Calcium-binding Protein Content Research Articles

Human growth hormone increases intestinal vitamin D-dependent calcium-binding protein in hypophysectomized rats.

We examined the effects of hypophysectomy and pituitary hormone replacement on vitamin D-dependent calcium-binding protein (CaBP) in rat small intestine. The concentration of immunoreactive CaBP per mg intestinal protein was decreased by at least 56% in hypophysectomized rats compared to that in intact pair-fed controls. Alkaline phosphatase and total protein also were reduced by hypophysectomy, but pair-feeding produced comparable decreases. Daily injections of 2, 10, or 50 micrograms human GH (hGH) for 9 days produced a dose-dependent increase in CaBP. At the highest hGH dose (50 micrograms), the content of CaBP was increased 2- to 4-fold to intact levels. By comparison, the increases in total protein and alkaline phosphatase were small (25% to 40% and 80% to 90%, respectively). The induction of CaBP preceded the other protein responses; half-maximal increases in CaBP occurred after 2 days of hGH (50 micrograms/day) treatment before statistically significant changes in total protein or alkaline phosphatase activity. hGH was the most potent pituitary hormone tested; ovine TSH (25 mU/day) had no effect on CaBP, and ovine PRL (10 or 50 micrograms/day) increased CaBP by only 25-27% (P = 0.014). These studies indicate that the vitamin D-dependent intestinal CaBP in hypophysectomized rats is regulated by GH and provide further evidence that the pituitary may be involved in regulating vitamin D-dependent intestinal adaptations.

In Rat Uterus 17β-Estradiol Stimulates a Calcium-Binding Protein Similar to the Duodenal Vitamin D-Dependent Calcium-Binding Protein

A calcium-binding protein (CaBP) similar to rat duodenal vitamin D-dependent CaBP was identified in rat uterus. Uterine CaBP and duodenal CaBP had the same mol wt (9,000-10,000), exhibited the same calcium-dependent electrophoretic mobility, and were immunologically identical. The localization of CaBP in the rat uterus was explored using indirect immunoperoxidase methods, and by CaBP RIA in the endometrium and myometrium after enzyme separation. In the endometrium CaBP was found in the cytoplasm of the stroma cells but not in the epithelium or in the glandular cells. In the myometrium, it was located inside the smooth myometrial fibers. Hormonal regulation of CaBP was shown to differ in the uterus and duodenum. Duodenal CaBP concentrations increased in response to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), and were not influenced by ovariectomy or sex steroids administration. By contrast, CaBP synthesis fell drastically in the uterus of ovariectomized rats, but was greatly enhanced by low physiological doses of 17 beta-estradiol. This effect of 17 beta-estradiol on uterine CaBP was dose dependent. Medroxyprogesterone and more especially 1,25(OH)2D3 exerted no such stimulating effect on uterine CaBP. In vitamin D-deficient ovariectomized rats, administration of 17 beta-estradiol alone restored the uterine CaBP concentrations to normal and this potency contrasted with the apparent inability of 1,25(OH)2D3 to affect the uterine CaBP concentrations. Our data suggest that, unlike duodenal CaBP regulation, the expression of the CaBP gene in rat uterus is predominantly controlled by 17 beta-estradiol.

Intestinal vitamin D-induced calcium-binding protein: time-course of immunocytological localization following 1,25-dihydroxyvitamin D3.

The vitamin D-induced calcium-binding protein (CaBP) was localized in histological sections of chick duodenum using the peroxidase-antiperoxidase immunocytochemical technique. The time-course of appearance of CaBP in rachitic chicks was investigated from 0 to 120 hr after stimulation by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). CaBP was not routinely detected at 0 hr after 1,25(OH)2D3 administration. CaBP was first noted in some, but not all, of the samples taken 2 hr following 1,25(OH)2D3 and was detected in all 2 1/2 hr samples. The number of CaBP-containing absorptive cells and the apparent CaBP concentration both increased to a maximum at about 16-24 hr. At later times, as CaBP free cells migrated up the villi, the CaBP-containing cells decreased in number, but even at 120 hr post 1,25(OH)2D3 dose there were significant numbers of CaBP-containing cells present. The relationships between time-course of CaBP location on intestinal villi, enterocyte migration rates, and the time-course of 1,25(OH)2D3 stimulated intestinal calcium transport are discussed.

