Estrogen therapy enhances calcium absorption and retention and diminishes bone turnover in young girls with Turner's syndrome: a calcium kinetic study

Normal bone density but altered geometry in girls with Turner syndrome

OBJECTIVES

Data on bone density and stability in Turner syndrome (TS) are contradictory. A confounding factor for interpretation is short stature. The aim was to measure bone density, geometry and stability in girls with TS compared to idiopathic short stature (ISS).

METHODS

From 1999 to 2008, 59 girls with TS (35 prepubertal) were evaluated by pQCT. Mean age was 8.9 in prepubertal and 17.3 years in adolescent girls. Mean height was -3.1 and -1.8 SDS in prepubertal treatment-free and in adolescent, formerly rhGH-treated girls. For comparison, 18 prepubertal ISS girls were studied (age 7.7 years; height -3.3 SDS). Examination of radius with pQCT (XCT 2000). Cortical (CD) and trabecular density (TD), total bone area (TBA), cortical area (CA), cortical thickness, muscle area and strength strain index (SSI) were determined and compared with height related references.

RESULTS

In prepubertal girls with TS, TD and CD were normal (0.55 and 0.90 SDS) and comparable to ISS (0.95 and 1.53 SDS). TBA was greater in girls with TS than in ISS (0.87 vs. -0.33 SDS) whereas CA was similar (1.48 vs. 1.43 SDS). The SSI was comparable (1.61 vs. 1.56 SDS). Adolescent girls with TS showed similar results with a TD of 0.48 SDS, a CD of -0.32, TBA of 1.99, a CA of -0.05 and an SSI of 0.88 SDS.

CONCLUSIONS

The observations are consistent with normal bone density and stability but altered bone geometry in prepubertal and substituted adolescent girls with TS. This peculiarity may reflect SHOX deficiency. We therefore think that timely and adequate estrogen substitution could prevent bone loss in TS.

Elevated Levels of 1,25-Dihydroxyvitamin D in Plasma as a Missing Risk Factor for Celiac Disease

The prevalence of celiac disease (CD) has increased significantly in some developed countries in recent decades. Potential risk factors that have been considered in the literature do not appear to provide a convincing explanation for this increase. This has led some researchers to hypothesize that there is a "missing environmental factor" that increases the risk of CD. Based on evidence from the literature, the author proposes that elevation in plasma levels of 1,25-dihydroxyvitamin D [1,25(OH)₂D] is a missing risk factor for CD, and relatedly that significant oral vitamin D exposure is a "missing environmental factor" for CD. First, elevated plasma levels of 1,25(OH)₂D are common in CD, especially in the newly diagnosed. Second, nine distinct conditions that increase plasma levels of 1,25(OH)₂D are either associated with CD or have indications of such an association in the literature. Third, a retrospective study shows that sustained oral vitamin D supplementation in infancy is associated with increased CD risk, and other studies on comorbid conditions support this association. Fourth, large doses of oral vitamin D upregulate many of the same cytokines, chemokines, and toll-like receptors that are upregulated in CD. Fifth, epidemiological evidence, such as the timing of the inception of a CD "epidemic" in Sweden, the increased prevalence of CD in Finland and the United States in recent decades, the unusually low prevalence of CD in Germany, and the differential in prevalence between Finnish Karelians and Russian Karelians, may all be explained by oral vitamin D exposure increasing CD risk. The same is true of some seemingly contradictory results in the literature on the effects of breastfeeding on CD risk. If future research validates this hypothesis, adjustments to oral vitamin D consumption among those who have genetic susceptibility may decrease the risk of CD in these individuals.

Bone mineral density in Turner's syndrome and the influence of pubertal development

AIM

To describe bone mineral density at the lumbar spine in a group of girls with Turner's syndrome and determine its relation to pubertal development.

METHODS

Girls with Turner's syndrome aged over 12 years were invited to participate in the study. All participants underwent auxology, pubertal assessment and laboratory evaluation. Bone mineral density was estimated by dual-energy X-ray absorptiometry. Bone mineral apparent density was then calculated, and the results were compared with age-matched Dutch reference data.

