Mechanisms of phosphate uptake into brush-border membrane vesicles from goat jejunum

A first model of the fate of dietary calcium and phosphorus in broiler chickens

To reduce P excretion and increase the sustainability of poultry farms, one needs to understand the mechanisms surrounding P metabolism and its close link with Ca metabolism to precisely predict the fate of dietary P and Ca and related requirements for birds. This study describes and evaluates a model developed to estimate the fate of Ca and P consumed by broilers. The Ca and P model relies on three modules: (1) digestion of Ca and P; (2) dynamics of Ca and P in soft tissue and feathers; and (3) dynamics of body ash. Exogenous phytase affects the availability of Ca and P; thus, to predict the absorption of those minerals, the model also accounts for the effect of phytase on Ca and P digestibility. We used a database to estimate the consequences of dietary Ca, P, and phytase over feed intake response. This study followed a four-step process: (1) Ca and P model development and its coupling with a growth broiler model; (2) model behavior assessment; (3) sensitivity analysis to identify the most influential parameters; and (4) external evaluation based on three databases. The proportion of P in body protein and the Ca to P ratio in bone are the most sensitive parameters of P deposition in soft tissue and bone, representing 91 and 99% of the total variation. The external evaluation results indicated that body water and protein had an overall mean square prediction error (rMSPE) of 7.22 and 12.3%, respectively. The prediction of body ash, Ca, and P had an rMSPE of 7.74, 11.0, and 6.56%, respectively, mostly errors of disturbances (72.5, 51.6, and 90.7%, respectively). The rMSPE for P balance was 13.3, 18.4, and 22.8%, respectively, for P retention, excretion, and retention coefficient, with respective errors due to disturbances of 69.1, 99.9, and 51.3%. We demonstrated a mechanistic model approach to predict the dietary effects of Ca and P on broiler chicken responses with low error, including detailed simulations to show the confidence level expected from the model outputs. Overall, this model predicts broilers' response to dietary Ca and P. The model could aid calculations to minimize P excretion and reduce the impact of broiler production on the environment. A model inversion is ongoing that will enable the calculation of Ca and P dietary quantities for a specific objective. This will simplify the use of the model and the feed formulation process.

Modulation of Intestinal Phosphate Transport in Young Goats Fed a Low Phosphorus Diet

The intestinal absorption of phosphate (P_i) takes place transcellularly through the active NaPi-cotransporters type IIb (NaPiIIb) and III (PiT1 and PiT2) and paracellularly by diffusion through tight junction (TJ) proteins. The localisation along the intestines and the regulation of P_i absorption differ between species and are not fully understood. It is known that 1,25-dihydroxy-vitamin D₃ (1,25-(OH)₂D₃) and phosphorus (P) depletion modulate intestinal P_i absorption in vertebrates in different ways. In addition to the apical uptake into the enterocytes, there are uncertainties regarding the basolateral excretion of P_i. Functional ex vivo experiments in Ussing chambers and molecular studies of small intestinal epithelia were carried out on P-deficient goats in order to elucidate the transepithelial P_i route in the intestine as well as the underlying mechanisms of its regulation and the proteins, which may be involved. The dietary P reduction had no effect on the duodenal and ileal P_i transport rate in growing goats. The ileal PiT1 and PiT2 mRNA expressions increased significantly, while the ileal PiT1 protein expression, the mid jejunal claudin-2 mRNA expression and the serum 1,25-(OH)₂D₃ levels were significantly reduced. These results advance the state of knowledge concerning the complex mechanisms of the P_i homeostasis in vertebrates.

Review: Regulation of gastrointestinal and renal transport of calcium and phosphorus in ruminants

In comparison to monogastric animals, ruminants show some peculiarities in respect to the regulation of mineral homeostasis, which can be regarded as a concerted interplay between gastrointestinal absorption, renal excretion and bone mobilisation to maintain physiological Ca and phosphate (Pi) concentrations in serum. Intestinal absorption of Ca or Pi is mediated by two general mechanisms: paracellular, passive transport dominates when luminal Ca or Pi concentrations are high and transcellular. The contribution of active transport becomes more important when dietary Ca or Pi supply is restricted or the demand increased. Both pathways are modulated directly by dietary interventions, influenced by age and regulated by endocrine factors such as 1,25-dihydroxyvitamin D3. Similar transport processes are observed in the kidney. After filtration, Ca and Pi are resorbed along the nephron. However, as urinary Ca and Pi excretion is very low in ruminants, the regulation of these renal pathways differs from that described for monogastric species, too. Furthermore, salivary secretion, as part of endogenous Pi recycling, and bone mobilisation participate in the maintenance of Ca and Pi homeostasis in ruminants. Saliva contains large amounts of Pi for buffering rumen pH and to ensure optimal conditions for the rumen microbiome. The skeleton is a major reservoir of Ca and Pi to compensate for discrepancies between demand and uptake. But alterations of the regulation of mineral homeostasis induced by other dietary factors such as a low protein diet were observed in growing ruminants. In addition, metabolic changes, for example, at the onset of lactation have pronounced effects on gastrointestinal mineral transport processes in some ruminant species. As disturbances of mineral homeostasis do not only increase the risk of the animals to develop other diseases, but are also associated with protein and energy metabolism, further research is needed to improve our knowledge of its complex regulation.

