Regulation of calciotropic hormones in vivo in the New Zealand white rabbit

Rabbits (Oryctolagus cuniculus) increase caecal calcium absorption at increasing dietary calcium levels

Hindgut fermenting herbivores from different vertebrate taxa, including tortoises, and among mammals some afrotheria, perissodactyla incl. equids, several rodents as well as lagomorphs absorb more calcium (Ca) from the digesta than they require, and excrete the surplus via urine. Both proximate and ultimate causes are elusive. It was suggested that this mechanism might ensure phosphorus availability for the hindgut microbiome by removing potentially complex-building Ca from the digesta. Here we use Ussing chamber experiments to show that rabbits (Oryctolagus cuniculus) maintained on four different diets (six animals/diet) increase active Ca absorption at increasing Ca levels. This contradicts the common assumption that at higher dietary levels, where passive uptake should be more prevalent, active transport can relax and hence supports the deliberate removal hypothesis. In the rabbits, this absorption was distinctively higher in the caecum than in the duodenum, which is unexpected in mammals. Additional quantification of the presence of two proteins involved in active Ca absorption (calbindin-D9K CB; vitamin D receptor, VDR) showed higher presence with higher dietary Ca. However, their detailed distribution across the intestinal tract and the diet groups suggests that other factors not investigated in this study must play major roles in Ca absorption in rabbits. Investigating strategies of herbivores to mitigate potential negative effects of Ca in the digesta on microbial activity and growth might represent a promising area of future research.

The impact of humic acid additives on mineral metabolism of rabbits in the postnatal period of ontogenesis

Humic acids are characterized by chelating properties due to which they are able to influence the mineral metabolism in animals. Rabbits have species-specific characteristics of mineral metabolism, which distinguishes them from most species of mammals. For the experiment, 16 rabbits of 45-day-old of Hyplus breed were divided into the experimental and control group of 8 rabbits in each. The animals were 15 days in the preparatory period. The rabbits of the experimental group received humic acid additive individually with water in the amount of 5 mg/kg, the rabbits of the control group received pure water in the same amount. The ability of humic acid additive to increase the content of calcium, ionized calcium, iron, to correct the content of copper and zinc, and also to increase the activity of alkaline phosphatase in the blood serum of rabbits of the experimental group has been proved. We observed the early effect of the humic acid additive on increasing the content of iron and calcium ionized and a later effect on the activity of alkaline phosphatase and an increase in the content of copper in the blood serum. The influence of humic acid additive on the distribution of calcium, phosphorus, manganese, copper and zinc in bone tissue is determined. According to a histological study, there was a significant increase in the number of layers of osteons and osteoblasts in the bone tissue of the femur, an increase in the number of osteons and osteoblasts in the bone tissue of the sternum and an increase in the number of columns of chondrocytes and an increase in the number of chondrocytes in the column in the cartilaginous tissue of the sternum. The results of a histological study of bone tissue, together with an increase in structural macroelements in it and redistribution of osteotropic trace elements in the blood and bone tissue suggest the possibility of using a humic acid additive to intensify the growth and mineralization of bone tissue, which can improve the state of bone tissue of rabbits in the postnatal period ontogenesis.

Diet is a main source of vitamin D in Finnish pet rabbits (Oryctolagus cuniculus)

