Biomedical subjects

R W Gray

Publications and source records attributed to R W Gray.

At least 37 records · Page 2Linked to original sources

PubMed1994

Regulated production and intracrine action of 1,25-dihydroxyvitamin D3 in the chick myelomonocytic cell line HD-11.

To better understand the extrarenal production of active vitamin D metabolites by cells of the monocyte/macrophage lineage, we investigated the 25-hydroxyvitamin D (25OHD)-1-hydroxylation reaction in the v-myc-transformed chick myelomonocytic cell line HD-11; the 1-hydroxylation reaction in this cell line has a high affinity for 25-hydroxylated vitamin D substrates, is localized to mitochondria, and is associated with cytochrome P450 activity. In this study we demonstrated that the HD-11 cell 1-hydroxylation reaction in vitro is not affected by the majority of extracellular regulatory factors that modulate expression of the renal 25OHD-1-hydroxylase in vivo. A 50% increase in extracellular calcium and phosphate concentrations, physiological inhibitory events for renal 1,25-dihydroxyvitamin D [1,25-(OH)2D] synthesis, did not decrease basal expression of the HD-11 cell 1-hydroxylation reaction, nor did a 50% decrease in extracellular calcium and phosphate concentrations, stimulatory signals for the 1-hydroxylase in vivo, increase 1,25-(OH)2D3 synthesis in vitro. Receptor-saturating concentrations of PTH and PTH-related peptide were similarly without effect. In contrast, the HD-11 1-hydroxylation reaction was significantly stimulated in a dose-dependent fashion by the macrophage stimulatory agents lipopolysaccharide [P < 0.001 at a maximum effective concentration (EC100) of 25 micrograms/ml] and interferon-gamma (P < 0.001 at EC100 of 1000 IU/ml) and by insulin-like growth factor-I (P < 0.01 at EC100 of 15 nM) with the rank order of stimulation being interferon-gamma > lipopolysaccharide > insulin-like growth factor-I. Dexamethasone (> or = 10 nM) and the cytochrome P450 inhibitors (EC100, 20 microM), ketoconazole, clotrimazole, and menadione, all significantly inhibited the HD-11 cell 1-hydroxylation reaction. The naphthoquinone menadione, which blocks electron transfer to the P450-associated enzyme, was the most effective inhibitor of the reaction in both intact cells (3 +/- 1% of basal expression; P < or = 0.002) and after reconstitution of HD-11 cell mitochondrial extracts with a ferredoxin, reductase, O2, and NADPH (5 +/- 1% of basal; P < or = 0.02). We have also shown that 1,25-(OH)2D3 produced from substrate 25OHD3 appears to exert an endogenous (intracrine) inhibitory effect on HD-11 cell growth; incubation of HD-11 cells with a concentration of ketoconazole (10 microM) known to reduce 1,25-(OH)2D3 production by roughly 50% restored 50% of the growth deficit induced by 1,25-(OH)2D3 (EC100, 100 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Intestinal malabsorption of 45calcium in young Gy mice, a second model for X-linked hypophosphatemia.

X-linked hypophosphatemia, a common metabolic bone disease in humans and mice (the Hyp and Gy mutations), is characterized by decreased plasma phosphate, decreased renal tubular reabsorption of phosphate, rickets, and osteomalacia. The question of whether intestinal malabsorption of calcium contributes to the bone disease is controversial. Intestinal absorption of 45Ca was studied in three different mouse colonies: Gy on B6C3H background, Hyp on B6C3H background, and Hyp on C57BL/6J background, all at 4 weeks of age. The duodenum was isolated by sutures, and 45Ca in a 150 mM NaCl and 2 mM CaCl2 solution at pH 7.2 was injected into the lumen. Absorption was measured by the amount of 45Ca remaining in the lumen and by the plasma isotope level. The Gy and Hyp mice of both sexes significantly malabsorbed 45Ca at 4 weeks of age compared to normal littermates. Following the 4 week study, intestinal absorption was measured at 2, 7-8, and 12 weeks of age in normal and Gy mice on the B6C3H background. At 2 and 7-8 weeks of age, the Gy males significantly malabsorbed 45Ca compared to their normal littermates. Serum 1,25-dihydroxyvitamin D was not significantly altered in Gy males at 4 weeks of age. This suggests the possibility of resistance of the intestine to stimulation. Malabsorption of calcium in young Gy and Hyp mice may exacerbate the low mineralization in their rachitic bone disease.

