Intestinal Calcium Transport Research Articles

1,25-Dihydroxyvitamin D3-mediated intestinal calcium transport. Biochemical identification of lysosomes containing calcium and calcium-binding protein (calbindin-D28K).

A variety of intestinal cell organelles and proteins have been proposed to mediate 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-stimulated calcium absorption. In the present study biochemical analyses were undertaken to determine the subcellular localization of 45Ca after calcium transport in vivo in ligated duodenal loops of vitamin D-deficient chicks injected with 1.3 nmol of 1,25-(OH)2D3 or vehicle 15 h prior to experimentation. Separation of Golgi, mitochondria, basal lateral membrane, and lysosome fractions in the epithelial homogenates was achieved by differential sedimentation followed by centrifugation in Percoll gradients and evaluation of appropriate marker enzyme activities. Both vitamin D-deficient and 1,25-(OH)2D3-treated chicks had the highest levels of 45Ca-specific activity in lysosomal fractions. The lysosomes were also the only organelles to exhibit a 1,25-(OH)2D3-mediated difference in calcium content, increasing to 138% of controls. Lysosomes prepared from 1,25-(OH)2D3-treated chicks also contained the greatest levels of immunoreactive calbindin-D28k (calcium-binding protein). Chloroquine, a drug known to interfere with lysosomal function, was tested and found to inhibit 1,25-(OH)2D3-stimulated intestinal calcium absorption. Neither 1,25-(OH)2D3 nor chloroquine affected [3H]2O transport. In additional experiments, microsomal membranes (105,000 X g pellets) were subjected to gradient centrifugation. The highest levels of 45Ca-specific activity and calcium-binding protein in material from 1,25-(OH)2D3-treated chicks were found in fractions denser than endoplasmic reticulum and may represent endocytic vesicles. In studies on intestinal mucosa of 1,25-(OH)2D3-treated birds fractionated after 30 min of exposure to lumenal Ca2+ or Ca2+ plus chloroquine, 45Ca was found to accumulate in lysosomes and putative endocytic vesicles, relative to controls. A mechanism involving vesicular flow is proposed for 1,25-(OH)2D3-mediated intestinal calcium transport. Endocytic internalization of Ca2+, fusion of the vesicles with lysosomes, and exocytosis at the basal lateral membrane complete the transport process.

Correction by 1-25-dihydroxycholecalciferol of the abnormal fluidity and lipid composition of enterocyte brush border membranes in vitamin D-deprived rats.

Weanling male Wistar rats were deprived of dietary and light sources of vitamin D for 11-18 weeks along with age-matched diet vitamin D-repleted controls to evaluate the role of lipid fluidity in the stimulatory effect of calcitriol on Ca transport. The "static" component of fluidity of proximal small intestine brush border membrane, as assessed by steady-state fluorescence techniques using the fluorophore 1,6-diphenyl-1,3,5-hexatriene, was similar between these two groups. In contrast, the "dynamic" component of fluidity, as assessed by DL-2-(9-anthroyl)-stearic acid and DL-12-(9-anthroyl)-stearic acid, was decreased in membranes of D-deprived animals. Lipid composition was analyzed to evaluate the potential mechanism mediating these fluidity changes. In vitamin D-deprived rats, linoleic (18:2) and arachidonic (20:4) acids of the phosphatidylcholine and phosphatidylethanolamine fractions of the membrane were decreased, whereas palmitic (16:0) and stearic (18:0) acids were increased in the phosphatidylethanolamine fraction of the membrane. These associated fatty acyl alterations could explain, at least in part, the differences in membrane fluidity between D-repleted and D-deprived rats. Membrane fluidity, lipid composition, and duodenal Ca transport were also analyzed 1, 2, and 5 h after the acute administration of 1-25-dihydroxycholecalciferol to D-deprived animals. In D-deprived rats, within 1-2 h, this hormone restored to levels of vitamin D-repleted controls the dynamic component of fluidity and concentrations of the same membrane phospholipid fatty acids. Since these changes temporally precede detectable increases in Ca absorption (demonstrable only during the 5th h), these data support the hypothesis that alterations in membrane fluidity and lipid composition may play an important role in the stimulation of intestinal calcium transport by calcitriol.

