Search

Estrogen deficiency-induced bone loss is associated with complex changes in the calcium fluxes that constitute calcium balance. We studied the effects of oophorectomy on calcium balance and its components within the first 9 wk after the operation. Six-day calcium balance studies were performed on 30-wk-old female Sprague-Dawley rats before either sham operation or oophorectomy (oophx) and at 3-wk intervals for 9 wk postoperation. The rats were fed a diet containing 0.4g Ca/100 g diet and 0.3 g P/100 g diet throughout the study. The postoperative changes in calcium balance (P < 0.05) and net calcium absorption (P < 0.02) were negative in the oophx group compared with the ovary-intact group. The oophx group excreted more calcium via both the kidney (urine Ca, P < 0.05) and the gastrointestinal tract (endogenous fecal Ca, P < 0.05). The postoperation endogenous fecal calcium was higher at 3 wk postoophorectomy than at later times (P < 0.05). Oophorectomy did not affect true calcium absorption up to 9 wk postoophorectomy. Oophorectomy stimulates bone metabolism and our findings indicate that within the first 9 wk after oophorectomy, bone mineral loss is associated with a transient increase in the excretion of calcium by the gastrointestinal tract and the kidney. KEY WORDS: * oophorectomy * rat * calcium balance * calcium excretion.

Calcium balance and its major components (true calcium absorption, urinary calcium excretion and intestinal calcium secretion) were assessed by a 6-d balance technique in young (6-wk-old) rats fed a diet containing 7.2 g Ca/kg diet. Following either oophorectomy (oophx) or sham operation, the balance study was repeated at 9, 12 and 15 wk of age. Calcium balance decreased with age but remained positive in each group (mean (pooled SEM): sham, (6 wk) 0.86 to (15 wk) 0.27 (0.03) mmol/d (P < 0.0001); oophx, (6 wk) 0.91 to (15 wk) 0.22 (0.03) mmol/d (P < 0.0001)). Analysis of variance of the change in calcium balance indicated that there was a significantly greater reduction in calcium balance in the oophx group compared with the sham-operated group. Intestinal calcium absorption decreased in each group with age (sham: (6 wk) 46.3% to (15 wk) 22.6% (1.5%) (P < 0.0001); oophx: (6 wk) 48.2% to (15 wk) 21.2% (1.5%) (P < 0.0001)). There was a marked rise in intestinal calcium secretion in the oophx group compared with the sham-operated group at 6 wk post-operation (12 wk of age) (oophx: 0.33 (0.02) mmol/d; sham: 0.23 (0.02) mmol/d (P < 0.01)). Urinary calcium excretion was not affected by either age or oophorectomy. We conclude that oophorectomy in young rats leads to a reduction in calcium accumulation, which is mainly the result of an increase in intestinal calcium secretion.

Objective To assess if a cell-based readout of androgen action in serum demonstrates a closer association with recognized classical parameters of androgen action in men than current measures of serum testosterone (T). Design To develop, validate and utilize a mammalian cell-based assay to measure specifically bioactive T and determine if this measure is a physiologically relevant fraction of serum T. Measurements and participants We have developed a specific serum T bioassay using human prostate cancer cells. A rapid 5-min exposure to 100% serum followed by serum withdrawal confers specificity of the assay to serum T and provides sufficient sensitivity to measure T in male serum samples. Matrix effects were experimentally discounted as a confounding issue. A total of 960 male serum samples from the Florey Adelaide Male Ageing Study (FAMAS) with previous comprehensive cohort data and serum measurements were utilized. Results Bioassay T measurement in the 960 FAMAS serum samples returned a median of 10·7 nmol/l (1·7–45·4), and was most closely related to immunoassayed total T, but not immunoassayed bioavailable T or calculated free T. Immunoassayed total T demonstrated a positive association with isometric grip-strength ( R2 = 0·127, P < 0·001), self-reported sexual desire ( R2 = 0·113, P < 0·001) and erectile function ( R2 = 0·085, P < 0·05) while bioassay T did not. Conclusions While cellular bioassays offer a rapid and sensitive means of identifying the androgenic potential of complex environmental compounds, the utility of such assays in defining a clinically relevant fraction of serum T distinct from total T needs further investigation. [ABSTRACT FROM AUTHOR]