During the winter time in Finland, sunlight is inadequate for vitamin D synthesis. Many pet rabbits live as house rabbits with limited outdoor access even during summer and may therefore be dependent on dietary sources of vitamin D. The aims of this study were to report the serum 25-hydroxyvitamin D concentrations in Finnish pet rabbits and to identify factors that influence vitamin D status. Serum 25-hydroxyvitamin D concentrations from 140 pet rabbits were determined using a vitamin D enzyme immunoassay (EIA) kit. Eleven rabbits were excluded from the statistical analysis because of unclear dietary data. The remaining 129 rabbits were divided into groups depending on outdoor access during summer (no access n = 26, periodic n = 57, regular n = 46) as well as daily diet: little or no hay and commercial rabbit food ≤1/2 dl (n = 12); a lot of hay and no commercial food daily (n = 23); a lot of hay and commercial food <1 dl (n = 59); a lot of hay and commercial food ≥1 dl (n = 35). The range of serum 25-hydroxyvitamin D concentration was from 4.5 to 67.5 ng/ml with a mean of 26.1 ng/ml. Statistical general linear model adjusted for weight, age and season indicated that diet was associated with vitamin D concentrations (p = 0.001), but outdoor access during summer was not (p = 0.41). Mean 25-hydroxyvitamin D concentration was significantly higher in the rabbits receiving a lot of hay and commercial food ≥1 dl (33.9 ± 13.2 ng/ml) than in rabbits in other diet groups (24.0 ± 8.5 ng/ml, 21.7 ± 8.1 ng/ml, and 22.2 ± 18.0 ng/ml, respectively). This investigation showed wide variation in 25-hydroxyvitamin D concentrations among Finnish pet rabbits. Diet remains a main source since outdoor access seems to be too limited to provide adequate vitamin D synthesis for most of them, and the use of vitamin D supplements is rare.

An update on clinical biochemical RIs of rabbits with special consideration for age, gender, and size

BACKGROUND

Literature regarding the impact of age and breed size on clinical biochemical measurands in rabbits is scarce.

OBJECTIVES

We aimed to establish clinical biochemical measurand RIs for rabbits bred and kept under standard conditions and to evaluate the impact of gender, age, and breed size on the results using a Nova CRT8 analyzer (Nova Biomedical GmbH) and an ABX Pentra 400 analyzer (ABX Horiba, Axonlab).

METHODS

Serum samples were available from 122 adult rabbits (56 males, 66 females) of 10 different breeds and crossbreds with a mean age of 264 ± 21 days. The impact of age was evaluated by sampling 48 rabbits at weaning (8 weeks of age) and when they reached adulthood.

RESULTS

Significantly higher median values were obtained for ALT, glutamate dehydrogenase (GLDH), and potassium in adult males compared with adult female rabbits. Total bilirubin, cholesterol, creatinine, and urea were significantly higher in adult females than adult males. Juvenile animals at weaning revealed significantly higher median values for ALP, cholesterol, GGT, GLDH, glucose, phosphate, and triglycerides compared with their adulthood values. In contrast, lower median albumin, ALT, chloride, creatinine, globulin, ionized calcium, magnesium, potassium, total protein, urea, and calcium-phosphate ratios were seen at the time of weaning compared with adulthood values. Significantly lower median CK, creatinine, and ALT were found in dwarf/small rabbit breeds compared with intermediate/large breeds.

CONCLUSIONS

These RIs are especially useful for rabbit production and experimental studies. Age should be considered when evaluating clinical biochemical measurands. Creatinine, CK, and ALT are affected by organ mass.

Effects of ultraviolet radiation produced from artificial lights on serum 25-hydroxyvitamin D concentration in captive domestic rabbits (Oryctolagus cuniculi)

OBJECTIVE

To determine the effects of UVB radiation produced by artificial lights on serum 25-hydroxyvitamin D concentrations in domestic rabbits (Oryctolagus cuniculi).

ANIMALS

9 juvenile domestic rabbits.

PROCEDURES

After an acclimation period, rabbits were anesthetized with isoflurane, and an initial blood sample was collected for determination of serum 25-hydroxyvitamin D concentration. Rabbits were randomly assigned to receive 12-hour exposure to UVB radiation produced by 2 compact fluorescent lights daily (n = 5) or no UVB supplementation (4) commencing on day 1. The UVB radiation emitted into the cage was measured at 9 points approximately 34 cm from the surface of the UVB light sources (representing the position of the rabbits in the cage) after 10 hours of exposure on days 1, 8, and 14. On day 14, another blood sample was collected from anesthetized rabbits for determination of serum 25-hydroxyvitamin D concentration.

RESULTS

The UVB radiation level was 8.3 to 58.1 μW/cm² for the exposed rabbits and consistently < 0.001 μW/cm² for the control rabbits. Mean ± SD serum 25-hydroxyvitamin D concentrations in the rabbits that were or were not provided supplemental UVB radiation for 14 days differed significantly (66.4 ± 14.3 nmol/L and 31.7 ± 9.9 nmol/L, respectively).