Age Factors↗

PubMed1993

Potassium causes calcium retention in healthy adults.

The administration of 60 mmol/d of KHCO3 to healthy adults reduced urinary calcium excretion by 0.9 mmol/d and caused calcium balance to become equivalently more positive. Other studies showed that 90 mmol/d of KHCO3 reduced both daily and fasting urinary calcium excretion rates, whereas deprivation of either KCl or KHCO3, using synthetic diets, was accompanied by increased daily and fasting urinary calcium excretion rates. A significant inverse relationship between the changes in urinary calcium and the changes in urinary potassium was observed: delta urinary Ca (mmol/d) = 0.29-0.015 delta urinary K (mmol/d); r = -0.65. Correlative evaluation of additional data suggested that the fall in urinary calcium during potassium administration may be related to the natriuretic effects of potassium, resulting in ECF-volume contraction or to potassium-induced phosphate retention and suppression of calcitriol synthesis, or to both mechanisms.

Animals↗

PubMed1992

Renal phosphate transport and vitamin D metabolism in X-linked hypophosphatemic Gy mice: responses to phosphate deprivation.

Two closely linked, nonallelic genes, Gy and Hyp, result in X-linked hypophosphatemia in mice. The present studies in Gy mice were undertaken to determine whether renal brush-border membrane Na(+)-phosphate cotransport kinetics and adaptive responses of renal phosphate transport and vitamin D metabolism to phosphate deprivation are comparable in the two mutant strains. Transport studies in purified brush-border membrane vesicles over a phosphate concentration range of 10-500 microM demonstrated that the apparent maximum velocity of the high affinity transport system is significantly decreased in Gy mice (420 +/- 110 vs. 710 +/- 100 pmol/mg protein.6 sec, Gy vs. normal; mean +/- SE; P less than 0.05), whereas the affinity of the cotransporter for phosphate is unchanged (apparent Km, 25 +/- 3 vs. 27 +/- 2 microM; NS). Feeding a low phosphate diet results in a significant fall in plasma phosphate and an increase in brush-border membrane Na(+)-phosphate cotransport in both normal (568 +/- 40 to 1416 +/- 139 pmol/mg protein.6 sec; P less than 0.01) and Gy mice (407 +/- 27 to 1236 +/- 132 pmol/mg protein.6 sec; P less than 0.01). While the low phosphate diet elicited a rise in plasma 1,25-dihydroxyvitamin D in normal mice (51 +/- 12 to 158 +/- 12 pM; P less than 0.01), a fall in plasma hormone levels was evident in phosphate-deprived Gy mice (90 +/- 22 to 23 +/- 11 pM; P less than 0.01). Phosphate deprivation decreased 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase), the first enzyme in the renal vitamin D catabolic pathway, in normal mice (117 +/- 21 to 69 +/- 8 fmol/mg protein.min), but increased enzyme activity in Gy mice (172 +/- 14 to 240 +/- 18 fmol/mg protein.min; P less than 0.05). Moreover, under both dietary conditions, 24-hydroxylase activity was significantly elevated in Gy mice. The present results demonstrate that hypophosphatemia in Gy mice can be attributed to a decrease in the maximum velocity of the high affinity Na(+)-phosphate cotransport process in renal brush-border membranes. Our results also show that while renal brush-border membrane phosphate transport is appropriately modulated by phosphate in Gy mice, phosphate regulation of vitamin D metabolism is apparently impaired in the mutant strain. The present findings provide evidence for phenotypic similarities between murine Gy and Hyp mutations.

Animals↗

PubMed1991

Hypercalciuria and stones.

