Role of 1,25-dihydroxyvitamin D3 on intestinal phosphate absorption in rats with a normal vitamin D supply

Acute High Dietary Phosphorus Following Low-Phosphorus Diet Acclimation Does Not Enhance Intestinal Fractional Phosphorus Absorption in Nephrectomized Male Rats

Dietary phosphorus restriction and phosphorus binders are commonly prescribed for patients with chronic kidney disease (CKD). However, occurrences of non-adherence to these interventions are common. As low-phosphorus (LP) diets have been consistently experimentally shown in vitro to increase intestinal phosphorus absorption efficiency, a bout of non-adherence to diet or binders may cause an unintended consequence of enhanced intestinal phosphorus absorption. Thus, we aimed to determine the effect of a single bout of high-phosphorus (HP) intake after acclimation to a LP diet. Male Sprague Dawley rats with 5/6 nephrectomy (n = 36) or sham operation (n = 36) were block-randomized to 1 of 3 diets: LP (0.1% P w/w), HP (1.2%), or LP followed by acute HP (LPHP 0.1% then 1.2%). Phosphorus absorption tests were conducted using 33P radioisotope administrated by oral gavage or intravenously (iv). Although the overall two-way ANCOVA model for intestinal fractional phosphorus absorption was non-significant, exploratory comparisons showed intestinal fractional phosphorus absorption efficiency tended to be higher in rats in the LP compared with HP or LPHP groups. Rats in the HP or LPHP groups had higher plasma phosphorus compared with rats in the LP group, but the LPHP group was not different from the HP group. Gene expression of the major intestinal phosphate transporter, NaPi-2b, was lower in the jejunum of rats in the LPHP group compared with rats in the HP group but not different in the duodenum. These results demonstrate that an acute HP load after acclimation to a LP diet does not lead to enhanced intestinal fractional phosphorus absorption efficiency in 5/6 nephrectomized male rats. These data provide evidence against the notion that dietary phosphorus restriction or binder use adversely increases absorption efficiency after a single instance of dietary or binder non-adherence. However, other adverse consequences of fluctuating dietary phosphorus intake cannot be ruled out. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Epidemiology, Pathophysiology, and Genetics of Primary Hyperparathyroidism

In this narrative review, we present data gathered over four decades (1980-2020) on the epidemiology, pathophysiology and genetics of primary hyperparathyroidism (PHPT). PHPT is typically a disease of postmenopausal women, but its prevalence and incidence vary globally and depend on a number of factors, the most important being the availability to measure serum calcium and parathyroid hormone levels for screening. In the Western world, the change in presentation to asymptomatic PHPT is likely to occur, over time also, in Eastern regions. The selection of the population to be screened will, of course, affect the epidemiological data (ie, general practice as opposed to tertiary center). Parathyroid hormone has a pivotal role in regulating calcium homeostasis; small changes in extracellular Ca++ concentrations are detected by parathyroid cells, which express calcium-sensing receptors (CaSRs). Clonally dysregulated overgrowth of one or more parathyroid glands together with reduced expression of CaSRs is the most important pathophysiologic basis of PHPT. The spectrum of skeletal disease reflects different degrees of dysregulated bone remodeling. Intestinal calcium hyperabsorption together with increased bone resorption lead to increased filtered load of calcium that, in addition to other metabolic factors, predispose to the appearance of calcium-containing kidney stones. A genetic basis of PHPT can be identified in about 10% of all cases. These may occur as a part of multiple endocrine neoplasia syndromes (MEN1-MEN4), or the hyperparathyroidism jaw-tumor syndrome, or it may be caused by nonsyndromic isolated endocrinopathy, such as familial isolated PHPT and neonatal severe hyperparathyroidism. DNA testing may have value in: confirming the clinical diagnosis in a proband; eg, by distinguishing PHPT from familial hypocalciuric hypercalcemia (FHH). Mutation-specific carrier testing can be performed on a proband's relatives and identify where the proband is a mutation carrier, ruling out phenocopies that may confound the diagnosis; and potentially prevention via prenatal/preimplantation diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Evaluation and Management of Primary Hyperparathyroidism: Summary Statement and Guidelines from the Fifth International Workshop

The last international guidelines on the evaluation and management of primary hyperparathyroidism (PHPT) were published in 2014. Research since that time has led to new insights into epidemiology, pathophysiology, diagnosis, measurements, genetics, outcomes, presentations, new imaging modalities, target and other organ systems, pregnancy, evaluation, and management. Advances in all these areas are demonstrated by the reference list in which the majority of listings were published after the last set of guidelines. It was thus, timely to convene an international group of over 50 experts to review these advances in our knowledge. Four Task Forces considered: 1. Epidemiology, Pathophysiology, and Genetics; 2. Classical and Nonclassical Features; 3. Surgical Aspects; and 4. Management. For Task Force 4 on the Management of PHPT, Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) methodology addressed surgical management of asymptomatic PHPT and non-surgical medical management of PHPT. The findings of this systematic review that applied GRADE methods to randomized trials are published as part of this series. Task Force 4 also reviewed a much larger body of new knowledge from observations studies that did not specifically fit the criteria of GRADE methodology. The full reports of these 4 Task Forces immediately follow this summary statement. Distilling the essence of all deliberations of all Task Force reports and Methodological reviews, we offer, in this summary statement, evidence-based recommendations and guidelines for the evaluation and management of PHPT. Different from the conclusions of the last workshop, these deliberations have led to revisions of renal guidelines and more evidence for the other recommendations. The accompanying papers present an in-depth discussion of topics summarized in this report. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Calcium, Phosphorus, and Vitamin D in Dogs and Cats: Beyond the Bones

Calcium, phosphorus, and vitamin D are nutrients that play a key role in maintaining normal organ, cell, and tissue function. Much is known about their role in bone metabolism, but these nutrients are also important in renal health, urinary tract disease, and multiple other organ systems. It is nutritionally important to meet the physiologic requirements for each of these nutrients, but the interplay between them should also be considered.

1,25(OH)2 vitamin D3 stimulates active phosphate transport but not paracellular phosphate absorption in mouse intestine

KEY POINTS

Intestinal absorption of phosphate proceeds via an active/transcellular route mostly mediated by NaPi-IIb/Slc34a2 and a poorly characterized passive/paracellular pathway. Intestinal phosphate absorption and expression of NaPi-IIb are stimulated by 1,25(OH)₂ vitamin D₃ but whether NaPi-IIb is the only target under hormonal control remains unknown. We report that administration of 1,25(OH)₂ vitamin D₃ to wild-type mice resulted in the expected increase in active transport of phosphate in jejunum, without changing paracellular fluxes. Instead, the same treatment failed to alter phosphate transport in intestinal-depleted Slc34a2-deficient mice. In both genotypes, 1,25(OH)₂ vitamin D₃ induced similar hyperphosphaturic responses and changes in the plasma levels of FGF23 and PTH. While urinary phosphate loss induced by administration of 1,25(OH)₂ vitamin D₃ did not alter plasma phosphate, further studies should investigate whether chronic administration would lead to phosphate imbalance in mice with reduced active intestinal absorption.