Mechanisms and regulation of epithelial phosphate transport in ruminants: approaches in comparative physiology

Ruminants have a unique utilization of phosphate (P_i) based on the so-called endogenous P_i recycling to guarantee adequate P_i supply for ruminal microbial growth and for buffering short-chain fatty acids. Large amounts of P_i enter the gastrointestinal tract by salivary secretion. The high saliva P_i concentrations are generated by active secretion of P_i from blood into primary saliva via basolateral sodium (Na⁺)-dependent P_i transporter type II. The following subsequent intestinal absorption of P_i is mainly carried out in the jejunum by the apical located secondary active Na⁺-dependent P_i transporters NaPi IIb (SLC34A2) and PiT1 (SLC20A1). A reduction in dietary P_i intake stimulates the intestinal P_i absorption by increasing the expression of NaPi IIb despite unchanged plasma 1,25-dihydroxyvitamin D₃ concentrations, which modulate P_i homeostasis in monogastric species. Reabsorption of glomerular filtrated plasma P_i is mainly mediated by the P_i transporters NaPi IIa (SLC34A1) and NaPi IIc (SLC34A3) in proximal tubule apical cells. The expression of NaPi IIa and the corresponding renal Na⁺-dependent P_i capacity were modulated by high dietary phosphorus (P) intake in a parathyroid-dependent manner. In response to reduced dietary P_i intake, the expression of NaPi IIa was not adapted indicating that renal P_i reabsorption in ruminants runs at a high level allowing no further increase when P intake is diminished. In bones and in the mammary glands, Na⁺-dependent P_i transporters are able to contribute to maintaining P_i homeostasis. Overall, the regulation of P_i transporter activity and expression by hormonal modulators confirms substantial differences between ruminant and non-ruminant species.

In vivo and in vitro methods for evaluating soil arsenic bioavailability: relevant to human health risk assessment

Arsenic (As) is the most frequently occurring contaminant on the priority list of hazardous substances, which lists substances of greatest public health concern to people living at or near U.S. National Priorities List site. Accurate assessment of human health risks from exposure to As-contaminated soils depends on estimating its bioavailability, defined as the fraction of ingested As absorbed across the gastrointestinal barrier and available for systemic distribution and metabolism. Arsenic bioavailability varies among soils and is influenced by site-specific soil physical and chemical characteristics and internal biological factors. This review describes the state-of-the science that supports our understanding of oral bioavailability of soil As, the methods that are currently being explored for estimating soil As relative bioavailability (RBA), and future research areas that could improve our prediction of the oral RBA of soil As in humans. The following topics are addressed: (1) As soil geochemistry; (2) As toxicology; (3) in vivo models for estimating As RBA; (4) in vitro bioaccessibility methods; and (5) conclusions and research needs.

Segmental diversity of phosphate transport along the intestinal axis in horses

For horses, distinct differences in intestinal phosphate transport have been postulated to account for the unique features of hind gut fermentation compared to other monogastric animals and ruminants. So far published data on mechanisms and underlying transport proteins involved in intestinal phosphate transport in the horse are still missing. Therefore we investigated intestinal phosphate transport in horses at both functional and molecular levels. Segmental diversity of intestinal phosphate transport along the intestinal axis was documented using the Ussing chamber technique. A transcellular phosphate secretion in the jejunum was confirmed. Furthermore, 2 sodium-dependent phosphate cotransporters, NaPiIIb and PiT1, were first detected in the equine intestine at mRNA level with PiT1 being expressed in both the small and large intestine, and NaPiIIb being solely expressed in the large intestine. In the colon, unidirectional net flux rates of phosphate were significantly greater compared to flux rates in other segments ( < 0.005) suggesting the colon as a major site for phosphate absorption in horses. Phosphate transport in the colon was mainly transcellular and mediated by a sodium-gradient as documented by Ussing chamber experiments and uptake of phosphate into colonic brush border membrane vesicles. In summary, the present study demonstrated mechanisms and transporters of intestinal phosphate transport in equine intestinal tissues with distinct differences between intestinal segments providing a new basis for a better understanding of intestinal phosphate transport in horses.

Gastrointestinal transport of calcium and glucose in lactating ewes

During lactation, mineral and nutrient requirements increase dramatically, particularly those for Ca and glucose. In contrast to monogastric species, in ruminants, it is rather unclear to which extend this physiological change due to increased demand for milk production is accompanied by functional adaptations of the gastrointestinal tract (GIT). Therefore, we investigated potential modulations of Ca and glucose transport mechanisms in the GIT of lactating and dried‐off sheep. Ussing‐chamber technique was applied to determine the ruminal and jejunal Ca flux rates. In the jejunum, electrophysiological properties in response to glucose were recorded. Jejunal brush‐border membrane vesicles (BBMV) served to characterize glucose uptake via sodium‐linked glucose transporter 1 (SGLT1), and RNA and protein expression levels of Ca and glucose transporting systems were determined. Ruminal Ca flux rate data showed a trend for higher absorption in lactating sheep. In the jejunum, small Ca absorption could only be observed in lactating ewes. From the results, it may be assumed that lactating ewes compensate for the Ca loss by increasing bone mobilization rather than by increasing supply through absorption from the GIT. Presence of SGLT1 in the jejunum of both groups was shown by RNA and protein identification, but glucose uptake into BBMV could only be detected in lactating sheep. This, however, could not be attributed to electrogenic glucose absorption in lactating sheep under Ussing‐chamber conditions, providing evidence that changes in jejunal glucose uptake may include additional factors, that is, posttranslational modifications such as phosphorylation.

