Evidence for calcium-dependent control of 1,25-dihydroxyvitamin D3 production by rat kidney proximal tubules

Calcium-Sensing Receptors Control CYP27B1-Luciferase Expression: Transcriptional and Posttranscriptional Mechanisms

25-hydroxyvitamin D 1α-hydroxylase (encoded by CYP27B1), which catalyzes the synthesis of 1,25-dihydroxyvitamin D₃, is subject to negative or positive modulation by extracellular Ca²⁺ (Ca²⁺_o) depending on the tissue. However, the Ca²⁺ sensors and underlying mechanisms are unidentified. We tested whether calcium-sensing receptors (CaSRs) mediate Ca²⁺_o-dependent control of 1α-hydroxylase using HEK-293 cells stably expressing the CaSR (HEK-CaSR cells). In HEK-CaSR cells, but not control HEK-293 cells, cotransfected with reporter genes for CYP27B1-Photinus pyralis (firefly) luciferase and control Renilla luciferase, an increase in Ca²⁺_o from 0.5mM to 3.0mM induced a 2- to 3-fold increase in firefly luciferase activity as well as mRNA and protein levels. Surprisingly, firefly luciferase was specifically suppressed at Ca²⁺_o ≥ 5.0mM, demonstrating biphasic Ca²⁺_o control. Both phases were mediated by CaSRs as revealed by positive and negative modulators. However, Ca²⁺_o induced simple monotonic increases in firefly luciferase and endogenous CYP27B1 mRNA levels, indicating that the inhibitory effect of high Ca²⁺_o was posttranscriptional. Studies with inhibitors and the CaSR C-terminal mutant T888A identified roles for protein kinase C (PKC), phosphorylation of T888, and extracellular regulated protein kinase (ERK)_1/2 in high Ca²⁺_o-dependent suppression of firefly luciferase. Blockade of both PKC and ERK_1/2 abolished Ca²⁺_o-stimulated firefly luciferase, demonstrating that either PKC or ERK_1/2 is sufficient to stimulate the CYP27B1 promoter. A key CCAAT box (−74 bp to −68 bp), which is regulated downstream of PKC and ERK_1/2, was required for both basal transcription and Ca²⁺_o-mediated transcriptional upregulation. The CaSR mediates Ca²⁺_o-dependent transcriptional upregulation of 1α-hydroxylase and an additional CaSR-mediated mechanism is identified by which Ca²⁺_o can promote luciferase and possibly 1α-hydroxylase breakdown.

Vitamin D regulates prostate cell metabolism via genomic and non-genomic mitochondrial redox-dependent mechanisms

Vitamin D deficiency has been associated with increased risk for aggressive prostate cancer (PCa). Prostate epithelium has a unique metabolism compared to other tissues. Normal prostate exhibits low levels of mitochondrial respiration and there is a metabolic switch to increased oxidative phosphorylation in PCa. 25-hydroxyvitamin D (25(OH)D) is the major circulating form of vitamin D and is used clinically to determine vitamin D status. Activation of 25(OH)D to the transcriptionally active form, 1,25(OH)₂D occurs via a reduction-oxidation (redox) reaction within the mitochondria that is catalyzed by the P450 enzyme, CYP27B1. We sought to determine if hydroxylation of 25(OH)D by CYP27B1 contributes to non-genomic activity of vitamin D by altering the redox-dependent state of the mitochondria in benign prostate epithelial cells. Exposure to 25(OH)D produced a transient pro-oxidant effect and change in mitochondrial membrane potential that was dependent on CYP27B1. Extended exposure ultimately suppressed mitochondrial respiration, consistent with a protective effect of 25(OH)D in supporting benign prostate metabolism. To model physiologically relevant changes in vitamin D, cells were cultured in constant 25(OH)D then changed to high or deficient concentrations. This model also incurred a biphasic effect with a pro-oxidant shift after short exposure followed by decreased respiration after 16 h. Several genes involved in redox cycling and Mitochondrial Health were regulated by 25(OH)D in these cells. These results indicate a secondary non-genomic mechanism for vitamin D to contribute to prostate cell health by supporting normal mitochondrial respiration.