ABSTRACT

Intestinal absorption of phosphate is stimulated by 1,25(OH)₂ vitamin D_3. At least two distinct mechanisms underlie phosphate absorption in the gut, an active transcellular transport requiring the Na⁺ /phosphate cotransporter NaPi-IIb/Slc34a2, and a poorly characterized paracellular passive pathway. 1,25(OH)₂ vitamin D₃ stimulates NaPi-IIb expression and function, and loss of NaPi-IIb reduces intestinal phosphate absorption. However, it is remains unknown whether NaPi-IIb is the only target for hormonal regulation by 1,25(OH)₂ vitamin D₃ . Here we compared the effects of intraperitoneal administration of 1,25(OH)₂ vitamin D₃ (2 days, once per day) in wild-type and intestinal-specific Slc34a2-deficient mice, and analysed trans- vs. paracellular routes of phosphate absorption. We found that treatment stimulated active transport of phosphate only in jejunum of wild-type mice, though NaPi-IIb protein expression was upregulated in jejunum and ileum. In contrast, 1,25(OH)₂ vitamin D₃ administration had no effect in Slc34a2-deficient mice, suggesting that the hormone specifically regulates NaPi-IIb expression. In both groups, 1,25(OH)₂ vitamin D₃ elicited the expected increase of plasma fibroblast growth factor 23 (FGF23) and reduction of parathyroid hormone (PTH). Treatment resulted in hyperphosphaturia (and hypercalciuria) in both genotypes, though mice remained normophosphataemic. While increased intestinal absorption and higher FGF23 can trigger the hyperphosphaturic response in wild types, only higher FGF23 can explain the renal response in Slc34a2-deficient mice. Thus, 1,25(OH)₂ vitamin D₃ stimulates intestinal phosphate absorption by acting on the active transcellular pathway mostly mediated by NaPi-IIb while the paracellular pathway appears not to be affected.

Race, Ancestry, and Vitamin D Metabolism: The Multi-Ethnic Study of Atherosclerosis

CONTEXT

A comprehensive characterization of racial/ethnic variations in vitamin D metabolism markers may improve our understanding of differences in bone and mineral homeostasis and the risk of vitamin D-related diseases.

OBJECTIVE

Describe racial/ethnic differences in vitamin D metabolism markers and their associations with genetic ancestry.

DESIGN, SETTING, PARTICIPANTS

In a cross-sectional study within the Multi-Ethnic Study of Atherosclerosis (MESA), we compared a comprehensive panel of vitamin D metabolism markers across self-reported racial/ethnic groups of Black (N = 1759), White (N = 2507), Chinese (N = 788), and Hispanic (N = 1411). We evaluated associations of proportion African and European ancestry with this panel of markers in Black and Hispanic participants using ancestry informative markers. Latent class analysis evaluated associations between patterns of vitamin D measurements with race/ethnicity.

RESULTS

Compared with Black participants, White participants had significantly higher serum concentrations of 25-hydroxyvitamin D and fibroblast growth factor-23; lower concentrations of parathyroid hormone and 1,25-dihydroxyvitamin D; circulating vitamin D metabolite ratios suggesting lower CYP27B1 and higher CYP24A1 activity; higher urinary concentrations of calcium and phosphorus with higher urinary fractional excretion of phosphorus; and differences in vitamin D binding globulin haplotypes. Higher percent European ancestry was associated with higher 25-hydroxyvitamin D and lower parathyroid hormone concentrations among Black and Hispanic participants. Latent classes defined by vitamin D measurements reflected these patterns and differed significantly by race/ethnicity and ancestry.

CONCLUSIONS

Markers of vitamin D metabolism vary significantly by race/ethnicity, may serve to maintain bone and mineral homeostasis across ranges of 25-hydroxyvitamin D production, and be attributable, at least partly, to genetic ancestry.

A review of phosphorus homeostasis and the impact of different types and amounts of dietary phosphate on metabolism and renal health in cats

Elevated concentrations of serum phosphate are linked with progression and increased case fatality rate in animals and humans with chronic kidney disease. Elevated concentrations of serum phosphate can be a risk factor for development of renal and cardiovascular diseases or osteoporosis in previously healthy people. In rodents, an excess intake of dietary phosphorus combined with an inverse dietary calcium : phosphorus ratio (<1 : 1) contributes to renal calcification. Renal injury also has occured in cats fed experimental diets supplemented with highly soluble phosphate salts, especially in diets with inverse calcium : phosphorus ratios. However, not all phosphorus sources contribute similarly to this effect. This review, which focuses on cats, summarizes the published evidence regarding phosphorus metabolism and homeostasis, including the relative impact of different dietary phosphorus sources, and their impact on the kidneys. No data currently shows that commercial cat foods induce renal injury. However, some diets contain high amounts of phosphorus relative to recommendations and some have inverse Ca : P ratios and so could increase the risk for development of kidney disease. While limiting the use of highly soluble phosphates appears to be important, there are insufficient data to support a specific upper limit for phosphate intake. This review also proposes areas where additional research is needed in order to strengthen conclusions and recommendations regarding dietary phosphorus for cats.

Development of experimental chronic kidney disease and vascular calcification alters diurnal variation of phosphate and its hormonal regulators

The mineral-bone axis is tightly regulated and dependent on renal function. In chronic kidney disease (CKD) progressive loss of renal capacity disrupts this axis over-time, with marked changes in circulating calcium, phosphate, PTH, and fibroblast growth factor-23 (FGF-23). These changes contribute to the development of cardiovascular disease, like vascular calcification (VC), which worsens morbidity and mortality in CKD. Although the chronic changes in these circulating factors and their relationships are well known, no experimental studies have examined how the progressive development of CKD and VC alter the circadian rhythms of these factors. An adenine-induced experimental model of CKD in rats was used to establish (i) general circulating trends, (ii) if renal dysfunction affects these observed trends, and (iii) identify potential changes in these trends caused by VC. This study clearly discerned patterns of daily variations in circulating minerals and hormones, finding that both phosphate and PTH follow modelable diurnal variations whereas calcium and FGF-23 maintain relative stability over 24-hr. Surprisingly, the development of CKD was not sufficient to disrupt these patterns of diurnal variation and only altered the magnitude of change; however, it was found that the diurnal rhythms of circulating phosphate and daily stability of calcium were only significantly altered in the setting of CKD with established VC.