Hypercalciuria, defined as the urinary excretion of more than 0.1 mmol Ca/kg/d (4 mg/kg/24 h), is observed in approximately 50% of patients with calcium oxalate/apatite nephrolithiasis and is one of the risk factors for stone formation. Urinary Ca excretion rates among such patients are higher than normal, despite comparable ranges of glomerular filtration rate (GFR) and serum ultrafiltrable Ca concentrations, and thus glomerular filtration of Ca, suggesting that hypercalciuria is the result of inhibition of net tubular Ca reabsorption. Although increased dietary NaCl or protein intake and reduced K intake increase urinary Ca excretion rates, urinary Ca excretion rates are higher among hypercalciuric stone formers than among normal subjects in relation to comparable ranges of urinary Na, SO4 (as a reflection of protein intake), or K excretion rates, indicating that these dietary factors are not primarily responsible for hypercalciuria. Hypophosphatemia is observed among a subset of hypercalciuric patients and consequent activation of 1,25-(OH)2-D synthesis increases intestinal Ca absorption and urinary calcium excretion. Other hypercalciuric patients exhibit augmented intestinal Ca absorption without elevated plasma 1,25-(OH)-2-D levels, suggesting that either the capacity of 1,25-(OH)2-D to upregulate its own receptor in the intestine or 1,25-(OH)2-D-independent intestinal Ca transport are responsible for increased Ca absorption and hypercalciuria. Hypercalciuric patients also exhibit accelerated radiocalcium turnover, negative Ca balances, reduced bone density, delayed bone mineralization, fasting hypercalciuria, and increased hydroxyproline excretion, all of which reflect participation of the skeleton and presumably a more generalized acceleration of Ca transport. Hypercalciuria may be familial.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium↗

PubMed1991

Potassium administration reduces and potassium deprivation increases urinary calcium excretion in healthy adults [corrected].

This study was undertaken to evaluate the effects of dietary K intake, independent of whether the accompanying anion is Cl- or HCO3-, on urinary Ca excretion in healthy adults. The effects of KCl, KHCO3, NaCl and NaHCO3 supplements, 90 mmol/day for four days, were compared in ten subjects fed normal constant diets. Using synthetic diets, the effects of dietary KCl-deprivation for five days followed by recovery were assessed in four subjects and of KHCO3-deprivation for five days followed by recovery were assessed in four subjects. On the fourth day of salt administration, daily urinary Ca excretion and fasting UCa V/GFR were lower during the administration of KCl than during NaCl supplements (delta = -1.11 +/- 0.28 SEM mmol/day; P less than 0.005 and -0.0077 +/- 0.0022 mmol/liter GFR; P less than 0.01), and lower during KHCO3 than during control (-1.26 +/- 0.29 mmol/day; P less than 0.005 and -0.0069 +/- 0.0019 mmol/liter GFR; P = 0.005). Both dietary KCl and KHCO3 deprivation (mean reduction in dietary K intake -67 +/- 8 mmol/day) were accompanied by an increase in daily urinary Ca excretion and fasting UCaV/GFR that averaged on the fifth day +1.31 +/- 0.25 mmol/day (P less than 0.005) and +0.0069 +/- 0.0012 mmol/liter GFR (P less than 0.005) above control. Both daily urinary Ca excretion and fasting UCaV/GFR returned toward or to control at the end of recovery. These observations indicate that: 1) KHCO3 decreases fasting and 24-hour urinary Ca excretion; 2) KCl nor NaHCO3, unlike NaCl, do not increase fasting or 24-hour Ca excretion and 3) K deprivation increases both fasting and 24-hour urinary Ca excretion whether the accompanying anion is Cl- or HCO3-. The mechanisms for this effect of K may be mediated by: 1) alterations in ECF volume, since transient increases in urinary Na and Cl excretion and weight loss accompanied KCl or KHCO3 administration, while persistent reductions in urinary Na and Cl excretion and a trend for weight gain accompanied K deprivation; 2) K mediated alterations in renal tubular phosphate transport and renal synthesis of 1.25-(OH)2-vitamin D, since KCl or KHCO3 administration tended to be accompanied by a rise in fasting serum PO4 and TmPO4 and a fall in fasting UPO4 V/GFR, a fall in serum 1,25-(OH)2-D and a decrease in fasting UCa V/GFR, while dietary KCl or KHCO3 deprivation were accompanied by a reverse sequence.

