Purification and cloning of the mitochondrial branched-chain amino acid aminotransferase from sheep placenta

This paper presents the first purification of the mitochondrial branched-chain amino acid aminotransferase (BCATm) from sheep placenta. It is a homodimer with an apparent subunit molecular mass of 41 kDa. The enzyme differs from those of the rat and human as it appears to form at least one intermolecular disulfide bond. The sheep BCATm cDNA (1.4 kb) encodes a mature polypeptide of 366 amino acids with a calculated molecular mass of 41 329 Da and a partial mitochondrial targeting sequence of seven amino acids. The sheep BCATm sequence shares higher identity with other mammalian BCATm isoenzymes (82-85%) than with the cytosolic isoenzymes (60%). By Northern blot analysis, a message of 1.7 kb was detected in sheep placenta and skeletal muscle. Measurements of BCAT activity, mRNA and BCATm protein in sheep placenta and skeletal muscle revealed that BCATm is the sole BCAT isoenzyme expressed in placenta, whereas it contributes 57 and 71% of the BCAT activity in tensor fascia latae and masseter muscles from weaned lambs respectively. Skeletal muscle, the main site of branched-chain amino acid transamination, exhibits significantly lower BCAT activity in sheep than in rat. Our results suggest that the low BCATm mRNA level probably accounts for the low BCAT activity in sheep skeletal muscle, and that the metabolic scheme for branched-chain amino acid catabolism is specific to each species.     

Regulation of branched-chain amino acid metabolism in the lactating rat

There is evidence that during lactation, uptake of the essential branched-chain amino acids (BCAA) by mammary glands exceeds their output in milk protein. In this study, we have measured the potential of lactating rats to catabolize BCAA. The activity, relative protein and specific mRNA levels of the first two enzymes in the BCAA catabolic pathway, branched-chain aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD), were measured in mammary gland, liver and skeletal muscle obtained from rat dams at peak lactation (12 d), from rat dams 24 h after weaning at peak lactation and from age-matched virgin controls. Western analysis showed that the mitochondrial BCATm isoenzyme was found in mammary gland. Comparison of lactating and control rats revealed that tissue BCATm activity, protein and mRNA were at least 10-fold higher in mammary tissue during lactation. Values were 1.3- to 1. 9-fold higher after 24 h of weaning. In mammary gland of lactating rats, the BCKD complex was fully active. In virgin controls and weaning dams, only about 20% of the complex was in the active state. Hypertrophy of the liver and mammary gland during lactation resulted in a 73% increase in total oxidative capacity in lactating rats. The results are consistent with increased expression of the BCATm gene in the mammary gland during lactation, whereas oxidation appears to be regulated primarily by changes in activity state (phosphorylation state) of BCKD.     

Hepatic histidase gene expression responds to protein rehabilitation in undernourished growing rats

We studied the effect of nutritional rehabilitation with a 6, 18 or 50% casein diet in undernourished rats on histidase (Hal) expression. Undernutrition was induced by feeding rats a 0.5% casein diet for 5 wk. Over this period, growth, serum total proteins and insulin-like growth factor-I (IGF-I) levels were significantly lower than those of rats that freely consumed an 18% casein diet. During this period, undernutrition also significantly reduced Hal activity and Hal-mRNA concentration. Nutritional rehabilitation for 21 d with a 6% casein diet did not change any of these variables. Nutritional rehabilitation with an 18 or 50% casein diet for 1 d initiated the restoration of Hal activity and mRNA concentration. After 10 d of consuming 18 or 50% casein diets, Hal activity was 5- and 14-fold, and mRNA concentration was 8.5- and 23-fold higher, respectively, than in the protein-undernourished group (PU). During this period, body weight, total serum proteins and IGF-I levels were also significantly (P < 0.05) higher than those of the PU group. At the end of 21 d of rehabilitation with an 18 or 50% casein diet, Hal activity was 14- and 31-fold higher and Hal mRNA concentration was 10- and 24-fold higher, respectively, than in the PU group. In conclusion, our data showed that rehabilitation of undernourished rats with a 6% casein diet was not sufficient to re-establish growth indicators, Hal activity or gene expression, and that nutritional rehabilitation with an 18 or 50% casein diet effectively re-established body weight , biochemical variables and the capacity of histidase gene expression to eliminate the excess of protein.     