RESULTS

We studied 32 girls with Turner's syndrome, mean (SD) [range] age 16.7 (2.6) [12.4-20.2] years and height 148.3 (9.0) [126-159.2] cm. Bone mineral apparent density lumbar spine values were -0.87 SD, significantly lower than in the reference population (p <0.001). Bone mineral apparent density values were positively and significantly associated with breast Tanner stages and postmenarcheal status, but not spontaneous puberty. However, no significant association was found between bone mineral apparent density and karyotype, growth hormone or timing of oestrogen therapy.

CONCLUSION

Girls with Turner's syndrome have lower bone mineral apparent density values at the lumbar spine, even after correcting for size, compared with an age- and sex-matched general population. Pubertal development has significant impact on bone mineral apparent density in girls with Turner's syndrome.

Sex hormone replacement in Turner syndrome

The cardinal features of Turner syndrome (TS) are short stature, congenital abnormalities, infertility due to gonadal dysgenesis, with sex hormone insufficiency ensuing from premature ovarian failure, which is involved in lack of proper development of secondary sex characteristics and the frequent osteoporosis seen in Turner syndrome. But sex hormone insufficiency is also involved in the increased cardiovascular risk, state of physical fitness, insulin resistance, body composition, and may play a role in the increased incidence of autoimmunity. Severe morbidity and mortality affects females with Turner syndrome. Recent research emphasizes the need for proper sex hormone replacement therapy (HRT) during the entire lifespan of females with TS and new hypotheses concerning estrogen receptors, genetics and the timing of HRT offers valuable new information. In this review, we will discuss the effects of estrogen and androgen insufficiency as well as the effects of sex HRT on morbidity and mortality with special emphasis on evidence based research and areas needing further studies.

The renin-angiotensin system, blood pressure, and heart structure in patients with hereditary vitamin D-resistance rickets (HVDRR)

Vitamin D deficiency has been linked to hypertension and an increased prevalence of cardiovascular risk factors and disease. Studies in vitamin D receptor knockout (VDR KO) mice revealed an overstimulated renin-angiotensin system (RAS) and consequent high blood pressure and cardiac hypertrophy. VDR KO mice correspond phenotypically and metabolically to humans with hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR). There are no data on the cardiovascular system in human HVDRR. To better understand the effects of vitamin D on the human cardiovascular system, the RAS, blood pressure levels, and cardiac structures were examined in HVDRR patients. Seventeen patients (9 males, 8 females, aged 6 to 36 years) with hereditary HVDRR were enrolled. The control group included age- and gender-matched healthy subjects. Serum calcium, phosphorous, creatinine, 25-hydroxyvitamin D [25(OH)D],1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ], parathyroid hormone (PTH), plasma rennin activity (PRA), aldosterone, angiotensin II (AT-II), and angiotensin-converting enzyme (ACE) levels were determined. Ambulatory 24-hour blood pressure measurements and echocardiographic examinations were performed. Serum calcium, phosphorus, and alkaline phosphatase values were normal. Serum 1,25(OH)(2) D(3) and PTH but not PRA and ACE levels were elevated in the HVDRR patients. AT-II levels were higher than normal in the HVDRR patients but not significantly different from those of the controls. Aldosterone levels were normal in all HVDRR patients. No HVDRR patient had hypertension or echocardiographic pathology. These findings reveal that 6- to 36-year-old humans with HVDRR have normal renin and ACE activity, mild but nonsignificant elevation of AT-II, normal aldosterone levels, and no hypertension or gross heart abnormalities.

Protein metabolism in Turner syndrome and the impact of hormone replacement therapy

BACKGROUND

Studies have documented an altered body composition in Turner syndrome (TS). Body fat is increased and muscle mass is decreased. Ovarian failure necessitates substitution with female hormone replacement therapy (HRT), and HRT induces favourable changes in body composition. It is unknown how HRT affects protein metabolism.

AIM

To test whether alterations in body composition before and after HRT in TS are a result of altered protein metabolism.

DESIGN

We performed a randomized crossover study with active treatment (HRT in TS and oral contraceptives in controls) or no treatment.

MATERIALS AND METHODS

We studied eight women (age 29.7 +/- 5.6 (mean +/- SD) years) with TS, verified by karyotype, and eight age-matched controls (age 27.3 +/- 4.9 years). All subjects underwent a 3-h study in the postabsorptive state. Protein dynamics of the whole body and of the forearm muscles were measured by an amino acid tracer dilution technique using [(15)N]phenylalanine and [(2)H(4)]tyrosine. Substrate metabolism was examined by indirect calorimetry.