The difference of biological activity among four diastereoisomers of 1α,25-dihydroxycholecalciferol-26,23-lactone

All four possible diastereoisomers of 1α,25-dihydroxycholecalciferol-26,23-1αctone (1α,25-(OH) 2D 3-26,23-lactone) were chemically synthesized and were compared to 1α,25-dihydroxycholecalciferol (1α,25(OH) 2D 3) in terms of their stimu1ation, in vivo, of intestinal calcium transport and mobilization of calcium from bone in vitamin D-deficient rats (the two classic vitamin D-mediated responses), and their relative binding to the chick intestinal cytosol receptor for 1α,25-(OH) 2D 3. The receptor binding affinity results are expressed as relative competitive index (RCI), where the RCI is defined as 100 for 1α,25(OH) 2D 3. The RCI obtained for 23(S)25(S)-1α,25(OH) 2D 3-26,23-lactone was 7.90, for 23(R)25(R)-1α, 25(OH) 2D 3-26,23-lactone was 2.27, 23(S)25(R)-1α,25(OH) 2D 3-26,23-lactone was 0.17, for 23(R)25(S)-1α, 25(OH) 2D 3-26,23-lactone 0.22 and for the in vivo produced 1α,25(OH) 2D 3-26,23-lactone the RCI was only 0.17. Also the four diastereoisomers of 1α,25(OH) 2D 3-26,23-lactone all stimulated intestinal calcium transport, reaching a maximum 8 h after administration. Compared with the stimulation of intestinal calcium transport by 1α,25(OH) 2D 3, 23(S)25(S)-1α,25(OH) 2D 3-26,23-lactone was 1 4 as effective, 23(R)25(R)-1α,25(OH) 2D 3-26,23-1actone was 1 20 as effective, 23(S)25(R)-1α,25(OH) 2D 3-26,23-lactone was 1 74 as effective and 23(R)25(S)-1α,25(OH) 2D 3-26,23-lactone was 1 53 as effective. Similarly, 23(S)25(S)-1α,25(OH) 2D 3-26,23-lactone and 23(R)25(R)-1α,25(OH) 2D 3-26,23-lactone were estimated to be 3 and 20 times less active than 1α,25-(OH) 2D 3 in elevation of serum calcium. However, 23(S)25(R)-1α, 25(OH) 2D 3-26,23-1αctone and 23(R)25(S)-1α,25(OH) 2D 3-26,23-lactone decreased the serum calcium levels 24 h after administration. 23(S)25(R)-1α,25(OH) 2D 3-26,23-lactone reduced serum calcium concentrations to a greater extent than 23(R)25(S)-1α,25(OH) 2D 3-26,23-lactone. These results indicate that the biological activities of the diastereoisomers of 1α,25(OH) 2D 3-26,23-lactone were quite different among four stereochemical configurations.

Effect of ethanol on intestinal calcium transport in chicks

To test the hypothesis that chronic ethanol exposure would alter the membrane fluidity of the intestinal brush-border membrane, which would lead to calcium malabsorption, we gave chicks 15% ethanol in their drinking water from hatching to 3 or 4 wk of age. Although such chicks grew less quickly than their hatchmates not ingesting ethanol, their ability to absorb calcium from the duodenum in vivo was unimpaired. However, when calcium accumulation by isolated duodenal brush-border membrane vesicles (BBMVs) was assessed, the BBMVs from chicks ingesting ethanol for 1–3 wk had a lower rate of uptake than the BBMVs from the controls at all calcium concentrations evaluated (0.1–25 mM). This difference could not be explained by differences in membrane fluidity as assessed by fluorescence depolarization, or by changes in intravesicular volume. Glucose uptake was not affected. The acute addition of ethanol (up to 1 M) in vitro to the control BBMVs increased both membrane fluidity and calcium accumulation. No difference in the fluidizing effect of ethanol in BBMVs between ethanol-ingesting chicks and control chicks could be demonstrated, although the acute effect of ethanol on calcium accumulation was blunted in the BBMVs from chicks ingesting ethanol. Increasing the temperature of the incubation medium also increased membrane fluidity and calcium accumulation in BBMVs from control and ethanol-ingesting chicks, with a greater increase in calcium uptake by the control BBMVs despite comparable increases in fluidity in BBMVs from the control and ethanol-ingesting chicks. We conclude that the chronic ingestion of ethanol by chicks, although markedly altering growth rates, has minimal impact on the intestinal absorption of calcium when assessed in vivo. However, chronic ethanol ingestion does appear to alter the intestinal brush-border membrane to make it less permeable to calcium and less susceptible to the stimulation by ethanol of calcium flux across this membrane; this adaptation may prevent increased flux of calcium across the brush-border membrane into the cell in the presence of ethanol.

