Separate effects of 1,25-dihydroxyvitamin D and calcium on renal calbindin-D28k and intestinal calbindin-D9k

The effects of normal, low and high plasma concentrations of 1,25-(OH)2vitamin D (1,25-(OH)2D) combined with normal, low and high concentrations of plasma calcium on renal calbindin-D28k and intestinal calbindin-D9k were examined in rats. We found that the expression of renal calbindin-D28k was significantly (P<0.05) increased by high levels of 1,25-(OH)2D, but was not affected by a 50% reduction in 1,25-(OH)2D. In contrast the intestinal calbindin-D9k responded significantly (P<0.005) to both high and low 1,25-(OH)2D levels. The effect of 1,25-(OH)2D on intestinal calbindin-D9k was modulated by plasma calcium concentrations. Increased plasma calcium levels did not affect the renal calbindin-D28k concentrations, but suppressed intestinal calbindin-D9k (P<0.05). The effect of calcium was not mediated by calciotropic hormones. This suggests the existence of a calcium-sensing mechanism in the proximal duodenum. It is concluded that intestinal calbindin-D9k is more sensitive than renal calbindin-D28k to changes in 1,25-(OH)2D and that intestinal calbindin-D9k in contrast to renal calbindin-D28k is sensitive to changes in plasma calcium concentrations.     

Separate Effects of 1,25‐Dihydroxyvitamin D and Calcium on Renal Calbindin‐D28k and Intestinal Calbindin‐D9k

Abstract: The effects of normal, low and high plasma concentrations of 1,25‐(OH)₂vitamin D (1,25‐(OH)₂D) combined with normal, low and high concentrations of plasma calcium on renal calbindin‐D28k and intestinal calbindin‐D9k were examined in rats. We found that the expression of renal calbindin‐D28k was significantly (P<0.05) increased by high levels of 1,25‐(OH)₂D, but was not affected by a 50% reduction in 1,25‐(OH)₂D. In contrast the intestinal calbindin‐D9k responded significantly (P<0.005) to both high and low 1,25‐(OH)₂D levels. The effect of 1,25‐(OH)₂D on intestinal calbindin‐D9k was modulated by plasma calcium concentrations. Increased plasma calcium levels did not affect the renal calbindin‐D28k concentrations, but suppressed intestinal calbindin‐D9k (P<0.05). The effect of calcium was not mediated by calciotropic hormones. This suggests the existence of a calcium‐sensing mechanism in the proximal duodenum. It is concluded that intestinal calbindin‐D9k is more sensitive than renal calbindin‐D28k to changes in 1,25‐(OH)₂D and that intestinal calbindin‐D9k in contrast to renal calbindin‐D28k is sensitive to changes in plasma calcium concentrations.     

COMPARISON OF PARATHYROID HORMONE AND CALCITONIN ON RAT RENAL CALCIUM AND MAGNESIUM TRANSPORT

1. Because in vitro adenylate cyclase activity studies suggest that parathyroid hormone (PTH) and calcitonin (CT) increase renal tubular calcium and magnesium reabsorption by stimulating the same transport mechanism, the separate and combined effects of these hormones on calcium and magnesium transport was assessed in the rat. 2. Fractional excretion of calcium was 9.5 +/- 0.5% in calcium-infused thyroparathyroidectomized (TPTX) rats and was reduced to 5.1 +/- 0.6 and 4.1 +/- 0.6% by maximal hypocalciuric concentrations of PTH and CT, respectively. 3. Combined administration of maximal PTH and CT produced an effect on fractional and absolute calcium excretion similar to that recorded with maximal CT alone. The combined administration of half-maximal concentrations of PTH and CT also produced a comparable fall in the fractional excretion of calcium to 3.0 +/- 0.7%, which was similar to that observed in the 'maximal CT' and 'maximal PTH plus CT' groups. The magnesium reabsorption data were comparable. 4. These results support biochemical data suggesting that PTH and CT act upon the same transport site, presumably within the thick ascending limb of Henle's loop, to facilitate calcium and magnesium reabsorption.     

