Modeling of transcellular Ca transport in rat duodenum points to coexistence of two mechanisms of apical entry

Water-soluble biopolymers calcium polymalate derived from fermentation broth of Aureobasidium pullulans markedly alleviates osteoporosis and fatigue

Osteoporosis is a prevalent condition characterized by bone loss and decreased skeletal strength, resulting in an elevated risk of fractures. Calcium plays a crucial role in preventing and managing osteoporosis. However, traditional calcium supplements have limited bioavailability, poor solubility, and adverse effects. In this study, we isolated a natural soluble biopolymer, calcium polymalate (PMACa), from the fermentation broth of the fungus Aureobasidium pullulans, to investigate its potential as an anti-osteoporosis therapeutic agent. Characterization revealed that linear PMA-Ca chains juxtaposed to form a porous, rod-like state, in the presence of Ca2+. In vivo mouse models demonstrated that PMA-Ca significantly promoted the conversion of serum calcium into bone calcium, and stimulated bone growth and osteogenesis. Additionally, PMA-Ca alleviated exercise fatigue in mice by facilitating the removal of essential metabolites, such as serum lactate (BLA) and blood urea nitrogen (BUN), from their bloodstream. In vitro studies further showed that PMA-Ca strengthened osteoblast cell activity, proliferation, and mineralization. And PMA-Ca upregulated the expression of some genes involved in osteoblast differentiation, indicating a potential correlation between bone formation and PMACa. These findings indicate that soluble PMA-Ca has the potential to be a novel biopolymer-based calcium supplement with sustainable production sourced from the fermentation industry.

Vitamins as regulators of calcium-containing kidney stones - new perspectives on the role of the gut microbiome

Calcium-based kidney stone disease is a highly prevalent and morbid condition, with an often complicated and multifactorial aetiology. An abundance of research on the role of specific vitamins (B6, C and D) in stone formation exists, but no consensus has been reached on how these vitamins influence stone disease. As a consequence of emerging research on the role of the gut microbiota in urolithiasis, previous notions on the contribution of these vitamins to urolithiasis are being reconsidered in the field, and investigation into previously overlooked vitamins (A, E and K) was expanded. Understanding how the microbiota influences host vitamin regulation could help to determine the role of vitamins in stone disease.

A model of calcium homeostasis in the rat

We developed a model of calcium homeostasis in the rat to better understand the impact of dysfunctions such as primary hyperparathyroidism and vitamin D deficiency on calcium balance. The model accounts for the regulation of calcium intestinal uptake, bone resorption, and renal reabsorption by parathyroid hormone (PTH), vitamin D₃, and Ca²⁺ itself. It is the first such model to incorporate recent findings regarding the role of the calcium-sensing receptor (CaSR) in the kidney, the presence of a rapidly exchangeable pool in bone, and the delayed response of vitamin D₃ synthesis. Accounting for two (fast and slow) calcium storage compartments in bone allows the model to properly predict the effects of bisphophonates on the plasma levels of Ca²⁺ ([Ca²⁺]_p), PTH, and vitamin D₃ Our model also suggests that Ca²⁺ exchange rates between plasma and the fast pool vary with both sex and age, allowing [Ca²⁺]_p to remain constant in spite of sex- and age-based hormonal and other differences. Our results suggest that the inconstant hypercalciuria that is observed in primary hyperparathyroidism can be attributed in part to counterbalancing effects of PTH and CaSR in the kidney. Our model also correctly predicts that calcimimetic agents such as cinacalcet bring down [Ca²⁺]_p to within its normal range in primary hyperparathyroidism. In addition, the model provides a simulation of CYP24A1 inactivation that leads to a situation reminiscent of infantile hypercalcemia. In summary, our model of calcium handling can be used to decipher the complex regulation of calcium homeostasis.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Casein phosphopeptides modulate proliferation and apoptosis in HT-29 cell line through their interaction with voltage-operated L-type calcium channels

At the intestinal level, proliferation and apoptosis are modulated by the extracellular calcium concentration; thus, dietary calcium may exert a chemoprotective role on normal differentiated intestinal cells, while it may behave as a carcinogenesis promoter in transformed cells. Calcium in milk is associated with casein and casein phosphopeptides (CPPs), hence is preserved from precipitation. CPPs were demonstrated to induce uptake of extracellular calcium ions by in vitro intestinal tumor HT-29 cells but only upon differentiation. Here, the hypothesis that CPPs could differently affect proliferation and apoptosis in undifferentiated and differentiated HT-29 cells through their binding with calcium ions was investigated. Results showed that CPPs protect differentiated intestinal cells from calcium overload toxicity and prevent their apoptosis favoring proliferation while inducing apoptosis in undifferentiated tumor cells. The CPP effect on undifferentiated HT-29 cells, similar to that exerted by ethyleneglycol-O, O'-bis(2-aminoethyl)-N, N, N', N'-tetraacetic acid (EGTA), is presumably due to the ability in binding the extracellular calcium. The effect on differentiated HT-29 cells is coupled to the interaction of CPPs with the voltage-operated L-type calcium channels, known to activate calcium entry into the cells under depolarization and to exert a mitogenic effect: the use of an agonist potentiates the cell response to CPPs, while the antagonists abolish the response to CPPs (36% of examined cells) or reduce both the percentage of responsive cells and the increase of intracellular calcium concentration. Taken together, these results confirm the potentialities of CPPs as nutraceuticals/functional food and also as modulators of cellular processes connected to the expression of a cancer phenotype.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