Blood levels of ionized calcium, inorganic phosphorus, 1,25-dihydroxycholecalciferol and gonadal hormones in hens laying hard-shelled or shell-less eggs.

The production of shell-less eggs was induced in hens to measure the effects of the high demands made by shell formation on the blood minerals and hormones whose concentrations change during egg formation. In control hens laying hard-shelled eggs, the concentration of ionized calcium in plasma decreased at the onset of shell formation, but no change was found in hens laying shell-less eggs. Total calcium concentrations in plasma decreased slightly throughout the ovulation cycle in both groups. Concentrations of inorganic phosphorus in the plasma were increased in the control group during the period of shell formation and decreased when calcification was suppressed. Finally, the concentrations of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) in plasma were significantly increased 16 and 20 h following an ovulation compared with 4 h after ovulation, or compared with the concentrations observed in hens laying shell-less eggs. The variations in the plasma concentrations of ionized calcium, inorganic phosphorus and 1,25-(OH)2D3 associated with egg formation were therefore absent in hens laying shell-less eggs demonstrating their direct link with shell calcification. On the other hand, suppression of shell production had no influence on the changes in the plasma concentrations of progesterone, oestradiol and testosterone which are associated with the normal ovulatory cycle. It is concluded that the increases in intestinal and uterine calcium transport and in 1,25-(OH)2D3 production which occur at the onset of egg production in hens are mainly controlled by factors involved in maintaining calcium homeostasis rather than by gonadal hormones.

Pathogenesis of hypercalcemia in nude mice bearing a human renal carcinoma.

When grown as sc tumors in the nude (nu/nu) mouse, cells of the established human renal carcinoma cell line 786-0 produce hypercalcemia; this has an apparent humoral basis because it is reversed by resection of the primary tumor. We have investigated the pathogenesis of hypercalcemia in this model. Tumor-bearing mice were hypercalcemic (13.4 +/- 0.9 vs. 9.52 +/- 0.13 mg/dl in control mice) and hypophosphatemic (10.0 +/- 0.8 vs. 13.8 +/- 1.5 mg/dl in control mice; all values are mean +/- SEM). The serum concentration of 1,25-dihydroxyvitamin D was increased in tumor-bearing animals (70.0 +/- 9.3 vs. 43.8 +/- 4.8 pg/ml in control animals). Urinary excretion of cAMP was similar in control (33.7 +/- 1.4 nmol/mg creatinine) and tumor-bearing mice (38.2 +/- 4.7 nmol/mg creatinine). However, in the latter, the acute response of urinary cAMP to PTH was blunted. Although intestinal calcium transport in everted duodenal sacs in vitro was increased in tumor-bearing mice, hypercalcemia was unaffected by feeding the animals for 8 days a diet containing less than 0.02% calcium. Hence, absorption of dietary calcium did not play a significant role in maintenance of hypercalcemia. In hypercalcemic animals, the calcium content of the humerus was decreased (2.95 +/- 0.08 vs. 3.29 +/- 0.13 mg in controls; P less than 0.05). Quantitative histomorphometric analysis of the distal femoral metaphysis disclosed a significant reduction in trabecular bone volume in tumor-bearing mice (12.0 +/- 1.1% vs. 16.1 +/- 1.1% in controls; P less than 0.02). A strong trend for increased osteoclast surface and number was observed, suggesting that bone resorption was increased. Osteoblast surface and number were also somewhat increased, as was the rate of mineral apposition (2.55 +/- 0.14 vs. 1.91 +/- 0.04 micron/day in controls; P less than 0.01). Thus, the decrease in trabecular bone volume was associated with high turnover of bone, with an apparent net increase in bone resorption. We conclude that hypercalcemia in the nude mouse bearing human renal carcinoma cells is associated with increased bone resorption, high bone turnover, hypophosphatemia, and increased serum levels of 1,25-dihydroxyvitamin D.

