Vitamin D: mode of action and biomedical applications

The Diverse Roles of 17β-Estradiol in Non-Gonadal Tissues and Its Consequential Impact on Reproduction in Laying and Broiler Breeder Hens

Estradiol-17β (E2) has long been studied as the primary estrogen involved in sexual maturation of hens. Due to the oviparous nature of avian species, ovarian production of E2 has been indicated as the key steroid responsible for activating the formation of the eggshell and internal egg components in hens. This involves the integration and coordination between ovarian follicular development, liver metabolism and bone physiology to produce the follicle, yolk and albumen, and shell, respectively. However, the ability of E2 to be synthesized by non-gonadal tissues such as the skin, heart, muscle, liver, brain, adipose tissue, pancreas, and adrenal glands demonstrates the capability of this hormone to influence a variety of physiological processes. Thus, in this review, we intend to re-establish the role of E2 within these tissues and identify direct and indirect integration between the control of reproduction, metabolism, and bone physiology. Specifically, the sources of E2 and its activity in these tissues via the estrogen receptors (ERα, ERβ, GPR30) is described. This is followed by an update on the role of E2 during sexual differentiation of the embryo and maturation of the hen. We then also consider the implications of the recent discovery of additional E2 elevations during an extended laying cycle. Next, the specific roles of E2 in yolk formation and skeletal development are outlined. Finally, the consequences of altered E2 production in mature hens and the associated disorders are discussed. While these areas of study have been previously independently considered, this comprehensive review intends to highlight the critical roles played by E2 to alter and coordinate physiological processes in preparation for the laying cycle.

Do males 30-50 years of age with chronic epilepsy and on long-term anticonvulsant medication have lower-than-expected risk of developing coronary heart disease?

PURPOSE

It has been claimed that long-term use of antiepileptic drugs (AEDs) brings about changes in the lipid profile which might resist development of cardiovascular disease. The aim of this study was to test the following hypothesis: Do males with chronic epilepsy and on long-term treatment with AEDs have a lower than expected risk of developing coronary heart disease?

METHODS

The coronary risk profile in 40 male patients with chronic epilepsy, aged 30-50 years, was explored, using an American individual coronary risk factor test program (the Cooper test). We made a survey of the patients' risk factors: total cholesterol, high density lipoprotein (HDL) cholesterol, triglycerides, glucose, blood pressure, body fat percentage, physical fitness, hereditary factors, smoking habits, and degree of stress. Each factor was given a score, and the sum of scores constituted the total coronary risk profile. Eighty men in the same age group, casually selected from an industrial medical service, served as controls.

RESULTS

There was no statistically significant difference in the total coronary risk profile between the epilepsy patients and the controls. Nor were there any significant differences among the individual risk factors except for physical fitness level, which was significantly lower among the epilepsy group than among the controls. In the epilepsy group, however, there was a trend toward higher levels of total cholesterol, higher levels of HDL-cholesterol, lower levels of total cholesterol/HDL-cholesterol ratio, increased body fat percentage, greater heredity and somewhat greater coronary risk score as compared with the healthy controls, but the differences did not reach statistical significance. The protective effect against coronary heart disease exercised by the enzyme-inducing AEDs seems to be counteracted by other factors, like reduced physical fitness and increased body fat percentage.

CONCLUSIONS

Our hypothesis was not confirmed, because we found that the 40 patients with chronic epilepsy tested had about the same risk of developing coronary heart disease as did the control group.

Effect of cimetidine on eggshell quality and plasma 25-hydroxycholecalciferol in laying hens