Villin promoter-mediated transgenic expression of transient receptor potential cation channel, subfamily V, member 6 (TRPV6) increases intestinal calcium absorption in wild-type and vitamin D receptor knockout mice

Transient receptor potential cation channel, subfamily V, member 6 (TRPV6) is an apical membrane calcium (Ca) channel in the small intestine proposed to be essential for vitamin D-regulated intestinal Ca absorption. Recent studies have challenged the proposed role for TRPV6 in Ca absorption. We directly tested intestinal TRPV6 function in Ca and bone metabolism in wild-type (WT) and vitamin D receptor knockout (VDRKO) mice. TRPV6 transgenic mice (TG) were made with intestinal epithelium-specific expression of a 3X Flag-tagged human TRPV6 protein. TG and VDRKO mice were crossed to make TG-VDRKO mice. Ca and bone metabolism was examined in WT, TG, VDRKO, and TG-VDRKO mice. TG mice developed hypercalcemia and soft tissue calcification on a chow diet. In TG mice fed a 0.25% Ca diet, Ca absorption was more than three-fold higher and femur bone mineral density (BMD) was 26% higher than WT. Renal 1α hydroxylase (CYP27B1) mRNA and intestinal expression of the natural mouse TRPV6 gene were reduced to <10% of WT but small intestine calbindin-D(9k) expression was elevated >15 times in TG mice. TG-VDRKO mice had high Ca absorption that prevented the low serum Ca, high renal CYP27B1 mRNA, low BMD, and abnormal bone microarchitecture seen in VDRKO mice. In addition, small intestinal calbindin D(9K) mRNA and protein levels were elevated in TG-VDRKO. Transgenic TRPV6 expression in intestine is sufficient to increase Ca absorption and bone density, even in VDRKO mice. VDR-independent upregulation of intestinal calbindin D(9k) in TG-VDRKO suggests this protein may buffer intracellular Ca during Ca absorption. © 2012 American Society for Bone and Mineral Research.

Dietary agents in the chemoprevention of prostate cancer

Nutritional factors have been estimated to contribute 20-60% of cancers around the globe, and almost one-third of deaths are being reported in Western countries. According to estimates by the American Cancer Society, during the year 2005 about 232,090 new cases of prostate cancer will be diagnosed alone in the United States and 30,350 men will die of this disease. The high incidence and long latency period of prostate cancer offer plenty of time to pursue strategies toward prevention and/or treatment to suppress or revert this disease. Epidemiological evidence suggests that plant-based dietary agents decrease the risk of some types of human cancer, including prostate cancer. Intake of 400-600 g/day of fruits and vegetables is associated with reduced risk of several cancers. The use of micronutrients and/or other phenolic agents in the diet or synthetic exogenous supplements to prevent neoplastic transformation of normal cells or to slow the progression of established malignant changes in cancer cells is termed "chemoprevention." Considerable attention has been devoted to identify plant-based dietary agents that may serve as natural inhibitors of prostate carcinogenesis. Much progress has been made in the last decade in this area of investigation through identification of pathways that play important roles in prostate tumorigenesis. This article summarizes epidemiological, clinical, and mechanistic studies and the significance of plant-derived dietary agents such as flavonoids, indoles, isothiocyanates, phenolics, monoterpenes, and complementary and alternative agents in the management of prostate cancer with recommendations for future studies to advance this area of research.

Gene profiling the effects of calcium deficiency versus 1,25-dihydroxyvitamin D induced hypercalcemia in rat kidney cortex

Determinants involved in the activation and repression of 1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)) synthesis in renal cortex by changes in extracellular Ca were studied. Cortical kidney RNA isolated from hypocalcemic (LC) rats generated by a low Ca diet, and hypercalcemic (HC) rats generated by a normal Ca diet and two sequential 1 microg doses of 1,25(OH)(2)D(3). Among the genes up-regulated were 1alpha-OHase (4.6-fold) in the LC group and high differential gene expression of VDR (4.0-fold) and 24-OHase (10.4-fold) in the HC group. Moreover, the exposure of renal cortex to LC versus HC conditions revealed a high differential expression of a PKA-dominated pathway involving CBP interacting protein, GATA-1 and CREB transcription factors in the LC model. In the HC model, elevated renal cortex gene expression of several growth factors, peptide receptors, and intracellular signaling molecules depicts a role for CaSR activation and receptor tyrosine kinase signaling in 1,25(OH)(2)D(3)-mediated gene activation and repression of 1alpha-OHase.