The synthesis and biological activity of 22-fluorovitamin D 3: A new vitamin D analog

We synthesized 22-fluorovitamin D 3 from (22S) cholest-5-ene-3β, 22-diol-3β-acetate 2 . Compound 2 was treated with diethylaminosulfur trifluoride to give 22-fluorocholest-5-en-3β-acetate 3 and (E) 22-dehydrocholest-5-en-3β-acetate. Compound 3 was treated with N-bromosuccinimide to give a mixture of the respective 5,7- and 4,6-dienes. The 5,7-diene of 3 was separated from the 4,6-diene using the dienophile 4-phenyl-1,2,4-triazoline-3,5-dione. 22-Fluoro-5α, 8α-(3,5-dioxo-4-phenyl-1,2,4-triazolino)-cholest-6-en-3β-acetate 4 was purified by flash chromatography and treated with lithium aluminum hydride to generate 22-fluorocholesta-5,7-dien-3β-ol 5 . Photolysis of the diene 5 , followed by thermal equilibration, resulted in the synthesis of 22-fluorovitamin D 3 7 . The vitamin 7 increased active intestinal calcium transport only at a dose of 50,000 pmol/rat, whereas vitamin D 3 increased intestinal calcium transport at a dose of between 50 and 500 pmol/rat. 22-Fluorovitamin D 3 7 did not mobilize bone and soft tissue calcium at a dose as high as 50,000 pmol/rat, whereas vitamin D 3 was successful in doing so at a dose of 500 pmol/rat. When tested in the duodenal organ culture system, 22-fluorovitamin D 3 7 had approximately 1/25th the potency of vitamin D 3. It did not antagonize the activity of 1,25-dihydroxyvitamin D 3. 22-Fluorovitamin D 3 7 bound to the rat plasma vitamin D binding protein less avidly than vitamin D 3. 22-Fluorovitamin D 3 was bound very poorly to the chick intestinal cytosol receptor for 1,25-dihydroxyvitamin D 3. We conclude that the introduction of fluorine at the C-22 position results in a vitamin D sterol with decreased biologic activity when compared to vitamin D 3. The presence of a fluorine group at C-22 position inhibits the binding of the vitamin to rat vitamin D binding protein when compared to the binding of its hydrogen analog, vitamin D 3.

Abnormal vitamin D metabolism, intestinal calcium transport, and bone calcium status in the spontaneously hypertensive rat compared with its genetic control.

Abnormalities of intestinal calcium absorption and the vitamin D axis in the spontaneously hypertensive rat (SHR) are controversial. The present report documents a reduction in circulating 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in the 12-14-wk-old male SHR with evidence of its functional significance. Both plasma 1,25(OH)2D3 and mucosa-to-serosa duodenal calcium flux (Jm-s), measured by the Ussing chamber, were significantly lower (approximately 60% of value in Wistar-Kyoto rats [WKY]) in SHR on both normal (1%) and low (0.1%) calcium diets than in corresponding control WKY. Low dietary calcium increased both 1,25(OH)2D3 and Jm-s by approximately 80% in SHR and WKY, with levels of both parameters rising in the SHR to levels found in the WKY under baseline conditions. The latter fact suggests the improbability of intestinal resistance to the action of 1,25(OH)2D3 in the SHR. Plasma 25-hydroxyvitamin D3 (25(OH)D3) was not significantly different between the strains. Intraperitoneal 1,25(OH)2D3 increased Jm-s in 12-14-wk-old SHR to levels observed in equivalent WKY. In 20-24-wk-old SHR, calcium deprivation was associated with significantly reduced Jm-s compared with equivalent WKY. Bone density and bone calcium content in 20-30-wk-old SHR were significantly reduced. In summary, we provide evidence that the SHR was unable to sustain appropriate circulating levels of 1,25(OH)2D3, an impairment which resulted in reduced duodenal calcium absorption.

Hypercalcemia inhibits the rapid stimulatory effect on calcium transport in perfused duodena from normal chicks mediated in vitro by 1,25-dihydroxyvitamin D3.