Modulation of intestinal calcium and phosphate transport in young goats fed a nitrogen- and/or calcium-reduced diet

Feeding ruminants a reduced N diet is a common approach to reduce N output based on rumino-hepatic circulation. However, a reduction in N intake caused massive changes in Ca and inorganic phosphate (Pi) homoeostasis in goats. Although a single dietary Ca reduction stimulated intestinal Ca absorption in a calcitriol-dependent manner, a concomitant reduction of Ca and N supply led to a decrease in calcitriol, and therefore a modulation of intestinal Ca and Pi absorption. The aim of this study was to examine the potential effects of dietary N or Ca reduction separately on intestinal Ca and Pi transport in young goats. Animals were allocated to a control, N-reduced, Ca-reduced or combined N- and Ca-reduced diet for about 6-8 weeks, whereby N content was reduced by 25 % compared with recommendations. In Ussing chamber experiments, intestinal Ca flux rates significantly decreased in goats fed a reduced N diet, whereas Pi flux rates were unaffected. In contrast, a dietary Ca reduction stimulated Ca flux rates and decreased Pi flux rates. The combined dietary N and Ca reduction withdrew the stimulating effect of dietary Ca reduction on Ca flux rates. The expression of Ca-transporting proteins decreased with a reduced N diet too, whereas Pi-transporting proteins were unaffected. In conclusion, a dietary N reduction decreased intestinal Ca transport by diminishing Ca-transporting proteins, which became clear during simultaneous N and Ca reduction. Therefore, N supply in young ruminant nutrition is of special concern for intestinal Ca transport.

Contrary seasonal changes of rates of nutrient uptake, organ mass, and voluntary food intake in red deer (Cervus elaphus)

Northern ungulates acclimatize to winter conditions with restricted food supply and unfavorable weather conditions by reducing energy expenditure and voluntary food intake. We investigated in a study on red deer whether rates of peptide and glucose transport in the small intestines are also reduced during winter as part of the thrifty phenotype of winter-acclimatized animals, or whether transport rates are increased during winter in order to exploit poor forage more efficiently. Our results support the latter hypothesis. We found in a feeding experiment that total energy intake was considerably lower during winter despite ad libitum feeding. Together with reduced food intake, mass of visceral organs was significantly lower and body fat reserves were used as metabolic fuel in addition to food. However, efficacy of nutrient absorption seemed to be increased simultaneously. Extraction of crude protein from forage was higher in winter animals, at any level of crude protein intake, as indicated by the lower concentration of crude protein in feces. In line with these in vivo results, Ussing chamber experiments revealed greater electrogenic responses to both peptides and glucose in the small intestines of winter-acclimatized animals, and peptide uptake into jejunal brush-border membrane vesicles was increased. We conclude that reduced appetite of red deer during winter avoids energy expenditure for unproductive search of scarcely available food and further renders the energetically costly maintenance of a large gut and visceral organs unnecessary. Nevertheless, extraction of nutrients from forage is more efficient in the winter to attenuate an inevitably negative energy balance.

Ontogenic Changes of Villus Growth, Lactase Activity, and Intestinal Glucose Transporters in Preterm and Term Born Calves with or without Prolonged Colostrum Feeding

Oral glucose supply is important for neonatal calves to stabilize postnatal plasma glucose concentration. The objective of this study was to investigate ontogenic development of small intestinal growth, lactase activity, and glucose transporter in calves (n = 7 per group) that were born either preterm (PT; delivered by section 9 d before term) or at term (T; spontaneous vaginal delivery) or spontaneously born and fed colostrum for 4 days (TC). Tissue samples from duodenum and proximal, mid, and distal jejunum were taken to measure villus size and crypt depth, protein concentration of mucosa and brush border membrane vesicles (BBMV), total DNA and RNA concentration of mucosa, mRNA expression and activity of lactase, and mRNA expression of sodium-dependent glucose co-transporter-1 (SGLT1) and facilitative glucose transporter 2 (GLUT2) in mucosal tissue. Additionally, protein expression of SGLT1 in BBMV and GLUT2 in crude mucosal membranes and immunochemical localization of GLUT2 in the enterocytes were determined. Villus height in distal jejunum was lower in TC than in T. Crypt depth in all segments was largest and the villus height/crypt depth ratio in jejunum was smallest in TC calves. Concentration of RNA was highest in duodenal mucosa of TC calves, but neither lactase mRNA and activity nor SGLT1 and GLUT2 mRNA and protein expression differed among groups. Localization of GLUT2 in the apical membrane was greater, whereas in the basolateral membrane was lower in TC than in T and PT calves. Our study indicates maturation processes after birth for mucosal growth and trafficking of GLUT2 from the basolateral to the apical membrane. Minor differences of mucosal growth, lactase activity, and intestinal glucose transporters were seen between PT and T calves, pointing at the importance of postnatal maturation and feeding for mucosal growth and GLUT2 trafficking.