Dietary vitamin D interacts with high phosphate-induced cardiac remodeling in rats with normal renal function

BACKGROUND

Vitamin D (VD) and phosphate (Pi) load are considered as contributors to cardiovascular disease in chronic kidney disease and the general population, but interactive effects of VD and Pi intake on the heart are not clearly illustrated.

METHODS

We fed normal male rats with three levels of dietary VD (100, 1100 or 5000 IU/kg chow) and Pi (0.2, 0.6 or 1.6%) (3X3 design) for 8 weeks and examined renal and cardiac function and histology.

RESULTS

High dietary Pi decreased plasma and renal Klotho and plasma 25-hydroxyvitamin D, and increased plasma Pi, fibroblast growth factor 23 and parathyroid hormone without affecting renal function, while low Pi increased plasma and renal Klotho. Both low and high VD diets enhanced high Pi-reduced Klotho expression. Low dietary VD reduced-plasma Klotho was rescued by a low Pi diet. High dietary Pi reduced-cardiac ejection fraction was not modified by a low or high VD diet, but the dietary VD effects on cardiac pathologic changes were more complex. High dietary Pi-induced cardiac hypertrophy was attenuated by a low VD and exacerbated by a high VD diet. In contrast, high dietary Pi -induced cardiac fibrosis was magnified by a low VD and attenuated by a high VD diet.

CONCLUSIONS

High Pi diet induces hypertrophy and fibrosis in left ventricles, a low VD diet accelerates high Pi-induced fibrosis, and a high VD diet exacerbated high Pi -induced hypertrophy. Therefore, cardiac phosphotoxicity is exacerbated by either high or low dietary VD in rats with normal kidney function.

FGF23 and its role in X-linked hypophosphatemia-related morbidity

Background

X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood.

Methods

The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH.

Results

The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations.

Conclusions

By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Effects of the long-term feeding of diets enriched with inorganic phosphorus on the adult feline kidney and phosphorus metabolism

Renal disease has a high incidence in cats, and some evidence implicates dietary P as well. To investigate this further, two studies in healthy adult cats were conducted. Study 1 (36 weeks) included forty-eight cats, stratified to control or test diets providing 1·2 or 4·8 g/1000 kcal (4184 kJ) P (0 or approximately 3·6 g/1000 kcal (4184 kJ) inorganic P, Ca:P 1·2, 0·6). Study 2 (29 weeks) included fifty cats, stratified to control or test diets, providing 1·3 or 3·6 g/1000 kcal (4184 kJ) P (0 or approximately 1·5 g/1000 kcal (4184 kJ) inorganic P, Ca:P 1·2, 0·9). Health markers, glomerular filtration rate (GFR) and mineral balance were measured regularly, with abdominal ultrasound. Study 1 was halted after 4 weeks as the test group GFR reduced by 0·4 (95 % CI 0·3, 0·5) ml/min per kg, and ultrasound revealed changes in renal echogenicity. In study 2, at week 28, no change in mean GFR was observed (P >0·05); however, altered renal echogenicity was detected in 36 % of test cats. In agreement with previous studies, feeding a diet with Ca:P <1·0, a high total and inorganic P inclusion resulted in loss of renal function and changes in echogenicity suggestive of renal pathology. Feeding a diet containing lower total and inorganic P with Ca:P close to 1·0 led to more subtle structural changes in a third of test cats; however, nephrolithiasis occurred in both diet groups, complicating data interpretation. We conclude that the no observed adverse effects level for total dietary P in adult cats is lower than 3·6 g/1000 kcal (4184 kJ), however the effect of inorganic P sources and Ca:P require further investigation.

Effect of dietary phosphorus intake and age on intestinal phosphorus absorption efficiency and phosphorus balance in male rats

Intestinal phosphorus absorption is an important component of whole-body phosphorus metabolism, and limiting dietary phosphorus absorption is particularly of interest as a therapeutic target in patients with chronic kidney disease to manage mineral bone disorders. Yet, mechanisms and regulation of intestinal phosphorus absorption have not been adequately studied and discrepancies in findings exist based on the absorption assessment technique used. In vitro techniques show rather consistent effects of dietary phosphorus intake level and age on intestinal sodium-dependent phosphate transport. But, the few studies that have used in vivo techniques conflict with these in vitro studies. Therefore, we aimed to investigate the effects of dietary phosphorus intake level on phosphorus absorption using the in situ ligated loop technique in three different aged rats. Male Sprague-Dawley rats (n = 72), were studied at 10-, 20-, and 30-weeks-of-age on a low (0.1%), normal (0.6%), or high (1.2%) phosphorus diet in a 3x3 factorial design (n = 8/group). Rats were fed their assigned diet for 2-weeks prior to absorption testing by jejunal ligated loop as a non-survival procedure, utilizing 33P radioisotope. Metabolic cages were used for determination of calcium and phosphorus balance over the final four days prior to sacrifice, and blood was collected at the time of sacrifice for biochemistries. Our results show that phosphorus absorption was higher in 10-week-old rats compared with 20- and 30-week-olds and this corresponded to higher gene expression of the major phosphate transporter, NaPi-2b, as well as higher whole-body phosphorus balance and net phosphorus absorption. Dietary phosphorus intake level did not affect jejunal phosphorus absorption or NaPi-2b gene expression. Our results contrast with studies utilizing in vitro techniques, but corroborate results of other rodent studies utilizing in situ or in vivo methods. Thus, there is need for additional studies that employ more physiological methods of phosphorus absorption assessment.

Phosphate wasting disorders in adults

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.

Intestinal Phosphorus Absorption in Chronic Kidney Disease

Chronic kidney disease (CKD) affects approximately 10% of adults worldwide. Dysregulation of phosphorus homeostasis which occurs in CKD leads to development of CKD-Mineral Bone Disorder (CKD-MBD) and contributes to increased morbidity and mortality in these patients. Phosphorus is regulated by multiple hormones (parathyroid hormone (PTH), 1,25-dihyxdroxyvitamin D (1,25D), and fibroblast growth factor 23 (FGF23)) and tissues (kidney, intestine, parathyroid glands, and bone) to maintain homeostasis. In health, the kidneys are the major site of regulation for phosphorus homeostasis. However, as kidney function declines, the ability of the kidneys to adequately excrete phosphorus is reduced. The hormonal changes that occur with CKD would suggest that the intestine should compensate for impaired renal phosphorus excretion by reducing fractional intestinal phosphorus absorption. However, limited studies in CKD animal models and patients with CKD suggest that there may be a break in this homeostatic response where the intestine fails to compensate. As many existing therapies for phosphate management in CKD are aimed at reducing absolute intestinal phosphorus absorption, better understanding of the factors that influence fractional and absolute absorption, the mechanism by which intestinal phosphate absorption occurs, and how CKD modifies these is a much-needed area of study.