Insulin-like growth factor-I stimulates renal 1, 25-dihydroxycholecalciferol synthesis in old rats fed a low calcium diet

The adaptive increase in renal proximal tubule 25-hydroxyvitamin D-alpha-hydroxylase activity (1-OHase) during dietary calcium restriction is mediated by an increase in parathyroid hormone (PTH) and is inhibited by aging. Recent studies in mature (3-4 mo) rats demonstrated that insulin-like growth factor-I (IGF-I) restored stimulation of renal 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)] production by low phosphorus diet (LPD), another major stimulus of 1-OHase. These studies were designed to determine whether IGF-I stimulates 1-OHase during low calcium intake in old rats. Male rats were fed a normal calcium diet (NCD, 6 g Ca/kg diet) or low calcium diet (LCD, 0.2 g Ca/kg diet) for 14 d, and recombinant human IGF-I [rhIGF-I, 1.4 mg/(24h 160 kg body wt)] or vehicle was administrated via miniosmotic pump for 72 h before killing. In 4-mo-old male Sprague-Dawley rats, LCD increased in vitro renal 1-OHase activity in the presence but not in the absence of rhIGF-I. LCD increased in vitro1-OHase activity in young (1-mo-old) but not old (24-mo-old) male Fischer 344 rats. RhIGF-I increased 1-OHase activity in 24 mo-old rats fed LCD to levels that were not different from those in 1-mo-old rats fed LCD. The results indicate that the adaptive increase in 1-OHase activity due to a LCD is lost by 4 mo in rats and can be restored by pharmacologic doses of rhIGF-I.

Effect of parathyroid hormone on renal calbindin-D28k

The present investigation was conducted to examine the effects of parathyroid hormone (PTH) and parathyroid hormone related peptide (PTHrP) on renal calbindin-D28k in rats. Four groups of studies were performed: (1) parathyroidectomy (PTX) or a sham operation followed by infusion of 1,25-dihydroxyvitamin D (1,25[OH]2D) or vehicle; (2) infusions of PTH(1-34), PTH(1-84), 1,25(OH)2D, or vehicle; (3) infusion of PTHrP(1-34), PTHrP (1-86), PTH(1-34), or vehicle; and (4) injections of calcium or vehicle. PTX reduced renal calbindin-D28k levels even when plasma concentrations of 1,25(OH)2D were kept constant by infusion of 1,25(OH)2D. Infusions of PTH(1-34), PTH(1-84), and 1,25(OH)2D all increased renal calbindin-D28k and plasma calcium, whereas PTHrP(1-34) and PTHrP(1-86) increased renal calbindin-D28k before an increase of plasma calcium took place. Hypercalcemia induced by the injection of calcium did not affect the levels of renal calbindin-D28k. The present data suggest that PTH and PTHrP exert a direct effect on renal calbindin-D28k, which is not mediated by changes of 1,25(OH)2D or calcium.

Calcium homeostasis in the laying hen. 1. Age and dietary calcium effects

An experiment was carried out to investigate the effects of age of laying hens (young = 22 wk vs old = 120 wk) in maintaining Ca homeostasis during periods of Ca depletion then repletion with Ca. Plasma Ca and P, tibia breaking strength and percentage ash, renal 25-hydroxycholecalciferol-1-hydroxylase (1 alpha-hydroxylase), and parathyroid hormone (PTH)-stimulated adenylate cyclase activities were studied during 28 d of Ca depletion on a .08% Ca diet (LC) and 28 d of Ca repletion on a 3.75% Ca diet (HC). When laying hens on a HC diet were placed on a LC diet, plasma Ca and P, tibia breaking strength and ash percentage, and renal PTH-dependent adenylate cyclase activity were significantly depressed, but renal 1 alpha-hydroxylase activity was significantly stimulated. These changes were greater in the young hens than in the older hens; therefore an interaction between age and dietary Ca was found. These changes were of a lesser magnitude at 28 d of Ca depletion, probably due to the cessation of egg laying and to the desensitization of hormone-mediated function. 1 alpha-Hydroxylase activity was significantly less during the repletion period. The age effect was most pronounced for 1 alpha-hydroxylase, with the younger birds expressing significantly higher activity and ability to respond to hypocalcemia. There was a significant increase in kidney weights in Ca-deficient groups at 14 and 28 d of Ca depletion. It is concluded that younger hens have greater adaptive responses to Ca restriction than do older hens.