Evidence for multiple effects of vitamin D3 on calcium absorption: response of rachitic chicks, with or without partial vitamin D3 repletion, to 1,25-dihydroxyvitamin D3.

The effects of vitamin D3 or 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or both, on the relationship among calcium absorption, vitamin D-induced calcium-binding protein (CaBP), and phospholipid metabolism were examined. When 1,25(OH)2D3 was injected intracardially into D3-deficient chicks, both the stimulation of calcium absorption and the induction of the synthesis of CaBP occurred 2-4 hr later. When 1,25(OH)2D3 was injected into chicks partially repleted with D3, an earlier increase in calcium absorption was observed without a significant change in the concentration of CaBP already present in the duodenal mucosa. Other early events were an increased uptake of calcium by the intestinal tissue and an alteration in phospholipid metabolism. These and other observations support the proposal that at least two phases of calcium absorption are influenced by 1,25(OH)2D3--permeation of calcium across the brush border, and transfer of calcium through and out of the cell. The first phase responds more rapidly to 1,25(OH)2D3 than does the second phase, correlates with changes in phospholipid metabolism, and might not be dependent on de novo protein synthesis. The second phase correlates with CaBP synthesis and therefore is dependent on protein synthesis. Either the first phase or the second phase can constitute the limiting step in calcium absorption.

The possible role of calcium-binding protein induced by 1 alpha,25-dihydroxyvitamin D3 in the intestinal calcium transport mechanism.

The relationship between the appearance of vitamin D-dependent calcium-binding protein (CaBP) and calcium absorption was studied in sequentially isolated duodenal mucosal preparations from vitamin D-deficient chicks and those supplemented with vitamin D3 or 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3]. The duodenal calcium absorption activity was significantly increased 6 h after a single iv injection of 0.625 microgram of 1 alpha,25-(OH)2D3, attained a maximum at 12 h, and declined gradually thereafter. On the other hand, only small amounts of CaBP appeared in the crypt and lower villus regions 6 h after 1 alpha,25-(OH)2D3 administration. At 12 h, the CaBP was found in the entire villus, but its content was still much higher in the crypt and lower villus. At 24 h and 48 h, the distribution of CaBP showed the opposite gradient, higher in the villus and lower in the crypt. At 72 h, CaBP was found only in the upper villus. The life time of duodenal mucosal cells was calculated to be 108 h as indicated by the cells labeled with [3H]thymidine. Thus, the movement of CaBP from the crypt to the villus tip was considered to be much faster than the cell migration, suggesting that both crypt and villus cells are capable of producing CaBP. Daily administration of vitamin D3 or 1 alpha,25-(OH)2D3 for 2 weeks resulted in a marked increase in CaBP levels mainly in the mid- and upper villus regions. The higher the intestinal calcium transport activity was, the higher the duodenal CaBP content. These results suggest that CaBP is not necessary in initiating intestinal calcium transport, but it plays an important role in maintaining the enhanced transport mechanism.

Pharmaeokinetics and Pharmacodynamics of 1,25-Dihydroxyvitamin D3in the Chick

To relate the disposition kinetics of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to its function, we injected a physiological dose of radiolabeled 1,25-(OH)2D3 (84.6 pmol) into the wing veins of vitamin D-deficient chicks. The concentration of calcium-binding protein in duodenal mucosal cytosol, the concentration of 1,25-(OH)2D3 in plasma and duodenal mucosa, and the total radioactivity in bone, kidney, liver, muscle, pancreas, spleen, plasma, and duodenal mucosa were determined in individual chicks from 1–72 h later. The uptake of 1,25-(OH)2D3 by the duodenal mucosa occurred quickly, reaching a peak concentration of 6.2 pmol/g wet wt at 3 h. This was followed by a rapid disappearance, the kinetics of which paralleled the terminal phase of plasma 1,25-(OH)2D3 disappearance. Intact 1,25-(OH)2D3 accounted for more than 75% of the total radioactivity in the duodenal mucosa for at least 36 h, but fell to 30% of the total radioactivity in plasma by 8 h. An unidentified metabolite which cochromatographed with 1,24...