CONCLUSIONS AND CLINICAL RELEVANCE

Exposure to UVB radiation produced by artificial light significantly increased serum 25-hydroxyvitamin D concentration in juvenile rabbits. Because vitamin D is an essential hormone in vertebrates, these findings suggested that the provision of supplemental UVB radiation to captive rabbits may be important.

Point-of-care blood gas and electrolyte analysis in rabbits

This article outlines a practical approach for the use of blood-gas analysis in pet rabbits using the I-STAT analyzer. Sampling techniques and a theoretic approach to diagnosis are described. The following 95% RI were obtained from venous samples of 45 healthy rabbits: pH (7.245-7.533), PCO(2) (28.9-52.9 mm Hg), HCO(3) (17.0-32.5 mmol/L), total CO(2) (18-34 mmol/L), BE(ecf) (-10-8 mmol/L), AnGap (11-26 mmol/L), Na (136-147 mmol/L), K (3.4-5.7 mmol/L), Cl (93-113 mmol/L), Glu (93-245 mg/dL), BUN (9-33 mg/dL). Results from 20 arterial samples were pH (7.358-7.502), PCO(2) (29.1-36.8 mm Hg), HCO(3) (17.5-27.6 mmol/L), BE(ecf) (-7-5 mmol/L), PO(2) (75-101 mm Hg), iCa (1.67-1.85). The article also includes some conclusions obtained comparing results from healthy and ill rabbits over an 18-month period.

Cloning, comparative sequence analysis and mRNA expression of calcium-transporting genes in horses

Epithelial calcium transport occurs by paracellular and transcellular mechanisms. Transcellular transport in intestinal and renal epithelia involves several transport proteins, including transient receptor potential vanilloid member 5 (TRPV5), member 6 (TRPV6), calbindin D9k (CB9), calbindin D28k (CB28), sodium calcium exchanger 1 (NCX1), plasma membrane calcium ATPase 1 (PMCA1), and the vitamin D receptor (VDR). We are interested in the horse because of its unique calcium physiology (high blood calcium, high intestinal calcium absorption, high renal excretion of calcium, low vitamin D concentrations), and because horses often have dysregulated calcium balance with various diseases. We cloned the mRNA for equine TRPV5, TRPV6, CB9, CB28, NCX1, PMCA1, and VDR, performed comparative mRNA and protein sequence analysis, and quantified their mRNA expression in the kidney and gastrointestinal tract. Sequence homology for the mRNAs and proteins was high among mammals (>75%), with fish having the lowest homology (<75%). TRPV5, TRPV6, and CB9 expression was higher in the duodenum and proximal jejunum and followed a similar expression pattern. CB28 expression was greatest in the kidney. PMCA1 and NCX1 expression was similar throughout the intestine, but in the kidney PMCA1 expression was higher. Based on our findings, the proximal small intestine is the main site for transcellular calcium transport, with TRPV6 and CB9 serving as the main transport proteins. In the kidney, TRPV6, CB28, and PMCA1 are likely more important. The low VDR expression in the equine small intestine and kidney relative to the large intestine, together with the reported high intestinal absorption and renal excretion of calcium, and low vitamin D concentrations suggests that epithelial calcium transport in horses is not as dependent on vitamin D as in other species.

Induction of calcification by serum depletion in cell culture: a model for focal calcification in aortas related to atherosclerosis

Background

Since aortic calcification has been shown to initiate in the lower zone of well-thickened plaques (LZP) adjacent to the aortic media of rabbits fed supplemental cholesterol diets, a restricted supply of serum to vascular cells could play a role in vascular calcification. This study was designed to use a cell culture model to support this hypothesis.

