geriatric patients, but was reduced to normal values in epileptic patients. Glucagon suppressibility by glucose was partly restored after vitamin D supplementation in epileptic patients but not in geriatric patients. In contrast to that observed in severely vitamin D-deficient rats or rabbits, correction of subclinical vitamin D deficiency failed to enhance insulin secretion or to improve glucose tolerance in man.