Regulation of calcium-binding protein in mouse placenta and intestine.

To study the developmental controls for both intestinal and placental calcium-binding protein (CaBP), we have altered mineral metabolism by feeding pregnant mice diets high in calcium and strontium. The effects of these dietary changes on CaBP content of maternal intestine and placenta and also on fetal mineral accumulation have been measured. Feeding high-calcium diets to pregnant mice decreased CaBP content in both maternal intestine and placenta (45 and 58%, respectively). Under these conditions of maternal hypercalcemia, fetal mineral accumulation was unchanged. In experiments with strontium-containing diets, CaBP content was reduced in both maternal intestine and placenta (66 and 67%, respectively), and fetal mineral accumulation was markedly reduced (70%). Administration of 1,25-dihydroxycholecalciferol [1,25(OH)2D] to the strontium-fed mice allowed the gestational rise in maternal intestinal CaBP to proceed to normal levels or above; however, the level of placental CaBP was not affected. Similarly, administration of 1,25(OH)2D to pregnant mice on a normal diet increased maternal intestinal CaBP, but had no effect upon placental CaBP. Thus, administration of 1,25(OH)2D to the mother, in amounts sufficient for a maternal intestinal response, was not a sufficient condition to increase placental production of CaBP.

Vitamin D3 metabolite injections to thyroparathyroidectomized pregnant rats: effects on calcium-binding proteins of maternal duodenum and of fetoplacental unit.

Fetomaternal relationships with respect to vitamin D metabolism were investigated in thyroparathyroidectomized (TPTX) pregnant rats, with or without treatment with different vitamin D3 metabolites. Calcium-binding protein (CaBP) in maternal duodenum was used as an index of 1,25-(OH)2D3 status of the mother. Pregnant rats were TPTX on day 12.5 and CaBP was measured on 21.5 days of gestation by RIA in maternal duodenal mucosa and in the fetoplacental unit (placenta, fetal membranes, and fetal intestine). In the duodenum of TPTX mothers, the CaBP concentration was reduced by 50%. This fall was associated with a decrease of 1,25-(OH)2D in maternal plasma. CaBP in maternal duodenum increased by the administration of 1,25-(OH)2D3 or 1,24,25-(OH)3D3. In contrast, 24,25-(OH)2D3 injections to TPTX mothers were ineffective. In both placenta and fetal membranes, CaBPs decreased by 20% in TPTX mothers and were normalized only in 1.25-(OH)2D3-treated TPTX mothers. In the fetal intestine, CaBP variations paralleled those of maternal duodenal CaBP. The data indicate that plasma levels of 1,25-(OH)2D in TPTX pregnant rats are partly under the control of maternal parathyroid glands, and they support that even in pregnancy, the CaBP concentration in maternal duodenum may well reflect the 1,25-(OH)2D status of the mother. The CaBP synthesis in placenta and fetal membranes are vitamin D-dependent, and their regulation differs from that of intestinal CaBP. It app]ears that 1 alpha-hydroxylase activities of the fetoplacental unit (placenta and fetal kidney) are blunted in TPTX animals and that CaBP synthesis in the fetus depends on the presence of 1 alpha-hydroxylated vitamin D3 metabolites in the mother.

Calcium-ion-binding activity in human small-intestinal mucosal cytosol. Purification of two proteins and interrelationship of calcium-binding fractions.

Ca2+-binding activity was investigated in human small-intestinal mucosal cytosol. Binding was detected in fractions with molecular weights of 28000 and about 900000, as determined by gel filtration. No binding was found at molecular weight 12000-13000 (the molecular weight of calcium-binding protein in lower mammalian species) until the cytosol had been subjected to a hollow-fibre-filtration step. The appearance of Ca2+-binding at molecular weight 12000-13000 was associated with a decline in the 28000-mol.wt. calcium-binding fraction. The 12000-13000-mol.wt. fraction contained two distinct calcium-binding proteins. One of these proteins had properties similar to those of pig calcium-binding protein. Antiserum to this protein reacted against the 28000-mol.wt calcium-binding fraction in cytosol from human small-intestinal mucosa and from human kidney. An immunoassay method for one of the calcium-binding proteins was established. In normal duodenal mucosa the concentration was 915 micrograms/g and in the ileum it was 443 micrograms/g of mucosa. A subject with hypercalcaemic sarcoidosis had 1200 micrograms/g of mucosa in the jejunum, and a subject with an undetectable concentration of plasma 25-hydroxycholecalciferol had concentrations of calcium-binding protein in the mucosa similar to those found in normal subjects.