Reduced kidney branched chain aminotransferase expression in puromycin aminonucleoside-induced nephrotic syndrome

Injection of puromycin aminonucleoside to rats induces nephrotic syndrome characterized by hypoalbuminemia, proteinuria and hypercholesterolemia. In these rats, a low protein diet (6% casein diet) increased serum albumin by 26.3%, decreased proteinuria by 39% and reduced total cholesterol by 32%. Branched chain aminotransferase activity in kidney mitochondria of nephrotic rats fed 20 or 6% casein diet decreased by 30 and 24% with respect to their pair-fed groups and it was not modified by the protein content of the diet. Mitochondrial branched chain aminotransferase mRNA expression decreased by 67.3 and 72.5% in nephrotic rats fed 20 and 6% casein diet in comparison to their pair-fed groups. Total serum branched chain amino acids concentration (leucine, isoleucine, valine) in nephrotic rats was 30% higher than their pair-fed groups and it was associated with a decrease in the branched chain aminotransferase activity and mRNA expression suggesting that the catabolism of branched chain amino acid is reduced to conserve body nitrogen.     

Cholestyramine and a fat-free diet lower apolipoprotein A-IV mRNA in jejunum and cholestyramine lowers apolipoprotein A-I mRNA in ileum of rats

To investigate the effect of bile acids or dietary lipid on the expression of intestinal apolipoproteins, the mRNA levels of apolipoprotein A-I and A-IV in the intestine from rats fed a diet containing cholestyramine or a fat-free diet were compared with those from rats fed a control diet containing 25% casein and 5% corn oil. Plasma total cholesterol concentration was lower after 16 h in rats fed a diet containing cholestyramine or a fat-free diet than in rats fed a control diet. In rats fed the fat-free diet, HDL cholesterol concentration also was lower than in those fed the control diet. The pool of bile acid in intestinal contents was significantly lower in rats fed cholestyramine than in both other groups. The relative abundance of jejunal apolipoprotein A-I mRNA did not differ between groups. Jejunal apolipoprotein A-IV mRNA abundance was significantly lower than in controls in rats fed the fat-free and cholestyramine-containing diets. Abundance of apolipoprotein A-I mRNA in ileal mucosa was comparable to controls in rats fed a fat-free diet but was significantly lower in rats fed cholestyramine. Ileal apolipoprotein A-IV mRNA tended to be lower in rats fed cholestyramine and a fat-free diet than in controls. We propose that decreased absorption of dietary lipid may modulate changes in jejunal apolipoprotein A-IV mRNA level and low levels of bile acids in the lumen may modulate changes in ileal apolipoprotein A-I mRNA level.     

Tissue-specific effects of chronic dietary protein restriction and gastrostomy on the insulin-like growth factor-I pathway in the liver and colon of adult rats

Dietary protein restriction decreases plasma concentrations of insulin-like growth factor-I (IGF-I) and reduces IGF-I mRNA levels in the liver. In addition to the actions of systemic IGF-I, locally produced IGF-I is thought to mediate autocrine and paracrine growth effects in the colon. The objectives of the present study were to investigate the IGF-I pathway in the colon and liver of adult rats under conditions of dietary protein restriction, surgical stress, and dietary protein repletion. Two groups of rats were placed on either a 20% or 2% casein diet for 19 days. Two additional groups of rats underwent gastrostomy after a 2% casein diet for 2 weeks, and then were either kept on the 2% casein diet or changed to a 20% casein diet until day 19. Dietary protein restriction reduced plasma concentrations of IGF-I and IGF-binding proteins (IGFBPs) and hepatic IGF-I mRNA content, while increasing colonic IGF-I receptor mRNA. Gastrostomy in protein-depleted animals had no effect on hepatic IGF-I mRNA, but led to a marked increase in colonic IGF-I mRNA levels. Dietary protein repletion resulted in a decrease in colonic IGF-I receptor mRNA. The distinct effects of dietary protein depletion and operative stress on the IGF pathway in the colon as compared with the liver may serve to maintain the level of IGF-I signaling in the colon by autocrine or paracrine mechanisms under these conditions.     