RESULTS

Energy expenditure was comparable among TS and controls, and did not change during active treatment. Whole-body phenylalanine and tyrosine fluxes were similar in the untreated situations, and did not change during active treatment. Amino acid degradation and protein synthesis were similar in all situations. Muscle protein breakdown was similar among groups, and was not affected by treatment. Muscle protein synthesis rate and forearm blood flow did not differ among groups or due to treatment.

CONCLUSION

Protein metabolism in TS is comparable to controls, and is not affected by HRT.

Endocrine control of body composition in infancy, childhood, and puberty

Body composition exhibits marked variations across the early human lifetime. The precise physiological mechanisms that drive such developmental adaptations are difficult to establish. This clinical challenge reflects an array of potentially confounding factors, such as marked intersubject differences in tissue compartments; the incremental nature of longitudinal intrasubject variations in body composition; technical limitations in quantitating the unobserved mass of mineral, fat, water, and muscle ad seriatim; and the multifold contributions of genetic, dietary, environmental, hormonal, nutritional, and behavioral signals to physical and sexual maturation. From an endocrine perspective (reviewed here), gonadal sex steroids and GH/IGF-I constitute prime determinants of evolving body composition. The present critical review examines hormonal regulation of body composition in infancy, childhood, and puberty.

The influence of estradiol and diet on small intestinal glucose transport in ovariectomized rats

Although gender differences exist for intestinal absorption of nutrients and drugs, the possible role estradiol may play in modulating nutrient transport has not been established. Therefore, small intestine glucose transport was measured 1 week after administering estradiol to ovariectomized rats fed diets high in carbohydrate (C) or protein (P). Rats treated with estradiol ate 21% less (P<0.05) and lost body mass (7%; P<0.05) but did not have smaller intestines. Administration of estradiol increased rates of glucose transport, but only when the rats were fed the C diet. These findings indicate that estradiol causes a disconnect between food intake and the dimensions and nutrient transport capacities of the small intestine. Furthermore, the responses to estradiol are influenced by diet composition, are not of the same magnitude for rats and dogs, and can be predicted to affect systemic availability of nutrients and drugs.

Importance of estrogen on bone health in Turner syndrome: a cross-sectional and longitudinal study using dual-energy X-ray absorptiometry

Osteoporosis and fractures are features in adults with Turner syndrome (TS). Using dual-energy x-ray absorptiometry, correcting bone mineral content (BMC) for height and lean mass (LTM) avoids misclassification of short children as osteopenic. Total body (TB), lumbar spine (LS), and femoral neck (FN) dual-energy x-ray absorptiometry scans were performed on 83 patients with TS (aged 4-24 yr). A prepubertal subgroup (n = 17) receiving GH was followed for 24 months. Age z-scores for height, TB BMC, LTM, the BMC/LTM ratio, and LS volumetric bone mineral density (vBMD) decreased significantly (P < 0.001) with age in prepubertal subjects (n = 51) but were constant in the combined pubertal and postmenarchal group (n = 32). Osteopenia was found in 14.5% (TB), 15.8% (LS), and 28.4% (FN) of patients. In the longitudinal subgroup, TB BMC z-scores decreased by -0.28 (0.31) in subjects remaining prepubertal (n = 11) but increased by 0.71 (0.56) in subjects entering puberty (n = 6; P = 0.007). The z-scores for height and LTM increased in both groups. Our results show a height-independent prepubertal decrease in bone mass accrual, which ceased with puberty. Optimizing bone mass in TS may require earlier induction of puberty than currently recommended. However, reduced FN volumetric bone mineral density and a dissociation of bone and muscle measures were age independent, suggesting an additional intrinsic bone defect.