Effect of low-level lifetime exposure to cadmium on calciotropic hormones in aged female rats

The effect of low-level lifetime exposure to cadmium (Cd) on calciotropic hormones and the possible association between the Cd-induced disorders in bone metabolism and these hormones were investigated on a female rat model of human environmental exposure in areas unpolluted by this metal. For this purpose, the concentrations of 25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D (1,25(OH)₂D), calcitonin (CT) and parathormone (PTH) were measured in the serum of control and Cd-exposed (1 mg Cd/l in drinking water for 24 months) female rats. Calcium (Ca) and inorganic phosphorus (P_i) serum concentrations, renal tubular reabsorption of Ca (TRCa) and phosphate (TRP) and the glomerular filtration rate (GFR) were estimated as well. Moreover, 1,25(OH)₂D, metallothionein (MT) and Cd were determined in the kidney. The exposure to Cd led to a decrease in the serum concentrations of 25OHD and 1,25(OH)₂D (by 50 and 31%, respectively) and the concentration of 1,25(OH)₂D in the kidney mitochondrial fraction (by 55%). The serum concentrations of CT and PTH increased (5.2-fold and by 29%, respectively) and those of Ca and P_i were unchanged, whereas the TRCa, TRP and GFR decreased due to the exposure to Cd. The results give evidence that the low lifetime exposure to Cd disturbs the metabolism of calciotropic hormones and damages the reabsorptive and filtrative function of the kidney in aged female rats. Numerous correlations noted between calciotropic hormones and the indices of kidney function, and indices of bone turnover and bone mineral status (bone mineral content and density) of these females indicate a relationship between these hormones and the kidney functional status and bone metabolism. The results of the present study together with our previous findings on the bone status in the experimental model allow for the conclusion that the low lifetime exposure to Cd by affecting the metabolism and proper function of calciotropic hormones may contribute to the advancement of bone damage at the elderly.     

Acute effects of parathyroid extract on renal vitamin D hydroxylases in Japanese quail.

The dose- and the time-response relationships of parathyroid hormone's modulation of in vitro 1,25-(OH)2D3 and 24,25-(OH)2D3 production in Japanese quail were investigated. 4-week-old female Japanese quail were injected intramuscularly with three different doses (30, 90 and 270 USP units/kg) of parathyroid extract (PTH). 4, 12 and 24 h after the in vivo administration of PTH, kidney homogenates were incubated with tritiated 25-(OH)D3. All three doses of PTH stimulated 1,25-(OH)2D3 and inhibited 24,25-(OH)2D3 production at 12 h relative to the vehicle-treated control group. The responses of the vitamin D hydroxylases were not detectable at 4 h and had returned to control levels by 24 h. Plasma calcium rose significantly at 4 h with the two higher doese of PTH (90 and 270 USP units/mg) but not with the lowest dose. By 12 h the plasma calcium had returned to control levels except at the highest dose of PTH. By 24 h all of the plasma calcium levels had returned to control values. Plasma inorganic phosphate levels were depressed at 4 h by all three PTH doses; they remained depressed at 12 h and returned to control levels by 24 h. It is concluded that the pharmacological effects of PTH on the renal vitamin D hydroxylases are slower in onset and shorter in duration when compared with those of estradiol.     

Hydroxycholecalciferols modulate parathyroid hormone and calcitonin sensitive adenylyl cyclase in bone and kidney of rats. A possible physiological role for 24,25-dihydroxy vitamin D3

In particulate fractions from rat bone cells, but not from kidney, 24,25-(OH)2 D3 inhibits in a dose dependent manner (1 nM and above) the parathyroid hormone (PTH)-activated adenylyl cyclase. In contrast, 24,25-(OH)2D3 enhances the calcitonin (CT) stimulated cyclase in bone, but attenuates the CT-induced cyclase response in kidney. In supranormal concentrations 1,25-(OH)2D3 is also able to reduce the PTH-stimulated adenylyl cyclase in bone. In comparison, neither vitamin D3 metabolite interferes with stimulation of adenylyl cyclase from pituitary cell membranes by thyroliberin (TRH) or vasoactive intestinal polypeptide (VIP). These findings may have important therapeutical consequences in preventing excessive PTH action and bone demineralization.     