Objective Plasma androgen levels are inversely associated with health in men, the age-related decline of which may result from factors other than ageing per se. This study aimed to determine the effects of demographic, physical and lifestyle factors on age-related androgen status in men. Design An observational survey of a regionally representative male population residing in the North West regions of Adelaide, Australia. Participants Study sample includes 1195 men aged 35–81 years with a response rate of 45·1%. Measurements Plasma levels of total testosterone (TT), bioavailable testosterone (BT), SHBG, insulin-like peptide 3 (INSL3), and gonadotrophins were measured along with an extensive list of demographic, physical and lifestyle factors including body composition, muscle strength and biomarkers of chronic diseases, physical activity, nutrition and smoking behaviour. Results Low TT was mostly associated with high abdominal fat and triglycerides and low muscle strength rather than ageing per se. Low BT was associated with increased age followed by high whole body fat percentage. BT and TT levels were higher in unmarried men and smokers. SHBG levels increased with age, but were also inversely associated with insulin and triglycerides. The Leydig cell specific factor INSL3 was the strongest biomarker associated with both TT and BT. Conclusions Factors associated with low androgen status variably include high body fat percentage, low muscle strength and biomarkers of the metabolic syndrome. Reducing exposure to factors that adversely affect androgen status may improve the general health of ageing men by mechanisms yet to be defined. [ABSTRACT FROM AUTHOR]

Abstract: Circulating levels of 1,25-dihydroxyvitamin D (1,25D) are determined by bioactivation catalyzed by the renal 25-hydroxyvitamin D 1α-hydroxylase (CYP27B1) and degradation through the action of the renal 25-hydroxyvitamin D 24-hydroxylase (CYP24). CYP27B1 and CYP24 are also present in bone cells, but little is known of their physiological role. The purpose of this study was to determine the changes that occur with aging on the expression of CYP27B1 and CYP24 mRNA in whole kidney and femora of female Sprague–Dawley rats. Real-time RT-PCR was used to measure CYP27B1, CYP24 and vitamin D receptor (VDR) mRNA levels in the kidneys and bones of animals aged between 3 weeks and 2 years. Circulating 1,25D levels decreased exponentially with age which was correlated with both reduced kidney CYP27B1 mRNA (R 2 = 0.72) and increased CYP24 mRNA levels (R 2 = 0.71). In the bone, CYP27B1 mRNA levels were maintained at their highest level throughout the ages of 3 to 15 weeks before decreasing in adult animals (P &#60; 0.05). Bone CYP24 mRNA levels were positively correlated with bone CYP27B1 mRNA and not circulating 1,25D levels (R 2 = 0.74). Levels of bone CYP27B1 mRNA were positively correlated with distal femoral epiphyseal trabecular number (Tb.N) (R 2 = 0.74) and negatively with the trabecular thickness (Tb.Th) (R 2 = 0.56) in animals aged between 12 weeks and 2 years. These findings indicate that the regulation of CYP27B1 and CYP24 mRNA expression in the bone is unique from that in the kidney. The synthesis of 1,25D in bone tissue regulates bone CYP24 expression and is associated with bone mineralization suggesting that vitamin D metabolism has an autocrine or paracrine function. [Copyright &y& Elsevier]