Experiments were conducted to investigate the effect of feeding cimetidine (CIMET), ranging from 0 to 750 mg/kg, on vitamin D3 metabolism and eggshell calcification in laying hens fed two levels of vitamin D3 (500 and 2,000 ICU/kg). Final BW and feed intake were not significantly affected by either CIMET or vitamin D3 level. Feeding 500 and 750 mg of CIMET significantly decreased total egg production in hens fed either level of vitamin D3, but no differences were observed at lower CIMET levels. Tibia ash decreased significantly in hens fed 150 to 750 mg of CIMET, regardless of the vitamin D3 level. Plasma Ca and inorganic P concentrations were decreased in hens fed high CIMET levels (500 and 750 mg/kg) at Week 2, but no differences were observed at Week 4. Feeding CIMET (500 and 750 mg/kg) significantly decreased plasma 25-hydroxycholecalciferol (25-OHD3) levels at Week 2 in hens fed both vitamin D3 diets but not at Week 4. Eggshell breaking force, shell thickness, and percentage shell weight were decreased significantly by CIMET in all experiments; however, in one experiment, shell quality recovered by Week 8. These results suggest that the CIMET-induced reduction in bone mineralization, eggshell quality, and plasma 25-OHD3 levels could be due to interference of CIMET with vitamin D3 metabolism in vitamin D3-replete laying hens. Shell quality decreased in CIMET-treated hens fed the higher vitamin D diet even though 250-HD3 plasma levels were three times higher than in hens fed the lower vitamin D diet, suggesting that CIMET affected shell quality through some mechanism other than inhibition of 250-HD3 synthesis.

Calcium metabolism and its control--a review

It is now established that avians can only utilize the cholecalciferol form of vitamin D, which must be converted to the hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to perform normal calcium metabolism. Although 1,25(OH)2D3 is the final active form of vitamin D, hens fed only this form of vitamin D do not have normal hatchability of eggs. The problem appears to be caused by abnormal calcification and development of the embryonic beak. This appears to be caused by inadequate transport of 1,25(OH)2D3 into the egg. Although 1,25(OH)2D3 is not incorporated into the egg adequately, its precursor, 25-hydroxyvitamin D3 (25-OH-D3), is. The developing embryo however, can utilize 1,25(OH)2D and does so at least as early as Day 10 of incubation. During periods of maximal shell calcification and high circulating estradiol levels, the hen produces high levels of 1,25(OH)2D3. The kidney hydroxylase responsible for the final hydroxylation of the vitamin D hormone can be further stimulated by in vivo or in vitro administration of estradiol and, to a lesser extent, prolactin and parathyroid hormone. When eggs are not produced, as in the senescent or prepubertal stages of life, plasma 1,25(OH)2D3 concentrations are less than half that occurring during periods of active lay. Hens selected for their ability to produce thin or thick shells have 1,25(OH)2D3 concentrations in plasma that are positively correlated to their ability to produce egg shell.

Pain threshold and morphine activity in vitamin D-deficient rats

In rats, vitamin D-deficiency increases basal pain threshold and the analgesic effect of morphine (hot plate test). Cholecalciferol (1000 I.U./Kg/day s.c.x 5 days) restores pain sensitivity in vitamin D-deficient rats and brings the analgesic effect of morphine back to normal. On the other hand, tolerance to morphine develops faster in vitamin D-deficient rats, this effect too being prevented by cholecalciferol treatment. These data suggest a role for vitamin D status in pain sensitivity and opiate activity.

Renal handling of calcium in hypoparathyroidism

Treatment of hypoparathyroidism usually requires the use of pharmacological doses of parent vitamin D or near physiological amounts of the hydroxylated metabolites, calcitriol or alphacalcidol. Vitamin D intoxication and hypercalcaemia may be a problem but can be minimised by the use of small doses of vitamin D or its metabolites combined with large amounts of oral calcium. The response to treatment can be easily monitored by measuring serum and urinary calcium and creatinine concentrations. This allows the derivation of two simple indices reflecting calcium load presented to the kidney (calcium excretion in mmol/l glomerular filtrate) and renal tubular calcium reabsorption (TmCa/GFR). These can be used to predict the requirement of calcium supplements and also identify those patients at particular risk of hypercalcaemia.

Vitamin D dependency: replacement therapy with calcitriol?