Elemental diets in the repair of small intestinal damage

We have investigated the possible benefits of elemental diets, especially a diet supplemented with L-glutamine, on maintenance of intestinal absorptive function in rat small intestine damaged by 5-fluorouracil. Although a standard rat diet sustained better body growth in control rats, each of the elemental diets and the diet containing intact casein in place of hydrolyzed casein was beneficial in promoting less body weight loss during the 3 d after 5-fluorouracil injection. The same significant benefit was seen in absorptive activity measured in small intestine in vitro 3 d after the cytotoxic injury. Glutamine supplementation, however, did not confer any significant advantages, although it did cause significant elevation of muscle glutamine pools. This elevation was substantially less than the corresponding increase in muscle glycine content after dietary supplementation with glycine.     

Role of branched-chain aminotransferase isoenzymes and gabapentin in neurotransmitter metabolism

Because it is well known that excess branched-chain amino acids (BCAAs) have a profound influence on neurological function, studies were conducted to determine the impact of BCAAs on neuronal and astrocytic metabolism and on trafficking between neurons and astrocytes. The first step in the metabolism of BCAAs is transamination with alpha-ketoglutarate to form the branched-chain alpha-keto acids (BCKAs). The brain is unique in that it expresses two separate branched-chain aminotransferase (BCAT) isoenzymes. One is the common peripheral form [mitochondrial (BCATm)], and the other [cytosolic (BCATc)] is unique to cerebral tissue, placenta, and ovaries. Therefore, attempts were made to define the isoenzymes' spatial distribution and whether they might play separate metabolic roles. Studies were conducted on primary rat brain cell cultures enriched in either astroglia or neurons. The data show that over time BCATm becomes the predominant isoenzyme in astrocyte cultures and that BCATc is prominent in early neuronal cultures. The data also show that gabapentin, a structural analogue of leucine with anticonvulsant properties, is a competitive inhibitor of BCATc but that it does not inhibit BCATm. Metabolic studies indicated that BCAAs promote the efflux of glutamine from astrocytes and that gabapentin can replace leucine as an exchange substrate. Studying astrocyte-enriched cultures in the presence of [U-14C]glutamate we found that BCKAs, but not BCAAs, stimulate glutamate transamination to alpha-ketoglutarate and thus irreversible decarboxylation of glutamate to pyruvate and lactate, thereby promoting glutamate oxidative breakdown. Oxidation of glutamate appeared to be largely dependent on the presence of an alpha-keto acid acceptor for transamination in astrocyte cultures and independent of astrocytic glutamate dehydrogenase activity. The data are discussed in terms of a putative BCAA/BCKA shuttle, where BCATs and BCAAs provide the amino group for glutamate synthesis from alpha-ketoglutarate via BCATm in astrocytes and thereby promote glutamine transfer to neurons, whereas BCATc reaminates the amino acids in neurons for another cycle.     