Diet effects on glucose absorption in the small intestine of neonatal calves: importance of intestinal mucosal growth, lactase activity, and glucose transporters

Colostrum (C) feeding in neonatal calves improves glucose status and stimulates intestinal absorptive capacity, leading to greater glucose absorption when compared with milk-based formula feeding. In this study, diet effects on gut growth, lactase activity, and glucose transporters were investigated in several gut segments of the small intestine. Fourteen male German Holstein calves received either C of milkings 1, 3, and 5 (d 1, 2, and 3 in milk) or respective formulas (F) twice daily from d 1 to d 3 after birth. Nutrient content, and especially lactose content, of C and respective F were the same. On d 4, calves were fed C of milking 5 or respective F and calves were slaughtered 2h after feeding. Tissue samples from duodenum and proximal, mid-, and distal jejunum were taken to measure villus size and crypt depth, mucosa and brush border membrane vesicles (BBMV) were taken to determine protein content, and mRNA expression and activity of lactase and mRNA expression of sodium-dependent glucose co-transporter-1 (SGLT1) and facilitative glucose transporter (GLUT2) were determined from mucosal tissue. Additionally, protein expression of SGLT1 in BBMV and GLUT2 in crude mucosal membranes and BBMV were determined, as well as immunochemically localized GLUT2 in the intestinal mucosa. Villus circumference, area, and height were greater, whereas crypt depth was smaller in C than in F. Lactase activity tended to be greater in C than in F. Protein expression of SGLT1 was greater in F than in C. Parameters of villus size, lactase activity, SGLT1 protein expression, as well as apical and basolateral GLUT2 localization in the enterocytes differed among gut segments. In conclusion, C feeding, when compared with F feeding, enhances glucose absorption in neonatal calves primarily by stimulating mucosal growth and increasing absorptive capacity in the small intestine, but not by stimulating abundance of intestinal glucose transporters.

The influence of the mycotoxin deoxynivalenol on jejunal glucose transport in pigs

The experiments were aimed at evaluating whether the activity of the glucose transporter in the pig's jejunum is affected by deoxynivalenol (DON)-contaminated feed. This is based on the ability of DON to inhibit the protein synthesis on cellular level which might also involve the synthesis of the intestinal glucose transporter.Two groups of six castrated male pigs were restrictively fed a control or a DON contaminated diet (=5.7 mg/kg). Immediately after slaughtering the pigs, the mid-jejunum was quickly dissected, washed, frozen and stored at -80°C until preparations. Brush border membrane vesicles (BBMV) were prepared by Mg(2+)-EGTA-precipitation and differential centrifugation. The activity of the glucose transporter was examined by measuring the uptake of [(3)H]-glucose into the BBMV. The glucose uptake was quantified by using the rapid filtration technique.The glucose uptake into BBMV followed Michaelis-Menten-kinetic. Comparison of the kinetic parameters Vmax (transport capacity) and Km (transport affinity) of the two treatment-groups showed no significant differences between feeding a DON-contaminated or control diet. It was concluded that dietary DON exposure (=5.7 mg/kg) for five weeks had no obvious influence on the intestinal glucose transporter and its activity in pigs.

Modulation of intestinal glucose transport in response to reduced nitrogen supply in young goats

The reduction of dietary protein is a common approach in ruminants to decrease the excretion of N because ruminants are able to recycle N efficiently by the rumino-hepatic circulation. In nonruminant species an impact on other metabolic pathways such as glucose metabolism was observed when dietary protein intake was reduced. However, an impact of dietary N reduction in goats on glucose metabolism especially on intestinal glucose absorption is questionable because ruminants have very efficient endogenous recycling mechanisms. Therefore, the aim of the present study was to characterize the intestinal absorption of glucose in growing goats kept on different N supply under isoenergetic conditions. The different CP concentrations (20, 16, 10, 9, and 7% CP) of the experimental diets were adjusted by adding urea to the rations. Intestinal flux rates of glucose were determined by Ussing chamber experiments. For a more mechanistic approach, the Na(+)-dependent uptake of glucose into intestinal brush-border membrane vesicles (BBMV) and the expression patterns of the Na(+)-dependent glucose transporter SGLT1 and the glucose transporter 2 (GLUT2) were determined. Reduced N intake resulted in a decrease of plasma glucose (P < 0.001) and insulin (P = 0.004) concentrations whereas the intestinal flux rates of glucose were elevated (P < 0.001), which were inhibited by phlorizin. However, the uptake of glucose into intestinal BBMV was not changed whereas the expression of SGLT1 on mRNA (P < 0.05) and protein abundance (P = 0.03) was decreased in response to a reduced N intake. The mRNA expression of GLUT2 was not affected. From these data, it can be concluded that the intestinal absorption of glucose was modulated by changes in dietary N intake. It is suggested that intracellular metabolism or basolateral transport systems or both might be activated during this feeding regimen because the apical located SGLT1 might not be involved. Therefore, an impact of dietary N reduction on glucose metabolism in growing goats occurred as in monogastric animals.

Intestinal glucose absorption in calves as affected by different carbohydrate sources

From numerous recent studies, it has been demonstrated that the development of the forestomach system in ruminants and thus microbial carbohydrate fermentation do not exclude the potential of the small intestines for enzymatic carbohydrate digestion and subsequent monosaccharide absorption. However, the role of regulatory nutritional factors is still under discussion. Therefore, we investigated the kinetic parameters of intestinal Na(+) -dependent glucose absorption and SGLT1 expression using isolated brush border membrane vesicles (BBMV) from the jejunum of 10-week-old calves kept on either hay, concentrate or corn silage-based diets in addition to milk replacer. While the maximal transport capacity was significantly higher for concentrate and corn silage-fed animals, SGLT1 protein expression was highest in BBMV isolated from hay-fed animals. This observation differs from the prevalent conception that induction of Na(+) -dependent glucose uptake via SGLT1 is based on an increased number of transporters at the brush border membrane.