Regulation of sodium, calcium and vitamin D metabolism in Dahl rats on a high-salt/low-potassium diet: genetic and neural influences

1. A dietary combination of high salt and low potassium (HSLK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders previously normotensive Dahl salt-resistant (DR) rats hypertensive. In both strains, the severity of hypertension correlates with urinary calcium loss. However, the magnitude of excretory calcium losses is significantly greater in DS rats and is potentiated by chemical sympathectomy in both strains. 2. We hypothesized that a defect in vitamin D metabolism may underlie the observed strain-dependent differences in calcium balance. 3. Arterial blood pressure (ABP), water and mineral balance and serum concentrations of 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3) and 25-hydroxyvitamin D3 (25(OH)D3) were measured in intact and chemically sympathectomized (6-hydroxydopamine; 6-OHDA) DS and DR rats after 8 weeks on a HSLK diet. 4. Chronic ingestion of this diet resulted in marked and moderate levels of hypertension in DS and DR rats, respectively. The hypertension was abated and eliminated by 6-OHDA in the DS and DR strains, respectively. Independent of treatment, DS rats had significantly higher urinary excretion of calcium and reduced intestinal absorption of the ion compared with DR rats. The DS rats had significantly higher serum levels of 1,25(OH)2 D3 and markedly lower serum levels of 25(OH)D3 than DR rats. Chemical sympathectomy tended to increase 1,25(OH)2 D3 and to decrease 25(OH)D3 levels in both strains. 5. These data indicate a genetic difference in vitamin D metabolism between DS and DR rats. The abnormally elevated levels of 1,25(OH)2 D3 in DS rats may be an appropriate compensatory response to excessive excretory calcium loss and reduced target organ sensitivity to the hormone and may, maladaptively, directly contribute to hypertension, by stimulating vascular smooth muscle contractility.     

Influence of cadmium on the metabolism of vitamin D3 in rats

In order to obtain further information on the effects of cadmium (Cd) on the mechanism of activation of vitamin D3 in the kidney, the serum concentrations of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3] were determined by radioimmunoassay techniques. The serum concentration of 1,25(OH)2D3 in Cd-exposed rats was always higher than that in control rats. The concentrations of serum 1,25(OH)2D3 in parathyroidectomized rats (PTX), both in the control and in the Cd-exposed groups, were markedly lower than those in non-PTX rats. On the other hand, the concentration of serum 24,25(OH)2D3 in Cd-exposed rats was less than that in control rats. In other words, the pathway from secretion of parathyroid hormone (PTH) to synthesis of 1,25(OH)2D3 or 24,25(OH)2D3 was normal, but the pathway from stimulation of PTH to secretion of PTH was abnormal. These results suggest that rats exposed to Cd for 90 days were in a state of either deficiency of 1,25(OH)2D3 or excess secretion of parathyroid hormone. Although hypophosphatemia occurred in the Cd-exposed rats, the 1,25(OH)2D3 serum level in rats was not increased by PTX. On the basis of these results, it is suggested that hypophosphatemia occurring after the exposure of rats to Cd is a secondary hypophosphatemia.     

Alterations in gene expression in vitamin D‐deficiency: Down‐regulation of liver Cyp7a1 and renal Oat3 in mice

The vitamin D‐deficient model, established in the C57BL/6 mouse after 8 weeks of feeding vitamin D‐deficient diets in the absence or presence of added calcium, was found associated with elevated levels of plasma parathyroid hormone (PTH) and plasma and liver cholesterol, and a reduction in cholesterol 7α‐hydroxylase (Cyp7a1, rate‐limiting enzyme for cholesterol metabolism) and renal Oat3 mRNA/protein expression levels. However, there was no change in plasma calcium and phosphate levels. Appraisal of the liver revealed an up‐regulation of mRNA expressions of the small heterodimer partner (Shp) and attenuation of Cyp7a1, which contributed to hypercholesterolemia in vitamin D‐deficiency. When vitamin D‐sufficient or D‐deficient mice were further rendered hypercholesterolemic with 3 weeks of feeding the respective, high fat/high cholesterol (HF/HC) diets, treatment with 1α,25‐dihydroxyvitamin D₃ [1,25(OH)₂D₃], active vitamin D receptor (VDR) ligand, or vitamin D (cholecalciferol) to HF/HC vitamin D‐deficient mice lowered the cholesterol back to baseline levels. Cholecalciferol treatment partially restored renal Oat3 mRNA/protein expression back to that of vitamin D‐sufficient mice. When the protein expression of protein kinase C (PKC), a known, negative regulator of Oat3, was examined in murine kidney, no difference in PKC expression was observed for any of the diets with/without 1,25(OH)₂D₃/cholecalciferol treatment, inferring that VDR regulation of renal Oat3 did not involve PKC in mice. As expected, plasma calcium levels were not elevated by cholecalciferol treatment of vitamin D‐deficient mice, while 1,25(OH)₂D₃ treatment led to hypercalcemia. In conclusion, vitamin D‐deficiency resulted in down‐regulation of liver Cyp7a1 and renal Oat3, conditions that are alleviated upon replenishment of cholecalciferol.     