A Controlled Increase in Dietary Phosphate Elevates BP in Healthy Human Subjects

Background Despite epidemiologic evidence for increased cardiovascular morbidity and mortality associated with both high dietary and serum phosphate in humans with normal renal function, no controlled phosphate intervention studies of systemic hemodynamics have been reported. Higher serum 25(OH) vitamin D levels are associated with better cardiovascular outcomes, but vitamin D increases intestinal phosphate absorption.Methods We conducted a prospective outpatient study with blinded assessment in 20 young adults with normal renal function randomized to high phosphate (regular diet plus 1 mmol/kg body wt per day of Na as neutral sodium phosphate) or low phosphate (regular diet plus lanthanum, 750 mg thrice/day, plus 0.7 mmol/kg body wt per day of Na as NaCl) for 11 weeks. After 6 weeks, all subjects received vitamin D₃ (600,000 U) by intramuscular injection. Outcome parameters were 24-hour ambulatory systolic and diastolic BP (SBP and DBP), pulse rate (PR), biomarkers, and measures of endothelial and arterial function.Results Compared with the low-phosphate diet group, the high-phosphate diet group had a significant increase in mean±SEM fasting plasma phosphate concentration (0.23±0.11 mmol/L); 24-hour SBP and DBP (+4.1; 95% confidence interval [95% CI], 2.1 to 6.1; and +3.2; 95% CI, 1.2 to 5.2 mm Hg, respectively); mean 24-hour PR (+4.0; 95% CI, 2.0 to 6.0 beats/min); and urinary metanephrine and normetanephrine excretion (54; 95% CI, 50 to 70; and 122; 95% CI, 85 to 159 µg/24 hr, respectively). Vitamin D had no effect on any of these parameters. Neither high- nor low-phosphate diet nor vitamin D affected endothelial function or arterial elasticity.Conclusions Increased phosphate intake (controlled for sodium) significantly increases SBP, DBP, and PR in humans with normal renal function, in part, by increasing sympathoadrenergic activity.

Coupling between phosphate and calcium homeostasis: a mathematical model

We developed a mathematical model of calcium (Ca) and phosphate (PO₄) homeostasis in the rat to elucidate the hormonal mechanisms that underlie the regulation of Ca and PO₄ balance. The model represents the exchanges of Ca and PO₄ between the intestine, plasma, kidneys, bone, and the intracellular compartment, and the formation of Ca-PO₄-fetuin-A complexes. It accounts for the regulation of these fluxes by parathyroid hormone (PTH), vitamin D₃, fibroblast growth factor 23, and Ca²⁺-sensing receptors. Our results suggest that the Ca and PO₄ homeostatic systems are robust enough to handle small perturbations in the production rate of either PTH or vitamin D₃ The model predicts that large perturbations in PTH or vitamin D₃ synthesis have a greater impact on the plasma concentration of Ca²⁺ ([Ca²⁺]_p) than on that of PO₄ ([PO₄]_p); due to negative feedback loops, [PO₄]_p does not consistently increase when the production rate of PTH or vitamin D₃ is decreased. Our results also suggest that, following a large PO₄ infusion, the rapidly exchangeable pool in bone acts as a fast, transient storage PO₄ compartment (on the order of minutes), whereas the intracellular pool is able to store greater amounts of PO₄ over several hours. Moreover, a large PO₄ infusion rapidly lowers [Ca²⁺]_p owing to the formation of CaPO₄ complexes. A large Ca infusion, however, has a small impact on [PO₄]_p, since a significant fraction of Ca binds to albumin. This mathematical model is the first to include all major regulatory factors of Ca and PO₄ homeostasis.

Effects of Proton Pump Inhibitor Administration and Intake of a Combination of Yogurt and Galactooligosaccharides on Bone and Mineral Metabolism in Rats

The aim of this study was to investigate the effects of proton pump inhibitor (PPI), the most potent acid-suppressing drug, administration and intake of a combination of yogurt and galactooligosaccharides (YG) on bone and mineral metabolism in adult rats. Twelve-week-old male Wistar rats were divided into three groups: a control group fed the control diet with vehicle administration, a PPI group fed the control diet with PPI administration and a YG + PPI group fed the YG diet with PPI administration. All of the groups received their respective experimental diets and daily subcutaneous injection of the vehicle or PPI for 12 weeks. The PPI group showed significantly lower bone mineral density (BMD) of the femur and the lumbar vertebrae and serum fibroblast growth factor 23 (FGF23) and significantly higher phosphorus absorption and serum 1,25-dihydroxyvitamin D (1,25(OH)2D) than the control group, although PPI did not affect calcium absorption. The PPI + YG group showed significantly higher BMD and serum FGF23 and significantly lower phosphorus absorption and serum 1,25(OH)2D than the PPI group. Furthermore, the PPI + YG group showed higher calcium absorption than the control group. These results suggest that although PPI administration did not affect calcium absorption, it adversely affected BMD and influenced phosphorus metabolism in adult rats. Furthermore, the YG diet beneficially affected BMD and attenuated the effects of PPI administration on phosphorus metabolism.

Phosphate: an old bone molecule but new cardiovascular risk factor

Phosphate handling in the body is complex and involves hormones produced by the bone, the parathyroid gland and the kidneys. Phosphate is mostly found in hydroxyapatite. however recent evidence suggests that phosphate is also a signalling molecule associated with bone formation. Phosphate balance requires careful regulation of gut and kidney phosphate transporters, SLC34 transporter family, but phosphate signalling in osteoblasts and vascular smooth muscle cells is likely mediated by the SLC20 transporter family (PiT1 and PiT2). If not properly regulated, phosphate imblanace could lead to mineral disorders as well as vascular calcification. In chronic kidney disease-mineral bone disorder, hyperphosphataemia has been consistently associated with extra-osseous calcification and cardiovascular disease. This review focuses on the physiological mechanisms involved in phosphate balance and cell signalling (i.e. osteoblasts and vascular smooth muscle cells) as well as pathological consequences of hyperphosphataemia. Finally, conventional as well as new and experimental therapeutics in the treatment of hyperphosphataemia are explored.