Soybean protein suppresses hepatic lipogenic enzyme gene expression in Wistar fatty rats

The effects of dietary soybean protein on lipogenic enzyme gene expression in livers of genetically fatty rats (Wistar fatty) have been investigated. When Wistar fatty rats and their lean littermates (7-8-wk old) were fed a casein or soybean protein isolate diet containing hydrogenated fat (4% hydrogenated fat plus 1% corn oil) or corn oil (5%) for 3 wk, the hepatic messenger RNA concentrations and activities of lipogenic enzymes were significantly lower in rats fed soybean protein than in those fed casein, regardless of genotype or dietary fat. The conversion rates of thyroxine to triiodothyronine by liver microsomes and plasma triiodothyronine concentrations were lower in the fatty rats than in the lean rats and were significantly greater in rats fed soybean protein than in those fed casein. Conversely, plasma and liver triacylglycerol concentrations were lower in soybean protein-fed fatty and lean rats than in those fed casein. The body weight was less in the fatty rats fed soybean protein than in those fed casein after 3 wk of feeding. Moreover, dietary polyunsaturated fatty acids suppressed lipogenic enzyme gene expression in the lean rats but did not in the fatty rats. Dietary soybean protein appeared to be useful for the reduction of obesity.     

Resistant proteins alter cecal short-chain fatty acid profiles in rats fed high amylose cornstarch

The objective of this study was to examine the physiologic importance of undigested protein on cecal fermentation in rats fed a low (LAS) and high (HAS) amylose cornstarch. In Experiment 1, rats were fed diets containing LAS (655 g/kg diet) with one of four protein sources: casein, rice (RP), potato (PP) or soybean protein (SP) at 250 g/kg diet for 15 d. Apparent digestibilities of casein, RP, SP and PP were 96, 94, 93 and 92%, respectively. In rats fed the LAS diet with casein, acetate, propionate and succinate were the major cecal organic acids. The succinate pools in rats fed RP or SP were significantly lower than in those fed casein, whereas butyrate did not differ. Butyrate was significantly higher in rats fed PP, but succinate was the same as in rats fed casein. In Experiment 2, rats were fed diets containing HAS (200 g/kg diet) with one of the four protein sources at 250 g/kg diet for 10 d. HAS was substituted for the same amount of LAS. In rats fed the HAS diet, succinate was the major acid in rats fed casein; in rats fed RP or PP, however, the pools of this acid were significantly lower than in those fed casein, whereas butyrate was significantly higher in rats fed RP or PP. Fecal starch excretion was significantly lower in rats fed RP or PP than in those fed casein. In Experiment 3, rats were fed the casein-HAS diet with graded levels of PP (0, 10, 30, 50, 100 and 250 g/kg diet) for 14 d. The PP was substituted for the same amount of casein. Cecal butyrate was low in rats fed up to 100 g of PP/kg diet and then rose with 250 g of PP/kg diet. In Experiment 4, ileorectostomized rats were used and fed the same diets described in Experiment 3 for 9 d. The ileal starch/nitrogen ratio declined with increasing dietary PP, due solely to greater nitrogen excretion, whereas starch excretion was unaffected. In Experiment 5, rats were fed the casein-HAS diet with or without 60 g of artificial resistant protein/kg diet for 10 d. The resistant protein (apparent digestibility, 63%) was substituted for the same amount of casein. Rats fed the casein-HAS diet with resistant protein had significantly greater cecal butyrate and lower succinate than those fed the casein-HAS diet. These data show that large bowel fermentation of starch is altered by dietary protein. They support the hypothesis that nondigested protein, namely, resistant protein, may control fermentation efficiency as well as the fermentation profile of HAS, possibly as a result of a change in microflora through the change in the ratio of starch to nitrogen in the cecum.     