Aflatoxicosis alters avian renal function, calcium, and vitamin D metabolism

Experiments were designed to determine the effects of aflatoxicosis on avian renal function, calcium (CA), inorganic phosphorous (Pi), and vitamin D metabolism, and to determine if the effects of aflatoxin are reversible upon discontinuation of toxin administration. Three-week-old male broiler chickens (n = 12 per treatment) received aflatoxin (AF; 2 mg/kg po) or an equal volume of corn oil, the AF carrier vehicle, for 10 consecutive days. After 10 d of treatment, half of the birds from each treatment group were anesthetized and prepared for renal function analysis, which included a 2-h phosphate loading period. Ten days after discontinuation of AF treatment, the remaining birds in each treatment group were anesthetized and prepared for renal function analysis. AF decreased plasma 25-hydroxy vitamin D [25(OH)D] and 1,25-dihydroxy vitamin D [1,25(OH)2D] levels after 5 d of treatment. After 10 d of treatment, urine flow rate (V), fractional sodium excretion (FENa), and fractional potassium excretion (FEK) were lower in AF-treated birds. In addition, total plasma Ca tended to be lower (p = .10) and fractional Ca excretion (FECa) tended to be higher (p = .10) in the AF-treated birds. Intravenous phosphate loading produced a sharp increase in urine hydrogen ion concentration ([H+]) in the AF-treated birds. Glomerular filtration rate (GFR) was reduced and plasma osmolality was increased in AF-treated birds 10 d after discontinuation of toxin administration. The results indicate that AF directly or indirectly affects Ca and Pi metabolism in avians. At the present time, the effects may be related to altered vitamin D and parathyroid hormone (PTH) metabolism. Aflatoxicosis may decrease endogenous PTH synthesis and the renal sensitivity to PTH. The AF-related increase in urine [H+] during phosphate loading is probably due to increased Na+/H+ counterport, suggesting that AF stimulates sodium reabsorption. Also, the decrease in GFR exhibited 10 d after toxin removal indicates that AF may cause prolonged alteration in renal function.     

Treatment with vitamin 24,25 (OH)2D3 does not change serum levels of 1,25 (OH)2D or urinary calcium excretion rate in man

40 healthy early postmenopausal women participated in a controlled therapeutic trial with the aim of examining whether treatment with 24,25 vitamin D3 changed the serum concentration of either 1,25 (OH)2D and/or the 24-hour urinary calcium excretion rate. The 40 women were randomized to treatment with either 24R,25 (OH)2D3 (10 micrograms daily) or placebo. Serum concentrations of calcium, 25 (OH) D, 1,25 (OH)2D, 24,25 (OH)2D3 and 24-hour urinary calcium excretion rate were measured before (t0) and after (t1) 6 months of treatment. In the 24,25 (OH)2D3 treated group there was a highly significant increase in the mean serum 24,25 (OH)2D3 concentration, whereas serum 25 (OH) D and 1,25 (OH)2D and serum and urinary calcium were unchanged during the trial. In the placebo group all values were similar before and after the trial. We conclude that treatment with 24,25 (OH)2D3 is not an alternative to conventional treatment of renal hypercalciuria.     