Effect of dietary nonphytate phosphorus on laying performance and small intestinal epithelial phosphate transporter expression in Dwarf pink-shell laying hens

This study examined the effects of various levels of dietary nonphytate phosphorus on laying performance and the expression patterns of phosphorus metabolism related genes in Dwarf pink-shell laying hens. A total of 405 28-week-old Dwarf pink-shell laying hens were fed the same corn-soybean basal meals but containing 0.20%, 0.25%, 0.30%, 0.35% or 0.40% nonphytate phosphorus. The results showed that feed intake, egg production, and average egg weights were quadratically correlated with dietary nonphytate phosphorus content (P < 0.05), and the highest egg production, feed intake and average egg weights were achieved when dietary nonphytate phosphorus was at 0.3% (P < 0.05). mRNA expression of intestinal sodium phosphorus co-transporter linearly decreased when dietary nonphytate phosphorus increased. mRNA and protein expression of intestinal calbindin and vitamin D receptor correlated quadratically with dietary nonphytate phosphorus, and the highest expression was found when dietary available phosphorus was at 0.25% to 0.3%. In conclusion, the ideal phosphorus requirement for Dwarf pink-shell layer hens is estimated to be 0.3% in a corn-soybean diet. With this level of phosphorus supplementation, calbindin and vitamin D receptor reached their highest expression.

Evidence for FGF23 involvement in a bone-kidney axis regulating bone mineralization and systemic phosphate and vitamin D homeostasis

Bone is involved in the maintenance of phosphate and vitamin D homeostasis via its production and secretion of FGF23 and serves as a reservoir for the storage and release of calcium and phosphate into the circulation. Alterations in mineralization of extracellular matrix and the remodeling activities of the skeleton are coupled to the kidney conservation of phosphate and production of 1,25(OH)2D via the regulation of FGF23 production by osteocytes through yet-to-be defined locally derived factors. In addition, FGF23 production is regulated by 1,25(OH)2D in a feedback loop where FGF23 stimulate Cyp24 mediated degradation of 1,25(OH)2D that serves to protect the organism from the toxic effects of vitamin D excess. In this chapter, we will review the regulation and function of FGF23.

Arterial calcification in chronic kidney disease: key roles for calcium and phosphate

Vascular calcification contributes to the high risk of cardiovascular mortality in chronic kidney disease (CKD) patients. Dysregulation of calcium (Ca) and phosphate (P) metabolism is common in CKD patients and drives vascular calcification. In this article, we review the physiological regulatory mechanisms for Ca and P homeostasis and the basis for their dysregulation in CKD. In addition, we highlight recent findings indicating that elevated Ca and P have direct effects on vascular smooth muscle cells (VSMCs) that promote vascular calcification, including stimulation of osteogenic/chondrogenic differentiation, vesicle release, apoptosis, loss of inhibitors, and extracellular matrix degradation. These studies suggest a major role for elevated P in promoting osteogenic/chondrogenic differentiation of VSMC, whereas elevated Ca has a predominant role in promoting VSMC apoptosis and vesicle release. Furthermore, the effects of elevated Ca and P are synergistic, providing a major stimulus for vascular calcification in CKD. Unraveling the complex regulatory pathways that mediate the effects of both Ca and P on VSMCs will ultimately provide novel targets and therapies to limit the destructive effects of vascular calcification in CKD patients.

The emergence of phosphate as a specific signaling molecule in bone and other cell types in mammals

Although considerable advances in our understanding of the mechanisms of phosphate homeostasis and skeleton mineralization have recently been made, little is known about the initial events involving the detection of changes in the phosphate serum concentrations and the subsequent downstream regulation cascade. Recent data has strengthened a long-established hypothesis that a phosphate-sensing mechanism may be present in various organs. Such a phosphate sensor would detect changes in serum or local phosphate concentration and would inform the body, the local environment, or the individual cell. This suggests that phosphate in itself could represent a signal regulating multiple factors necessary for diverse biological processes such as bone or vascular calcification. This review summarizes findings supporting the possibility that phosphate represents a signaling molecule, particularly in bone and cartilage, but also in other tissues. The involvement of various signaling pathways (ERK1/2), transcription factors (Fra-1, Runx2) and phosphate transporters (PiT1, PiT2) is discussed.

Osteocyte: the unrecognized side of bone tissue

INTRODUCTION

Osteocytes represent 95% of all bone cells. These cells are old osteoblasts that occupy the lacunar space and are surrounded by the bone matrix. They possess cytoplasmic dendrites that form a canalicular network for communication between osteocytes and the bone surface. They express some biomarkers (osteopontin, beta3 integrin, CD44, dentin matrix protein 1, sclerostin, phosphate-regulating gene with homologies to endopeptidases on the X chromosome, matrix extracellular phosphoglycoprotein, or E11/gp38) and have a mechano-sensing role that is dependent upon the frequency, intensity, and duration of strain.

DISCUSSION

The mechanical information transmitted into the cytoplasm also triggers a biological cascade, starting with NO and PGE(2) and followed by Wnt/beta catenin signaling. This information is transmitted to the bone surface through the canalicular network, particularly to the lining cells, and is able to trigger bone remodeling by directing the osteoblast activity and the osteoclastic resorption. Furthermore, the osteocyte death seems to play also an important role. The outcome of micro-cracks in the vicinity of osteocytes may interrupt the canalicular network and trigger cell apoptosis in the immediate surrounding environment. This apoptosis appears to transmit a message to the bone surface and activate remodeling. The osteocyte network also plays a recognized endocrine role, particularly concerning phosphate regulation and vitamin D metabolism. Both the suppression of estrogen following menopause and chronic use of systemic glucocorticoids induce osteocyte apoptosis. On the other hand, physical activity has a positive impact in the reduction of apoptosis. In addition, some osteocyte molecular elements like sclerostin, connexin 43, E11/gp38, and DKK1 are emerging as promising targets for the treatment of various osteo-articular pathologies.

Phosphate sensing

PURPOSE OF REVIEW

To discuss findings suggesting the presence of a phosphate-sensing mechanism in the various organs and the presence of a novel intestinal effector that alters renal phosphate excretion after the ingestion of a phosphate-containing meal.

RECENT FINDINGS

Although phosphate homeostasis is controlled by a variety of hormones (such as parathyroid hormone and 1,25-dihydroxyvitamin D), peptides (the phosphatonins - fibroblast growth factor 23, secreted frizzled-related protein-4, matrix extracellular phosphoglycoprotein) and small molecules (dopamine) that regulate the efficiency of phosphate absorption in the intestine and phosphate excretion in the renal tubule, recent data suggest that postcibal changes in renal phosphate excretion following a meal containing phosphate are mediated by signals generated within the intestine that alter the efficiency of phosphate excretion in the kidney. The intestine detects luminal phosphate and signals to the kidney via the release of the mediator that increases renal phosphate excretion.

SUMMARY

Such information would imply the existence of a phosphate-sensing mechanism within the intestine and the presence of intestinal factors that influence renal phosphate handling.