The nondigestible disaccharide epilactose increases paracellular Ca absorption via rho-associated kinase- and myosin light chain kinase-dependent mechanisms in rat small intestines

We previously showed that epilactose, a nondigestible disaccharide, increased calcium (Ca) absorption in the small intestines of rats. Here, we explored the mechanism(s) underlying the epilactose-mediated promotion of Ca absorption in a ligated intestinal segment of anesthetized rats. The addition of epilactose to the luminal solution increased Ca absorption and chromium (Cr)-EDTA permeability, a paracellular indicator, with a strong correlation (R = 0.93) between these changes. Epilactose induced the phosphorylation of myosin regulatory light chains (MLCs), which is known to activate the paracellular route, without any change in the association of tight junction proteins with the actin cytoskeleton. The epilactose-mediated promotion of the Ca absorption was suppressed by specific inhibitors of myosin light chain kinase (MLCK) and Rho-associated kinase (ROCK). These results indicate that epilactose increases paracellular Ca absorption in the small intestine of rats through the induction of MLC phosphorylation via MLCK- and ROCK-dependent mechanisms.

Inhibitory effect of aluminium on calcium absorption in small intestine of rats with different thyroid hormone status

To analyse the influence of thyroid status on the effect of aluminium (Al) upon intestinal calcium (Ca) absorption, adult male Wistar rats with experimentally altered thyroid hormones circulating levels, were orally treated (o.g.) with 0 (control), or 50 mg elemental Al (as chloride)/kg body weight (b.w.) per day, for a 14 d period. Hyper- and hypo-thyroid conditions were respectively achieved by means of administration of either sodium levothyroxine (50 microg/kg b.w. per day, o.g.) or methimazole, a thyroxine synthesis inhibitor (1mg/kg b.w. per day, o.g.). In duodenum-jejunum segments, in vitro mucosa-to-serosa (45)Ca flux (JCa(ms)) and kinetics of (45)Ca uptake in isolated enterocytes, were determined. In serum, concentrations of thyroxine (T4) and triiodothyronine (T3) were measured by chemiluminescent enzyme immunoassay. Unlike non-Al-treated rats, JCa(ms) of Al-exposed rats decreased as serum levels of T4 and T3 increased, showing a significant inverse correlation in both cases (T4: r(2)=0.414, P=0.024; T3: r(2)=0.443, P=0.018). Enterocytes isolated from rats treated with Al plus thyroxine showed a reduction of both maximum Ca uptake (4.86+/-0.44 vs. 6.85+/-1.04 nmol Ca/mg protein, P<0.05) and K(m) (0.84+/-0.18 vs. 1.05+/-0.36 mM, P<0.05) when compared to control. The observed variability in the Al effect on Ca transport with thyroid status of rats could be reflecting a negative interaction of Al with thyroid hormone action mechanisms on intestinal Ca absorption, which would take place mainly at Ca entry into enterocyte from lumen.

Recent developments in intestinal calcium absorption

Calcium absorption proceeds by transcellular and paracellular flux, with the latter accounting for most absorbed calcium when calcium intake is adequate. Vitamin D helps regulate transcellular calcium transport by increasing calcium uptake via a luminal calcium channel and by inducing the cytosolic calcium transporting protein, calbindinD(9k). Recent studies utilizing knockout mice have challenged the functional importance of the channel and calbindin. To integrate the new findings with many previous studies, the function of the two molecules must be evaluated in the calcium transport and economy of mice. When calcium intake is high, transcellular calcium transport contributes little to total calcium absorption. Therefore, increasing calcium intake seems the most effective nutritional approach to ensure adequate absorption and prevent bone loss.

Mobile trap algorithm for zinc detection using protein sensors

We present a mobile trap algorithm to sense zinc ions using protein-based sensors such as carbonic anhydrase (CA). Zinc is an essential biometal required for mammalian cellular functions although its intracellular concentration is reported to be very low. Protein-based sensors like CA molecules are employed to sense rare species like zinc ions. In this study, the zinc ions are mobile targets, which are sought by the mobile traps in the form of sensors. Particle motions are modeled using random walk along with the first passage technique for efficient simulations. The association reaction between sensors and ions is incorporated using a probability (p1) upon an ion-sensor collision. The dissociation reaction of an ion-bound CA molecule is modeled using a second, independent probability (p2). The results of the algorithm are verified against the traditional simulation techniques (e.g., Gillespie's algorithm). This study demonstrates that individual sensor molecules can be characterized using the probability pair (p1,p2), which, in turn, is linked to the system level chemical kinetic constants, kon and koff. Further investigations of CA-Zn reaction using the mobile trap algorithm show that when the diffusivity of zinc ions approaches that of sensor molecules, the reaction data obtained using the static trap assumption differ from the reaction data obtained using the mobile trap formulation. This study also reveals similar behavior when the sensor molecule has higher dissociation constant. In both the cases, the reaction data obtained using the static trap formulation reach equilibrium at a higher number of complex molecules (ion-bound sensor molecules) compared to the reaction data from the mobile trap formulation. With practical limitations on the number sensors that can be inserted/expressed in a cell and stochastic nature of the intracellular ionic concentrations, fluorescence from the number of complex sensor molecules at equilibrium will be the measure of the intracellular ion concentration. For reliable detection of zinc ions, it is desirable that the sensors must not bind all the zinc ions tightly, but should rather bind and unbind. Thus for a given fluorescence and with association-dissociation reactions between ions and sensors, the static trap approach will underestimate the number of zinc ions present in the system.