Overview on the effects of progestins on bone

In view of the fact that fractures are the clinically relevant events, risk factors for fractures are discussed first. Bone mineral density (BMD) appears to be a much less important risk factor for the most severe hip fractures than the risk of falling. No results of experimental studies on hormones and fractures at advanced age are available. An overview of the effects of progestins on bone is given. Effects of progestins on bone have been studied by in vitro experiments using cell lines and by more relevant clinical observations. Prospective studies have been conducted following the use of progestins contained in oral contraceptives, alone or in combination with oestrogens; long-term contraception by injection of depot preparations; so-called "add-back" hormonal therapy attempting to reverse the adverse effects of gonadotropin releasing hormone agonists on bone and after different regimens of hormone replacement therapy (HRT) in postmenopausal women. From the data there are no indications that the various progestins, used in clinical practice, have either a bone-protective or an oestrogen antagonistic activity. Progestins do not add or subtract much of the protective action of oestrogens on the bones.

Osteoporosis in men: pathophysiology, evaluation, and therapy

Osteoporosis is a serious health problem for men. An advance in our understanding of the pathophysiology and treatment of this disorder has resulted in the possibility of a gender-specific approach to screening, diagnosis, and treatment. Here we review the data on osteoporosis in men, discuss controversies regarding whom to screen, whom to treat, and how to treat. Recent treatment data as they relate to men are reviewed, and a clinical treatment algorithm is presented.

Marked disproportionality in bone size and mineral, and distinct abnormalities in bone markers and calcitropic hormones in adult turner syndrome: a cross-sectional study

Most women with Turner syndrome (TS) have no gonadal activity and thus lack estrogen. Bone mineral density (BMD) is often reduced, leading to an increased risk of osteoporosis and fractures. However, growth retardation with reduced final height and other endocrine disturbances may compromise interpretation of skeletal measurements. The aim of the present study was to explore skeletal findings, bone metabolism, and calcium homeostasis in TS. Sixty women with TS (age, 37 +/- 9 yr) and 181 normal age-matched female controls were studied. Bone area (A; square centimeters), bone mineral content (BMC; grams), area-adjusted BMD (aBMD; grams/square centimeter), and volumetric BMD (vBMD; grams/cubic centimeter) were measured at lumbar spine, femoral neck, and forearm using dual energy x-ray absorptiometry. Twenty-eight percent had osteopenia, and 23% had osteoporosis, according to World Health Organization criteria. At the lumbar spine, A, BMC, aBMD, and vBMD were reduced by 18, 27, 11, and 6%, respectively; at the femoral neck, A, BMC, and aBMD were reduced by 2, 10, and 8%, respectively, whereas the 9% reduction in vBMD was insignificant (P = 0.07); and in the forearm, A, BMC, and aBMD were reduced by 53, 55, and 9%, respectively. Bone markers indicated an enhanced bone resorption (21 and 23% increase in C-terminal and N-terminal cross-linking telopeptides of type I collagen/creatinine, respectively) with unchanged (osteocalcin, procollagen I N-terminal propeptide) or reduced (54% reduction in bone alkaline phosphatase) bone formation. Plasma levels of calcium and 25-hydroxyvitamin D (26%) were reduced, and PTH levels increased (74%) in TS. IGF-I (30%), IGF binding protein 3 (18%), testosterone (50%), and SHBG (40%) were reduced in TS. In summary, A, BMC, and aBMD were found to be universally reduced in TS, whereas vBMD was slightly reduced in the spine. Increased resorption of bone was present, with normal or blunted bone formation, suggesting uncoupling or imbalance in bone remodeling. Skeletal changes may be induced by chromosome abnormalities or by secondary endocrine or metabolic changes related to a relative estrogen deficiency, testosterone deficiency, reduced IGF-I, low vitamin D status, and secondary hyperparathyroidism.