Results

Rabbit aortic smooth muscle cells were grown to confluence in a culture media containing 10 % fetal bovine serum (FBS). The confluent cells were then exposed to the media for 2 hrs with or without serum at a Ca × P ion product range of 4.5–9.4 mM². In contrast to the cells cultured in the presence of FBS, confluent cells in its absence displayed marked mineral-positive alizarin red staining and infrared absorption of mineral phosphate. A kinetic parameter C_1/2 was used to designate the concentration of serum or its protein constituents needed to reduce the deposition of Ca and P by half. The C_1/2 for FBS and rabbit serum was 0.04–0.07 % The C_1/2 value for rabbit serum proteins was 13.5 μg/ml corresponding to the protein concentration in 0.06 % of serum. This C_1/2 was markedly smaller than 86.2 μg/ml for bovine serum albumin present in 0.37 % serum (p < 0.05). Serum depletion also caused marked membrane translocation as evidenced through a specific apoptosis dye uptake by cells. The proteomic analysis of calcifying vesicles, which can be released by serum depletion, revealed several calcification-related proteins.

Conclusion

The aortic smooth muscle cell culture model suggests that serum depletion may play a role in the initiation of aortic calcification. The serum exhibits remarkable ability to inhibit cell-mediated calcification.

Hormonal regulation and calcium metabolism in the rabbit

Calcium is an important mineral in homeostasis in all vertebrate animals. It is the most abundant mineral in the body, and is the major component of bones and teeth. In addition, calcium is involved in various vital physiologic processes, including blood coagulation, muscle contraction, membrane permeability, nerve conduction, enzyme activity, and hormone release. Calcium metabolism in the rabbit differs in several respects from that of most mammals. This article discusses calcium metabolism and its hormonal regulation in the rabbit, as compared with normal calcium metabolism in other mammalian species. Medical disorders associated with this unique metabolism are introduced briefly.

Dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism

This review examines the dynamics of parathyroid hormone secretion in health and in various causes of secondary hyperparathyroidism. Although most studies of parathyroid hormone and calcium have focused on the modification of parathyroid hormone secretion by serum calcium, the relationship between parathyroid hormone and serum calcium is bifunctional because parathyroid hormone also modifies serum calcium. In normal animals and humans, factors such as phosphorus and vitamin D modify the basal parathyroid hormone level and the maximal parathyroid hormone response to hypocalcemia. Certain medications, such as lithium and estrogen, in normal individuals and sustained changes in the serum calcium concentration in hemodialysis patients change the set point of calcium, which reflects the serum calcium concentration at which parathyroid hormone secretion responds. Hypocalcemia increases the basal/maximal parathyroid hormone ratio, a measure of the relative degree of parathyroid hormone stimulation. The phenomenon of hysteresis, defined as a different parathyroid hormone value for the same serum calcium concentration during the induction of and recovery from hypo- and hypercalcemia, is discussed because it provides important insights into factors that affect parathyroid hormone secretion. In three causes of secondary hyperparathyroidism--chronic kidney disease, vitamin D deficiency, and aging--factors that affect the dynamics of parathyroid hormone secretion are evaluated in detail. During recovery from vitamin D deficiency, the maximal parathyroid hormone remains elevated while the basal parathyroid hormone value rapidly becomes normal because of a shift in the set point of calcium. Much remains to be learned about the dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism.

Calcium deficiency-induced secondary hyperparathyroidism and osteopenia are rapidly reversible with calcium supplementation in growing rabbit pups