Prolactin is an important regulator of intestinal calcium transport

Prolactin has been shown to stimulate intestinal calcium absorption, increase bone turnover, and reduce renal calcium excretion. The small intestine, which is the sole organ supplying new calcium to the body, intensely expresses mRNAs and proteins of prolactin receptors, especially in the duodenum and jejunum, indicating the intestine as a target tissue of prolactin. A number of investigations show that prolactin is able to stimulate the intestinal calcium transport both in vitro and in vivo, whereas bromocriptine, which inhibits pituitary prolactin secretion, antagonizes its actions. In female rats, acute and long-term exposure to high prolactin levels significantly enhances the (i) transcellular active, (ii) solvent drag-induced, and (iii) passive calcium transport occurring in the small intestine. These effects are seen not only in pregnant and lactating animals, but are also observed in non-pregnant and non-lactating animals. Interestingly, young animals are more responsive to prolactin than adults. Prolactin-enhanced calcium absorption gradually diminishes with age, thus suggesting it has an age-dependent mode of action. Although prolactin's effects on calcium absorption are not directly vitamin D-dependent; a certain level of circulating vitamin D may be required for the basal expression of genes related to calcium transport. The aforementioned body of evidence supports the hypothesis that prolactin acts as a regulator of calcium homeostasis by controlling the intestinal calcium absorption. Cellular and molecular signal transductions of prolactin in the enterocytes are largely unknown, however, and still require investigation.

Mechanisms and regulation of calcium absorption from the gastrointestinal tract in pigs and ruminants: comparative aspects with special emphasis on hypocalcemia in dairy cows

Adequate blood calcium (Ca) concentrations are a prerequisite to maintain several physiological functions of mammals such as pig and ruminants. Thus, blood Ca levels have to be regulated within very close limits. This is basically ensured through the coordinated effects of the calcitropic hormones parathyroid hormone (PTH) and calcitriol (1,25-dihydroxyvitamin D3). If plasma Ca decreases, one main effect of subsequently secreted PTH is the stimulation of renal reabsorption of Ca from the glomerular filtrate to reduce urinary Ca loss. Another important action of PTH is the induction of the renal enzyme 1[@@@]\rmalpha [@@@]-hydroxylase, which is responsible for the production of calcitriol. In most monogastric species, so far investigated, one of the most important effects of calcitriol is to stimulate active absorption of Ca from the gastrointestinal tract, particularly the upper small intestines, via a vitamin D-receptor-mediated genomic action. Whether this concept can be transferred without any constrictions to ruminants is still under discussion. Marked interspecies differences have to be recognized with respect to ruminant or non-ruminant animals, particularly with respect to individual segments along the gastrointestinal axis and with respect to vitamin D sensitivity of Ca transport mechanisms. This review will elucidate some of the current concepts related to the mechanisms and sites of Ca absorption in pigs and ruminants with special emphasis on dairy cows where Ca homeostasis is occasionally compromised at the time of parturition.

Calcium absorption by Cav1.3 induces terminal web myosin II phosphorylation and apical GLUT2 insertion in rat intestine

Glucose absorption in rat jejunum involves Ca(2+)- and PKC betaII-dependent insertion of GLUT2 into the apical membrane. Ca(2+)-induced rearrangement of the enterocyte cytoskeleton is thought to enhance paracellular flow. We have therefore investigated the relationships between myosin II regulatory light chain phosphorylation (RLC(20)), absorption of glucose, water and calcium, and mannitol clearance. ML-7, an inhibitor of myosin light chain kinase, diminished the phloretin-sensitive apical GLUT2 but not the phloretin-insensitive SGLT1 component of glucose absorption in rat jejunum perfused with 75 mM glucose. Western blotting and immunocytochemistry revealed marked decreases in RLC(20) phosphorylation in the terminal web and in the levels of apical GLUT2 and PKC betaII, but not SGLT1. Perfusion with phloridzin or 75 mM mannitol, removal of luminal Ca(2+), or inhibition of unidirectional (45)Ca(2+) absorption by nifedipine exerted similar effects. ML-7 had no effect on the absorption of 10 mM Ca(2+), nor clearance of [(14)C]-mannitol, which was less than 0.7% of the rate of glucose absorption. Water absorption did not correlate with (45)Ca(2+) absorption or mannitol clearance. We conclude that the Ca(2+) necessary for contraction of myosin II in the terminal web enters via an L-type channel, most likely Ca(v)1.3, and is dependent on SGLT1. Moreover, terminal web RLC(20) phosphorylation is necessary for apical GLUT2 insertion. The data confirm that glucose absorption by paracellular flow is negligible, and show further that paracellular flow makes no more than a minimal contribution to jejunal Ca(2+) absorption at luminal concentrations prevailing after a meal.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Prolactin directly enhanced Na+/K+- and Ca2+-ATPase activities in the duodenum of female rats