Aspects of the treatment of Turner syndrome

Several issues have to be considered when taking care of girls and women with Turner syndrome. During childhood, short stature is the primary concern and treatment with growth hormone (GH) is now widely used, often in conjunction with the androgen, oxandrolone. Recent studies indicate that doses used previously in the treatment of short stature have been too small. Induction of puberty should be performed at an appropriate age with reference to the peers of the patient. In adulthood, female sex hormone substitution should be offered to possibly prevent the increased morbidity seen in Turner syndrome, which consists of increased risk of fractures and osteoporosis, a clustering of diseases like ischaemic heart disease, hypertension, stroke and Type 2 diabetes, the latter entities being involved in the insulin resistance syndrome. Furthermore, hypothyreosis are often seen and the risk of Type 1 diabetes may also be increased. Congenital malformations of the heart are frequently seen in Turner syndrome, possibly increasing the risk of dissecting aorta aneurism. Liver enzymes are often elevated in Turner syndrome and there may be an increased risk of cirrhosis of the liver. Mortality does seem to be increased in Turner syndrome and women with the 'pure' 45,X karyotype do seem to be most severely affected. In the clinical practice of Turner syndrome, a careful monitoring of glucose and bone metabolism, weight, thyroid function and blood pressure should be performed. A cardiovascular risk profile should be determined and the patient informed concerning risks and benefits from sex hormone replacement therapy. Based on the available literature, sex hormone replacement therapy is highly recommended, although at present there are no longitudinal data documenting the long-term positive effect of sex steroid substitution. However, hypogonadism is expected to explain at least part of the decreased lifespan found in Turner syndrome. Since general physicians encounter Turner patients infrequently, it is recommended that the care and treatment of Turner syndrome is centralised.

Choosing an oestrogen replacement therapy in young adult women with Turner syndrome

OBJECTIVE

Hormone replacement therapy (HRT) is prescribed to most patients with Turner syndrome (TS) although its use in adult TS patients has not been scientifically evaluated. The present study was performed to compare the short-term effects in adult women with Turner syndrome of low-dose oral conjugated oestrogen (0.625 mg, CE) with relatively high dose ethinyl oestradiol (30 microg, EE2); both combined with an oral progestin.

DESIGN AND PATIENTS

After 4 months off HRT, 17 young, otherwise healthy women with TS were enrolled in a random, unblinded, crossover study of the two oestrogenic preparations, each given for 6 months.

MEASUREMENTS

We compared parameters of oestrogenic activity that would cover immediate changes in hormone levels, biochemistry, bone turnover, uterine and cardiac variables, which constitute risk factors for later development of diabetes, atherosclerosis, osteoporosis and aortic dissection.

RESULTS

Serum FSH returned to normal follicular phase levels only on the EE2 regimen. The hypotrophic endometria normalized with either of the two oestrogen regimens with no excessive hypertrophy. Hyperinsulinaemia was suppressed to normal by both EE2 and CE. PTH and 1,25-dihydroxyvitamin D levels increased on HRT (EE2 > CE), and phosphorus decreased. Alkaline phosphatase, osteocalcin and urinary deoxypyridinoline cross-links (DPD) were high off therapy; the former two suppressed to high-normal levels on the EE2 regimen, but not on CE, and DPD did not normalize with either HRT. Lipid profiles in these young TS patients were normal. Liver enzymes were mildly elevated off therapy and suppressed to normal levels on both regimens, but more so with EE2.

CONCLUSIONS

The risk factors embodied in hyperinsulinaemia and enhanced bone turnover which, ultimately, have consequences for TS morbidity, are minimized by HRT. In the short term, neither regimen is effective for bone turnover in adult women with Turner syndrome.

Gonadal dysgenesis and bone metabolism

Gonadal dysgenesis is defined as congenital hypogonadism related to abnormalities of the sex chromosomes. Because sex steroids play a central role in the acquisition and maintenance of bone mass, studies have been done to investigate bone status in patients with gonadal dysgenesis, particularly Turner's syndrome and Klinefelter's syndrome, which are the two most common types. The severe estrogen deficiency characteristic of Turner's syndrome (44, X0) is associated with a significant bone mass decrease ascribable to increased bone turnover, as shown by histological studies and assays of bone turnover markers. Estrogen therapy is followed by a significant bone mass gain and a return to normal of bone turnover markers, suggesting that it is the estrogen deficiency rather than the chromosomal abnormality that causes the bone mass deficiency, although abnormalities in the renal metabolism of vitamin D have been reported. Combined therapy with estrogens and growth hormone seems beneficial during the prepubertal period. In Klinefelter's syndrome (47XXY), serum testosterone levels are at the lower end of the normal range and dihydrotestosterone levels are low. Histological studies show depressed osteoblast function and a decrease in 5-alpha-reductase activity responsible for partial tissue resistance to androgens. Assays of bone turnover markers show evidence of increased bone turnover. The bone deficiency is most marked at the femoral neck and seems correlated with serum testosterone and estradiol levels. Androgen therapy has favorable effects on the bone only if it is started before puberty. Recent data suggest that estrogens may contribute to the development of demineralization in KS and that bisphosphonate therapy may be beneficial.