Localization of the nephron site of gentamicin-induced hypercalciuria in the rat: a micropuncture study

In vivo renal micropuncture techniques were used to locate the nephron site of hypercalciuria induced by acute gentamicin infusion in anaesthetized Sprague Dawley rats. Three series of experiments were conducted. The effect of gentamicin on calcium reabsorption in the proximal tubule (Series I) and loop of Henle (Series II) was investigated using in vivo microperfusion whereas the effect on distal calcium handling (Series III) was studied using in vivo microinfusion. In all three experimental series, acute systemic gentamicin infusion at 0.28 mg kg(-1) min(-1) caused significant hypercalciuria within 30 min of commencing drug infusion. Gentamicin had no effect on the rates of urine flow or sodium excretion. Acute gentamicin infusion had no effect on unidirectional calcium reabsorption in the proximal tubule or loop of Henle despite a simultaneous and highly significant hypercalciuria at the whole kidney level. Net fluid reabsorption was also unaffected by the drug in these nephron segments. Acute gentamicin infusion significantly increased the urinary recovery of calcium following microinfusion into early distal tubules, whereas urinary calcium recovery was decreased after microinfusion into late distal tubules. We conclude that acute gentamicin-induced hypercalciuria is mediated by a decrease in calcium reabsorption in the early distal tubule. Thus, the acute hypercalciuric effect of gentamicin occurs at a different nephron site to the nephrotoxic effects associated with longer-term administration of the drug. It is, therefore, unlikely that gentamicin-induced hypercalciuria is involved in the pathogenesis of subsequent proximal tubular cell injury.     

Effects of verapamil and dexamethasone on the 1,25-dihydroxyvitamin D3-mediated calcium absorptive mechanism in the organ-cultured embryonic chick duodenum

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) is known to induce the biosynthesis of a specific, calcium-binding protein (CaBP) and to stimulate calcium transport in the organ-cultured embryonic chick duodenum. The biosynthesis of CaBP has been shown previously to exhibit an absolute dependence on the ambient calcium concentration of the culture medium. Verapamil, a calcium-channel blocker, decreased calcium influx into the organ-cultured duodenum and inhibited the induction of CaBP by 1,25(OH)2D3. Raising ambient calcium concentrations to as high as 10 mM did not prevent or reverse the inhibitory actions of verapamil. Dexamethasone, known to augment CaBP biosynthesis and calcium uptake in the organ-cultured duodenum in response to 1,25(OH)2D3, largely prevented inhibition of CaBP by verapamil. The actions of verapamil and dexamethasone were correlated with altered steady-state calcium concentrations of the organ-culture duodenum, strongly supporting a regulatory role of calcium in the 1,25(OH)2D3-mediated, intestinal calcium absorptive mechanism.     

Effects of 1,25-dihydroxicolecalciferol and dietary calcium-phosphate on distribution of lead to tissues during growth

The susceptibility to the toxic effects of lead (Pb) is mainly mediated by age and nutritional and hormonal status, and children are among the most vulnerable to them. During growth, an increase in calcium, phosphate and vitamin D in diet is recommended to enhance calcium and phosphate intestinal absorption and bone deposit. Calcium and phosphate reduce lead intestinal absorption, and 1,25-dihydroxicolecalciferol (1,25(OH)2D3) (active metabolite of vitamin D) increases both lead and calcium intestinal absorption. However, the effects of 1,25(OH)2D3 on lead bone deposit and redistribution to soft tissues are not well known. In this study, we examined the effects of calcium-phosphate diet supplementation and the administration of 1,25(OH)2D3 on Pb distribution to soft tissue and bone in growing rats exposed to Pb. Rats (21 days old) were exposed for 28 days to 100 ppm of Pb solution in drinking water. Calcium and phosphate in diet were increased from 1 to 2.5% and from 0.65 to 1.8%, respectively, and 1,25(OH)2D3 was administrated by intraperitoneal injection of 7.2 ng/kg every 7 days. Between 21 and 49 days, the body weight increased about 5 times. The results showed that high calcium-phosphate diet led to lower Pb concentration in blood and in bone, but Pb liver and kidney concentrations increased, which indicates that absorption and bone deposit redistribution of Pb decreased. On the other hand, no effect of this diet rich in calcium-phosphate in Pb concentration was observed in brain. Blood and bone Pb concentrations increased even more when the high calcium-phosphate diet included 1,25(OH)2D3. In the rats treated only with 1,25(OH)2D3, blood and bone Pb concentrations were lower. Higher concentrations of lead in the soft organs were observed also in rats treated under a high calcium-phosphate diet plus 1,25(OH)2D3 administration. The above mentioned results suggested that 1,25(OH)2D3 induces an increased absorption and redistribution of Pb, and therefore, it may enhance systemic damage in Pb-exposed growing animals.     