Adult↗

PubMed1991

Intestinal vitamin D-dependent calbindin-D9k and alkaline phosphatase in spontaneously hypertensive rats.

Intestinal Ca2+ malabsorption has been described in spontaneously hypertensive rats (SHRs), but the molecular basis for this defect is unknown. In this study, we measured intestinal alkaline phosphatase and vitamin D-dependent Ca(2+)-binding protein (calbindin-D9k), two proteins implicated in the active pathway of intestinal Ca2+ absorption. Both proteins were measured in the small intestines of SHRs and their normotensive controls, Wistar-Kyoto rats, before, during, and after development of hypertension (4, 9, 14, 18, and 28 wk of age). At all ages, alkaline phosphatase activity in duodenum (0-6 cm) was decreased by 30-57% (P less than 0.001) and by 47-75% in the 2nd intestinal segment (6-12 cm) (P less than 0.001-0.05). Calbindin-D9k was decreased similarly. The decreases of calbindin were statistically significant (P less than 0.001-0.05) in the duodena at 4, 14, 18, and 28 wk (9-30% decreases) and in the 2nd segment at 4, 14, and 18 wk (38-69% decreases; P less than 0.001-0.005). Decreased calbindin in SHRs was documented in animals from two suppliers. The deficiencies of calbindin-D9k and alkaline phosphatase could not be attributed to malnutrition or to a generalized brush-border defect as indicated by body weights and the intestinal marker enzyme sucrase. Although calbindin-D9k was decreased in young SHRs, the serum 1,25-dihydroxycholecalciferol [1,25(OH)2D3] was increased by 59 and 129% in 4- and 9-wk-old SHRs (P less than 0.001), respectively; by contrast, serum 1,25(OH)2D3 was unchanged or decreased in older SHRs.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1990

Ketoconazole reduces elevated serum levels of 1,25-dihydroxyvitamin D in hypercalcemic sarcoidosis.

The antifungal drug ketoconazole, a cytochrome P450 inhibitor, has been shown to inhibit renal 1,25-dihydroxyvitamin D production in vitro and to lower serum 1,25-dihydroxyvitamin D levels in normal subjects and in patients with primary hyperparathyroidism. To assess the usefulness of this drug in the hypercalcemia of sarcoidosis, a condition thought to result from overproduction of 1,25-dihydroxyvitamin D by sarcoid-involved tissues, two men with sarcoidosis, hypercalcemia, and elevated serum levels of 1,25-dihydroxy-vitamin D were given ketoconazole, 600-800 mg per day, for four to six days. Serum 1,25-dihydroxyvitamin D levels were markedly reduced (by approximately 40%) in both patients during ketoconazole administration, but serum calcium was not affected. In both patients, renal function deteriorated during ketoconazole treatment. We conclude that ketoconazole administration can lower the elevated serum 1,25-dihydroxyvitamin D levels in sarcoidosis. However, deterioration of renal function during ketoconazole administration as well as failure of hypercalcemia to be affected during short-term ketoconazole treatment suggest that this drug might not be appropriate for acute treatment of hypercalcemic sarcoidosis.

Adult↗

PubMed1990

Induction of 25-OH-vitamin D3 24- and 23-hydroxylase activities in partially purified renal extracts from pigs given exogenous 1,25-(OH)2D3.