Prolactin has recently been shown to directly stimulate 2 components of the active duodenal calcium transport in female rats, i.e., solvent drag-induced and transcellular-active calcium transport. Since the basolateral Na+/K+- and Ca2+-ATPases, respectively, play important roles in these 2 transport mechanisms, the present study aimed to examine the direct actions of prolactin on the activities of both transporters in sexually mature female Wistar rats. The results showed that 200, 400, and 800 ng/mL prolactin produced a significant increase in the total ATPase activity of duodenal crude homogenate in a dose-dependent manner within 60 min (i.e., from a control value of 1.53 ± 0.13 to 2.29 ± 0.21 (p < 0.05), 2.68 ± 0.19 (p < 0.01), and 3.92 ± 0.33 (p < 0.001) µmol Pi·(mg protein)–1·min–1, respectively). Activity of Na+/K+-ATPase was increased by 800 ng/mL prolactin from 0.17 ± 0.03 to 1.18 ± 0.29 µmol Pi·(mg protein)–1·min–1 (p < 0.01). Prolactin at doses of 400 and 600 ng/mL also significantly increased the activities of Ca2+-ATPase in crude homogenate from a control value of 0.84 ± 0.03 to 1.75 ± 0.29 (p < 0.05), and 2.30 ± 0.37 (p < 0.001) µmol Pi·(mg protein)–1·min–1. When the crude homogenate was purified for the basolateral membrane, the Na+/K+-ATPase activities were elevated 10-fold. In the purified homogenate, 800 ng/mL prolactin increased Na+/K+-ATPase activity from 1.79 ± 0.38 to 2.63 ± 0.44 µmol Pi·(mg protein)–1·min–1 (p < 0.05), and Ca2+-ATPase activity from 0.08 ± 0.14 to 2.03 ± 0.23 µmol Pi·(mg protein)–1·min–1 (p < 0.001). Because the apical calcium entry was the first important step for the transcellular active calcium transport, the brush border calcium uptake was also investigated in this study. We found that, 8 min after being directly exposed to 800 ng/mL prolactin, the brush border calcium uptake into the duodenal epithelial cells was increased from 0.31 ± 0.02 to 0.80 ± 0.28 nmol·(mg protein)–1 (p < 0.05). It was concluded that prolactin directly and rapidly enhanced the brush border calcium uptake as well as the activities of the basolateral Na+/K+- and Ca2+-ATPases in the duodenal epithelium of female rats. These findings explained the mechanisms by which prolactin stimulated duodenal active calcium absorption.

Absorption of uranium through the entire gastrointestinal tract of the rat

The aim was to determine the gastrointestinal segments preferentially implicated in the absorption of uranium. The apparent permeability to uranium (233U) was measured ex vivo in Ussing chambers to assess uranium passage in the various parts of the small and large intestines. The transepithelial electrical parameters (potential difference, short-circuit current, transepithelial resistance and tissue conductance) were also recorded for each segment. Determination of in vivo uranium absorption after in-situ deposition of 233U in digestive segments (buccal cavity, ileum and proximal colon) and measurements of uranium in peripheral blood were then made to validate the ex vivo results. In addition, autoradiography was performed to localize the presence of uranium in the digestive segments. The in vivo experiments indicated that uranium absorption from the digestive tract was restricted to the small intestine (with no absorption from the buccal cavity, stomach or large intestine). The apparent permeability to uranium measured with ex vivo techniques was similar in the various parts of small intestine. In addition, the experiments demonstrated the existence of a transcellular pathway for uranium in the small intestine. The study indicates that uranium absorption from the gastrointestinal tract takes place exclusively in the small intestine, probably via a transcellular pathway.

Les hypercalciuries

The frequency of hypercalciuria is increasing in western countries with an incidence of nephrolithiasis which can reach 13%. Hypercalciuria appears as an alteration of the calcium transport system (kidney, bowel, bone) which is regulated by calcitriol and parathormone. The aim of this review was to screen etiologies of hypercalciuria taking into account recent genetic advances (calcium epithelial channel and calcium sensing receptor). Hypercalciuria may be favored by nutritional causes (diet rich in calcium, sodium, carbohydrates, proteins, poor in phosphates and potassium). It may also be related to an increase in calcium absorption (vitamin D excess, primary hyperparathyroidism, sarcoidosis, lymphoma, estrogens, and certain genetic causes), an increase in osteoresorption (bone metastasis, myeloma, Paget, hyperthyroidism, immobilization, hypercortisolism and corticosteroid therapy), or a decrease of kidney tubular resorption (diuretics, Cacci and Ricci, acromegally, Bartter, familial dominant hypocalcemia, Fanconi, Dent, familial hypomagnesemia-hypercalciuria syndrome, type 1 distal tubular acidosis, pseudohypoaldosteronism, diabetes). If no cause is identified, persistence of hypercalciuria after instituting a correct diet is defined as idiopathic hypercalciuria. Treatment of the cause is essential in secondary hypercalciuria, in addition to diet (low sodium intake, normocalcic diet, hydration), associated with thiazide diuretics and biphosphonates if necessary.

Calcium-modulated S100 protein-phospholipid interactions. An NMR study of calbindin D9k and DPC

The cellular functions of several S100 proteins involve specific interactions with phospholipids and the cell membrane. The interactions between calbindin D(9k) (S100D) and the detergent dodecyl phosphocholine (DPC) were studied using NMR spectroscopy. In the absence of Ca(2+), the protein associates with DPC micelles. The micelle-associated state has intact helical secondary structures but no apparent tertiary fold. At neutral pH, Ca(2+)-loaded calbindin D(9k) does not associate with DPC micelles. However, a specific interaction is observed with individual DPC molecules at a site close to the linker between the two EF-hands. Binding to this site occurs only when Ca(2+) is bound to the protein. A reduction in pH in the absence of Ca(2+) increases the stability of the micelle-associated state. This along with the corresponding reduction in Ca(2+) affinity causes a transition to the micelle-associated state also in the presence of Ca(2+) when the pH is lowered. Site-specific analysis of the data indicates that calbindin D(9k) has a core of three tightly packed helices (A, B, and D), with a dynamic fourth helix (C) more loosely associated. Evidence is presented that the Ca(2+)-binding characteristics of the two EF-hands are distinctly different in a micelle environment. The role of calbindin D(9k) in the cell is discussed, along with the broader implications for the function of the S100 protein family.

