1,25-Dihydroxycholecalciferol regulates salivary phosphate secretion in cattle

Dietary phosphorus restriction affects bone metabolism, vitamin D metabolism and rumen fermentation traits in sheep

Homeostasis of calcium (Ca) and phosphate (P_i ) is maintained by a concerted interplay of absorption and reabsorption via the gastrointestinal tract and the kidney and by storage and mobilization from the bone regulated mainly by parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol and calcitonin. The present study aimed at characterizing the effects of dietary P restriction on bone, vitamin D metabolism and rumen fermentation traits reflecting the endogenous P cycle maintaining the ruminal P supply for microbial metabolism. The experiments were done in eleven female, non-pregnant, non-lactating four- to nine-year-old Black Headed Mutton sheep allotted to two feeding groups: "P-restricted" (0.11% P/kg DM and 0.88% Ca/kg DM) and "Control" (0.38% P/kg DM and 0.88% Ca/kg DM). Dietary P restriction did not lead to hypophosphataemia, probably due to a compensation by bone mobilization, demonstrated by increased serum concentrations of a resorption marker and altered gene expression in bone tissue. In addition, the RNA expression of fibroblast growth factor 23, a bone-derived factor involved in the regulation of vitamin D metabolism, was significantly reduced with dietary P restriction. Furthermore, several genes related to vitamin D metabolism and plasma concentrations of 1,25-(OH)₂ D were associated with serum concentrations of phosphate (P_i ). In the parotid gland, the expression of the P_i transporter NaPi2b was negatively associated with serum P_i and positively with parathyroid PTH expression. Although P_i concentrations in saliva and the gastrointestinal tract were significantly reduced, we found no adverse effects of the P-restricted ration on the production of short chain fatty acids, but slight differences in the production of butyrate as well as its relationship to rumen P_i and ammonia concentrations that might indicate an impact on ruminal fermentation.

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.

The kinetics of inorganic phosphate excretion in the acidotic rabbit during intravenous phosphate loading: a pseudo-ruminant model

The rabbit is a much-used experimental animal in renal tubule physiology studies. Although a monogastric mammal, the rabbit is a known hindgut fermenter. That ruminant species excrete inorganic phosphate (Pi) mainly through the digestive system while non-ruminants eliminate surplus phosphate primarily through the renal system are acknowledged facts. To understand phosphate homeostasis in the acidotic rabbit, anaesthetized animals were infused with hydrochloric acid, after which they underwent intravenous phosphate loading. Biofluids were collected during the infusion process for analysis. Plasma Pi increased (7.9 ± 1.7 mmoles.Litre⁻¹ (N = 5) vs 2.2 ± 0.4 mmoles.Litre⁻¹ (N = 10) pre-infusion, (p < 0.001)), while urinary phosphate excretion was also enhanced (74.4 ± 15.3 from a control value of 4.7 ± 3 µmol.min⁻¹ (N = 9), pre-infusion, p < 0.001)) over an 82.5 minute Pi loading period. However, the fractional excretion of Pi (FePi) only increased from 14.2 ± 5.4% to a maximum of 61.7 ± 19% (N = 5) over the infusion period. Furthermore, the renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmPi/GFR) computed to 3.5 mmol.L⁻¹, while a reading of 23.2 µmol.min⁻¹.Kg.^0.75 was obtained for the transport maximum for Pi (TmPi). The high reabsorptivity of the rabbit nephrons coupled with possibly a high secretory capacity of the salivary glands for Pi, may constitute a unique physiological mechanism that ensures the rabbit hindgut receives adequate phosphate to regulate caecal pH in favour of the resident metabolically - active microbiota. The handling of Pi by the rabbit is in keeping with the description of this animal as a monogastric, pseudo-ruminant herbivore.