Dietary salt regulates expression of Tamm-Horsfall glycoprotein in rats

BACKGROUND: Tamm-Horsfall glycoprotein (THP) is a unique protein that is produced exclusively by cells of the thick ascending limb of Henle's loop (TALH). This study examined whether dietary salt altered renal THP production. METHODS: Male Sprague-Dawley rats were examined on days 1, 4, and 15 following placement in metabolic cages on diet that contained 0.3%, 1.0% or 8.0% NaCl. THP expression was quantified using Northern hybridization and Western blotting analysis. RESULTS: An increase in dietary salt produced sustained increases in relative steady-state mRNA and protein levels of THP in the kidney. Addition of furosemide, but not chlorothiazide, to animals on the 8.0% NaCl diet further augmented steady-state mRNA levels of THP. CONCLUSIONS: An increase in dietary salt and the loop diuretic, furosemide, increased expression of THP in the rat. The data support the involvement of this unique protein in the function of the TALH during changes in dietary salt. These findings also suggest that restriction of dietary salt may be beneficial in cast nephropathy in multiple myeloma and recurrent nephrolithiasis, two diseases in which THP can play an important pathogenetic role.     

Regulation of hypothalamic preprogrowth hormone-releasing factor messenger ribonucleic acid expression in food-deprived rats: a role for histaminergic neurotransmission

To determine the component(s) of dietary protein that regulates GH-releasing factor (GRF) synthesis, we measured hypothalamic prepro-GRF mRNA by solution hybridization/nuclease protection analysis in food-deprived rats refed protein-free diets (PF) supplemented with individual amino acids. Adult male Sprague-Dawley rats were allowed free access to food (Fed), food deprived for 72 h (FD), or FD then refed for 72 h with a normal (NF) diet, a protein-free (PF) diet, or PF diets containing tyrosine, tryptophan (Trp), glutamic acid, or histidine (His). Food-deprived rats displayed the expected 80% reduction in hypothalamic prepro-GRF mRNA. Upon refeeding, levels were normalized in rats refed a normal diet, but not in those refed a PF diet alone or with tyrosine, Trp, or glutamic acid. In contrast, prepro-GRF mRNA was restored to 70% of Fed values by a PF diet with His. Supplementing a PF diet with His was sufficient to maintain hypothalamic prepro-GRF mRNA expression, as 3 days of feeding replete rats with PF diet or PF diet with added Trp resulted in a 50% reduction in prepro-GRF mRNA, whereas levels were reduced 25% by feeding animals a PF diet with His. Groups of rats allowed free access to food were treated for 72 h with two daily injections of 100 mg/kg alpha-fluoremethylhistidine, a specific irreversible inhibitor of histidine decarboxylase, to determine if the effect of His on prepro-GRF mRNA depended on neural conversion to histamine. alpha-Fluoremethylhistidine-treated rats showed a 40% reduction in hypothalamic prepro-GRF mRNA, with no concomitant change in preproneuropeptide-Y or preprosomatostatin. These data indicate that decreased hypothalamic prepro-GRF mRNA in FD rats is due in part to the lack of dietary and provide clear evidence for a role of the histaminergic neural system in the regulation of hypothalamic GRF expression.     

Three-dimensional triple-quantum-filtered 23Na imaging of the dog head in vivo

Uninephrectomized rats with diet-induced hypercholesterolemia develop interstitial inflammation and fibrosis after 8 to 12 weeks. Fibrosis has been associated with the accumulation of lipid peroxidation products within the tubulointerstitium, along with increased renal mRNA levels for transforming growth factor beta-1 (TCF-beta 1), some matrix proteins, and the tissue inhibitor of metalloproteinases (TIMP-1). However, mRNA levels for urokinase-type plasminogen activator (uPA) have been found to be decreased. The purpose of the present study was to determine whether antioxidant therapy could attenuate interstitial fibrosis in hypercholesterolemic rats and to determine changes in the pattern of renal gene expression induced by antioxidant therapy. Three groups of uninephrectomized rats were studied after 12 weeks of feeding standard rat chow, an atherogenic diet (standard chow plus 4% cholesterol/1% cholic acid), or an atherogenic diet supplemented with high doses of the antioxidants probucol and vitamin E. Rats fed the atherogenic diet developed hypercholesterolemia and a 56% increase in total kidney collagen compared with rats fed standard chow. In comparison, the hypercholesterolemic rats treated with antioxidants had normal levels of renal lipid peroxidation products and a normal kidney collagen content. In contrast, there were no significant differences in urinary albumin excretion rates or the number of interstitial macrophages between the two hypercholesterolemic groups. Compared with the untreated hypercholesterolemic group, antioxidant therapy induced significant reductions in renal mRNA levels for procollagen III (to 60% of untreated levels), collagen IV (60%), and TIMP-1 (20%), while uPA levels were significantly increased (to 210%). Paradoxically, antioxidant therapy was associated with a significant increase in renal TGF-beta 1 mRNA levels (to 150%), although TGF-beta 1 protein expression shifted from interstitial to tubular epithelial cells in predominance. The results of the present study demonstrate the efficiency of antioxidant therapy in preventing renal interstitial fibrosis in hypercholesterolemic rats with a single kidney. Based on changes in renal gene expression at the mRNA level, impaired matrix protein synthesis and increased intrarenal activity of the metalloproteinases and uPA/plasmin may play a role in the attenuation of fibrosis.     