A renal mitochondrial cytochrome P 450 preparation from pigs treated with exogenous 1,25-(OH)2D3 was reconstituted with an NADPH-generating system, adrenodoxin and adrenodoxin reductase. The reconstituted system catalyzed the conversion of the substrate, 25-OH-D3, to metabolites comigrating with authentic 23,25-(OH)2D3 and 24,25-(OH)2D3 in both straight- and reverse-phase high-performance liquid chromatography systems, which achieve separation of these metabolites from each other as well as from other vitamin D metabolites. The putative 23,25-(OH)2D3 product was resistant to periodate treatment, while the 24,25-(OH)2D3 product was sensitive, providing additional evidence for the identity of the products. Although induction of 24-hydroxylase activity has been studied using renal homogenates from several species, only recently have techniques become available to study the activity of the enzyme in a solubilized and reconstituted form. Using these techniques, the present study shows that production of 24,25-(OH)2D3 was increased more than 80-fold with 1,25-(OH)2D3 treatment compared with untreated controls, an effect much greater than that previously observed with homogenates. In addition, production of both 23,25-(OH)2D3 and 24,25-(OH)2D3 varied with substrate concentration and was consistent with a monooxygenase-linked enzyme reaction.

Animals↗

PubMed1990

Elevated 1,25-dihydroxyvitamin D3 and intestinal calbindin-D9k in the toothless rat.

The toothless (tl) rat is a nonlethal osteopetrotic mutation characterized by systemic skeletal sclerosis, growth plate morphology suggestive of rickets, and morphological evidence of reduced osteoclastic bone resorption. Vitamin D metabolites, serum calcium and phosphorus levels, and the developmental appearance of vitamin D-dependent intestinal calcium binding protein (calbindin-D9k) was studied in normal and mutant rats of tl stock from 7 to 35 days of age. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] was found to be significantly elevated in mutant animals by 7 days of age (71 +/- 9 pM, tl/tl vs. 24 +/- 8 pM, +/?) and continued to increase to a peak of 428 pM at the time of weaning. This was 240% higher than normals at this period. The elevated levels of 1,25-(OH)2D3 stimulated a significant and precocious appearance of intestinal calbindin-D9k in mutants, beginning by 14 days of age and reaching their peak levels at 21 days postpartum (25.6 +/- 1.7 micrograms/mg protein, tl/tl vs. 16.4 +/- 1.5 micrograms/mg protein, +/?). The cause of the elevated circulating levels of 1,25-(OH)2D3 in tl rats is unknown but may be due to the low serum phosphorus levels present in these animals.

Aging↗

PubMed1989

Solubilization and reconstitution of kidney 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylases from vitamin D-replete pigs.

Pig kidney mitochondrial 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylase activities have been solubilized with cholate/Emulgen 911 and reconstituted with NADPH, ferredoxin reductase and ferredoxin. All three of these components are required for full catalytic activity of both enzymes. Both products were identified by co-chromatography with authentic metabolites on both normal and reverse-phase h.p.l.c. using solvent systems which were shown to separate 10-oxo-19-nor-25-hydroxyvitamin D3 from 1,25-dihydroxyvitamin D3 [1,25-(OH)2-D3]. In addition, periodate treatment of the 24,25-(OH)2-D3 product resulted in complete loss of the product as measured by protein-binding assay. Further purification by p-chloroamphetamine-Sepharose chromatography of a solubilized extract from a pig fed a normal diet increased the specific content of the cytochrome P-450 from 0.019 to 0.239 nmol/mg and the 1 alpha-hydroxylase activity from 4.75 to 268 pmol/h per mg. Activity of the 24-hydroxylase in the crude solubilized extract was 6.3 pmol/h per mg, but was undetectable after partial purification by a p-chloroamphetamine-Sepharose column. However, further fractionation of this material by DEAE-Sepharose chromatography resulted in a further increase in 1 alpha-hydroxylase activity to 430 pmol/h per mg and detection of 24-hydroxylase in a separate fraction at a level of 53 pmol/h per mg. Production of 1,25-(OH)2-D3 was linear with time up to 2 h and was dependent upon ferredoxin concentration as well as cytochrome P-450 concentration in the range of 0-40 nM. In the presence of excess ferredoxin and adequate amounts of cytochrome P-450, 1,25-(OH)2-D3 production was also dependent upon substrate concentrations in the range of 0.25 to 2.5 microM yielding an estimated Km of 1 microM. In the presence of excess substrate and ferredoxin, the catalytic-centre activity of the enzyme was estimated to be 1 h-1.