Vitamin D-dependent calcium-binding protein. Immunochemical studies and synthesis by placental tissue in vitro.

A protein similar to rat intestinal calcium-binding protein (CaBP) has been identified in both mouse placenta and mouse small intestine. The mouse protein had a molecular weight of approximately 10,000, exhibited cation-binding properties, and demonstrated immunologic identity with vitamin D-dependent rat CaBP. Under normal dietary conditions, the concentrations of CaBP in mouse placenta and intestine increased 6- and 3-fold, respectively, during the third trimester of pregnancy in parallel with the fetal demands for skeletal mineral. Studies of in vitro protein synthesis indicated that CaBP was synthesized by placental tissue. Slices of mouse or rat placental tissue (12-18-day gestation) were incubated with [3H]leucine and the biosynthesis of placental CaBP was quantified by an immunoprecipitation method using rabbit antiserum to rat intestinal CaBP. Sodium dodecyl sulfate gel electrophoresis of the radioactive immune complex revealed a single 3H-labeled peak corresponding to the molecular weight of rat and mouse CaBP (10,050). The amount of CaBP synthesized by mouse placental tissue was dependent upon gestational age of the placenta and reflected the in vivo changes in placental CaBP content observed during gestation. These data indicate that CaBP is synthesized by placenta and provide an in vitro model for studying the developmental control of placental CaBP synthesis.

Relationships between Cholecalciferol Metabolism and Growth in Chicks as Modified by Age, Breed and Diet

Fast-growing heavy (White Rock) chicks, fed a vitamin D-deficient diet, exhibited a higher activity of kidney 25-hydroxycholecalciferol-1-hydroxylase (1-hydroxylase) than slow-growing light (White Leghorn × Rhode Island Red) chicks fed the same diet. 1-Hydroxylase and duodenal calcium-binding protein (CaBP) declined with age. Feeding of low energy diets with or without vitamin D resulted in a slower rate of growth and reduced 1-hydroxylase activity and CaBP concentration. Severe dietary restriction of either calcium or phosphorus resulted in a lower growth rate as well as a duodenal CaBP as compared to a moderate mineral restriction. The severe dietary calcium restriction also resulted in a lower 1-hydroxylase activity than that resulting from a moderate restriction. The results clearly indicate that high 1-hydroxylase activity and a high intestinal CaBP are associated with a high growth rate.Vitamin D deficiency calcium-binding protein

Effect of Ca intake on saturable and nonsaturable components of duodenal Ca transport.

Calcium absorption was studied by an in situ ligated-loop procedure in 9-wk-old male Wistar rats that had been placed from weaning on one of three semisynthetic regimens, 0.17% Ca, 0.44% Ca, or 0.44% Ca plus lactose. Lactose was added because it is known to increase intestinal calcium retention. When the amount of calcium absorbed was expressed as a function of calcium instilled in the loop, it became possible to describe absorption as the sum of a hyperbolic and a linear function, equivalent to a saturable and a nonsaturable process, respectively. The slope of the nonsaturable component was independent of prior calcium intake, while the maximum saturable flux (Jmax) decreased as calcium intake increased. Analysis of the duodenal content of the vitamin D-dependent calcium-binding protein (CaBP, Mr congruent to 10(4)) revealed a positive relation between Jmax and CaBP. Thus, vitamin D appears to be implicated in the saturable, but not in the nonsaturable, component of calcium absorption.

Effects of diabetes of calcium-binding protein in the cecum and colon of the rat.

Calcium-binding protein (CaBP) was measured in the mucosa of cecum and colon of pair-fed control and diabetic rats. After extraction from mucosa, CaBP was purified by Bio-gel P-100 chromatography, and eluted fractions were analyzed by Chelex assay to define the peak of CaBP. Total CaBP content of mucosa in cecum was slightly depressed in diabetics compared to controls, but was decreased to one-third the control value in colon. Specific activity (CaBP per g mucosa and per mg protein) of diabetics was reduced to two-thirds the control value in both segments. Thus, in diabetes, the response of large intestinal CaBP is similar, but of lower magnitude than that of duodenum.