Ex vivo intestinal studies on calcium and phosphate transport in growing goats fed a reduced nitrogen diet

In ruminant feeding, the reduction of dietary protein is an effective approach for decreasing the excretion of N. In non-ruminant species, the intestinal absorption of Ca was affected when dietary protein was reduced. Therefore, it was the aim of the present study to characterise the intestinal absorption of Ca and inorganic phosphate (Pi) in goats fed different N and Ca diets. Intestinal flux rates of Ca and Pi were determined in goats fed a reduced N and Ca diet by Ussing chamber experiments. For a more mechanistic approach, the uptake of Ca and Pi in intestinal brush-border membrane vesicles (BBMV), the expression levels of the epithelial Ca channel transient receptor potential vanilloid channel type 6 (TRPV6), the sodium-dependent Pi transporter (NaPi) IIb and the vitamin D receptor (VDR) were measured. In goats fed a reduced N and Ca diet, the intestinal flux rates of Ca and Pi were elevated. However, the reduced N and Ca diet had no effect on the uptake of Ca and Pi in intestinal BBMV, while the expression of TRPV6 and NaPi IIb protein in the corresponding intestinal segments was even decreased. The mRNA expression of NaPi IIb and VDR was not affected. Therefore, a post-transcriptional regulation of TRPV6 and NaPi IIb protein was suggested in goats fed a reduced N and Ca diet. From these data, it can be concluded that the intestinal absorption of Ca and Pi in growing goats was affected by changes in dietary N and Ca intake like those in single-stomached animals but differently modulated.

Modulation of electrolyte homeostasis by dietary nitrogen intake in growing goats

In goats, the combination of dietary N and Ca reduction caused hypocalcaemia and further changes in Ca homeostasis. The aim of the present study was to characterise the effects of dietary N reduction under normocalcaemia on mineral and bone metabolism in young goats. Young male goats of the Saanen breed were fed a diet reduced in N (8 %) for about 7 weeks (ten animals per group) and were compared with goats fed with an adequate N (14 %) diet. When N intake was reduced in young goats, plasma urea concentrations as well as renal elimination of urea were reduced. This was inversely related to creatinine in plasma and urine, which increased during a dietary N reduction as a function of reduced renal activity to save urea during N scarcity. During this decrease in renal function, associated with declined insulin-like growth factor 1 concentrations, a reduction in calcidiol and calcitriol concentrations could be observed. Meanwhile, carboxyterminal cross-linked telopeptide of type I collagen values and activity of total alkaline phosphatase were both elevated, indicating some bone remodelling processes taking place during a reduced N diet in young goats. The concentrations of inorganic phosphate (Pi) and total Ca were changed in several body fluids, indicating that Pi and Ca homeostasis was perturbed in goats fed a reduced N diet. Therefore, more research is needed to find the balance between reduction of environmental N pollution by reducing dietary N in ruminant feeding and maintaining the animal's health.

Hormonal regulation of phosphate homeostasis in goats during transition to rumination

Regulatory processes in phosphorus (P) homeostasis in small ruminants are quite different compared to monogastric animals. Adaptive responses of modulating hormones [parathyroid hormone (PTH) and calcitriol] to feeding variable amounts of P are lacking. Therefore, the aim of this study was to examine the influence of high dietary P intake (control diet: 4 g kg(-1) dry matter; high-P diet: 8 g kg(-1) dry matter) on the expression levels of PTH receptor (PTHR), vitamin D receptor (VDR) and Na+-dependent Pi transporters (NaPi II) in kidney and jejunum of goats starting rumination. After 3 months of feeding, plasma phosphate (Pi) and PTH concentrations were increased in the high-P diet group, whereas calcium and calcitriol were not changed. The intestinal Na+-dependent Pi transport capacity was not influenced by a high-P diet and the expression of jejunal VDR, PTHR and NaPi IIb was not modified. Interestingly, renal Na+-dependent Pi transport capacity was significantly reduced and concomitantly the expression of PTHR and NaPi IIa was decreased. In conclusion, the adaptive response of renal Pi reabsorption in goats, which were in transition from non-ruminant to ruminant stage was comparable to that of monogastric animals. In contrast, the modulation of the intestinal Pi absorption was like in adult ruminants.

Sodium-dependent phosphate transport across the apical membrane of alveolar epithelium in caprine mammary gland

NaPi IIb cotransporter is expressed in various tissues including mammary glands of mice. The physiological role of NaPi IIb in lactating mammary glands is still unclear. Therefore, it was the aim of the study to detect and to localize NaPi IIb protein in lactating goat mammary glands by Western analysis and immunohistochemistry. Furthermore, Na(+)-dependent P(i) uptake into apical membrane vesicles isolated from goat milk was determined using rapid filtration technique. NaPi IIb protein could specifically be detected in the apical membranes of lactating alveolar epithelial cells. Na(+)-dependent P(i) uptake into apical membrane vesicles could be measured, which was inhibited by phosphonoformic acid. The kinetic parameters were V(max) with 0.9 nmol/mg protein/10 s and K(m) with 0.22 mmol/L for P(i) affinity, K(m) value for Na(+) affinity 11 mmol/L. Stoichiometry of this mammary gland Na(+)/P(i) transport across the apical membranes seemed to be 1:1 P(i):Na(+) without cooperativity in P(i) and Na(+) binding as assessed by Scatchard and Hill plots. These features of Na(+)/P(i) transport suggest that it could be mediated by NaPi IIb. The quantitative role of this P(i) transport which is directed from the alveolar lumen into the epithelial cell of goat mammary gland will be the topic of further investigations.