The reversibility of osteopenia secondary to isolated Ca deficiency (CaDef) is still not clear. We studied the effect of severe CaDef on Ca homeostasis and bone accrual in a ‘hypercalcaemic’ animal, the rabbit, during the post-weaning period and its reversibility on Ca supplementation. Male Belgian 5-week-old rabbit pups were fed CaDef diet (0·026% Ca) for 10 weeks. As compared with those fed with a normal chow diet (0·45% Ca), CaDef pups developed significant hypocalcaemia (P<0·05), hypocalciuria (urinary Ca 76 (sem 12) V 17 (sem 1) mg/l;P<0·005), hypophosphataemia (serum inorganic P 100 (sem 6) V. 65 (sem 4) mg/l; P<0·005), secondary hyperparathyroidism (SHPT) (serum intact parathyroid hormone human equivalent 18·2 (sem 1·4) V. 125·0 (sem 4·5) pg/ml; P<0·001) and elevated serum calcitriol levels (34·0 (sem 3·9) V. 91·0 (sem 1·0) pg/ml; v<0·005). Elevated urinary C-terminal telopeptide of class I collagen (P<0·005) and total serum alkaline phosphatase (P<0·005) suggested increased bone turnover. There was a significantly lower gain in bone mineral density (BMD) and bone mineral content (BMC) in the whole body and lumbar spine in vivo, and various sub-regions of the femur and tibia in vitro.Supplementation of adequate Ca (0·45% Ca) after 15 weeks on the normal diet resulted in rapid catch-up growth, and resolution of SHPT. Rapid gain in various BMD and BMC parameters continued at 30 weeks of age, and both were comparable with those in rabbits on a normal diet. We conclude that Ca deficiency-induced SHPT and poor bone accrual in growing rabbit pups are rapidly reversible with Ca supplementation. The present study indicates that early intervention may be a more appropriate window period for human nutritional corrective measures.

Nutritional secondary hyperparathyroidism in rabbits

The present study was designed to document the effect of a low (0.6%) calcium-high (1.2%) phosphorus (LCaHP) diet on the development of parathyroid gland hyperplasia in rabbits and to describe the dynamics of parathyroid function (PTH-Ca2+ curves) in rabbits with nutritional secondary hyperparathyroidism (N2HPT). Parathyroid gland weight, parathyroid cell proliferation (measured as percentage of cells in S-phase), and parathyroid calcium (CaRmRNA) and Vitamin D (VDRmRNA) receptor expression were measured in normal rabbits and in rabbits with N2HPT. The PTH-Ca2+ curve was studied in normal rabbits (Group I) and in rabbits with N2HPT at two stages: 2-3 weeks (Group IIA) and 5-6 weeks (Group IIB) after being fed LCaHP diet. An increase in parathyroid gland weight and percentage of cells in S-phase was detected in the course of N2HPT. After receiving a LCaHP diet for 6 weeks rabbits had decreased levels of CaRmRNA but VDRmRNA remained unchanged. A progressive increase in the concentrations of plasma PTH (Group IIA=167+/-14 pg/ml and Group IIB=377+/-54 pg/ml, P<0.05 versus Group I=27+/-3 pg/ml) was detected in the rabbits fed a LCaHP diet. This was accompanied by an increase in maximal and minimal PTH, reductions in plasma Ca2+ and calcitriol and elevations in plasma phosphate and creatinine. In conclusion, feeding a LCaHPD results in a rapid induction of N2HPT in rabbits. After 6 weeks on the LCaHPD rabbits develop parathyroid hyperplasia characterized by increases in PTH secretion, glandular weight and proliferation and by a decrease in CaRmRNA.

Parathyroid gland function in the uremic rabbit

Rabbits with renal failure have been reported to be hypercalcemic and to have decreased parathyroid hormone (PTH) concentrations. Thus, it would seem that uremic rabbits are resistant to secondary hyperparathyroidism (HPT). The work reported here was designed to investigate parathyroid gland function in uremic rabbits and the effect of diets with different calcium (Ca) and phosphorus (P) content. The relationship between PTH and ionized calcium (Ca2+), parathyroid gland size and parathyroid cell cycle were studied in three groups of rabbits: Group I, rabbits with normal renal function on a standard diet (Ca = 1.2%, P = 0.6%); Group II, partially nephrectomized rabbits on a standard diet; and Group III, partially nephrectomized rabbits on a low Ca (0.6%)-high P (1.2%) diet. Group I rabbits had baseline Ca2+ = 1.71 +/- 0.05 mmol/l and PTH = 26.9 +/- 3.2 pg/ml. During hypo- and hypercalcemic stimulation PTH reached maximal values (PTHmax) of 94.4 +/- 5.5 pg/ml and minimal concentrations (PTHmin) of 3.2 +/- 0.2 pg/ml. Rabbits from Group II were hypercalcemic (baseline Ca2+ = 2.03 +/- 0.06 mmol/l) and had very low PTH levels (1.7 +/- 0.5 pg/ml); however, they reached a PTHmax that was similar to Group I, 92 +/- 8.7 pg/ml. Group III rabbits were hypocalcemic (baseline Ca2+ = 1.22 +/- 0.08 mmol/l) and had very high basal PTH levels (739 +/- 155 pg/ml). Their PTHmax and PTHmin were 801 +/- 169.4 pg/ml and 102.2 +/- 22.2 pg/ml, respectively. Both parathyroid gland size and parathyroid cell proliferation were increased in Group III. In conclusion, our results show that the Ca and P content of the diet markedly influence PTH secretion in the uremic rabbit and that when placed on a low Ca-high P diet uremic rabbits develop secondary HPT.