Loss of parathyroid hormone-stimulated 1,25-dihydroxyvitamin D3 production in aging does not involve protein kinase A or C pathways

Intestinal calcium absorption declines with aging as a result of decreased renal 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] biosynthesis. At least part of the decline in 1,25-(OH)2D3 may be due to acquired resistance to parathyroid hormone (PTH) stimulation of renal 25-hydroxyvitamin D1-hydroxylase (1-OHase) activity. To test whether aging rats can increase 1,25-(OH)2D3 production in response to PTH, male rats of the same litter were fed a normal Ca diet and were sacrificed at 175-225 g (young rats) or 3 months later at 350-425 g (aging rats). At sacrifice, basal serum 1,25-(OH)2D3 levels (88 +/- 16 versus 49 +/- 8 pg/ml, P < 0.05) and in vitro renal proximal tubule 1-OHase activity (178 +/- 15 versus 77 +/- 5 pmol/mg protein/5 minutes, n = 6, P < 0.001) were lower in aging animals. rPTH-(1-34) (10(-11) or 10(-7) M) increased in vitro 1,25-(OH)2D3 secretion by perifused renal proximal tubules from young but not aging rats. For young and aging rats, rPTH-(1-34) (10(-7) M) increased proximal tubule cAMP-dependent protein kinase (PKA) activity, and lower concentrations (10(-11) M) stimulated translocation of protein kinase C (PKC) activity from cytosolic to soluble membrane proximal tubule cell fractions. The results of this study show that PTH activation of 1,25-(OH)2D3 production may involve both signaling pathways, with the PKC pathway responsive to lower concentrations of the hormone. The acquired resistance to PTH stimulation of 1,25-(OH)2D3 production in aging appears not to involve the hormonal activation of PKA or PKC.

Response of genetic hypercalciuric rats to a low calcium diet

A fundamental mechanism for hypercalciuria in genetic hypercalciuric rats appears due to a primary increase in intestinal calcium absorption. However previous studies could not exclude additional mechanisms to account for the hypercalciuria. To determine if enhanced bone mineral dissolution either as a primary abnormality or secondary to a defect in renal tubule calcium reabsorption is responsible for a component of the augmented calcium excretion we studied rats continually inbred for hypercalciuria. Nineteenth generation adult female idiopathic hypercalciuric (IH) and non-inbred control (Ctl) rats were fed 13 g/day of a normal calcium diet (0.6% calcium, NCD) for 10 days. Urine calcium excretion over the last seven days was greater in IH (34 +/- 2 mg/7 day) than in Ctl (2.9 +/- 0.3, P < 0.01) rats. Some rats in each group were continued on the same diet while others were fed a low calcium diet (0.02% calcium, LCD) for an additional 10 days; balance measurements were made over the final seven days. With LCD, urine calcium excretion was approximately 8-fold higher in IH compared to Ctl (13 +/- 2 mg/7 day vs. 1.6 +/- 0.1, IH vs. Ctl, respectively, P < 0.01). In IH rats percent calcium absorption was greater (59 +/- 3% vs. 45 +/- 3, IH vs. Ctl, P < 0.01), however calcium retention was negative (-1.9 +/- 2.0 mg/7 day vs. 6.5 +/- 0.5, IH vs. Ctl, P < 0.01) compared to Ctl rats. The fall in urine calcium excretion when IH rats are fed LCD indicates that enhanced intestinal calcium absorption is a primary mechanism of the hypercalciuria.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypocalcemia, but not PTH or hypophosphatemia, induces a rapid increase in 1,25(OH)2D3 levels in rats