Adaptation of epithelial sodium-dependent phosphate transport in jejunum and kidney of hens to variations in dietary phosphorus intake

The objective of this study was to explore the homeostatic response of jejunal and renal epithelia regarding the inorganic phosphate (P(i)) transport capacities to variations in dietary total phosphorus (tP) supply in hens. Adaptive processes were determined by quantitative measures of intake and excretion, P(i) transport studies across brush border membranes, and semiquantitative detection of sodium-dependent phosphate transporters (NaPi II) based on mRNA expression in the jejunum and kidney. Twelve hens (4/group) were adapted to 3 tP feeding levels in a pair-fed manner (60 g/d): low P diet with 0.073% tP, medium P diet with 0.204% tP, and high P diet with 0.343% tP. Excretion was measured during the last 5 d of a 16-d feeding period. After slaughtering, jejunal mucosa and renal cortex were removed. Tissues were used for (32)P uptake studies in brush-border membrane vesicles by rapid filtration technique and NaPi II mRNA expression studies by northern analyses. Plasma P(i) concentrations were additionally measured. The NaPi II transporter mRNA could specifically be detected in chicken jejunum and kidney. Functional parameters of Na(+)-dependent P(i) transport indicated that these transporters were involved in chicken P(i) transport across the apical membranes of jejunal and renal epithelia. Increased tP intake resulted in an increased overall tP excretion. Correlating individual data from all animals by linear regression highlighted that the adaptive decrease of renal P(i) transport capacity and NaPi IIa mRNA expression was associated with an increase in plasma P(i) levels and resulted in a higher tP excretion. Jejunal P(i) transport capacity and NaPi IIb mRNA expression did not react to variations in dietary tP supply. In conclusion, the homeostatic response was mainly based on the adaptive capacity of the kidney in hens.

Influence of dietary calcium and phosphorus supply on epithelial phosphate transport in preruminant goats

P homeostasis affected by high or low Ca and/or P supply in preruminant goats was characterized by balance studies in vivo. The main excretion pathway was the renal P(i) excretion whose extent was modulated by variations in dietary P and/or Ca supply. Faecal P excretion remained low irrespective of dietary regimen. The balance data were combined with respective in vitro data on P(i) transport properties and their adaptation in response to changes in dietary Ca and/or P intake. Therefore, P(i) transport capacities were determined by P(i) uptake into brush border membrane vesicles of jejunum and kidney. Epithelial P(i) transporters were determined semiquantitatively by northern and western blot analyses in jejunum, kidney and salivary gland. Renal P(i) transport was downregulated by doubling dietary P supply while doubling both, Ca and P as well as restrictive Ca at unchanged P led to slight, but not significant reductions in renal P(i) transport. Jejunal P(i) transport was reduced by P excess (doubling P and doubling both, Ca and P), but only NaPi IIb protein expression was significantly diminished. In conclusion, the significance of epithelial adaptation to dietary Ca and P supply for P homeostasis is discussed in preruminant goats.

The effects of dietary phosphorus deficiency on surface pH and membrane composition of the mucosa epithelium in caprine jejunum

In ruminants, the uptake of inorganic phosphate (P(i)) across the intestinal mucosa epithelium by Na-dependent and Na-independent mechanisms is a main regulatory factor in P homeostasis. The aim of the study was to elucidate to which extent Na-independent mechanisms, including pH effects or composition of mucosal brush-border membranes, could be involved in positive stimulation of P(i) absorptive processes seen under the P deficient condition. Therefore, luminal, surface and intracellular pH of the jejunal epithelial cells in control and P depleted goats were compared and biochemical analyses of membrane phospholipids in the apical membrane of the jejunal epithelium were performed. Dietary P depletion resulted in decreased plasma P(i) levels. While pH in jejunal ingesta was not significantly changed, P depletion resulted in a significantly lower surface pH in the crypt region compared to control animals (7.62 +/- 0.02 vs. 7.77 +/- 0.04, n = 4, P < 0.01). Inhibition of apical Na(+)/H(+)-exchange resulted in an increase of the jejunal surface pH in P depleted animals by 0.07 +/- 0.01 (n = 6, P < 0.01) and 0.05 +/- 0.01 (n = 6, P < 0.01) for the villus and the crypt region, respectively. This increase were inversely correlated with the initial surface pH prior to inhibition. In contrast to surface pH, intracellular pH of the jejunal epithelium and the phospholipid composition of the apical jejunal membrane were not affected by P depletion. Although the data suggest the existence of a Na(+)/H(+)-exchange mechanism at the luminal surface of goat jejunum they do not support the hypothesis that adaptational processes of active P(i) absorption from goat jejunum in response to low dietary P could be based on "non P(i) transporter events".