Effect of age, vitamin D, and calcium on the regulation of rat intestinal epithelial calcium channels

Transepithelial transport of calcium involves uptake at the apical membrane, movement across the cell, and extrusion at the basolateral membrane. Active vitamin D metabolites regulate the latter two processes by induction of calbindin D and the plasma membrane ATPase (calcium pump), respectively. The expression of calbindin D and the calcium pump declines with age in parallel with transepithelial calcium transport. The apical uptake of calcium is thought to be mediated by the recently cloned calcium channels-CaT1 (or ECaC2, TRPV6) and CaT2 (or ECaC1, TRPV5). The purpose of these studies was to determine whether there were age-related changes in intestinal calcium channel regulation and to identify the dietary factors responsible for their regulation. Young (2 months) and adult (12 months) rats were fed either a high calcium or low calcium diet for 4 weeks. The low calcium diet significantly increased duodenal CaT1 and CaT2 mRNA levels in both age groups, but the levels in the adult were less than half that of the young. The changes in calcium channel expression with age and diet were significantly correlated with duodenal calcium transport and with calbindin D levels. To elucidate the relative roles of serum 1,25(OH)2D3 and calcium in the regulation of calcium channel expression, young rats were fed diets containing varying amounts of calcium and vitamin D. Dietary vitamin D or exogenous 1,25(OH)2D3 more than doubled CaT1 mRNA levels, and this regulation was independent of dietary or serum calcium. These findings suggest that the apical calcium channels, along with calbindin and the calcium pump, may play a role in intestinal calcium transport and its modulation by age, dietary calcium, and 1,25(OH)2D3.

Dietary calcium deficiency increases Ca2+ uptake and Ca2+ extrusion mechanisms in chick enterocytes

Ca2+ uptake and Ca2+ extrusion mechanisms were studied in enterocytes with different degree of differentiation from chicks adapted to a low Ca2+ diet as compared to animals fed a normal diet. Chicks adapted to a low Ca2+ diet presented hypocalcemia, hypophosphatemia and increased serum 1,25(OH)2D3 and Ca2+ absorption. Low Ca2+ diet increased the alkaline phosphatase (AP) activity, independently of the cellular maturation, but it did not alter gamma-glutamyl-transpeptidase activity. Ca2+ uptake, Ca2+-ATPase and Na(+)/Ca2+ exchanger activities and expressions were increased by the mineral-deficient diet either in mature or immature enterocytes. Western blots analysis shows that vitamin D receptor (VDR) expression was much higher in crypt cells than in mature cells. Low Ca2+ diet decreased the number of vitamin D receptor units in both kinds of cells. In conclusion, changes in Ca2+ uptake and Ca2+ extrusion mechanisms in the enterocytes by a low Ca2+ diet appear to be a result of enhanced serum levels of 1,25(OH)2D3, which would promote cellular differentiation producing cells more efficient to express vitamin D dependent genes required for Ca2+ absorption.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Calcium absorption and bone mineral density in celiacs after long term treatment with gluten-free diet and adequate calcium intake

Calcium malabsorption, hypocalcemia and skeletal demineralization are well-recognized features of untreated celiac disease. This study investigates calcium absorption and bone mineral density (BMD) after a prolonged, over 4 years, treatment with a gluten-free diet. Twenty-four adult females with treated celiac disease and twenty age- and sex-matched control subjects were studied. Mean body mass index (MBI), energy intake, serum calcium, and serum 25(OH)D concentrations in treated celiacs did not differ from controls. However, while both dietary calcium and protein intake were significantly higher in celiacs (P<0.012), fractional calcium absorption was lower (mean percentage+/-SD; treated 39.8+/-12 versus controls 52.3+/-10, P<0.001). Thus, after adjusting for calcium intake, the estimated amount of calcium absorbed daily was similar in both groups. Whole body, spine and trochanter BMD were significantly lower in treated celiac patients compared with controls (P<0.05). There were significant inverse correlations between: serum parathyroid hormone (PTH) and femoral neck or total body BMD (P<0.01), PTH and duration of gluten-free diet (P=0.05), and fractional calcium absorption and alkaline phosphatase (P=0.022). Increased calcium intake could potentially compensate for the reduced fractional calcium absorption in treated adult celiac patients, but may not normalize the BMD. In addition, the inverse correlation between PTH and time following treatment is suggestive of a continuing long-term benefit of gluten withdrawal on bone metabolism in celiac patients.

Prolactin directly stimulated the solvent drag-induced calcium transport in the duodenum of female rats