This study determined whether acute decreases in plasma ionized calcium (Ca2+) levels regulate plasma 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels independent of changes in parathyroid hormone (PTH) secretion and plasma phosphate levels. Chronically catheterized rats were subjected to a hypocalcemic clamp (mean decrement of Ca2+ levels 0.38 +/- 0.04 mM), a rat PTH-(1-34) infusion, and a PTH vehicle infusion for 2 h. Plasma NH2-terminal immunoreactive PTH levels were elevated 3.2- and 8.7-fold during hypocalcemia and PTH infusion, respectively. Plasma phosphate decreased by 23 +/- 4 and 42 +/- 3% during hypocalcemia and PTH infusion, respectively. In response to hypocalcemia, plasma 1,25(OH)2D3 levels increased promptly, were significantly elevated by 15 min (56 +/- 23% increase), and continued to increase until the end of the experiment at 5 h (350 +/- 30% increase). In contrast, no changes in plasma 1,25(OH)2D3 levels occurred during the PTH infusion, but levels were elevated by 5 h, i.e., 3 h after the end of the infusion (360 +/- 20% increase). No significant changes in 25(OH)D3 or 24,25(OH)2D3 levels occurred in any protocol. Thus hypocalcemia rapidly elevates 1,25(OH)2D3 levels in rats, but the increase is not caused by elevated PTH secretion, hypophosphatemia, or elevated 25(OH)D3 levels. Furthermore, the increase in 1,25(OH)2D3 levels by hypophosphatemia does not occur rapidly. These studies show that there is a calcium-dependent mechanism that is independent of changes in PTH secretion and that results in the rapid elevation of plasma 1,25(OH)2D3 levels to counteract hypocalcemia.

Deficiency of vitamin D metabolites directly stimulates renal 25-hydroxyvitamin D3-1-hydroxylase activity in rats

Renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) enzyme activity in rats is known to be increased by parathyroid hormone (PTH), hypophosphatemia, and hypocalcemia. Thus, enzyme activity is markedly increased in vitamin D-deficient states, but whether this stimulation is a direct response to the vitamin D deficiency or only occurs following the associated changes in plasma calcium, phosphate, or PTH is unclear. We tested whether vitamin D deficiency per se influences 1-hydroxylase activity in renal cortical slices using a normocalcemic rat model of vitamin D deficiency. Weanling male rats were fed one of the following three diets: (A) 0.8% Ca, 0.5% P, 2.2 IU vitamin D3/g; or vitamin D-deficient diets containing, (B) 0.8% Ca, 0.5% P; and (C) 2.0% Ca, 1.25% P, 20% lactose. Vitamin D-deficient rats fed diet B were hypocalcemic with elevated PTH at both test periods, and 1-hydroxylase activity was increased more than 100-fold compared with rats fed diet A. Plasma calcium, phosphate, and PTH levels were the same in groups A and C, but 1-hydroxylase activity was also substantially elevated in group C versus group A rats (104- and 17-fold increases after 10 and 19 diet weeks, respectively). These data lead to the important conclusion that severe deficiency of vitamin D metabolites per se provides a strong and independent stimulus to renal 1-hydroxylase activity in rats, perhaps due to the absence of 1,25(OH)2D3-mediated enzyme inhibition.

Vitamin D metabolism in rats with adjuvant-induced arthritis

Adjuvant-induced arthritis in rats shares many of the features of humans with rheumatoid arthritis, including the development of osteopenia in areas distal to erosive joint disease. We established adjuvant arthritis in male and female Sherman strain rats and then studied external calcium balances and vitamin D metabolism during the period of acute active clinical, serologic, and pathologic arthritis and osteopenia and in the preclinical period. While ingesting a calcium-sufficient vitamin D-replete diet (0.6% calcium, 0.65% phosphorus, and 2.2 IU D3 per g food), female rats with arthritis demonstrated reduced calcium balance (arthritic, 36 +/- 8 versus control, 169 +/- 13 mg per 6 days, p less than 0.02) because of inefficient gastrointestinal absorption of calcium (arthritic 9.7% versus control 37%). This was associated with calcitriol deficiency (arthritic 52 +/- 7 versus control 70 +/- 10 pg/ml) and reduced osteocalcin levels. Male rats with arthritis demonstrated an inability to raise serum calcitriol levels to the same degree as control rats (200 +/- 30 versus 440 +/- 70, respectively) while ingesting a calcium-deficient diet (0.002% calcium, 0.34% phosphorus, and 2.2 IU D3 per g food) and also had reduced balance (59 +/- 7 versus 85 +/- 10 mg per 6 days, respectively) due in part to decreased efficiency of absorption (55 versus 67%). No abnormalities in calcium balance or in serum calcitriol levels on the sufficient diet were present in the preclinical period. Physiologic calcitriol replacement to arthritic female rats increased osteoid available for mineralization and increased mineral apposition rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Avian kidney mitochondrial hemeprotein P-4501 alpha: isolation, characterization and NADPH-ferredoxin reductase-dependent activity