Estrogen agonist (zeranol) treatment in a castrated male lamb model: effects on growth and bone mineral accretion

The mechanism of estrogen's action on bone mineralization in children has received little attention. Our objective was to determine the effect of time (developmentally) and duration of exposure to an estrogen agonist (zeranol) on bone growth and mineralization using a castrated male lamb model. At birth, 40 male lambs were castrated and within 14 days of birth (day = 0) they were assigned (n = 10 per group) to age-matched control lambs (C-AGE) or to receive a 12.5-mg zeranol implant as follows: E-0, implanted on days 0, 45, 90, and 135; E-90, implanted on days 90 and 135; and E-0, 90, implanted on days 0, 90, and 135. Lambs were studied for 163 days. Serum was collected on days 28, 73, 118, 135, and 163 and analyzed for minerals (Ca, P, and Mg), markers of bone remodeling (bone alkaline phosphatase [ALP] and tartrate resistant acid phosphatase [TRAP]), 1,25-dihydroxyvitamin D [1,25(OH)2D], growth hormone (GH), and insulin-like growth factor I (IGF-I). Whole-body bone mineral content (BMC), bone mineral density (BMD), fat mass, and lean mass were determined by dual energy X-ray absorptiometry (DEXA) on days 28, 73, 118, and 163. There was a linear increase in growth at all time points. Whole-body BMC, weight, and lean mass of C-AGE and E-90 lambs were less than E-0, and E-0, 90 lambs at all time points. Whole-body BMD of C-AGE and E-90 lambs was less than E-0 and E-0, 90 lambs at 28 days and 73 days; however, after implantation at day 90 whole-body BMD of E-90 lambs was similar to E-0 and E-0, 90 lambs at day 118 and day 163 and all three were greater than C-AGE lambs. There was no effect of treatment on calcium absorption, serum minerals, hormones, or markers of bone remodeling. We conclude from these data that treatment of growing castrated lambs with an estrogen agonist from birth augments growth, whereas delaying estrogen agonist treatment does not facilitate growth but appears to augment bone mineral accretion. We suggest these observations may have clinical relevance, and deserve consideration when treating children with delays in growth and bone mineral accretion.

Insulin-like growth factor I and growth hormone (GH) treatment in GH-deficient humans: differential effects on protein, glucose, lipid, and calcium metabolism

We examined the effects of recombinant human (rh) insulin-like growth factor I (IGF-I) vs. rhGH in a variety of metabolic paths in a group of eight severely GH-deficient young adults using an array of contemporary tools. Protein, glucose, and calcium metabolism were studied using stable labeled tracer infusions of L-[1-13C]leucine, [6,6-2H2]glucose, and 42Ca and 44Ca; substrate oxidation rates were assessed using indirect calorimetry; muscle strength was determined by isokinetic and isometric dynamometry of the anterior quadriceps, as well as growth factors, hormones, glucose, and lipid concentrations in plasma before and after 8 weeks of rhIGF-I (60 microg/kg, sc, twice daily), followed by 4 weeks of washout, then 8 weeks ofrhGH (12.5 microg/kg-day, sc); the treatment order was randomized. In the doses administered, rhIGF-I and rhGH both increased fat-free mass and decreased the percent fat mass, with a more robust decrease in the percent fat mass after rhGH; both were associated with an increase in whole body protein synthesis rates and a decrease in protein oxidation. Neither hormone affected isokinetic or isometric measures of skeletal muscle strength. However, rhGH was more potent than rhIGF-I at increasing lipid oxidation rates and improving plasma lipid profiles. Both hormones increased hepatic glucose output, but rhGH treatment was also associated with decreased carbohydrate oxidation and increased glucose and insulin concentrations, indicating subtle insulin resistance. Neither hormone significantly affected bone calcium fluxes, supporting the concept that these hormones, by themselves, are not pivotal in bone calcium metabolism. In conclusion, rhIGF-I and rhGH share common effects on protein, muscle, and calcium metabolism, yet have divergent effects on lipid and carbohydrate metabolism in the GH-deficient state. These differences may allow for better selection of treatment modalities depending on the choice of desired effects in hypopituitarism.