Prolactin has been reported to stimulate the calcium absorption of the duodenum where three components of the active calcium transport, namely transcellular active, voltage-dependent and solvent drag-induced calcium transport, were identified. It was known that the transcellular active, but not the voltage-dependent, duodenal calcium transport was directly stimulated by prolactin. The present study thus aimed to evaluate the direct action of prolactin on the solvent drag-induced duodenal calcium transport by using the Ussing chamber technique. The jejunum was used as a reference for the existence of solvent drag and the widening of tight junction induced by cytochalasin E. Results showed that the solvent drag-induced calcium transport existed in both intestinal segments, but the magnitude was significantly greater in the duodenum (29.27+/-2.27 vs. 17.31+/-1.65 nmol h(-1) cm(-2), P<0.001). We further demonstrated that 200, 600 and 800, but not 1000 ng/ml, prolactin significantly promoted the solvent drag-induced duodenal calcium transport in a dose-response manner, i.e. from the control value of (nmol h(-1) cm(-2)) 24.31+/-2.36 to 45.42+/-3.47 (P<0.01), 63.82+/-5.28 (P<0.001) and 53.93+/-5.41 (P<0.01), respectively. However, prolactin did not manifest any effect on the jejunum. Because the paracellular transport was suggested to be size-selective as well as charge-selective, further experiments were designed to evaluate the mechanism by which prolactin stimulated the solvent drag-induced calcium transport. The duodenum was exposed to 20 microM cytochalasin E, 600 ng/ml prolactin or the combination of both in the presence of a paracellular marker 3H-mannitol, while the jejunum was a positive reference. The results showed that, in the jejunum, cytochalasin E alone and cytochalasin E plus prolactin significantly increased the mannitol fluxes from (micromol h(-1) cm(-2)) 0.29+/-0.04 to 0.49+/-0.03 (P<0.05) and 0.48+/-0.05 (P<0.05), respectively, while having no effect on the calcium fluxes. Prolactin alone had no effect on the jejunal calcium flux. In the duodenum, neither mannitol nor calcium fluxes were enhanced by cytochalasin E, however, prolactin still increased the solvent drag-induced calcium flux from 27.74+/-2.41 to 51.03+/-4.35 nmol h(-1) cm(-2) (P<0.001). It was concluded that prolactin directly stimulated the solvent drag-induced duodenal calcium transport in a dose-response and biphasic manner without the widening of tight junction.

A role for Ca(v)1.3 in rat intestinal calcium absorption

Active Ca(2+) absorption through epithelial Ca(2+) channels TRPV5/6 in duodenum is activated by hyperpolarisation. However, when diet and Ca(2+) are plentiful, digestion products cause depolarisation. We therefore used homology-based PCR from a rat jejunal mucosal cDNA preparation to reveal the presence of the neuroendocrine L-type isoform Ca(v)1.3alpha(1). Immunocytochemical labelling and immunoblotting localised Ca(v)1.3 alpha(1) protein in apical membrane from proximal jejunum to mid ileum. Perfusion studies in vivo with 1.25 mM luminal Ca(2+) revealed L-type channel activity. Inhibition of glucose absorption with phloridzin strongly inhibited 45Ca(2+) absorption; absorption was inhibited by nifedipine and Mg(2+) and activated by Bay K 8644, none of which affect TRPV5/6. At 10mM Ca(2+), nifedipine inhibited 45Ca(2+) absorption with a time course similar to that at 1.25 mM Ca(2+): absorption was therefore channel-mediated rather than paracellular. We suggest that in times of dietary sufficiency, Ca(v)1.3 may mediate a significant route of Ca(2+) absorption into the body.

Thyroid hormones stimulate calcium transport systems in rat intestine

Thyroid hormone status influences calcium metabolism. To elucidate the mechanism of action of thyroid hormones on transcellular transport of calcium in rat intestine, Ca(2+) influx and efflux studies were carried out in brush border membrane vesicles (BBMV) and across the basolateral membrane (BLM) of enterocytes, respectively. Steady-state uptake of Ca(2+) into BBMV as well as Ca(2+) efflux from the BLM enterocytes was significantly increased in hyperthyroid (Hyper-T) rats and decreased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Kinetic studies revealed that increase in steady state Ca(2+) uptake into BBMV from hyper-T rats was fraternized with decrease in Michaelis Menten Constant (K(m)), indicating a conformational change in Ca(2+) transporter. Further, this finding was supported by significant changes in transition temperature and membrane fluidity. Increased Ca(2+) efflux across enterocytes was attributed to sodium-dependent Ca(2+) exchange activity which was significantly higher in Hyper-T rats and lower in Hypo-T rats as compared to Eu-T rats. However, there was no change in Ca(2+)-ATPase activity of BLMs of all groups. Kinetic studies of Na(+)/Ca(2+) exchanger revealed that alteration in Na(+)-dependent Ca(2+) efflux was directly associated with maximal velocity (V(max)) of exchanger among all the groups. cAMP, a potent activator of Na(+)/Ca(2+) exchanger, was found to be significantly higher in intestinal mucosa of Hyper-T rats as compared to Eu-T rats. Therefore, the results of this study suggest that Ca(2+) influx across BBM is possibly modulated by thyroid hormones by mediating changes in membrane fluidity. Thyroid hormones activated the Na(+)/Ca(2+) exchange in enterocytes possibly via cAMP-mediated pathway.

Mechanisms and functional aspects of intestinal calcium absorption

Calcium absorption, in terms of mechanisms and function, is well adapted to meet the calcium needs of mammals. When calcium levels in the food are low, the active, mediated transcellular calcium transport assumes primary importance. This process is vitamin D-dependent, largely localized in the duodenum, and involves three steps: entry across the brush border, mediated by a molecular structure, CaT1, with two components; a facilitated transport that saturates at low luminal calcium concentration; and a channel component through which most calcium enters the cell at the higher luminal concentrations. Intracellular diffusion is assured by a small, cytosolic calcium binding molecule, calbindinD(9k), which carries more than 90% of the calcium that traverses the duodenal cell, thus also serving as a buffer. Extrusion is by the CaATPase and is not a limiting step. Calcium entry is reduced by more than 90% in the absence of vitamin D, with biosynthesis of calbindinD(9k) totally vitamin D-dependent. Active transport is upregulated on low calcium intake and downregulated at high calcium intake, when paracellular calcium transport through the tight junctions of the intestine becomes the dominant process. The amount of calcium absorbed paracellularly is a function of the calcium gradient between lumen and plasma and of the time the chyme spends at a given intestinal site. The coexistence of mediated and nonmediated transport processes assures the organism of an adequate calcium supply, yet prevents excessive calcium absorption.