25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗

PubMed1989

Parabiosis suggests a humoral factor is involved in X-linked hypophosphatemia in mice.

Reduced renal tubular reabsorption of phosphate of unknown etiology is characteristic of X-linked hypophosphatemia in both humans and mice. To test whether a humoral abnormality is involved in the renal effect, parabiosis was performed between Hyp and normal mice at 4 weeks of age. The normal mice joined to Hyp mice showed a progressive diminution of plasma phosphate over the next 3 weeks to approach the level of the Hyp mice. These normal mice had a greater renal phosphate excretion index (urine P/plasma P/urine creatinine) than normal-normal pairs, thus suggesting a reduced renal tubular reabsorption of phosphate. At the same time the expected rises in plasma calcium and plasma 1,25-dihydroxyvitamin D did not occur. There was a significant reduction in their femoral mineral content but not in their femoral length or body growth relative to normal-normal pairs. This change in renal handling of phosphate was specific since the urinary losses of potassium and magnesium were not significantly changed. Separation of normal-Hyp pairs 3 or 6 weeks after parabiosis caused the normal mice to achieve normal plasma phosphate levels within 24 h. At 48 h and 7 days after separation these normal mice had plasma phosphate levels higher than normal mice separated from normal-normal pairs. In summary, the data suggest the presence of a phosphaturic factor in the Hyp mice that can cross a parabiotic union into normal mice and induce many of the symptoms of X-linked hypophosphatemia.

Animals↗

PubMed1989

Idiopathic hypercalciuria.

Idiopathic hypercalciuria, defined as the urinary excretion of more than 300 mg. calcium per day in men or more than 250 mg. calcium per day in women, or more than 4 mg. calcium per kg. per day, is observed in about 50 per cent of the patients with calcium oxalate/apatite nephrolithiasis and is one of the risk factors for stone formation. These patients do not exhibit hypercalcemia, elevated serum parathyroid hormone concentrations or urinary cyclic adenosine monophosphate excretion nor clinical evidence of sarcoidosis, other granulomas or a malignancy. Hypophosphatemia may be present. Augmented rates of intestinal absorption of dietary calcium account for most of the increments in urinary calcium. Serum 1,25-dihydroxyvitamin D concentrations are in the upper normal range or elevated among many patients and are normal but not suppressed in the others. Activation of 1,25-dihydroxyvitamin D formation may be secondary to hypophosphatemia or other, as yet undefined, factors. Since, 1,25-dihydroxyvitamin D apparently can up-regulate its own receptor, small increments in its synthesis and blood levels could amplify the effect of the hormone to stimulate intestinal calcium absorption. Calcium balances are slightly but significantly negative and urinary hydroxyproline excretion may be increased so that a generalized disorder of calcium homeostasis also involving bone may be present. Additional studies are required to determine the genetic basis for the occurrence of idiopathic hypercalciuria in families, the cause of greater expression of idiopathic hypercalciuria in men and whether environmental factors (high dietary sodium chloride, protein and purified carbohydrate intakes) contribute to the expression of idiopathic hypercalciuria. Although thiazide diuretics, inorganic phosphate, magnesium hydroxide and potassium citrate have provided effective therapy, prospective studies are needed to determine optimum therapy and the optimum duration of treatment.

Calcitriol↗

PubMed1989

Potassium bicarbonate, but not sodium bicarbonate, reduces urinary calcium excretion and improves calcium balance in healthy men.