Background: It is assumed that calcium absorption decreases with age, but this is not well documented. We report a study that addresses this issue. Objective: The aim was to establish the extent and timing of any age-related change in calcium absorption in postmenopausal women. Design: We measured radiocalcium absorption (α) in 262 healthy postmenopausal women aged 40 - 87 y. We also measured the serum vitamin D metabolites, parathyroid hormone (PTH), and other biochemical variables. Results: Radiocalcium absorption decreased with age (P = 0.018); it was 28% lower in the 25 women aged >75 y than in the rest (P < 0.001). Itwas significantly related to serum 1,25-dihydroxyvitamin D [1,25(OH)2D] in the whole set and in both the younger and older subsets, but it was not related to either 25-dihydroxyvitamin D [25(OH)D] or PTH or to any other measured variable. No decrease in 1,25(OH)2D was seen with age to account for the decrease in calcium absorption, so radiocalcium absorption corrected for serum 1,25(OH)2D decreased significantly after age 75 y. On multivariate analysis, the serum 1,25(OH)2D concentration was a positive function of 25(OH)D (P < 0.001), albumin (P = 0.010), and PTH (P = 0.012)andanegative function of serum creatinine (P = 0.003).PTH was a negative function of calculated ionized calcium (P = 0.004) and 25(OH)D (P = 0.009) and a positive function of weight (P = 0.011) and age (P = 0.028). Conclusions: A late age-related decrease in calcium absorption is seen in postmenopausal women in addition to the decline that occurs at menopause. This decrease could be due to a decline in either the active calcium transport or diffusion component of the calcium absorption system. [ABSTRACT FROM AUTHOR]

Details of the molecular mechanisms determining levels of the secosteroid, 1,25-dihydroxyvitamin D3 (1,25D) remain to be elucidated. The current paradigm for the control of serum 1,25D levels is the tight regulation of renal 25-hydroxyvitamin D-1α-hydroxlase (CYP27B1) activity by a number of physiological factors. 1,25D production is also regulated by the cytochrome P450 enzyme, 25-hydroxyvitamin D-24-hydroxylase (CYP24), which through side chain hydroxylation reactions, inactivates 1,25D. We have recently demonstrated that renal CYP27B1 and CYP24 expression contribute equally to regulating serum 1,25D levels. We now describe the contribution of renal Vitamin D receptor (VDR) expression in determining serum 1,25D levels. Serum 1,25D levels were decreased when the dietary calcium intake was increased. We measured mRNA levels for CYP27B1, CYP24 and VDR receptor in kidney RNA extracts from animals fed diets containing different levels of calcium, ranging from 0.05 to 1%. Serum 1,25D levels were negatively correlated with renal CYP24 mRNA levels (R2=0.35, P<0.01) while renal VDR is positively correlated with renal CYP24 mRNA (R2=0.80, P<0.001). However, only renal VDR mRNA remained a significant determinant of renal CYP24 expression when both these variables were included in multiple linear regression analysis (multiple R2=0.89, P<0.001). These findings suggest that kidney CYP24 activity acts in concert with kidney CYP27B1 to control serum 1,25D levels and that serum 1,25D stimulates renal CYP24 expression by acting through the renal VDR. [Copyright &y& Elsevier]

Abstract: Precise determination of Vitamin D-dependent intestinal calcium absorption in longitudinal studies is problematic. We have assessed Vitamin D-dependent intestinal calcium absorption by 45Ca gavage. Rats were gavaged with a 1mL solution containing 45Ca (CaCl2, 9.3MBq/mL) maintained at 37°C. Total Ca concentration of the gavage fluid was optimised by comparing the absorption curves for fluids made up to 0.025, 2.025, 4.025 and 40.025mmol/L with 40CaCl2. The effect of varying dietary Ca on fractional Ca absorption was determined in rats fed semi-synthetic diets containing either 0.05%, 0.2%, 0.4% or 1.0% Ca for 50 days. Serum 1,25 dihydroxyvitamin D (1,25D) was determined by radioimmunoassay. Total gavage Ca of 0.025mmol/L achieved the highest peak fractional absorption and was adopted for all future experiments. Fifty days after allocation to the diets both fractional Ca absorption and 1,25D were highest in rats fed 0.05% Ca and lowest in those fed 1.0% Ca (absorption, P <0.05 and 1,25D, P <0.05). There was a strong logarithmic relationship between 1,25D and fractional Ca absorption (R 2 0.69, P <0.001). Weekly repetition of the procedure did not cause a fall in haematocrit over 7 weeks. Radiocalcium (45Ca) absorption by gavage provides a simple measure of Vitamin D-dependent Ca absorption for repetitive use in longitudinal studies. [Copyright &y& Elsevier]