Nine patients with vitamin D-dependency type I were studied. We observed that treatment with large doses of vitamin D altered the phenotypic expression of the disease, thus making a delayed diagnosis difficult. At the time of entry, eight children had hypocalcemia, and seven had hypophosphatemia. Elevated serum immunoreactive parathyroid hormone and low (less than 3 SD from control mean) 1 alpha,25-dihydroxyvitamin D values were constant findings, with no vitamin D deficiency. Despite the elevated serum iPTH, three children had normal urinary phosphate excretion and five had normal urinary cAMP excretion. In the five children tested before treatment, there was no significant change in renal phosphate excretion during an acute parathyroid hormone infusion, although in all a significant rise of urinary cAMP occured. Treatment with calcitriol (0.25 to 2 microgram/day) returned all the biochemical values to normal within four months. In two patients, both supplemented with vitamin D, histomorphometric analysis of iliac crest biopsies revealed severe osteomalacia. After nine and ten months of treatment with calcitriol, there was histologic evidence for improvement of bone mineralization. Since calcitriol requirements may vary during the course of treatment, careful monitoring of biochemical variables is essential.

Regulation of intestinal calcium-binding protein in rats: role of parathyroid hormone

Intestinal calcium-binding protein (CaBP) levels of rats fed a high (1.5%) Ca diet were the same whether the animals were parathyroidectomized (PTX), sham-operated controls pair-fed with the PTX animals, or sham-operated controls fed ad libitum. Consequently, a given base level of CaBP seems to be parathyroid hormone independent and not closely related to feed intake. On the other hand, whereas the ad libitum fed controls more than doubled their intestinal CaBP in response to a 2-day low-calcium (0.02%) regimen, neither the parathyroidectomized animals nor the pair-fed sham-operated controls were able to do so. Since the latter two groups consumed less feed and therefore less vitamin D than the ad libitum fed animals, the inability to increase CaBP in response to a low-calcium diet may have been caused by a restricted vitamin D intake rather than by the absence of parathyroid hormone.

Normal plasma-1,25-(OH)2-vitamin-D concentrations in nutritional osteomalacia

Three patients with advanced untreated nutritional osteomalacia had low plasma concentrations of 25-OHD and 24, 25-(OH)2D, and a raised concentration of immuno-assayable parathyroid hormone. The plasma concentration of 1, 25-(OH)2D was normal in all 3 patients.

The management of renal osteodystrophy

Hyperparathyroidism is the main cause of renal bone disease. At the moment its progress can be retarded by controlling plasma calcium and phosphate. But the prevention and cure of hyperparathyroidism without surgery eludes us. There is a suggestion that the administration of some metabolite of Vitamin D may be more successful. Osteomalacia on the other hand does not appear to be an important problem and is easily prevented and cured.

Vitamin-D deficiency in the osteomalacia of chronic renal failure

The plasma level of 25-hydroxy-vitamin D (25-OHD) has been measured and the histological appearances of bone examined in 22 patients with stable chronic renal failure. The results show that osteomalacia occurred only in those patients with relatively low levels of 25-OHD. It is concluded that the osteomalacia of chronic renal failure results from a lack of 25-hydroxy-vitamin D3 superimposed on an existing deficiency of 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) rather than from lack of 1,25-(OH)2D3 alone.

Radioligand receptor assay for 25-hydroxyvitamin D2/D3 and 1 alpha, 25-dihydroxyvitamin D2/D3