Previous studies demonstrated that the administration of NaHCO3 or sodium citrate had either only a small effect to reduce urinary Ca excretion or no effect, but that potassium citrate significantly reduced urinary Ca excretion. In order to further evaluate and compare the effects of NaHCO3 and of KHCO3, we performed ten metabolic balances in healthy men during 18 control days, 12 days of NaHCO3, 60 mmol/day and 12 days of KHCO3, 60 mmol/day. Six subjects were fed a low Ca diet (5.2 +/- 0.7 SD mmol/day) and three of these were also given calcitriol (0.5 microgram 6-hourly). Four subjects ate a normal Ca diet (19.5 +/- 1.3 mmol/day). For all 10 subjects, KHCO3 administration reduced urinary Ca excretion from control by -0.9 +/- 0.7 mmol/day, P less than 0.001. Net intestinal Ca absorption did not change detectably so that Ca balances became less negative by a +0.9 +/- 0.9 mmol/day; P = 0.01. KHCO3 administration was also accompanied by more positive PO4 and Mg balances. NaHCO3 administration had no significant effect on urinary Ca excretion or Ca balance. NaHCO3 and KHCO3 administration were accompanied by equivalently more positive Na or K balances, respectively and equivalently more negative acid balances (HCO3 retention). Neither NaHCO3 or KHCO3 altered fasting serum HCO3 concentrations, blood pH, serum 1,25-(OH)2-D or PTH concentrations. We conclude that KHCO3 promotes more positive Ca balances by either enhancing renal Ca retention or skeletal Ca retention or both.

Adult↗

PubMed1989

Epidermal growth factor increases intestinal calbindin-D9k and 1,25-dihydroxyvitamin D receptors in neonatal rats.

Epidermal growth factor (EGF) has been reported to increase intestinal calcium absorption in suckling rats. The mechanism of this effect is unknown, as are the roles of vitamin D-dependent and independent pathways. The present studies were undertaken to investigate the ability of EGF to accelerate the postnatal induction of the vitamin D-dependent intestinal calcium-binding protein, calbindin-D9k. Subcutaneous administration of EGF increased duodenal calbindin-D9k in suckling rats by more than 100% (P less than 0.001). The effect of EGF was not seen in older weaned animals or when EGF was given to suckling rats by gavage. Administration of EGF simulated the changes of normal development. 1) It increased calbindin-D9k, and the effect was greater in proximal than distal duodenum. 2) EGF increased alkaline phosphatase activity to the same extent in proximal and distal duodenum. 3) EGF increased sucrase more markedly in distal than in proximal epithelium. Maximal and half-maximal effects of EGF on each of these proteins were observed at twice daily doses of 0.1 and 0.04 microgram/g BW, respectively. 4) EGF at the maximally effective dose produced a small (30%) but statistically significant (P less than 0.005) increase in serum 1,25-dihydroxyvitamin D. 5) Most importantly, EGF treatment resulted in a 2-fold increase in intestinal 1,25-dihydroxyvitamin D receptors (VDR) in the proximal segments of the small intestine (P less than 0.001). EGF effects on calbindin-D9k and VDR were specific for the intestine, as EGF did not change kidney calbindin-D9k or kidney VDR. Thus, EGF was able to prematurely initiate a complex series of molecular changes that occur during normal development. The mechanism of EGF's action to stimulate calcium absorption appears to involve a maturation effect on the vitamin D-dependent pathway.

Alkaline Phosphatase↗

PubMed1989

Oxalate is overestimated in alkaline urines collected during administration of bicarbonate with no specimen pH adjustment.

We compared measurements of daily urine oxalate excretion in urines collected at the prevailing urine pH with measurements of urine oxalate excretion in urines collected into 20 mL of 6 mol/L HCl. We studied eight healthy adults fed constant diets. Urines were collected during control conditions and, in each subject, during the administration of NaCl, KCl, NaHCO3, or KHCO3, 90 mmol/day. Daily urine oxalate excretion calculated for collections made in acid averaged 271 (SD 79) mumol/day and did not vary with any of the salt supplements. When urines were collected at ambient urine pH (average 5.94, SD 0.23) during control conditions, and during the administration of NaCl or KCl, urine oxalate excretion averaged 263 (SD 88) mumol/day, a value not different from that for collections in acid. However, when urine was collected with no pH adjustment during NaHCO3 or KHCO3 administration (average pH 6.90, SD 0.14), apparent urine oxalate excretion averaged 398 (SD 132) mumol/day, significantly (P less than 0.025) exceeding the mean observed when urines were collected in acid. Moreover, the percentage increase in apparent oxalate excretion increased with urinary pH. These observations reinforce recommendations that urine specimens for measurement of oxalate be collected in acid to avoid the increase in apparent oxalate content that occurs during collection of alkaline urines. This increase presumably results from the well-known in vitro nonenzymatic conversion of ascorbate to oxalate.