Calcium transporter 1 and epithelial calcium channel messenger ribonucleic acid are differentially regulated by 1,25 dihydroxyvitamin D3 in the intestine and kidney of mice

We examined the expression of calcium transporter 1 (CaT1) and epithelial calcium channel (ECaC) mRNA in the duodenum and kidney of mice. Intestinal CaT1 mRNA level increased 30-fold at weaning, coincident with the induction of calbindin-D(9k) expression. In contrast, renal CaT1 and ECaC mRNA expression was equal until weaning when ECaC mRNA is induced and CaT1 mRNA levels fall 70%. Long- and short-term adaptation to changes in dietary calcium (Ca) level and 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] injection strongly regulated duodenal calbindin D(9k) and CaT1 mRNA. Following a single dose of 1,25(OH)(2)D(3), induction of CaT1 mRNA occurred rapidly (within 3 h, peak at 6 h of 9.6 +/- 0.8-fold) and preceded the induction of intestinal Ca absorption (significantly increased at 6 h, peak at 9 h). Neither renal CaT1 nor ECaC mRNA were strongly regulated by dietary calcium level or 1,25(OH)(2)D(3) injection. Our data indicate that CaT1 and ECaC mRNA levels are differentially regulated by 1,25(OH)(2)D(3) in kidney and intestine and that there may be a specialized role for CaT1 in kidney in fetal and neonatal development. The rapid induction of intestinal CaT1 mRNA expression by 1,25(OH)(2)D(3), and the marked induction at weaning, suggest that CaT1 is critical for 1,25(OH)(2)D(3)-mediated intestinal Ca absorption.

Calcium nutrition and metabolism

An adequate calcium intake throughout life is essential for maintenance of the skeleton, by far the largest body reservoir of calcium. Appropriately high calcium intake is particularly important in the first two decades, when the body calcium mass increases to near maximum. In subsequent decades, because calcium absorption is relatively modest, typically 25% or less, calcium intake must be kept near 1000 mg per day in order to minimize the possibility that the skeleton will be mined for its mineral content. The amount of calcium needed for signaling and to maintain the extracellular calcium constant is relatively small; however, skeletal turnover is enhanced in calcium deficiency, the increased turnover representing the body's attempt to preserve skeletal calcium.

A nonlinear compartmental model of Sr metabolism. II. Its physiological relevance for Ca metabolism

We have studied the peculiarities of the nonlinear compartmental model for human Sr metabolism (Staub JF, Foos E, Courtin B, Jochemsen R, and Perault-Staub AM. Am J Physiol Regul Integr Comp Physiol 284: R819-R834, 2003), including its physiological reliability in the context of Sr-Ca similarity-dissimilarity. We found it to be relevant to Ca metabolism, except for discrimination against Sr relative to Ca at urinary and intestinal levels. The main findings are as follows: 1) the saturable part of intestinal absorption, shared by Sr and Ca, does not seem to be responsible for the discrimination of the transcellular pathway; 2) although there is little discrimination in bone, the physicochemical behaviors of Sr and Ca at the bone surface differ, at least quantitatively; and 3) Sr behaves as a "tracer" for Ca metabolic pathways and, under non-steady-state conditions, can also reveal self-regulatory processes. It is suggested that they depend on Ca2+ (cationic)-sensing receptors that are apparently more sensitive to Sr than to Ca. Acting on gastrointestinal and osteoblast lineage cells, these slow processes might contribute to adaptive, rather than homeostatic, regulation of Ca metabolism. Understanding these features could help clarify the pharmacological and therapeutic effects of oral Sr.

Vitamin D receptor (VDR) knockout mice reveal VDR-independent regulation of intestinal calcium absorption and ECaC2 and calbindin D9k mRNA

To study the role of calbindin D(9k) (CaBP) and epithelial calcium channel ECaC2 in intestinal calcium (Ca) absorption, vitamin D receptor knockout (KO) and wild-type (WT) mice were fed either 0.5% Ca or a 2.0% Ca rescue diet starting at 21 d of age. Ca absorption and parameters involved in this process were measured at 60 or 90 d of age. Compared with WT, KO mice fed the 0.5% Ca diet had higher plasma parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], and lower plasma Ca and insulin-like growth factor-I (IGF-I). Duodenal Ca absorption (% Ca absorbed) in KO mice was reduced 71% relative to WT mice and was associated with 55% lower CaBP mRNA, 47% lower CaBP protein and 95% lower ECaC2 mRNA levels. Compared with WT mice, the percentage of Ca absorbed in KO mice fed the 0.5% Ca diet was inappropriately low for the level of duodenal CaBP. The 2% Ca rescue diet normalized plasma Ca, prevented osteomalacia, increased growth and plasma IGF-I levels, but did not normalize plasma PTH or 1,25(OH)(2)D(3) in KO mice. In addition, the relationship between CaBP protein and the percentage of Ca absorbed was normalized, whereas ECaC2 mRNA fell to near zero. Our data demonstrate that higher CaBP levels do not ensure high rates of duodenal Ca absorption and that transcellular Ca absorption can occur even when ECaC2 gene expression is very low. In addition, our data suggest that the 2% Ca diet promotes a vitamin D receptor-independent anabolic effect on bone formation and calcium absorption, leading to improved calcium balance even in the presence of high PTH levels.