Intestinal calcium-binding protein. A protein indicator of enterocyte maturation associated with the terminal web.

The vitamin D-dependent calcium-binding protein (CaBP) was studied in relation to the age of the cell, in isolated epithelial cell populations removed from rat duodenum. Alkaline phosphatase and thymidine kinase activities were used as markers to characterize differentiated villus cells and undifferentiated (mitotically active) crypt cells, respectively. CaBP distribution along the length of the villus, as established by radioimmunoassay, appears as a gradient increasing from the crypt to the tip of the villus. CaBP concentration in cells is shown to be (i) negatively correlated with the thymidine kinase activity of cells, and (ii) positively correlated with the alkaline phosphatase activity of cells. This indicates that CaBP is absent in crypt cells and appears in differentiated cells with the development of the brush border. Thus CaBP, like alkaline phosphatase, can be considered as an indicator of enterocyte maturation. These data were also confirmed by studying the cellular localization of the protein. In addition both indirect immunofluorescence and immunoperoxidase staining methods reveal that antibody against CaBP decorates the terminal web, but not the microvilli of the brush border of mature absorptive cells. The results suggest that CaBP may act as a modulator of some Ca2+-mediated biochemical processes at the level of the enterocyte brush border.

Effect of age on the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 by kidney of rat.

The decreased absorption of calcium by the small intestine of the adult may reflect changes in vitamin D metabolism with age. The purpose of this study was to compare the capacity of young (1.5 mo of age) and adult (12 mo of age) vitamin D-deficient rats to convert 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, the physiologically active form of vitamin D. Young rats responded to an oral dose of 25-hydroxyvitamin D3 with significantly increased intestinal absorption of calcium and a three-fold increase in the intestinal content of vitamin D-stimulated calcium-binding protein. Adult rats showed no significant increase in these parameters. The conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 was measured in the whole animal by administering a dose of tritiated 25-hydroxyvitamin D3 and determining the appearance of tritiated metabolites in plasma and small intestine. In the adult rat, only 2.1 +/- 0.6% of the plasma radioactivity was in the form of 1,25-dihydroxyvitamin D3 after 24 h compared with 20.8 +/- 3.0% in the young. The conversion of tritiated 25-hydroxyvitamin D3 to its products was also measured directly in isolated slices of renal cortex. 1,25-Dihydroxyvitamin D3 production by adult renal slices was found to be less than one-tenth that of slices from the young. These results indicate that there is a marked decrease in the capacity of the vitamin D-deficient adult rat to convert 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3. This is probably due to the decreased capacity of the adult kidney to 1-hydroxylate 25-hydroxyvitamin D3. These studies also demonstrate the usefulness of renal slices in measuring changes in the renal conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 in the mammal.

Effect of cadmium administration on intestinal calcium absorption and vitamin D-dependent calcium-binding protein

The effects of cadmium on intestinal calcium absorption and calcium-binding protein (CaBP) were investigated in chicks by means of the in situ ligated duodenal loop technique. Dietary cadmium, administered in the feed or by gastric intubation, resulted in significant declines in intestinal calcium absorption and mucosal calcium-binding protein concentrations. Cadmium chloride injected directly into the ligated loop of naive chicks also diminished calcium absorption and CaBP concentrations in an apparently dose-response related fashion. No adverse effects of cadmium administration on either the 25- or 1α-hydroxylation reactions of vitamin D were observed. While the general effect of cadmium administration was a reduction in intestinal calcium absorption, plasma calcium levels were consistently elevated in Cd-treated chicks, with the exception of those also maintained on diets low in Ca. The results indicate that cadmium toxicity exerts at least two effects on Ca metabolism, one at the intestinal level and another at the level of the bone, kidney, or both.