Selenium regulation of selenium-dependent glutathione peroxidases in animals and transfected CHO cells

Glutathione peroxidase (GPX1) was the first identified selenium-dependent enzyme, and this enzyme has been most useful as a biochemical indicator of selenium (Se) status and the parameter of choice for determining Se requirements. We have continued to study Se regulation of GPX1 to better understand the underlying mechanism and to gain insight into how cells themselves regulate nutrient status. In progressive Se deficiency in rats, GPX1 activity, protein and mRNA all decrease in a dramatic, coordinated and exponential fashion such that Se-deficient GPX1 mRNA levels are 6-15% of Se-adequate levels. mRNA levels for other Se-dependent proteins are far less decreased in the same animals. The mRNA levels for a second Se-dependent peroxidase, phospholipid hydroperoxide glutathione peroxidase (GPX4), are little affected by Se deficiency, demonstrating that Se regulation of GPX1 is unique. Se regulation of GPX1 activity in growing male and female rats shows that the Se requirement is 100 ng/g diet, based on liver GPX1 activity; use of GPX1 mRNA as the parameter indicates that the Se requirement is nearer to 50 ng Se/g diet in both male and female rats. This approach will readily detect an altered dietary Se requirement, as shown by the incremental increases in dietary Se requirement by 150, 100 or 50 ng Se/g diet in Se-deficient rat pups repleted with Se for 3, 7 or 14 d, respectively. Studies with CHO cells stably transfected with recombinant GPX1 also show that overexpression of GPX1 does not alter the minimum level of media Se necessary for Se-adequate levels of GPX1 activity or mRNA. We hypothesize that classical GPX1 has an integral biological role in the mechanism used by cells to regulate Se status, making GPX1 an especially useful and effective parameter for determining Se requirements in animals.     

The quantity of dietary protein affects brain protein synthesis rate in aged rats

The purpose of this study was to determine whether the quantity of dietary protein affects the rate of brain protein synthesis in aged rats. Experiments were conducted on three groups of 30-wk-old rats fed diets containing 0, 5 or 20 g casein/100 g for 10 d. The fractional rates of protein synthesis in brain, liver and kidney declined with a decrease in quantity of dietary protein. In brain, liver and kidney, RNA activity [g protein synthesized/(g RNA.d)] was significantly correlated with the fractional rate of protein synthesis. The RNA concentration (mg RNA/g protein) was not related to the fractional rate of protein synthesis in any organ. The results suggest that the rate of protein synthesis in the brain declines with a decrease in quantity of dietary protein in aged rats, and that RNA activity is at least partly related to the fractional rate of brain protein synthesis.     