Adult↗

PubMed1988

Elevated levels of vitamin D-dependent calcium-binding protein (calbindin-D9k) in the osteosclerotic (oc) mouse.

The osteosclerotic (oc) mouse is an osteopetrotic mutation that has recently been identified as having rickets associated with its osteopetrosis. The presence of this rachitic lesion, unexplainable from a nutritional standpoint, prompted an investigation into the vitamin D endocrine system in these animals. The developmental appearance of vitamin D-dependent calcium-binding protein (calbindin-D9k) and alkaline phosphatase was studied in oc mutant and normal mice from birth to weaning, as were serum concentrations of 25-hydroxyvitamin D3 (25OHD3), 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], calcium, and phosphorus. Intestinal and renal calbindin-D9k levels were markedly and precociously elevated (4- to 9-fold) in young suckling, but not newborn, mutant mice compared to values in normal controls. Serum 25OHD3 levels were very low to undetectable in 2-week-old mutant mice compared to normal values, while 1,25-(OH)2D3 levels were 6 times higher in mutants. The exact cause of this premature induction in mutants is unknown, but may be due to elevated circulating levels of 1,25-(OH)2D. Alkaline phosphatase activity was similar between phenotypes at all ages. These studies indicate that the rachitic lesion present in oc mutants may be the result of some inherited disorder in vitamin D metabolism in these animals. Alternatively, these data are also consistent with a normal appropriate response to hypocalcemia and hypophosphatemia resulting from decreased osteoclastic bone resorption.

Aging↗

PubMed1987

Evidence that low plasma 1,25-dihydroxyvitamin D causes intestinal malabsorption of calcium and phosphate in juvenile X-linked hypophosphatemic mice.

X-linked hypophosphatemic (Hyp) mice are a model for human sex-linked vitamin D-resistant rickets. We have reported intestinal malabsorption of calcium in young Hyp mice, and in this report we have explored the mechanism for it. To test for resistance of the intestine to 1,25(OH)2 vitamin D3, this hormone was continually infused via osmotic minipumps into 4-week-old normal and Hyp mice at 0, 17, 50 or 150 ng/kg/day. After 3 days, 45Ca and inorganic 32P were administered by gavage, and the mice were sacrificed on the fifth day. The Hyp mice showed responses to the hormone equivalent to the normal mice in terms of increased intestinal absorption of both 45Ca and 32P, increased plasma isotope levels, increased femoral isotope content, and increased duodenal and renal 9 kD vitamin D-dependent calcium-binding protein (calbindin-D9K; CaBP). Plasma 1,25(OH)2D was measured in these mice. There were significant correlations of plasma 1,25(OH)2D to the intestinal absorption of 45Ca and 32P and to duodenal and renal CaBP. Plasma 1,25(OH)2D was also measured in stock normal and Hyp mice and was found to be lower in 4-week-old Hyp mice than in 4-week-old normal mice (113 +/- 10 pM (n = 18) vs. 67 +/- 10 (n = 20), normal vs. Hyp, p less than .01), but unchanged at 13 weeks of age (77 +/- 13 (n = 13) vs. 70 +/- 15 (n = 15), NS). This observed difference in plasma 1,25(OH)2D between normal and Hyp mice at 4 weeks of age was sufficient to explain the observed normal-to-Hyp differences in intestinal absorption of 45Ca and duodenal and renal CaBP. It also explained 72 +/- 18% of the observed difference in 32P absorption. We conclude that Hyp mouse intestine is not resistant to 1,25(OH)2D and that the lower plasma 1,25(OH)2D of 4-week-old Hyp mice causes intestinal malabsorption of calcium and phosphate.

Animals↗