Effects of calcitriol on the immune system: new possibilities in the treatment of glomerulonephritis

1. 1,25-Dihydroxyvitamin D3 (1,25(OH)2D3), the hormonal form of vitamin D, is widely appreciated to play a central role in calcium and phosphate homeostasis. However, it is becoming increasingly clear that the sterol also play an important role in the regulation of cellular growth, haematopoietic tissues and the immune system, as well as in the modulation of hormone secretion by several endocrine glands. 2. In the present review, some of the mechanisms by which 1,25(OH)2D3 regulates immune function are highlighted. Moreover, a number of studies on the effects of calcitriol in several experimental animal models of renal disease are reported, suggesting new possibilities in the therapy of glomerulonephritis.     

Impairment of calcium homeostasis by hexachlorobenzene (HCB) exposure in Fischer 344 rats

Human exposure to hexachlorobenzene (HCB) has resulted in demineralization of bone and development of osteoporosis. Experiments were undertaken to investigate the effects of HCB on the homeostatic mechanism of calcium metabolism. Fischer 344 rats were dosed with 0, 0.1, 1.0, 10.0, or 25.0 mg HCB/kg body weight 5 d/wk for 5 wk while being fed normal rat diet or vitamin D3-deficient diet. Rats receiving the normal diet had a dose-related decrease in body weight gain and increased liver weight when compared to their controls. Serum cholesterol, alanine aminotransferase (ALT), 1, 25-dihydroxy-vitamin D3 [1,25-(OH)2D3], and parathyroid hormone (PTH) were significantly elevated when compared to control values. In the vitamin D3-deficient diet group, there was a dose related increase in liver weight, liver-to-body weight ratio and kidney-to-body weight ratio. Serum cholesterol and 1,25-(OH)2D3 were significantly elevated. Urinary calcium decreased significantly with increasing HCB dosage, indicating conservation of calcium. The data from this study indicate that HCB does affect calcium metabolism by altering the concentrations of two primary controlling factors in calcium homeostasis.     

Is 24,25-dihydroxycholecalciferol a calcium-regulating hormone in man?

Small doses (1-10 microgram daily) of 24,25-dihydroxycholecalciferol (24,25-(OH)2D3), a renal metabolite of vitamin D of uncertain function, increased intestinal absorption of calcium in normal people and in patients with various disorders or mineral metabolism, including anephric subjects. In five of six patients studied, calcium balance increased, but, unlike 1,25-dihydroxycholecalciferol, 24,25-(OH)2D3 did not increase plasma or urinary calcium concentrations. These results suggest that 24,25-(OH)2D3 may be an important regulator of skeletal metabolism in man with potential value as a therapeutic agent.     

The Effect of 1,25-Vitamin D3 on Calbindin-D and Calcium-Metabolic Variables in the Rat

Abstract: Intraperitoneal injection of l,25-(OH)₂D₃ 4 μg/kg was given to 84 calcium- and vitamin D-repleted Wistar rats and samples of plasma, duodenal mucosa and renal tissue were taken after 0, 3, 6, 12, 24, 48 and 96 hr (n = 12 at each time interval). Plasma-ionized Ca increased after 6 hr, reached a maximum after 24 hr and returned to the initial values after 96 hr. The concentrations of renal ealbindin-D28k and intestinal calbindin-D9k did not increase until 48 hr after injection and remained elevated until 96 hr after. Therefore, significantly elevated concentrations of the cytosolic calbindinD were found at a time with normal values of plasma Ca. The present data suggest that calbindin-D does not alone increase the transcellular Ca transport and, therefore, supports the view that calbindin-Ds may serve as Ca buffer proteins.