Tissue-specific responses of branched-chain alpha-ketoacid dehydrogenase activity in metabolic acidosis

In adrenalectomized rats, acidosis does not increase whole-body leucine oxidation unless a physiologic amount of glucocorticoids (dexamethasone) is also provided; an equivalent dose of dexamethasone without acidosis does not change leucine catabolism. Because the influences of acidification and glucocorticoids on branched-chain amino acid metabolism in specific organs are unknown, the function of branched-chain alpha-ketoacid dehydrogenase (BCKAD), the rate-limiting enzyme in branched-chain amino acid catabolism, in adrenalectomized rat skeletal muscle and liver, the two major tissues that degrade branched-chain amino acid was measured. In muscle of acidotic adrenalectomized rats receiving dexamethasone, basal and total BCKAD activities were increased 2.6- (P < 0.05) and 2.8-fold (P < 0.05), respectively. Neither acidosis nor dexamethasone alone increased these activities. BCKAD E1alpha subunit mRNA in muscle of acidotic rats given dexamethasone was increased 1.89-fold (P < 0.05) in parallel with the change in BCKAD activity; BCKAD E2 subunit mRNA was increased by acidosis, dexamethasone, or a combination of both stimuli. In contrast, basal BCKAD activity in liver of rats with acidosis or dexamethasone was nearly threefold lower (P < 0.05) and changes in enzyme activity reflected reduced subunit mRNA. Thus, there are reciprocal, tissue-specific changes in BCKAD function in response to acidosis.     

Effect of dietary magnesium level on nephrocalcinosis and growth in rats

We studied the extent of kidney calcification by varying dietary levels of Mg, based on pathological examinations and calcium (Ca) and magnesium (Mg) balance tests. AIN-76 diets containing varying levels of Mg--0.3 (-M), 1.3 (1/20M), 2.4 (1/10M), 9.2 (1/5M), 19 (control), 38 (2M), 102 (5M), and 187 (10M) mmol/kg diet--were fed to 3-week-old male Fischer-344 rats for 14d. Although the magnitude of abnormality was highest in kidney of rats fed the -M diet, the damage was normalized as the dietary level of Mg increased, with increasing serum Mg concentration and urinary excretion of Mg. We found almost no deposition of Ca in rats fed the 10M diet. The mechanism by which the high dietary Mg induces these effects most likely involves a competition between Mg and Ca for reabsorption in proximal and/or distal tubules, since these diets increased the urinary excretion of Ca. However, these high Mg diets decreased food intake and body weight gain compared with the control diet, although these indices were not decreased in rats fed the 2M diet. The results suggest that a dietary magnesium level approximately twice the normal level effectively reduces kidney calcification while maintaining normal growth in rats.     

Parathyroid hormone gene expression in Hyp mice fed a low-phosphate diet

BACKGROUND: The murine analogue of X-linked hypophosphataemia is the Hyp mouse; it has chronic phosphate depletion from an inherited defect of renal tubular reabsorption. Phosphate directly regulates the parathyroid (PT) in normal rats and it is of interest whether this regulation is intact in Hyp mice. METHODS: Hyp mice were fed either a low-phosphate diet or control diet and PTH mRNA levels were measured. In addition changes in NMR-visible kidney and muscle intracellular phosphate potentials in normal and Hyp mice were determined. Mice were maintained on a low-phosphate (0.02%) or normal-phosphate (0.6%) diet for 24 and 72 h. RESULTS: On the normal diet, Hyp mice had hypophosphataemia, normocalcaemia, and normal PTH mRNA levels. Phosphate deprivation for 72 h led to a profound fall in plasma phosphate, a slight but significant rise in plasma calcium, and a dramatic decrease in PTH mRNA, similar to that of normal mice fed this diet. Changes in kidney and muscle intracellular phosphate measured by NMR spectroscopy were not affected by diet or genotype. CONCLUSION: Dietary phosphate deprivation decreased Hyp mice PTH mRNA levels and caused no change in intracellular phosphate potentials. Therefore Hyp mice parathyroids' adapt appropriately to phosphate deprivation albeit at a lower threshold compared to normal mice.