A competitive protein binding assay for measurement of the plasma concentration of 1 alpha, 25-dihydroxyvitamin D3 [1alpha, 25-(OH)2D3] has been extended to include the immediate precursor of this hormone, 25-hydroxyvitamin D3 (25-OHD3). In addition, the assay system is capable of measuring the two metabolic products of ergocalciferol, namely. 25-hydroxyvitamin D2 (25-OHD2) and 1alpha, 25-dihydroxyvitamin D2 [1alpha, 25-(OH)2D2]. The target tissue assay system consists of a high affinity cytosol receptor protein that binds the vitamin D metabolites and a limited number of acceptor sites on the nuclear chromatin. By utilizing a series of chromatographic purification steps, a single plasma sample can be assayed for any of the four vitamin D metabolites either individually or combined. Therefore, the assay procedure allows for both the quantitative and qualitative assessment of the total active vitamin D level in a given plasma sample. To show that the binding assay was capable of measuring 1alpha, 25-(OH)2D2 as well as 1alpha, 25 (OH)2D3, two groups of rats were raised. One group, supplemented with vitamin D3, produced assayable material that represented 1alpha, 25-(OH)2D3. The other group, fed only vitamin D2 in the diet, yielded plasma containing only 1alpha, 25-(OH)2D2 as the hormonal form of the vitamin. The circulating concentrations of the two active sterols were nearly identical (15 ng/100 ml) in both groups, indicating that the competitive binding assay can be used to measure both hormonal forms in plasma. In a separate experiment, 1alpha, 25-(OH)2D2 was generated in an in vitro kidney homogenate system using 25-OHD2 as substrate. Comparison of this sterol with 1alpha, 25-(OH)2D3 in the assay system showed very similar binding curves; the D2 form was slightly less efficient (77%). Comparison of the respective 25-hydroxy forms (25-OHD2 vs. 25-OHD3) at concentrations 500-fold that of 1alpha, 25-(OH)2D3, again suggested that the binding of the D2 metabolite was slightly less efficient (71%). Finally, the assay was employed to measure the total active vitamin D metabolite pools in the plasma of normal subjects and patients with varying degrees of hypervitaminosis D. The normal plasma levels of 25-OHD and 1alpha, 25-(OH)2D measured in Tucson adults were 25-40 ng/ml and 2.1-4.5 ng/100 ml, respectively. Both sterols were predominately (greater than 90%) in the form of vitamin D3 metabolites in this environment. Typical cases of hypervitaminosis D exhibited approximately a 15-fold increase in the plasma 25-OHD concentration, and a dramatic changeover to virtually all metabolites existing in the form of D2 vitamins. In contrast, the circulating concentration of 1alpha, 25-(OH)2D was not substantially enhanced in vitamin D-intoxicated patients. We therefore conclude that hypervitaminosis D is not a result of abnormal plasma levels of 1alpha, 25-(OH)2D but may be cuased by an excessive circulating concentration of 25-OHD.

Ischemic heart disease in patients with epilepsy

It has been suggested that patients with epilepsy and particularly those on long-term anticonvulsant medication may have a lower than expected risk of ischemic heart disease. The records of a cohort of patients with epilepsy in Rochester, Minnesota were reviewed to ascertain their rates of occurrence of ischemic heart disease. The results did not show any relative decrease in the incidence or mortality rates due to ischemic heart disease among men or women with epilepsy. The numbers of ischemic heart disease incidence and mortality cases were 25 and 15, respectively, relative to corresponding expected values of 15.0 and 15.7 new and fatal events. The use of anticonvulsant medications did not appear to influence the rates of ischemic heart disease among the patients with epilepsy. Subgroups of the epilepsy patients, by etiology and types of epilepsy, were not found to account for a disproportionate share of the ischemic heart disease. The survivorship of epilepsy patients after the initial manifestations of ischemic heart disease was comparable to that expected among all ischemic heart disease patients.

The assay of 1alpha,25-dihydroxyvitamin D3: physiologic and pathologic modulation of circulating hormone levels

A sensitive radioreceptor assay for 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3) is utilized to quantitate the circulating concentration of this sterol in experimental animals and humans. When weanling rats are grown for 2 weeks on low calcium or low phosphate diets, limited availability of either ion elicits a five-fold increase in the plasma level of 1alpha,25-(OH)2D3. The enhancement of 1alpha,25-(OH)2D3 in calcium deficiency is dependent upon the presence of the parathyroid and/or thyroid glands, which is consistent with parathyroid hormone (PTH) mediation of this effect. In contrast, the response to phosphate deficiency is independent of these glands and may result from a direct action of low phosphate on the renal synthesis of 1alpha,25-(OH)2D3. Studies in humans indicate that the normal level of 1alpha,25-(OH)2D is 2.1--4.5 ng/100 ml plasma. Patients with chronic renal failure have markedly lower circulating 1alpha,25-(OH)2D and this kidney hormone is undetectable in anephric subjects, but returns to normal within 1 day after successful renal transplantation. Hypoparathyroidism and pseudohypoparathyroidism are associated with reduced plasma 1alpha,25-(OH)2D while patients with primary hyperparathyroidism have significantly elevated sterol hormone levels. Thus, from measurements in rats and humans, it appears that circulating 1alpha,25-(OH)2D3 is regulated by PTH and/or phosphate and that abnormal plasma 1alpha,25-(OH)2D3 is a part of the pathophysiology of renal osteodystrophy and parathyroid disorders.