Contributions of different NaPi cotransporter isoforms to dietary regulation of P transport in the pyloric caeca and intestine of rainbow trout

The anatomical proximity and embryological relationship of the pyloric caeca (PC) and small intestine of rainbow trout has led to the frequent assumption, on little evidence, that they have the same enzymes and transporters. In trout, the PC is an important absorptive organ for dietary nutrients, but its role in dietary P absorption has not been reported. We found that apical inorganic phosphate (Pi) transport in PC comprises carrier-mediated and diffusive components. Carrier-mediated uptake was energy- and temperature-dependent, competitively inhibited and Na(+)-independent, and greater than the Na(+)-dependent intestinal uptake. Pi uptake in PC was pH-sensitive in the presence of Na(+). Despite the active Pi transport system in PC, high postprandial luminal Pi concentrations ( approximately 20 mmol l(-1)) indicate that diffusive uptake represents approximately 92% of total Pi uptake in PC of fed fish. The nucleotide sequence of a sodium-phosphate cotransporter (NaPi-II) isoform isolated from PC was approximately 8% different from the intestinal NaPi cotransporter. PC-NaPi mRNA was abundant in PC but rare in the intestine, whereas intestinal NaPi mRNA was abundant in the intestine but scarce in PC. Dietary P restriction reduced serum and bone P concentrations, increased intestine-type, but not PC-type, NaPi mRNA in PC, and increased Pi uptake in intestine but not in PC. Intestine-type NaPi expression may be useful for predicting dietary P deficiency.

Ontogenesis of epithelial phosphate transport systems in goats

The rapid development of precocial goats in the first weeks after birth requires an adequate adaptation of phosphate transport systems to maintain the P homeostasis at each developmental stage. Here we examined the age-related development of Na+-Pi transport systems in small intestines, kidneys, and parotid glands of goats. Kinetic parameters were determined by brush-border membrane vesicle uptake studies, and relative expression of NaPi type II mRNA and protein was recorded by molecular biological methods. High intestinal Pi transport capacity was already present on the first day of life. Within the first 3 wk of life there seemed to be a change in the type of Na+-dependent Pi transporter, and NaPi IIb was expressed increasingly up to the fifth month of life. Renal Na+-Pi transport capacity was also high at birth, and this was associated with high expression levels of NaPi IIa mRNA, indicating the important role of this transporter for renal Pi reabsorption. At weaning an increase in both intestinal and renal Na+-Pi transport balanced the increasing requirements for Pi to establish the endogenous Pi cycle. Salivary Pi concentration and parotid NaPi II mRNA rose markedly to guarantee an adequate Pi supply for rumen microbes. We concluded that the high demand for Pi in young goats was assured by high basal Na+-Pi transport capacity of small intestines and kidney expressed continuously during ontogenesis.

Functional characterization of the human intestinal NaPi-IIb cotransporter in hamster fibroblasts and Xenopus oocytes

The recently cloned NaPi-IIb cotransporter is an apical membrane protein that is involved in the absorption of phosphate in the intestine. To expedite functional and structural studies, the human intestinal NaPi-IIb cotransporter was stably expressed in hamster fibroblast (PS120) cells. The hNaPi-IIb cDNA stably transfected cells exhibited a 1.8-fold higher sodium-dependent phosphate uptake than vector DNA transfected cells, and had a K(m) for Pi of approximately 106 microM and a K(m) for Na(+) of approximately 34 mM. The hNaPi-IIb cotransporter was also expressed in Xenopus oocytes and it exhibited a K(m) for Pi of approximately 113 microM and a K(m) for Na(+) of approximately 65 mM. The hNaPi-IIb cotransporter expressed in both PS120 cells and oocytes was inhibited by high external pH. Furthermore, the phosphate uptake mediated by the hNaPi-IIb cotransporter was inhibited by 5 mM phosphonoformic acid (PFA), 1 mM arsenate and 100 nM phorbol myristate acetate (PMA). These results demonstrate that the human intestinal NaPi-IIb cotransporter is functional when expressed in hamster fibroblasts, and that this model system may be useful in the future to identify NaPi-IIb cotransporter-specific inhibitors.

Phosphate transport in the duodenum and jejunum of goats and its adaptation by dietary phosphate and calcium

Endogenous P(i) recycling is a characteristic feature of the P homeostasis in ruminants. A pronounced salivary P(i) secretion into the rumen is balanced by a high intestinal P(i) absorption and an almost complete renal P(i) reabsorption. In monogastric animals, the major P(i) transport mechanism across the apical membrane of the enterocyte is an Na(+)-dependent transport mediated by NaPi cotransporter type IIb. In ruminants, an Na(+)-, as well as an H(+)-dependent, P(i) transport system seems to exist in the small intestines. Therefore, morphological localization, type of ionic dependence, and ability to adapt to dietary P or Ca restriction of duodenal and jejunal P(i) transport were characterized in goats. In the duodenum, there was an H(+)-dependent, Na(+)-sensitive P(i) transport system that did not belong to the NaPi type II family and was not influenced by dietary P or Ca restriction. In contrast, in the jejunum, there was an Na(+)-dependent, H(+)-sensitive P(i) transport mainly mediated by NaPi IIb. P restriction stimulated the NaPi IIb protein expression, resulting in higher P(i) transport capacity.