Growth hormone, insulin-like growth factor I and sex hormones: effects on protein and calcium metabolism

The availability of non-invasive tracer technologies has greatly facilitated the study of the metabolic effects of nutrients and hormones, particularly in children. This brief review examines recent work on the effects of growth hormone (GH), insulin-like growth factor I (IGF-I), testosterone and oestrogen on rates of protein synthesis and degradation and lipolysis, as well as on body composition and bone calcium fluxes in young children and adults. GH acts indirectly on whole body protein pools via IGF-I, but GH appears to act directly on lipolysis. Testosterone stimulates protein synthesis independently of changes in GH concentrations and acts synergistically with GH to enhance whole body metabolism. Oestrogens and androgens both modulate calcium fluxes, enhancing calcium absorption and retention, and thereby underscoring the importance of both groups of hormones in bone calcium metabolism. Further understanding of the physiological role of these hormones during the critical years of adolescence will give us better tools with which to treat disorders of puberty and growth.

Combination growth hormone and estrogen increase bone mineralization in girls with Turner syndrome

Turner syndrome is characterized by osteopenia and impaired skeletal growth. Neither feature is normalized by current modes of hormone therapy. The purpose of this study was to determine whether GH would increase protein anabolism and provide additional benefit to a regimen of estrogen replacement on calcium metabolism in girls and women with Turner syndrome. Using stable isotopes of calcium and leucine, we determined calcium absorption, urinary calcium loss, calcium retention, deposition into bone, leucine rate of appearance from protein, leucine incorporation into protein, and leucine oxidation in seven girls (10-17 y of age) and four adult females (16-34 y of age) with Turner syndrome, before and after 3 mo of GH treatment. All adults were treated with estrogen (ethinyl estradiol, 50 micrograms/d) and progesterone before and throughout the study. Three girls received no estrogen, and four girls were treated with low-dose estrogen (ethinyl estradiol, 5 micrograms/d) in combination with GH. The addition of estrogen to GH treatment resulted in a significant increase in calcium absorption and deposition in girls. GH did not affect calcium kinetics in adults already receiving estrogen/progesterone replacement therapy, nor did GH alone affect calcium kinetics in girls, and neither GH nor estrogen affected protein metabolism. These data suggest that the addition of low-dose estrogen to a regimen of GH improves bone deposition and calcium metabolism in girls with Turner syndrome and that estrogen is facultative for GH effects on bone.

Estrogen increases 1,25-dihydroxyvitamin D receptors expression and bioresponse in the rat duodenal mucosa

Menopause and estrogen deficiency are associated with apparent intestinal resistance to vitamin D, which can be reversed by estrogen replacement. The in vivo influence of estrogens on duodenal vitamin D receptor (VDR) was studied in three groups of rats: ovariectomized (OVX), sham-operated, and ovariectomized rats treated daily with estrogen (40 microg/kg BW) for 2 weeks (OVX + E). Estrogen administration to OVX rats resulted in a 2-fold increase in VDR messenger RNA transcripts. 1,25(OH)2D3 was shown to bind specifically to one class of receptors in duodenal mucosal extracts, with a dissociation constant of 0.03 nM. Binding was significantly increased in duodenal extracts from OVX + E rats, compared with OVX rats (735 +/- 81 vs. 295 +/- 26 fmol/mg protein; P < 0.001); a comparable, 1.5- to 2-fold increase in VDR protein expression was observed in Western blot analyzes of the duodenal mucosa. Markers of VDR activity were increased in estrogen-exposed rats: calbindin-9k messenger RNA transcript content was 1.4- to 1.6-fold higher, and alkaline phosphatase activity was 1.4- to 3-fold higher in sham-operated and OVX + E, respectively, compared with OVX. 25(OH)D, 1,25(OH)2D, or PTH levels were not altered by estrogen treatment. Cumulatively, these findings suggest that estrogen up-regulates VDR expression in the duodenal mucosa and concurrently increases the responsiveness to endogenous 1,25(OH)2D. Modulation of intestinal VDR activity by estrogen, and subsequent influence on intestinal calcium absorption, could be one of the major protective mechanisms of estrogen against osteoporosis.