Plasma 1,25-dihydroxyvitamin D, insulin-like growth factor-1, calcium, magnesium and phosphorus concentrations in pregnant beef cows and calves from a herd with a known history of congenital joint laxity and dwarfism

An investigation was undertaken to ascertain the possibility of a relationship between calcium, inorganic phosphorus, magnesium, 1,25-dihydroxyvitamin D [1,25(OH)2D], and insulin-like growth factor-1 (IGF-1) concentrations in blood plasma and occurrence of congenital joint laxity and dwarfism (CJLD) in young cattle. Pregnant cows were fed hay (30 cows) or grass silage (122 cows) during winter months (October 15 to calving in March). Blood samples were taken from cows on seven occasions during the experiment and 48 hours after calving, and from calves at birth, and at seven, 14 and 56 days old. Five per cent of calves born (six of 122) to cows fed grass silage and none born to cows fed hay were affected by CJLD. The diet and health status of calves were not significantly (P greater than 0.05) associated with the plasma concentration of 1,25(OH)2D. The plasma calcium concentration declined with age of the calves (P less than 0.05) but was not affected by the occurrence of CJLD. Plasma phosphorus and magnesium concentrations in calves born to cows fed silage were higher (P less than 0.05) than in those born to cows fed hay. At birth and seven days old, plasma phosphorus concentrations were higher (P less than 0.05) in CJLD-affected calves than in healthy calves but the plasma concentration of IGF-1 was not different (P greater than 0.05). It was concluded that the high plasma phosphorus concentrations in CJLD-affected calves and their dams could be related to the aetiology of the CJLD condition in calves.

Effect of phosphate-regulating hormones on plasma composition, cardiovascular function, and parotid salivary phosphate secretion in red kangaroos (Macropus rufus)

The effects of administration of phosphate-regulating hormones on plasma composition, cardiovascular function, and secretion of phosphate and other electrolytes in parotid saliva were investigated in anesthetized red kangaroos. Plasma [PO4] was elevated by intravenous injections of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) at 5 or 12.5 nmol/12 hr for 72 hr but was unaltered by intravenous or intracarotid infusion of either salmon or porcine calcitonins at rates up to 3.2 IU min-1 for 60 min or by intracarotid infusions of the 1-34 amino acid fragments of rat, human, or bovine parathyroid hormones (PTH(1-34) at 350-460 pmol/min for 60 min. Plasma [Ca] fell during high-rate calcitonin infusion and rose during 1,25(OH)2D3 administration. PTH(1-34) infusion did not alter plasma [Ca] but did lower plasma [K] and arterial blood pressure and elevated heart rate and hematocrit. Salivary [PO4] and [Ca] and secretion rates were unaffected by the calcitonin infusions, by PTH(1-34) infusions, or by 1,25(OH)2D3 injection. Plasma and salivary concentrations of other ions were unaltered. These data provide evidence that kangaroo tissue can recognize and respond to all three types of phosphate-regulating hormones despite the peptides being foreign; however, the parotid gland of kangaroos, unlike the parotids of rats and sheep, did not respond and presumably lacks some component of the receptor-secretion couplings for these hormones. This independence of salivary PO4 secretion from hormonal regulation may be one of several adaptations which ensure relatively stable and adequate phosphate delivery to the foregut microorganisms despite an unreliable phosphorus intake in the natural diet.

Plasma osteocalcin concentrations in cattle under various pathophysiological conditions

Plasma osteocalcin (BGP) concentrations were measured using an homologous radioimmunoassay in plasma samples (n = 6-14 per group) from fetal and newborn calves, 16-month-old heifers and bulls, and pregnant lactating cows. The highest values (nM) (62 +/- 3) were measured in fetal calves and the lowest (15 +/- 3) in 9-year-old pregnant and lactating cows. No significant relationship could be demonstrated between plasma BGP and somatomedin C, or 1,25(OH)2D or calcium concentrations. In eight normally calving cows, parturition was followed by a progressive increase in plasma BGP concentration, maximum 3 days after calving. It returned to prepartum values 10 days later. In six parturient hypocalcaemic and paretic cows, hypocalcaemia occurring within 12 h following calving was associated with a prompt and very transient increase in plasma BGP concentrations. These results indicate that osteocalcin might play a role in the regulation of bone metabolism in cattle.