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 (P(i)) in goats fed different N and Ca diets. Intestinal flux rates of Ca and P(i) 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 P(i) 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 P(i) 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 P(i) were elevated. However, the reduced N and Ca diet had no effect on the uptake of Ca and P(i) 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 P(i) 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.

Effects of a reduced nitrogen diet on calcitriol levels and calcium metabolism in growing goats.

For monogastric animals, changes in dietary protein content modulate calcium (Ca) metabolism by changing parathyroid hormone and calcitriol concentrations. However, the effects of dietary nitrogen (N) restriction on Ca metabolism are not known in ruminants. Since ruminants express endogenous recycling mechanisms very efficiently to save N, it is known that these recycling mechanisms protect ruminants against N depletion in times of dietary N restriction. Therefore, consequences on Ca metabolism induced by reduction of dietary N supply as observed in monogastric animals should not occur in ruminants. Due to this specific metabolic feature, a reduction of dietary N intake can be used to diminish environmental N pollution. The aim of the present study was to determine the consequences of a reduced N intake on Ca homeostasis and respective regulatory hormone concentrations in ruminants. Growing goats fed with a reduced N diet showed a decrease in ionised calcium (Ca2+) and total Ca concentrations while bone resorption marker carboxyterminal cross-linked telopeptide of type I collagen increased in plasma. Unexpectedly, despite hypocalcemia, concentrations of calcitriol were decreased in the animals of the N reduction group whereas calcidiol levels were not affected. From this data, it can be concluded that the Ca metabolism of growing goats can be modulated by changes of dietary N content like in monogastric animals.

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.

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.