We have previously reported that vascular perfusion of normal vitamin D3-replete chick duodena with physiological amounts of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] increases the movement of 45Ca2+ from the lumen to the venous effluent within 14 min under conditions of normal Ca2+ (0.9 mM) concentration in both the lumen and vascular perfusate. The present studies were designed to further explore details of this rapid 1,25-(OH)2D3 effect as a function of seco-steroid concentration and under conditions where the free Ca2+ concentrations in the perfusate were varied from 0.54-1.26 mM. Concentrations of 1,25-(OH)2D3 in the vascular perfusate ranging from 30-650 pM elicited an increasing stimulation of Ca transport, as judged by 45Ca levels in the venous effluent. At 0.98-6.5 nM 1,25-(OH)2D3, progressive inhibition of Ca transport was observed, yielding a biphasic dose-response curve. The optimal concentration of 650 pM 1,25-(OH)2D3 was used in subsequent experiments designed to study the effects of vascular Ca2+ levels on 45Ca transport mediated by the seco-steroid. The basal Ca2+ transport ratio, in the absence of 1,25-(OH)2D3, did not change when the divalent cation of the vascular perfusate was varied over the range 0.54-1.26 mM free calcium. However, the effect of 1,25-(OH)2D3 on 45Ca2+ transport was completely abolished in the group treated with 1.21 mM Ca2+ in the perfusate, but not in the groups treated with concentrations less than 1.17 mM Ca2+. These results suggest that the rapid intestinal calcium transport response to 1,25-(OH)2D3 may be modulated locally in part by the prevailing ionized Ca concentration of the vascular perfusate.

Intestinal calcium transport in the spontaneously hypertensive rat: response to calcium depletion.

We previously found intestinal Ca2+ transport to be lower in the spontaneously hypertensive (SH) as compared with the Wistar-Kyoto control (WKY) rat. These animals were fed a relatively high (1%) Ca2+ diet, and the concentration of 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] in serum was the same in both groups. In the present experiment we tested the possibility that the lower Ca2+ transport in the SH rat was the result of unresponsiveness to 1 alpha,25(OH)2D3. We fed diets high and low in Ca2+ and measured serum 1 alpha,25(OH)2D3 and Ca2+ transport. Serum 1 alpha,25(OH)2D3 increased in response to Ca2+ depletion at both 5 and 12 wk in both the WKY and SH rat. With high-Ca2+ diet, Ca2+ transport was lower in SH than in WKY when studied 1) in vitro in duodenum at 5 wk of age, and 2) in vivo in proximal and distal small intestine at 12 wk of age. Ca2+ transport increased in SH in response to Ca2+ depletion, but not in WKY, except in distal small intestine in vivo at 12 wk. In summary, although Ca2+ transport is lower in the SH as compared with the WKY rat when vitamin D activity is basal through feeding a high-Ca2+ diet, Ca2+ transport increases in the SH rat in response to the increase in 1 alpha,25(OH)2D3 produced by feeding a low-Ca2+ diet. We conclude that 1) the vitamin D-regulated component of mediated Ca2+ transport is intact in the SH rat and is unrelated to hypertension, and 2) mediated Ca2+ transport under basal conditions, i.e., nonvitamin D-regulated, differs in the SH and WKY rats and may be related to hypertension.

Skeletal response to dietary calcium in a rat model simulating weightlessness.

Unweighting the hindlimbs of a rat by tail suspension leads to a decrease in bone in the unweighted hindlimbs, but not in the normally weighted forelimbs. We evaluated whether increments in dietary calcium could prevent this. Growing rats were fed diets ranging in calcium content from 0.1% to 2.4%. After the rats were suspended for two weeks, we found no differences between suspended and control animals fed the same diet with respect to calcium transport or serum levels of calcium, phosphorus, 1,25-dihydroxyvitamin D, and parathyroid hormone. In both groups, increasing dietary calcium reduced active intestinal calcium transport and serum 1,25-dihydroxyvitamin D levels. The calcium content of the tibia and lumbar vertebra (but not the humerus) was reduced in suspended rats compared to control rats fed the same diet. However, increasing dietary calcium increased the calcium content of all bones in both suspended and control animals. The bone formation rate at the tibiofibular junction (measured by double-label tetracycline) was reduced in the suspended animals compared to controls and was not altered by dietary calcium. However, the marrow area of the tibia, an indication of bone resorption, did not differ between suspended and control animals and was equally reduced in both groups when dietary calcium was increased. Our data suggest that the deleterious effects of skeletal unweighting on bone formation cannot be explained by changes in the calciotropic hormones and are not reversed by increments in dietary calcium. However, increasing dietary calcium can increase bone calcium, even in unweighted limbs, by decreasing bone resorption.