60] Radioimmunoassay for chick intestinal calcium-binding protein

This chapter discusses radioimmunoassay for chick intestinal calcium-binding protein (CaBP). The most widely used methods for quantitating chick CaBP have been the relatively insensitive Chelex binding assay and the radial immunodiffusion assay, which uses antisera produced in the rabbit against highly purified chick intestinal CaBP. This chapter discusses about a radioimmunoassay that has been developed for chick intestinal CaBP. The assay system is based on the quantitative measurement of the displacement of radioactive antigen CaBP from its binding antibodies by the unlabeled antigen— that is, there is competitive binding of radioactive and unlabeled CaBP to the specific antiserum. Because there is a fixed, limited amount of labeled CaBP, antibody and varying amounts of unlabeled CaBP, the percentage of label that is bound is inversely related to the concentration of unlabeled CaBP. The chick intestinal CaBP radioimmunoassay utilizes the double-antibody technique, which reduces background to a minimum and maximizes sensitivity. Sensitivity, precision, and accuracy of the radioimmunoassay for chick intestinal CaBP is discussed in detail.

Reversal of prednisolone-induced inhibition of intestinal calcium-binding protein synthesis by 1 alpha-hydroxycholecalciferol in chicks.

Effects of prednisolone on the biochemical indices of blood serum, ash content in bones, calcium absorption and calcium-binding protein (CaBP) in the intestinal mucosa were studied. The oral administration of prednisolone (10 mg/kg) for 2 weeks reduced CaBP content and calcium absorption to the level typical for D-avitaminosis. The oral administration of 1 alpha-hydroxycholecalciferol (390 pmoles/day) in combination with prednisolone for a week returned calcium absorption and CaBP to the control value.

Effects of the Sex Steroid Hormones and Vitamin D3 on Calcium-Binding Proteins in the Chick Shell Gland1

The effects of diethylstilbestrol (DES), progesterone, testosterone and vitamin 03, alone or in various combinations, on the calcium-binding protein (CaBP) concentration, wet weight and histology of the shell gland were investigated in 7.5-10-week-old chicks. Chicks, which were either vitamin D deficient or vitamin D replete, were pretreated with DES to differentiate the shell gland. DES administration was then discontinued for 10 days and then chicks were treated with the hormones to be tested. In this secondary treatment period, DES increased the levels of CaBP (measured using a specific radioimmunodiffusion assay) in both vitamin D deficient and vitamin D replete chicks, suggesting an effect of DES on CaBP largely independent of vitamin 0. Progesterone alone had no effect on CaBP levels, but it inhibited the DES stimulated increase in CaBP in every case tested. In contrast to this antagonism of CaBP level, the increases in shell gland wet weight and hypertrophy of the tubular gland cells induced by DES were not inhibited by concomitant progesterone administration. Vitamin D3 alone did not increase CaBP concentration in the shell gland and, when administered with DES to vitamin D deficient chicks, it elicited little if any increase in CaBP level above that evoked by DES alone. Testosterone had little effect on CaBP level in the shell gland. Despite chronic injections with estrogen, however, the maximum concentration of CaBP induced in the DES-differentiated shell gland was only one-sixth the concentration found in the shell gland of the egg laying hen. These results indicate that CaBP, which is found in numerous organs, may be regulated by factors in addition to vitamin D3, the predominant regulator in the intestine, and that estrogen and progesterone may play important roles in modulation of CaBP concentration in the shell gland of the hen.

Intestinal calcium binding protein in uremia.

In order to assess the usefulness of intestinal biopsies as indicators of end-organ responsiveness to vitamin D in uremic patients, calcium binding activity and calcium binding protein (CaBP) content were measured in intestinal biopsies from 12 uremic patients (glomerular filtration rate less than 5.0 ml/min) and 12 adult controls. Values for both were found to vary with the site of biopsy, highest values being obtained in the duodenal bulb, with lower values distally. Values for activity correlated with values for CaBP content in both normals and uremics and no difference was observed between groups. Levels of calcium binding activity and content of CaBP did not correlate with serum immunoreactive parathormone levels, but were directly related to circulating 25-hydroxycholecalciferol (25-OHD) levels. The data show that intestinal CaBP is normal in activity, quantity, and affinity for calcium in malabsorbing uremic patients, and are consistent with the hypothesis that calcium malabsorption in uremia is unrelated to deficiency of intestinal calcium binding protein.