Novel mechanisms in the regulation of phosphorus homeostasis

Phosphorus plays a critical role in diverse biological processes, and, therefore, the regulation of phosphorus balance and homeostasis are critical to the well being of the organism. Changes in environmental, dietary, and serum concentrations of inorganic phosphorus are detected by sensors that elicit changes in cellular function and alter the efficiency by which phosphorus is conserved. Short-term, post-cibal responses that occur independently of hormones previously thought to be important in phosphorus homeostasis may play a larger role than previously appreciated in the regulation of phosphorus homeostasis. Several hormones and regulatory factors such as the vitamin D endocrine system, parathyroid hormone, and the phosphatonins (FGF-23, sFRP-4, MEPE) among others, may play a role only in the long-term regulation of phosphorus homeostasis. In this review, we discuss how organisms sense changes in phosphate concentrations and how changes in hormonal factors result in the conservation or excretion of phosphorus.

Hypophosphatemia: the common denominator of all rickets

Rickets is a disease of the hypertrophic chondrocytes in the growth plate and is caused by hypophosphatemia-a derived defect in terminal chondrocyte apoptosis. This highlights the critical role of phosphorous in cartilage and bone metabolism. This review shows the role of phosphorous metabolism, transport and function in maintaining phosphorous supply to the growth plate, bone osteoblast and the kidney. Given that phosphorous is the common denominator of all rickets, this review proposes a new classification for the differential diagnosis of rickets, which is based on the mechanisms leading to hypophosphatemia-high PTH activity, high FGF23 activity or renal phosphaturia.

Endocrine functions of bone in mineral metabolism regulation

Given the dramatic increase in skeletal size during growth, the need to preserve skeletal mass during adulthood, and the large capacity of bone to store calcium and phosphate, juxtaposed with the essential role of phosphate in energy metabolism and the adverse effects of hyperphosphatemia, it is not surprising that a complex systems biology has evolved that permits cross-talk between bone and other organs to adjust phosphate balance and bone mineralization in response to changing physiological requirements. This review examines the newly discovered signaling pathways involved in the endocrine functions of bone, such as those mediated by the phosphaturic and 1,25(OH)2D-regulating hormone FGF23, and the broader systemic effects associated with abnormalities of calcium and phosphate homeostasis.

1alpha,25-Dihydroxyvitamin D3 upregulates FGF23 gene expression in bone: the final link in a renal-gastrointestinal-skeletal axis that controls phosphate transport

Fibroblast growth factor (FGF)23 is a phosphaturic hormone that decreases circulating 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and elicits hypophosphatemia, both of which contribute to rickets/osteomalacia. It has been shown recently that serum FGF23 increases after treatment with renal 1,25(OH)(2)D(3) hormone, suggesting that 1,25(OH)(2)D(3) negatively feedback controls its levels by inducing FGF23. To establish the tissue of origin and the molecular mechanism by which 1,25(OH)(2)D(3) increases circulating FGF23, we administered 1,25(OH)(2)D(3) to C57BL/6 mice. Within 24 h, these mice displayed a dramatic elevation in serum immunoreactive FGF23, and the expression of FGF23 mRNA in bone was significantly upregulated by 1,25(OH)(2)D(3), but there was no effect in several other tissues. Furthermore, we treated rat UMR-106 osteoblast-like cells with 1,25(OH)(2)D(3), and real-time PCR analysis revealed a dose- and time-dependent stimulation of FGF23 mRNA concentrations. The maximum increase in FGF23 mRNA was 1,024-fold at 10(-7) M 1,25(OH)(2)D(3) after 24-h treatment, but statistically significant differences were observed as early as 4 h after 1,25(OH)(2)D(3) treatment. In addition, using cotreatment with actinomycin D or cycloheximide, we observed that 1,25(OH)(2)D(3) regulation of FGF23 gene expression occurs at the transcriptional level, likely via the nuclear vitamin D receptor, and is dependent on synthesis of an intermediary transfactor. These results indicate that bone is a major site of FGF23 expression and source of circulating FGF23 after 1,25(OH)(2)D(3) administration or physiological upregulation. Our data also establish FGF23 induction by 1,25(OH)(2)D(3) in osteoblasts as a feedback loop between these two hormones that completes a kidney-intestine-bone axis that mediates phosphate homeostasis.

Secreted frizzled-related protein-4 reduces sodium-phosphate co-transporter abundance and activity in proximal tubule cells

The phosphatonin, secreted frizzled-related protein-4 (sFRP-4), induces phosphaturia and inhibits 25-hydroxyvitamin D 1alpha-hydroxylase activity normally induced in response to hypophosphatemia. To determine the mechanism by which sFRP-4 alters renal phosphate (P(i)) transport, we examined the effect of sFRP-4 on renal brush border membrane (BBMV) Na(+)-dependent P(i) uptake, and the abundance and localization of the major Na(+)-P(i)-IIa co-transporter in proximal tubules and opossum kidney (OK) cells. Infusion of sFRP-4 increased renal fractional excretion of P(i) and decreased renal beta-catenin concentrations. The increase in renal P(i) excretion with sFRP-4 infusion was associated with a 21.9 +/- 3.4% decrease in BBMV Na(+)-dependent P(i) uptake (P < 0.001) compared with a 39.5 +/- 2.1% inhibition of Na(+)-dependent P(i) transport in renal BBMV induced by PTH (P < 0.001). sFRP-4 infusion was associated with a 30.7 +/- 4.8% decrease in Na(+)-P(i)-IIa co-transporter protein abundance (P < 0.01) assessed by immunoblotting methods compared to a 45.4 +/- 8.8% decrease induced by PTH (P < 0.001). In OK cells, sFRP-4 reduced surface expression of a heterologous Na(+)-P(i)-IIa co-transporter. We conclude that sFRP-4 increases renal P(i) excretion by reducing Na(+)-P(i)-IIa transporter abundance in the brush border of the proximal tubule through enhanced internalization of the protein.