Mechanisms of intestinal calcium absorption

Calcium is absorbed in the mammalian small intestine by two general mechanisms: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. The transcellular process involves three major steps: entry across the brush border, mediated by a molecular structure termed CaT1, intracellular diffusion, mediated largely by the cytosolic calcium-binding protein (calbindinD(9k) or CaBP); and extrusion, mediated largely by the CaATPase. Chyme travels down the intestinal lumen in approximately 3 h, spending only minutes in the duodenum, but over 2 h in the distal half of the small intestine. When calcium intake is low, transcellular calcium transport accounts for a substantial fraction of the absorbed calcium. When calcium intake is high, transcellular transport accounts for only a minor portion of the absorbed calcium, because of the short sojourn time and because CaT1 and CaBP, both rate-limiting, are downregulated when calcium intake is high. Biosynthesis of CaBP is fully and CaT1 function is approximately 90% vitamin D-dependent. At high calcium intakes CaT1 and CaBP are downregulated because 1,25(OH)(2)D(3), the active vitamin D metabolite, is downregulated.

Intestinal calcium absorption: Molecular vitamin D mediated mechanisms

Rickets and hyperparathyroidism caused by a defective Vitamin D receptor (VDR) can be prevented in humans and animals by high calcium intake, suggesting that intestinal calcium absorption is critical for 1,25(OH)(2) vitamin D [1,25-(OH)(2)D(3)] action on calcium homeostasis. We assessed the rate of serum (45)Ca accumulation within 10 min after oral gavage in two strains of VDR-knock out (KO) mice (Leuven and Tokyo KO) and observed a threefold lower area under the curve in both KO-strains. Moreover, we evaluated the expression of intestinal candidate genes, belonging to a new class of calcium channels (TRPV), involved in transcellular calcium transport. The calcium transport protein ECaC2 was more abundantly expressed at mRNA level than ECaC1 in duodenum, but both were considerably reduced (ECaC2 > 90%, ECaC1 > 60%) in the two VDR-KO strains on a normal calcium diet. Calbindin-D(9K) expression was only significantly decreased in the Tokyo KO, whereas PMCA(1b) expression was normal in both VDR-KOs. In Leuven wild type mice, a high calcium diet inhibited (> 90%), and 1,25(OH)(2)D(3) or low calcium diet induced (sixfold) duodenal ECaC2 expression and, to a lesser degree, ECaC1 and calbindin-D(9K) expression. In Leuven KO mice, however, high or low calcium intake decreased calbindin-D(9K) and PMCA(1b) expression, whereas both ECaC mRNA expressions remained consistently low on any diet. These results suggest that the expression of the novel duodenal epithelial calcium channels (in particular ECaC2 or TRPV6) is strongly vitamin D dependent and that calcium influx, probably interacting with calbindin-D(9K), should be considered as a rate-limiting step in the process of vitamin D dependent active calcium absorption.

Calcium-selective ion channel, CaT1, is apically localized in gastrointestinal tract epithelia and is aberrantly expressed in human malignancies

CaT1 is a highly selective calcium entry channel that has been proposed to be responsible for apical calcium entry in the vitamin D-regulated transcellular pathway of Ca(2+) absorption; however, the lack of a CaT1 antibody suitable for immunohistochemistry has prevented the direct testing of this hypothesis by the localization of CaT1 protein in the gastrointestinal tract and other tissues. In this study, we developed two CaT1 antibodies and have used them to establish for the first time that CaT1 localizes to the apical membrane of intestinal absorptive cells, thereby providing the first direct evidence that this protein is in fact an apical entry channel in the gastrointestinal tract. In addition, we found that CaT1 protein is highly expressed in a number of exocrine organs including pancreas, prostate, and mammary gland, suggesting an, as yet, unrecognized role in secretory epithelia. Finally, we found CaT1 protein to be present at elevated levels in comparison with normal tissues in a series of prostate, breast, thyroid, colon, and ovarian carcinomas, consistent with previous reports of up-regulation of CaT1 mRNA in prostate cancer tissues. Our findings indicate that CaT1 is likely to serve as a component of transcellular calcium transport mechanisms in many tissues and epithelial cancers.

A family of calcium-permeable channels in the kidney: distinct roles in renal calcium handling

PURPOSE OF REVIEW

Calcium is an essential intracellular messenger and a major component of the mineral phase of the skeleton. Calcium is absorbed in the intestine and reabsorbed in the kidney. The underlying transepithelial calcium transport mechanisms play crucial roles in calcium homeostasis. In this review, we present new developments in the area of calcium transport at the apical membrane, the first step in transepithelial calcium transport.

RECENT FINDINGS

Recently, a group of transient receptor potential (TRP)-related calcium-permeable channels has been identified. Several of these channels serve as important epithelial calcium entry mechanisms and possibly also as osmolarity sensors.

SUMMARY

Calcium channels in the kidney play important roles in maintaining total body calcium homeostasis. Their dysfunction may be associated with several human diseases such as hypercaliuric nephrolithiasis, certain forms of osteoporosis, Gitelman's disease and Bartter's syndrome.

Vectorial Transcellular Calcium Transport in Intestine: Integration of Current Models

In spite of decades of research, the exact subcellular pathway for calcium transport in intestine has not been elucidated. In this mini-review, we present three models for vectorial movement of calcium across the cell: facilitated (cytoplasmic) diffusion, vesicular/lysosomal transport, and tunneling through the endoplasmic reticulum compartment. We conclude by offering one way to integrate elements of these three models.