Impaired intestinal calcium transport in the spontaneously hypertensive rat.

The spontaneously hypertensive rat (SHR) exhibits a variety of abnormalities in its handling of calcium (1–3) and phosphorus (4,5) and calcium supplementation has been shown to reduce its arterial blood pressure (3,6). The relationship between disturbed calcium and phosphorus metabolism and hypertension, however, remains unclear. Several investi­gators have studied intestinal calcium transport in the SHR and have reported increased (7,8), unchanged (9) or decreased (10) absorption compared with its genetic control, the Wistar­Kyoto rat (WKY). In these studies, Ca2+ absorption has been determined by means of intestinal perfusion (10) or in situ uptake (8) in vivo,everted gut sacs in vitro (7,9,10), or by balance studies (8). The variety of experimental systems employed may in part explain the diversity in these findings. We have recently addressed this issue utilising the modified Ussing apparatus (11) which permits continuous monitoring of tissue viability and the study of active intestinal calcium transport under electrically controlled conditions.KeywordsShort Circuit CurrentUssing ChamberIntestinal PerfusionIntestinal Calcium TransportRenal Phosphate ExcretionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Effect of hypophysectomy on the occupied and unoccupied binding sites for 1,25-dihydroxyvitamin D 3 in rat intestine

It is not known why the intestinal active transport of calcium per unit of mucosal mass is not affected by hypophysectomy (HX) even though serum 1,25-dihydroxyvitamin D 3 [1,25(OH) 2D 3 and intestinal calcium-binding protein are decreased. In order to study the effect of HX on the quantity of intestinal receptor of 1,25(OH) 2D 3 and its binding characteristics, the intestinal total occupied and unoccupied binding sites for 1,25(OH) 2D 3 were measured, by the use of the mercurial reagent mersalyl, in the intestine of HX and age-matched control rats. In addition, the effect of bovine growth hormone (bGH) replacement on the quantity of both binding states was examined in the HX rats. Results of Scatchard analysis and sucrose density gradients showed that the 3.5S receptor of the HX rat intestine was not distinguishable from that seen in the intact rat intestinal cytosol. Under vitamin D-supplemented conditions, HX was shown to reduce the levels of occupied receptors when the data were expressed on the basis of cytosolic protein. The reduced occupied sites could, in fact, have resulted from the reduction in plasma 1,25(OH) 2D 3 levels. No synthesis rates were determined. The unoccupied and total binding sites for 1,25(OH) 2D 3 per length of intestine were lower in the HX group than in the intact group. Administration of bGH resulted in an increase of endogenously occupied binding sites without affecting the total binding activity. Under vitamin D-depleted (-D) conditions, the total binding activity (intestinal) for 1,25(OH) 2D 3 was increased in the intact but not in the -DHX rats. Administration of bGH to the -DHX rat resulted in no effect on the binding levels of 1,25(OH) 2D 3 receptor. The current study demonstrates that HX does not affect the binding affinity and the concentration of intestinal total binding receptor 1,25(OH) 2D 3. The decrease in endogenously occupied binding sites for 1,25(OH) 2D 3 without an affect on the intestinal active transport of calcium could be interpreted to mean that HX affects other factors controlling calcium absorption than 1,25(OH) 2D 3.

Abnormal Vitamin D3 Metabolism and Intestinal Calcium Transport in the Spontaneously Hypertensive Rat (SHR)

Abnormal Vitamin D3 Metabolism and Intestinal Calcium Transport in the Spontaneously Hypertensive Rat (SHR)

Inhibition by theophylline of CaBP-mediated active calcium transport in rat intestine

Inhibition by theophylline of CaBP-mediated active calcium transport in rat intestine

Analysis of calcium transport in rat intestine.