We describe the isolation of cytochrome P-4501 alpha from chick-kidney mitochondria. Although, gel permeation HPLC yielded 41% of the total amount of P-450 present in cholate-solubilized hemeproteins, it produced a highly purified mixture from which the P-4501 alpha could be purified to homogeneity in a final detergent-free state by a single-step application of hydrophobic interaction HPLC using hydroxypropyl silica. The purified P-4501 alpha traveled as a single band in SDS gel electrophoresis with an apparent Mr = 57,000. The absolute spectrum of the P-4501 alpha (Fe3+) form gave a lambda max at 403 nm. This characteristic lends support to the anomalous high-spin heme electron paramagnetic resonance spectrum and the heme structure of P-4501 alpha which we have previously reported (Ghazarian et al. (1980) J. Biol. Chem. 255, 8275-8281; Pedersen et al. (1976) J. Biol. Chem. 251, 3933-3941). In reconstitution experiments with ferredoxin-dependent NADPH-cytochrome c (P-450) reductase complexes, P-4501 alpha catalyzed the hydroxylation of 25-hydroxy-9,10-secocholesta-5,7,10(19)-trien-3 beta-ol at the C-1 position exclusively with a turnover number of 0.03 min-1. This number is identical to that obtained from measurements of the catalytic activity in intact mitochondria, indicating that only one major species of cytochrome P-450 occurs in chick-kidney mitochondria. The complete responsiveness of cytochrome P-450 concentrations in intact mitochondria to the vitamin D status of chicks provided additional evidence that the major cytochrome P-450 species present in renal mitochondria is uniquely associated with vitamin D metabolism.

Acidosis inhibits 1,25-(OH)2D3 but not cAMP production in response to parathyroid hormone in the rat

Parathyroid hormone (PTH) is a major activator of renal proximal tubule 25-hydroxyvitamin D3-1-hydroxylase (1-OHase). Chronic metabolic acidosis (CMA) inhibits 1-OHase and reduces circulating 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] levels in rats fed a low-Ca diet (LCD, 0.002% Ca). To examine the cellular mechanism whereby CMA inhibits 1-OHase, PTH-dependent renal 1-OHase activity and cAMP were measured in proximal tubules isolated from rats fed LCD for 14 days and made acidotic by the addition of 1.5% ammonium chloride to the drinking water. Serum 1,25-(OH)2D3 and proximal tubule 1-OHase activity and cAMP content were lower in acidotic rats. hPTH-(1-34) (10(-7) M) in vitro increased cAMP content to equivalent concentrations in tubules from rats with CMA and from nonacidotic controls; however, PTH increased 1-OHase activity only in tubules from nonacidotic animals. Although forskolin increased tubule cAMP content to equivalent levels in tubules from acidotic and nonacidotic rats, 1-OHase activity declined in tubules from nonacidotic rats and remained suppressed in acidotic tubules. The results suggest that chronic metabolic acidosis inhibits the PTH activation of 1-OHase through alteration of one or more steps in a cAMP-independent messenger system. PTH and forskolin can increase cAMP production by acidotic and nonacidotic proximal tubules; however, 1-OHase activity is not restored to normal in acidotic tubules and nonacidotic tubule 1-OHase may be inhibited.

Regulation of 1,25-dihydroxyvitamin D3 by calcium in the parathyroidectomized, parathyroid hormone-replete rat

Parathyroid hormone (PTH) is a major stimulus for the renal production of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Elevated arterial blood ionized calcium ([Ca2+]) depresses serum 1,25-(OH)2D3 in nonparathyroidectomized rats even when serum PTH is maintained at high levels by infusion. However, suppression by [Ca2+] of endogenous PTH, causing the fall in 1,25-(OH)2D, cannot be excluded. To determine whether [Ca2+] regulates 1,25-(OH)2D3 in the absence of a variation in PTH, we parathyroidectomized (PTX) rats (post-PTX calcium levels less than 7.0 mg/dl), inserted arterial and venous catheters, and then replaced PTH using an osmotic pump. We varied [Ca2+] by infusing either 75 mM sodium chloride (control), 0.61 mumol/min of EGTA (EGTA), or calcium chloride at 0.61 mumol/min (low calcium) or 1.22 mumol/min (high calcium) for 24 h 5 days after surgery. Blood was then drawn from the rat through the arterial catheter. Compared with the control, [Ca2+] fell with EGTA, remained constant with the low-calcium infusion, and rose with the high-calcium infusion. 1,25-(OH)2D3 was correlated inversely with [Ca2+] in all four groups together (r = -0.635, n = 34, p less than 0.001), within the control group alone (r = -0.769, n = 11, p less than 0.002), and within the EGTA group alone (r = -0.774, n = 10, p less than 0.003). Serum phosphorus, PTH, and arterial blood pH were not different in any group, and none correlated with serum 1,25-(OH)2D3. We conclude that 1,25-(OH)2D3 levels are regulated by [Ca2+] independently of serum PTH, phosphorus, and acid-base status, all of which support the hypothesis that [Ca2+] is a principal regulator of serum 1,25-(OH)2D3 in the rat.