Parathyroid hormone, haematological and biochemical parameters in relation to dental disease and husbandry in rabbits

During a two-year period between 1995 and 1997, over 80 blood samples were collected from pet rabbits in order to investigate an apparent osteodystrophy affecting the skulls of rabbits with acquired dental disease. A series of haematological and biochemical analyses relating to calcium metabolism were performed and samples were taken for parathyroid hormone (PTH) assay. The rabbits were categorised according to the condition of their teeth and the manner in which the pets were kept. PTH concentrations were higher and calcium concentrations lower in hutch-kept rabbits with advanced dental disease in comparison with those kept in free-range conditions. No dental problems were detected in the free-range rabbits on radiological or clinical examination. During the course of the study, differences in haematological pictures and albumin values emerged among rabbits kept under the different husbandry regimes. Complete blood counts from free-range rabbits were comparable with laboratory reference ranges, whereas there were significantly lower red cell and lymphocyte counts in rabbits exhibiting advanced dental disease. Serum albumin values were significantly higher in rabbits kept in free-range conditions than in those with advanced dental disease or those unaffected by dental disease but kept in hutches. Rabbits kept in hutches showed trends towards anaemia and lymphopenia. Results indicated that acquired dental disease of pet rabbits is related to husbandry and is associated with alterations in calcium metabolism.

Preliminary investigation of the vitamin D status of pet rabbits

The plasma concentrations of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) (vitamin D3) were measured in blood samples taken from one wild rabbit and 13 pet rabbits at different times of the year. Some pet rabbits had low or undetectable plasma concentrations of 1,25-(OH)2D3 especially if they were kept in hutches. Rabbits with more access to sunlight had higher concentrations of 1,25-(OH)2D3.

Protein folding coupled to disulphide bond formation

Protein folding that is coupled to disulphide bond formation has many experimental advantages. In particular, the kinetic roles and importance of all the disulphide intermediates can be determined, usually unambiguously. This contrasts with other types of protein folding, where the roles of any intermediates detected are usually not established. Nevertheless, there is considerable confusion in the literature about even the best-characterized disulphide folding pathways. This article attempts to set the record straight.

Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium

The extracellular Ca2+ (Ca(0)2+)-sensing receptor (CaR) recently cloned from mammalian parathyroid, kidney, brain, and thyroid plays a central role in maintaining near constancy of Ca(0)2+. We previously showed that the hypercalcemia normally present in New Zealand white rabbits is associated with an elevated set point for Ca(02+)-regulated PTH release (the level of Ca(0)2+ half-maximally inhibiting hormonal secretion). This observation suggested an alteration in the Ca(02+)-sensing mechanism in the rabbit parathyroid, a possibility we have now pursued by isolating and characterizing the rabbit homolog of the CaR. The cloned rabbit kidney CaR (RabCaR) shares a high degree of overall homology (> 90% amino acid identity) with the bovine, human, and rat CaRs, although it differs slightly in several regions of the extracellular domain potentially involved in binding ligands. By Northern analysis and/or immunohistochemistry, a similar or identical receptor is also expressed in parathyroid, thyroid C cells, small and large intestine, and in the thick ascending limb and collecting ducts of the kidney. When expressed transiently in HEK293 cells and assayed functionally through CaR agonist-evoked increases in Ca(i)2+, the rabbit CaR shows apparent affinities for Ca(0)2+, Mg(0)2+, and Gd(0)3+ that are indistinguishable from those observed in studies carried out concomitantly using the human CaR. Therefore, at least as assessed by its ability to increase Ca(i)2+ when expressed in HEK293 cells, the intrinsic functional properties of the rabbit CaR cannot explain the hypercalcemia observed in vivo in the New Zealand white rabbit.