Abstract: Vitamin D depletion in rats causes osteopenia in at least three skeletal sites. However it is unclear whether modulation of dietary calcium intake impacts on the relationship between the level of serum 25-hydroxyvitamin D (25D) and bone loss. Nine-month-old female Sprague–Dawley rats (n =5–6/group) were pair-fed a semi-synthetic diet containing either 0 or 20IU vitamin D3/day with either low (0.1%) or high (1%) dietary Ca for 6 months. At 15 months of age, fasting bloods were collected for biochemical analyses. Serum 25D levels were lowest in the animals fed 0IU vitamin D and 0.1% Ca. The animals fed 1% Ca had significantly higher serum 25D levels when compared to animals fed 0.1% Ca (P <0.05). The major determinants of serum 25D were dietary vitamin D and dietary calcium (Multiple R =0.75, P <0.05). Animals fed 0.1% Ca had higher renal CYP27B1 mRNA expression and 12–18-fold increased levels of serum 1,25D. Hence, the reported effects of low calcium diets on bone loss may be, in part, due to the subsequent effects of 25D metabolism leading to reduction in vitamin D status. Such an interaction has significant implications, given the recent evidence for local synthesis of active vitamin D in bone tissue. [ABSTRACT FROM AUTHOR]

Abstract: Circulating 1α,25-dihydroxyvitamin D3 (1,25D) derives from renal conversion of 25-hydroxyvitamin D3 (25D), by the 25D 1α-hydroxylase (CYP27B1). Blood 25D levels, but not 1,25D levels, are the best indicator of vitamin D status and predict fracture risk in the elderly. We examined the extent to which osteoblasts can metabolize 25D. Well-characterized human primary osteoblasts and osteosarcoma (OS) cell lines were examined for the expression and regulation of genes associated with vitamin D metabolism, using real-time PCR. Primary osteoblasts and OS cell lines were found to express CYP27B1 mRNA and secreted detectable 1,25D in response to 25D. Of the OS cell lines tested, HOS expressed the most CYP27B1 mRNA and secreted the highest levels of 1,25D. All osteoblastic cells examined up-regulated expression of the catabolic regulator of 1,25D, the 25-hydroxyvitamin D-24-hydroxylase (CYP24), when incubated with either 1,25D or 25D. Exposure to physiological levels of 25D resulted in up-regulated transcription of the 1,25D responsive genes, osteocalcin (OCN), osteopontin (OPN) and RANKL. Specific knockdown of CYP27B1 in HOS cells using siRNA resulted in up to 80% reduction in both 1,25D secretion and the transcription of OCN and CYP24, strongly implying that the 25D effect in osteoblasts is preceded by conversion to 1,25D. Incubation with 25D, like 1,25D, inhibited primary osteoblast proliferation and promoted in vitro mineralization. Finally, we detected expression by osteoblasts of receptors for vitamin D binding protein (DBP), cubilin and megalin, suggesting that osteoblasts are able to internalize DBP–25D complexes in vivo. Together, our results suggest that autocrine, and perhaps paracrine, pathways of vitamin D3 metabolism may regulate key osteoblast functions independently of circulating, kidney derived 1,25D. Our results are therefore consistent with the reported benefits of maintaining a healthy vitamin D status in the elderly to reduce the risk of fractures. [Copyright &y& Elsevier]