Fibroblast growth factor-23 relationship to dietary phosphate and renal phosphate handling in healthy young men

The renal handling of inorganic phosphate (Pi) is controlled not only by PTH, but also by hitherto undetermined mechanisms dependent on phosphate intake. Recently, fibroblast growth factor (FGF)-23 was identified as a novel phosphaturic factor in tumor-induced osteomalacia and autosomal-dominant hypophosphatemic rickets. We hypothesized that phosphate intake could influence FGF-23 concomitantly to the changes in renal Pi handling. Twenty-nine healthy males were subjected to a 5-d low-phosphate diet and a phosphate binder, followed by a high-phosphate diet including supplements. Concomitant modifications in calcium intake allowed minimizing PTH changes in response to dietary phosphate. Serum FGF-23 levels significantly decreased on the low-phosphate diet, then increased with the oral phosphate load. Changes in FGF-23 were positively correlated with changes in 24-h urinary Pi excretion and negatively correlated with changes in the maximal tubular reabsorption of Pi and 1,25(OH)(2)D(3) (calcitriol), whereas PTH was not. In multivariate analysis, changes in FGF-23 remained the most significantly correlated to changes in 1,25(OH)(2)D(3) and maximal tubular reabsorption of Pi. Moreover, FGF-23 was positively correlated to serum osteocalcin, a marker of osteoblastic activity. In summary, FGF-23 was inversely related to renal Pi transport and serum calcitriol levels in healthy young men. These data suggest that FGF-23 may be implicated in the physiological regulation of Pi homeostasis in response to dietary phosphate changes, independent of PTH.

Renal phosphate wasting disorders: clinical features and pathogenesis

Rickets and osteomalacia are associated with hypophosphatemia in several disease states, including X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and tumor-induced osteomalacia. Recent advances in the understanding of these diseases include discovery of mutations in the genes encoding human phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and fibroblast growth factor 23 (FGF-23) and the finding of overproduction of FGF-23 and other proteins including matrix extracellular phosphoglycoprotein (MEPE) and frizzled-related protein 4 (FRP-4) in tumor-induced osteomalacia. Research is ongoing to better define how these proteins relate to each other and to the sodium-phosphate cotransporter in both normal and abnormal phosphate metabolism. New and improved therapies for disorders of phosphate metabolism, osteomalacia, and rickets will develop as our knowledge of phosphate metabolism grows.

Vitamin D reduces renal NaPi-2 in PTH-infused rats: complexity of vitamin D action on renal P(i) handling

Acute administration of dihydroxycholecalciferol [1,25(OH)(2)D(3)] blunts phosphaturia and increases urinary cAMP excretion in parathyroid hormone (PTH)-infused parathyroidectomized (PTX) rats. Because chronic administration of 1,25(OH)(2)D(3) enhances the phosphaturic response to exogenous parathyroid hormone despite blunting of urinary cAMP excretion, we have examined the expression of the renal cortex type II Na-P(i) cotransporter (NaPi-2) mRNA and protein in 1) chronic PTX Sabra rats, 2) PTX rats receiving a physiological dose of 1,25(OH)-2-D(3), 3) PTX rats receiving 35 ng/h of PTH, and 4) rats receiving both PTH and 1,25(OH)(2)D(3), for 7 days via osmotic minipumps. Our results confirm that there is increased phosphaturia in the PTH+1,25(OH)(2)D(3)-infused animals despite blunting of urinary cAMP excretion, a reduced filtered load of phosphate, and lack of a phosphaturic effect by 1,25(OH)(2)D(3) alone. Both PTH and 1,25(OH)(2)D(3) significantly reduced expression of renal cortex NaPi-2 mRNA and NaPi-2 protein, and the administration of PTH together with 1,25(OH)(2)D(3) had additive effects in further decreasing NaPi-2 mRNA and NaPi-2 protein levels. Expression of two other epithelial transporters, type 1 Na-sulfate and type 1 Na-glucose cotransporters, were not different between the groups, suggesting specificity of the effects of PTH and 1,25(OH)(2)D(3) on phosphate transport. The effect of chronic administration of 1,25(OH)(2)D(3) has not been noted previously, and the cellular mechanisms and signaling processes that mediate the decrease in NaPi-2 remain to be determined.

Disorders of phosphate metabolism

Correct identification of the disorders of hypophosphatemia and hyperphosphatemia is important for determining therapy. Further research will provide insights into normal phosphate homeostasis, a complex and fascinating process.

Effects of oral calcitriol on bone mineral density in patients with end-stage renal failure

Evolution of bone mineral density (BMD) at various skeletal sites and the influence of calcitriol on BMD are still poorly documented in patients with terminal renal failure. Using dual photon absorptiometry, we investigated the changes in BMD at the levels of lumbar spine, femoral neck and midfemoral shaft in 21 patients with end-stage renal failure (ESRF) treated with calcitriol (mean dosage +/- SEM: 0.21 +/- 0.02 microgram/day) and compared them to 25 patients with ESRF but not treated with calcitriol (control group) over a period of 20.3 +/- 1.5 and 17.2 +/- 1.2 months, respectively. Lumbar spine BMD increased by 7.7 +/- 3.2%/year in the treated group and decreased by 2.5 +/- 1.3%/year in the control group (P < 0.005). Femoral shaft BMD increased more in treated than in control group (+ 6.7 +/- 2.3 vs. + 1.4 +/- 2.0%/year; P < 0.05) and femoral neck BMD remained stable. PTH levels increased by 92 +/- 121 and 1033 +/- 254 pmol/year (P < 0.01) in the treated group and the controls, respectively. Osteocalcin changes were -2.7 +/- 3.7 and +20.1 +/- 11.7 micrograms/liter (P < 0.05) per year in the same groups. These results indicate that low doses of oral calcitriol in patients with end-stage renal failure were associated with an increase in BMD at the levels of lumbar spine and femoral shaft, and with a stabilization of serum PTH and osteocalcin concentrations.

Is vitamin D3 essential for mineral metabolism in the Damara mole-rat (Cryptomys damarensis)?

The Damara mole-rat (Cryptomys damarensis) leads a strictly subterranean existence in an extensive maze of plugged burrows and eats a strictly herbivorous diet. An obvious source of vitamin D3 (D3), an important hormone in mineral homeostasis, therefore is lacking in this mammal. The effects of orally ingested D3 on mineral balance were investigated. In the normal D' status group of mole-rats, the apparent fractional intestinal absorption of calcium (91.1%), magnesium (91.2%), and inorganic phosphorous (91.2%) was high and increased slightly, (calcium, 94.5%; magnesium, 93.2%; inorganic phosphorous, 92.9%) after D3 administration. The amount of element absorbed was positively correlated with the amount ingested (P less than 0.001), suggesting that absorption occurs via a nonsaturable process. This might be an adaptation to the sparseness of natural food. The observed positive retention of calcium (94%), magnesium (85%), and inorganic phosphorous (99.9%) is attributed to their evergrowing teeth. These teeth are constantly worn down during digging and it is speculated that they represent a mineral "sink," assisting in the maintenance of mineral homeostasis. It is also hypothesized that mole-rats have adapted to an environment devoid of D3 and can attain sufficient calcium for their needs via nonvitamin D3-dependent processes. Although mole-rats have the "machinery" to use D3 and D3 can affect mineral balance, this effect may be disadvantageous if invoked.