A mouse model of human familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism

Mice lacking the calcium-sensing receptor (Casr) were created to examine the receptor's role in calcium homeostasis and to elucidate the mechanism by which inherited human Casr gene defects cause diseases. Casr+/- mice, analogous to humans with familial hypocalciuric hypercalcemia, had benign and modest elevations of serum calcium, magnesium and parathyroid hormone levels as well as hypocalciuria. In contrast, Casr-/- mice, like humans with neonatal severe hyperparathyroidism, had markedly elevated serum calcium and parathyroid hormone levels, parathyroid hyperplasia, bone abnormalities, retarded growth and premature death. Our findings suggest that Casr mutations cause these human disorders by reducing the number of functional receptor molecules on the cell surface.

Protection from hypoxemia-induced renal dysfunction by the thiophosphate WR-2721

The acute renal effects of hypoxemia-reoxygenation and the putative protective action of WR-2721 [S-, 2-(3-aminopropyl-amino)-, etylphosphosphorothioic acid], a drug with specific properties such as PTH-secretion inhibiting, calcium lowering and free radical scavenging activities, were investigated in anesthetized-ventilated rabbits. Glomerular filtration rate (GFR) and renal blood flow (RBF) were assessed by the clearance of inulin and para-aminohippuric acid, respectively. Each animal acted as its own control. Normoxemic control rabbits showed no changes in renal hemodynamics and function during 150 minutes. The administration of WR-2721 (75 mg/kg body wt i.v.) to normoxemic animals induced a significant decrease in MBP, RBF and diuresis, without affecting GFR. It significantly reduced plasma levels of PTH, decreased calcemia and increased urinary calcium excretion. In untreated hypoxemic-reoxygenated rabbits, 45 minutes of severe hypoxemia (PO2 around 35 mm Hg) induced a significant fall in MBP, GFR, RBF and diuresis, which persisted during the 60-minute reoxygenation period. The administration of WR-2721 before hypoxemia or reoxygenation prevented the hypoxemia-induced decrease in GFR and diuresis. Filtration fraction increased significantly. The renal functional improvement observed in hypoxemic rabbits administered WR-2721 could be mediated by its effect on calcium metabolism and/or its oxygen free radical scavenging properties.

Effect of dietary calcium on bone density in growing rabbits

Reductions in peak bone mass at skeletal maturity may increase the risk for the subsequent development of osteoporosis. Although changes in calcium intake can modify the rate of decline in bone density in the mature skeleton, longitudinal assessments of the effect of dietary calcium supplementation during skeletal growth on peak bone mass have not been done in humans or experimental animals. Thus quantitative computed tomography (QCT) was used to monitor changes in vertebral bone density at 6-wk intervals during growth from 8 wk of age until skeletal maturity at 35 wk in male New Zealand White rabbits maintained on diets containing 0.15% (low Ca), 0.45% (normal Ca), or 1.35% (high Ca) calcium. Serum parathyroid hormone (PTH) and calcitriol levels increased, and renal calcium excretion decreased in low Ca compared with normal Ca; in contrast, serum calcitriol levels decreased and renal calcium excretion increased from control values in high Ca. Vertebral bone density by QCT did not differ during growth between high Ca and normal Ca, and peak values at epiphyseal closure also did not differ in these two groups. Vertebral bone density was lower, however, throughout the study in low Ca, and peak values at epiphyseal closure remained below those in either normal Ca or high Ca. Quantitative bone histology revealed decreases in cortical thickness in the third lumbar vertebra in low Ca, whereas trabecular bone area did not differ among groups; there was no histological evidence of osteomalacia in low Ca. Thus dietary calcium restriction during growth reduces peak bone mass at skeletal maturity, but raising dietary calcium intake above normal levels does not increase peak bone mass in this experimental model.