Transmural calcium absorption is the resultant of two processes: a saturable, regulatable process that traverses the cell and a non-saturable, non-regulated process that is largely paracellular. This article will summarize findings and analyses in support of this generalization and provide details of the action of vitamin D, the primary regulator of active calcium absorption.KeywordsCalcium TransportDuodenal LoopFractional Calcium AbsorptionIntestinal Calcium TransportActive Calcium TransportThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Studies on organic fluorine compounds. L. Synthesis and biological activity of 2 alpha-fluorovitamin D3.

Reaction of 3, 6β-diacetoxycholest-2-ene (2) with cesium fluoroxysulfate gave 6β-acetoxy-2α-fluorocholestan-3-one (3), which was successfully converted to 2α-fluorovitamin D3 (10). A single dose of 500 ng of 10 produced an intestinal calcium transport response and a bone calcium mobilization response in vitamin D-deficient rats equivalent to those of vitamin D3.

Somatostatin and intestinal calcium transport in the rat

In intact rats we studied the influence of low doses of intravenously (i.v.) administered somatostatin (SRIF) on the net absorption and the bidirectional fluxes (lumen-to-plasma, LP; plasma-to-lumen, PL) of calcium in the duodenum, jejunum, ileum and caecum. In the duodenum SRIF inhibited the LP-flux and the net absorption of Ca significantly at infusion rates of 0.75 and 1.0 μg SRIF·kg −1·h −1. The PL-flux was not altered by any of the SRIF doses administered. In the other gut segments studied (jejunum, ileum, caecum) neither the net absorption nor the bidirectional Ca fluxes were changed by i.v. SRIF. It is concluded that (1) SRIF in the plasma levels achieved in this study has an influence on the duodenal calcium absorption (CaA) of the rat; (2) questions regarding the mechanisms of this action as well as the physiological significance of our findings are as yet unresolved.

Biological activities and binding properties of 23,23-difluoro-25-hydroxyvitamin D 3 and its 1α-hydroxy derivative

23,23-Difluoro-25-hydroxyvitamin D 3 is 5–10 times less active than 25-hydroxyvitamin D 3 in stimulating intestinal calcium transport, bone calcium mobilization, increasing serum phosphorus, mineralization of rachitic bone, and binding to the plasma transport protein in rats. It is converted to 23,23-difluoro-1α,25-dihydroxyvitamin D 3 by chick renal 25-hydroxyvitamin D-1-hydroxylase. This compound is one-seventh as active as 1,25-dihydroxyvitamin D 3 in binding to the chick intestinal receptor for 1,25-dihydroxyvitamin D 3. Thus, fluoro substitution on carbon-23 of vitamin D has an unexpected and unexplained suppressive action on plasma binding and biological activity. However, since this substitution does not block the biological response of 25-hydroxyvitamin D 3, these results provide additional evidence that 23-hydroxylation of vitamin D is not involved in biological function.

Cholecalcin (a 9-kDa cholecalciferol-induced calcium-binding protein) messenger RNA. Distribution and induction by calcitriol in the rat digestive tract.

In view of the possible physiological importance of the 9-kDa cholecalcin (a 9000-Mr cholecalciferol-induced calcium-binding protein) in the intestinal transport of calcium in mammals, the gene expression of this protein has been analysed. Its regulation in the digestive tract of the growing rat by calcitriol (1,25-dihydroxycholecalciferol) was studied using a specific cloned [32P]cDNA to 9-kDa cholecalcin. Northern hybridisation studies show that the cDNA sequence hybridises to a single 500-600-nucleotide species throughout the digestive tract and therefore demonstrate identical 9-kDa-cholecalcin mRNA processing in the whole of the intestine and caecum. The highest concentrations of cholecalcin mRNA occur in the duodenum, proximal jejunum and caecum. The observed differences in 9-kDa-cholecalcin mRNA levels correlate well with both the in vivo variations in cholecalcin itself and with the known intestinal sites of calcium absorption. The whole intestine is able to respond to exogenous calcitriol but the response of the distal intestine and caecum, as measured by the increase in cholecalcin mRNA and corresponding protein, was proportionally higher than in the duodenum. The rapid production of fully functional cholecalcin mRNA, which was detectable as early as 1 h after a single dose of calcitriol to vitamin-D-deficient rats, provides convincing evidence that calcitriol increases 9-kDa cholecalcin production by increasing cholecalcin gene expression at the transcriptional level.