Calcium and inorganic phosphate secretion of rat ileum in vitro. Influence of uremia and 1,25 (OH)2D3 inhibition

The unidirectional ileal transport of calcium (Ca) and inorganic phosphorus (Pi) of rats was measured in vitro with the modified Ussing technique. Animal preparation included 5/6 nephrectomy and EHDP treatment. They were compared to controls as well as to 1,25 (OH)2D3 supplemented rats. The results show that the ileum is a secretory organ for Ca and Pi, the serosa to mucosa transport (Jsm) exceeds the mucosa to serosa transport (Jms). Ca and Pi transport in sm direction is 1,25 (OH)2D3 independent but exhibits a mutual strong correlation. Our observations together with published data are in favor of mainly paracellular, non electrogenic sm transport of both ions. However, the factor controlling sm transport of Ca and Pi remains unidentified. The mucosa to serosa transport (Jms) in the ileum is low for both ions. Ca ms is stimulated by 1,25 (OH)2D3, Pi ms is unchanged.

Calcium absorption in the rat colon measured in vivo

Calcium absorption in the large intestine was studied under physiological conditions in the rat. When 45Ca was administered directly into the caecum, the isotope was significantly absorbed. This absorption was modulated by dietary Ca and Pi as well as by the 1,25(OH) D3 status. On the other hand, calcium coming from the food, either as 45Ca or as 40Ca, was not absorbed to a significant amount. When the absorption of 45Ca from the caecal content was assessed in duodenal loops, less 45Ca was absorbed when the isotope was administered orally than when it was added to the caecal content. Thus, the fact that oral Ca is not absorbed in the colon despite the efficient Ca transport system of this part of the intestine may be due to the transformation in the digestive tract of Ca from an absorbable into a non-absorbable form.

Evidence against the role of calcium deficiency in genetic hypertension

Epidemiological studies suggest an association between reduced calcium uptake and hypertension, while clinical trials and rat experiments indicate a small but significant hypotensive effect with oral calcium supplements. These data imply that calcium deficiency has a role in genetic hypertension. We reasoned that if the hypothesis is correct, the hypertension should be aggravated by further reducing calcium balance but attenuated by augmenting calcium balance. We tested this hypothesis by evaluating the blood pressure response in spontaneously hypertensive rats (SHR) as calcium balance was decreased by dietary restriction of calcium or increased by supplementation with magnesium or 1 alpha, 25-dihydroxycholecalciferol (calcitriol). A low calcium diet within the physiological range did not accentuate the hypertension in SHR during the 11 weeks of treatment, even though calcium balance was reduced by half. Similar results were obtained with dietary calcium restriction in parathyroidectomized SHR, which excludes any offsetting effects of changes in parathyroid hormone levels. Conversely, 7 weeks of a high magnesium diet, which increased calcium balance without reducing PO4 balance, did not correct the hypertension of SHR. Similarly, long-term administration of calcitriol failed to reduce the blood pressure of parathyroidectomized SHR and normotensive Wistar-Kyoto (WKY) controls, despite the presence of increased serum calcium levels comparable to those produced by oral calcium loading. Finally, external calcium balance was measured directly in 25-day-old, prehypertensive SHR. As a result of the increased calcium absorption and reduced calcium excretion, SHR retained more calcium than did the normotensive WKY, which directly refutes the existence of calcium deficiency at this normotensive stage. These data do not support the role of calcium deficiency in genetic hypertension.

Gender differences in antacid-induced phosphate deprivation in rats

To determine whether an animal model could be used to study the susceptibility of women to antacid-induced phosphate deficiency, 6-wk-old male and female rats were given basic aluminum carbonate gel (Basaljel) (1 ml/100 g body wt) or distilled water by gastric intubation daily for 3 wk. Rats were fed either ad libitum (group 1) or by pair-feeding (group 2) with pelleted rat food containing 0.74% phosphorus. In group 1, baseline, 1-wk, and 3-wk values for serum phosphorus in Basaljel-treated females were 7.7 +/- 0.2, 6.3 +/- 0.2, and 6.2 +/- 0.2 mg/dl, respectively. Corresponding values for control females were 7.8 +/- 0.3, 7.0 +/- 0.2, and 7.3 +/- 0.2 mg/dl. Values for treated females were significantly lower (p less than 0.02) than values for control females by 1 wk of treatment. Basaljel-treated males did not differ from controls. The pattern for group 2 was similar. Intestinal absorption and intramuscular stores of phosphate were assessed in group 1. After 3 wk of treatment, [32P]phosphate assimilation from the duodenum into the body was lower in Basaljel-treated females than in controls (33.8% +/- 1.9% vs. 49.8% +/- 6.2% of the luminally administered dose, p less than 0.05). This was due to increased retention of [32P]phosphate in the intestine of treated females (19.9% +/- 2.0% vs. 11.9% +/- 2.4% for control females, p less than 0.02). Results in jejunum were similar. Total intramuscular phosphate in females was significantly lower (p less than 0.005) than in males both before and after antacid treatment. Thus hypophosphatemia in the female rat during antacid administration is probably secondary to the additive effects of decreased assimilation and decreased soft tissue stores of phosphate.

Mechanism of stimulation of renal phosphate transport by 1,25-dihydroxycholecalciferol

Vitamin D has been shown to stimulate renal phosphate transport and to alter membrane phospholipid composition. The present studies examine the possibility that the effects of 1,25(OH)2D3 on phosphate transport are related to its effects on membrane lipids. Arrhenius plots, which relate maximum rates of sodium dependent phosphate uptake into brush-border membrane vesicles to temperature were constructed. Phosphate transport was studied using brush-border membrane vesicles from normal, vitamin D-deficient, and physiologically replete (15 pmol/100 g body weight per 24 h) rats. These plots were triphasic with characteristic, lipid-dependent, slopes (M1,M2,M3) representing activation energies and transition temperatures (T1,T2). Physiologic 1,25(OH)2D3 repletion normalized these plots by stimulating phosphate transport at all temperatures, increasing T2 from 18 +/- 0.7 to 23.5 +/- 0.9 degrees C and decreasing M2 and M3 from -5.8 +/- 0.2 and -10.2 +/- 0.4 to -4.5 +/- 0.4 and -7.7 +/- 0.3, respectively. Pharmacologic (1.2 nmol/100 g per 3 h) 1,25(OH)2D3 treatment resulted in a change in the Arrhenius plot of phosphate transport to a biphasic one with a transition temperature of 30 degrees C. This effect was not blocked by cycloheximide. The Arrhenius plots of glucose transport were triphasic and unchanged with vitamin D repletion. These data support a liponomic mechanism of action for 1,25(OH)2D3 on phosphate transport.