Characterization of primary cell cultures derived from rat renal proximal tubules

Proximal tubules were prepared from rat kidney cortex by collagenase digestion and purified by Percoll gradient centrifugation. Their enrichment was estimated by comparing the specific activities of various cell-specific enzymes in homogenates of renal cortex and of the isolated tubules. The tubules were cultured in a 50:50 mixture of Dulbecco's modified Eagle's and Ham's F12 media supplemented with insulin, transferrin, epidermal growth factor, hydrocortisone, and prostaglandin E1. After 2 to 3 d an extensive outgrowth of epithelial cells developed from the attached tubules. After 5 to 7 d near confluent monolayers were obtained. Hormonal responsiveness, marker enzyme activities, and transport properties were determined to further characterize the primary cultures. The cultured cells exhibited increased cyclic AMP production in response to parathyroid hormone but not calcitonin or vasopressin, consistent with the absence of cells derived from distal and collecting tubules. The cells also retained significant levels of 25-hydroxyvitamin D3-1 alpha-hydroxylase, alkaline phosphatase, and gamma-glytamyl-transpeptidase, three enzymes that are primarily associated with the proximal tubule. The cultured epithelial cells also exhibit a Na+-dependent phosphate and glucose transport systems. Therefore, the cells retain many functional properties that are characteristic of proximal tubules. Thus, the primary cultures should be suitable for the study of processes that occur specifically within this segment of the rat nephron.

Metabolic clearance rate and production rate of calcitriol in uremia

We have previously demonstrated that while both normal humans and dogs tightly control serum calcitriol levels after 25(OH)D administration, anephric humans and 5/6 nephrectomized dogs significantly increase circulating 1,25(OH)2D when supraphysiological concentrations of 25(OH)D are reached in serum. Plasma 1,25(OH)2D level is determined not only by its rate of production but also by its rate of degradation. To further characterize the mechanisms involved in the responses to 25(OH)D therapy in normal circumstances and in chronic uremia, we measured metabolic clearance rate (MCR) and production rate (PR) of 1,25(OH)2D in normal dogs and in dogs with moderate and severe renal failure, at normal and supraphysiological serum concentrations of 25(OH)D. Basal MCR in uremic dogs, either with moderate or with severe renal failure, did not differ significantly from normals (6.7 +/- 0.7, 6.8 +/- 0.4 and 6.8 +/- 0.3 ml/min, respectively). Oral 25(OH)D administration for two weeks did not affect MCR either in normal animals or in both groups of uremic dogs. 25(OH)D treatment did not affect production rates in normal dogs and in animals with moderate renal failure (with normal basal values of 1,25(OH)2D), but significantly increased 1,25(OH)2D production from 0.13 +/- 0.01 to 0.25 +/- 0.04 micrograms/day (P less than 0.05) in dogs with severe renal insufficiency. These data suggest that it is the basal level of 1,25(OH)2D which regulates the synthesis of 1,25(OH)2D in response to 25(OH)D administration in normal and uremic animals.

Effect of hypercalcemia-producing tumor on 1,25(OH)2D3 biosynthesis in athymic mice

Serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels are low in patients with malignancy-associated hypercalcemia (MAH), whereas murine models of MAH have high circulating 1,25(OH)2D3. To determine the effects of a hypercalcemia-producing tumor on circulating 1,25(OH)2D3, in vitro 25-hydroxyvitamin D1-hydroxylase (1OHase) activity was measured in kidneys from BALB/c athymic mice implanted with a hypercalcemia-producing human lung tumor. Twelve days of low-phosphorus diet (LPD) in control animals lowered serum phosphorus to levels found in tumor-bearing mice fed normal phosphorus diet (NPD; 4.1 +/- 0.3 vs. 4.4 +/- 0.7 mg/dl, P = NS) and increased 1OHase activity (1.6 +/- 0.2 vs. 3.9 +/- 0.7 pmol.mg protein-1.5 min-1, NPD vs. LPD, P less than 0.05). 1OHase activity was greater in tumor-bearing animals fed NPD compared with control animals fed LPD (8.4 +/- 0.6 vs. 3.9 +/- 0.7 pmol.mg protein-1.5 min-1, P less than 0.01). High-phosphorus intake suppressed 1OHase activity in both control and tumor-bearing animals. Seven days of parathyroid hormone infusion in control animals fed NPD raised serum calcium (9.4 +/- 0.2 vs. 13.3 +/- 1.6 mg/dl, P less than 0.05) and suppressed 1OHase activity (0.25 +/- 0.02 vs. 0.02 +/- 0.002 pmol.mg protein-1.5 min-1, P less than 0.001). The inverse relationship of serum phosphorus and 1OHase activity was much steeper in the tumor-bearing animals, with greater enzyme activity at comparable levels of serum phosphorus. The present study indicates that 1) factors produced by the tumor stimulate 1OHase activity, and 2) hypophosphatemia is required for expression of enhanced enzyme activity.

Extra-renal production of calcitriol in chronic renal failure

Renal 1-alpha-hydroxylase activity is tightly regulated in normal humans and intact animals. No significant changes in serum 1,25(OH)2D levels occur in response to vitamin D challenge. However, conflicting reports have appeared in the literature with regard to stimulation of 1,25(OH)2D production after 25(OH)D administration in uremia. To provide further insight into this issue, 25(OH)D at a dose of 100 micrograms every other day for two weeks followed by 50 micrograms every other day for the next two weeks was given orally to seven uremic mongrel dogs. After two weeks of 25(OH)D therapy, 1,25(OH)2D levels increased from 16.4 +/- 0.9 to 28.0 +/- 1.9 pg/ml (P less than 0.001) in parallel with a fourfold increase in 25(OH)D concentrations from a basal of 50.1 +/- 6.5 to 203.2 +/- 18.1 ng/ml. No significant changes in serum i-PTH, ICa or P were observed. Linear regression analysis of the relationship between serum concentrations of 1,25(OH)2D versus 25(OH)D, for each dog during this period, showed highly significant correlation coefficients. To evaluate the possibility that extra-renal sites contribute to the described enhanced 1,25(OH)2D net synthesis after 25(OH)D treatment, similar studies were performed in four anephric patients undergoing hemodialysis. Basal serum 1,25(OH)2D levels were 5.5 +/- 2.4 pg/ml and increased to 19.6 +/- 5.0 pg/ml after 25(OH)D administration. A significant correlation was also found for the relationship between serum levels of 1,25(OH)2D and 25(OH)D in anephrics (r = 0.72, P less than 0.001). The same therapy in four normal volunteers showed no significant changes in serum 1,25(OH)2D concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone-independent regulation of 1,25(OH)2D in response to inhibition of bone resorption

Both plasma level of 1,25-dihydroxyvitamin D [1,25(OH)2D] and intestinal Ca absorption increase after biphosphonate-induced inhibition of bone resorption. Parathyroid hormone (PTH) has been considered a key mediating element of this homeostatic response. In the present work, the role of PTH was assessed by studying the influence of 1-hydroxypentane-1,1-bisphosphonate (HPeBP) on vitamin D and Ca metabolism in both intact and thyroparathyroidectomized (TPTX) rats. In intact rats, HPeBP given at 0.1 mg P/kg body wt sc for 10 days strongly inhibited bone resorption without affecting bone formation. This effect was associated with a marked stimulation of intestinal Ca absorption and Ca balance. In this condition, HPeBP caused a marked rise in plasma 1.25(OH)2D without affecting the level of 25-hydroxyvitamin D. In TPTX rats, HPeBP given at same dose also inhibited bone resorption and enhanced plasma 1,25(OH)2D, intestinal Ca absorption and Ca balance. In summary, this study shows that bisphosphonates such as HPeBP with prevailing inhibitory activity on bone resorption induce a marked stimulation of both 1,25(OH)2D production and intestinal Ca absorption. This homeostatic response is not attenuated after PTH removal. Thus, as previously shown for the response to low Ca diet, PTH does not appear to be an essential mediating factor for stimulating 1,25(OH)2D production in response to an increase in bone mineral retention.