Age-dependent regulation of rat intestinal type IIb sodium-phosphate cotransporter by 1,25-(OH)(2) vitamin D(3)

The current studies were designed to characterize type IIb sodium-inorganic phosphate (P(i)) cotransporter (NaP(i)-IIb) expression and to assess the effect of 1,25-(OH)(2) vitamin D(3) on NaP(i)-IIb gene expression during rat ontogeny. Sodium-dependent P(i) absorption by intestinal brush-border membrane vesicles (BBMVs) decreased with age, and NaP(i)-IIb gene expression also decreased proportionally with age. 1,25-(OH)(2) vitamin D(3) treatment increased intestinal BBMV P(i) absorption by approximately 2.5-fold in suckling rats and by approximately 2.1-fold in adult rats. 1,25-(OH)(2) vitamin D(3) treatment also increased NaP(i)-IIb mRNA abundance by approximately 2-fold in 14-day-old rats but had no effect on mRNA expression in adults. Furthermore, in rat intestinal epithelial (RIE) cells, 1,25-(OH)(2) vitamin D(3) increased NaP(i)-IIb mRNA abundance, an effect that was abolished by actinomycin D. Additionally, human NaP(i)-IIb gene promoter activity in transiently transfected RIE cells showed approximately 1.6-fold increase after 1,25-(OH)(2) vitamin D(3) treatment. In conclusion, we demonstrate that the age-related decrease in intestinal sodium-dependent P(i) absorption correlates with decreased NaP(i)-IIb mRNA expression. Our data also suggest that the effect of 1,25-(OH)(2) vitamin D(3) on NaP(i)-IIb expression is at least partially mediated by gene transcription in suckling rats.

The absorption of phosphate ions from the ovine reticulorumen

The reticulorumen is now recognised to be an important site of net absorption of phosphate ions from ruminal fluid containing phosphate concentrations appropriate to those found in normal farming practice. These rates of absorption were measured in vivo from solutions placed in the washed reticulorumen, isolated in situ, in conscious, trained sheep. Reducing the ruminal sodium concentration led to reduced absorption of phosphate, suggestive that phosphate and sodium fluxes across the apical wall of the ruminal epithelial cell are linked, as they are in the kidney. Increased absorption of short chain fatty acids led to enhanced absorption of phosphate ions. Conversely, inhibition of carbonic anhydrase activity, by the addition of 1 mM acetazolamide to the ruminal fluid, led to a reduction in phosphate absorption. An increase in the acidity of the ruminal fluid also increased the absorption of phosphate, as did an increase in the ruminal Ca(2+) concentration over the range 1-4 mmol per litre. It is suggested that these effects can be accounted for by a Na(+)/H(+) antiporter coupled with a phosphate/proton symporter in the apical membrane of the ruminal epithelial cell.

Functional expression and characterization of the wild-type mammalian renal cortex sodium/phosphate cotransporter and an 215R mutant in Saccharomyces cerevisiae

The wild-type and an R215E mutant of the rat renal cortex sodium/phosphate cotransporter type 2 (NaPi-2) were functionally expressed in the yeast Saccharomyces cerevisiae strain MB192, a cell line lacking the high-affinity endogenous H+/P(i) cotransporter. The expression of the mRNA molecules and corresponding proteins was confirmed by Northern and Western blot analysis, respectively. As detected by indirect immunofluorescence and antibody capture assay, both wild-type and mutant NaPi-2 proteins are expressed in the yeast plasma membrane in comparable amounts. In the presence of 5 microM phosphate, Na+ promotes phosphate uptake into yeast cells expressing the wild-type NaPi-2 with a K(0.5) of 5.6 +/- 1.1 mM. The maximum uptake of phosphate (649 +/- 30 pmol/10 min) is approximately 8-fold higher than the uptake obtained with nontransformed cells (76.8 +/- 8 pmol/10 min). Yeast cells expressing the R215E mutant of NaPi-2 accumulate 213 +/- 9 pmol of phosphate/10 min under the same conditions. The K(0.5) for the stimulation of phosphate uptake by Na+ is 4.2 +/- 0.8 mM for the R215E mutant and thus not significantly different from the value obtained with cells expressing the wild-type cotransporter. The reduced level of accumulation of phosphate in yeast cells expressing the R215E mutant is probably due to a reduction of the first-order rate constant k for phosphate uptake: while cells expressing wild-type NaPi-2 accumulate phosphate with a k of 0.06 min(-1), the rate for phosphate uptake into cells expressing the R215E mutant (k) is 0.016 min(-1) and therefore about 4-fold lower. In comparison, the rate for phosphate uptake into nontransformed cells (k) is 0.0075 min(-1). Phosphate uptake into yeast cells that express the wild-type NaPi-2 in the presence of 150 mM NaCl is promoted by extracellular phosphate with a K(0.5) of 45 +/- 4 microM. A phosphate-dependent phosphate accumulation is also observed with cells expressing the R215E mutant, but the K(0.5) is twice as high (86 +/- 5 microM) as that obtained with the wild-type cotransporter. We conclude that the yeast expression system is a useful tool for the investigation of structure-function relationships of the renal sodium/phosphate cotransporter and that (215)R, although not involved in Na+ recognition, is a part of the structure involved in phosphate recognition and considerably influences the rate of phosphate uptake by the NaPi-2 cotransporter.