Calcium uptake in isolated brush-border vesicles from rat small intestine

Enhancement of intestinal calcium transport by short-chain fatty acids: roles of Na+/H+ exchanger 3 and transient receptor potential vanilloid subfamily 6

Small organic molecules in the intestinal lumen, particularly short-chain fatty acids (SCFAs) and glucose, have long been postulated to enhance calcium absorption. Here, we used 45Ca radioactive tracer to determine calcium fluxes across the rat intestine after exposure to glucose and SCFAs. Confirming previous reports, glucose was found to increase the apical-to-basolateral calcium flux in the cecum. Under apical glucose-free conditions, SCFAs (e.g., butyrate) stimulated the cecal calcium fluxes by approximately twofold, while having no effect on proximal colon. Since SCFAs could be absorbed into the circulation, we further determined whether basolateral SCFA exposure rendered some positive actions. It was found that exposure of duodenum and cecum on the basolateral side to acetate or butyrate increased calcium fluxes. Under butyrate-rich conditions, cecal calcium transport was partially diminished by Na+/H+ exchanger 3 (NHE3) inhibitor (tenapanor) and nonselective transient receptor potential vanilloid subfamily 6 (TRPV6) inhibitor (miconazole). To confirm the contribution of TRPV6 to SCFA-stimulated calcium transport, we synthesized another TRPV6 inhibitor that was demonstrated by in silico molecular docking and molecular dynamics to occlude TRPV6 pore and diminish the glucose- and butyrate-induced calcium fluxes. Therefore, besides corroborating the importance of luminal molecules in calcium absorption, our findings provided foundation for development of more effective calcium-rich nutraceuticals in combination with various absorptive enhancers, e.g., glucose and SCFAs.NEW & NOTEWORTHY Organic molecules in the intestinal lumen, e.g., glucose and short-chain fatty acids (SCFAs), the latter of which are normally produced by microfloral fermentation, can stimulate calcium absorption dependent on transient receptor potential vanilloid subfamily 6 (TRPV6) and Na+/H+ exchanger 3 (NHE3). A selective TRPV6 inhibitor synthesized and demonstrated by in silico docking and molecular dynamics to specifically bind to the pore domain of TRPV6 was used to confirm a significant contribution of this channel. Our findings corroborate physiological significance of nutrients and SCFAs in enhancing calcium absorption.

CFTR-mediated anion secretion across intestinal epithelium-like Caco-2 monolayer under PTH stimulation is dependent on intermediate conductance K+ channels

Parathyroid hormone (PTH), a pleiotropic hormone that maintains mineral homeostasis, is also essential for controlling pH balance and ion transport across renal and intestinal epithelia. Optimization of luminal pH is important for absorption of trace elements, e.g., calcium and phosphorus. We have previously demonstrated that PTH rapidly stimulated electrogenic [Formula: see text] secretion in intestinal epithelial-like Caco-2 monolayers, but the underlying cellular mechanism, contributions of other ions, particularly Cl^- and K⁺, and long-lasting responses are not completely understood. Herein, PTH and forskolin were confirmed to induce anion secretion, which peaked within 1-3 min (early phase), followed by an abrupt decay and plateau that lasted for 60 min (late phase). In both early and late phases, apical membrane capacitance was increased with a decrease in basolateral capacitance after PTH or forskolin exposure. PTH also induced a transient increase in apical conductance with a long-lasting decrease in basolateral conductance. Anion secretion in both phases was reduced under [Formula: see text]-free and/or Cl^--free conditions or after exposure to carbonic anhydrase inhibitor (acetazolamide), CFTR inhibitor (CFTRinh-172), Na⁺/H⁺ exchanger (NHE)-3 inhibitor (tenapanor), or K⁺ channel inhibitors (BaCl₂, clotrimazole, and TRAM-34; basolateral side), the latter of which suggested that PTH action was dependent on basolateral K⁺ recycling. Furthermore, early- and late-phase responses to PTH were diminished by inhibitors of PI3K (wortmannin and LY-294002) and PKA (PKI 14-22). In conclusion, PTH requires NHE3 and basolateral K⁺ channels to induce [Formula: see text] and Cl^- secretion, thus explaining how PTH regulated luminal pH balance and pH-dependent absorption of trace minerals.

Gastro-intestinal transport of calcium and cadmium in fresh water and seawater acclimated trout (Oncorhynchus mykiss)

Transport of calcium (Ca) and cadmium (Cd) was examined along the gastro-intestinal tract (GIT) of freshwater and seawater Oncorhynchus mykiss irideus (FWT and SWTies respectively) using in vitro and in vivo experiments. Based on known physiological differences between FWT and SWT which aid in regulating ion levels and osmolarity, we hypothesized that SWT would have lower rates of Ca uptake. Also, we predicted that Cd rates would also be lower because Cd is known to share a common transport mechanism with Ca. Kinetics of Ca and Cd transport were determined using mucosal salines of varying concentrations [1, 10, 30, 60, and 100 (mmolL(-1) for Ca, μmolL(-1) for Cd)]. Linear and saturating relationships were found for Ca for FWT and SWT, but overall SWT had lower rates. Linear and/or saturating relationships were also found for Cd uptake, but rates varied little between fish types. Elevated Ca had no inhibitory effect on Cd transport, and Ca channel blockers nifedipine and verapamil had little effect on Ca or Cd uptake. However, lanthanum reduced Ca transport into some compartments. A 21 day in vivo feeding experiment was also performed where FWT and SWT were exposed to control diets or Cd-spiked diets (552 μg Cd g(-1) food). Whole body Cd uptake between fish types was similar, but the majority of Cd in SWT remained in the posterior intestine tissue, while FWT transported more Cd through their gut wall. Overall it appears that large differences in Ca and Cd uptake between FWT and SWT exist, with SWT generally having lower rates.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

In vitro characterization of calcium transport along the gastrointestinal tract of freshwater rainbow trout Oncorhynchus mykiss

Using an in vitro gut-sac technique, this study examined the mechanisms of calcium (Ca) uptake along the gastrointestinal tract (GIT) of rainbow trout Oncorhynchus mykiss. Ca uptake into three different compartments (mucous-bound, mucosal epithelium and blood space) of four distinct GIT segments (stomach, anterior intestine, mid intestine and posterior intestine) was monitored after luminal exposure to 10 mM Ca saline (radiolabelled with (45) Ca). Ca transport was determined to be both time-dependent and concentration-dependent. The concentration-dependent kinetics of Ca uptake was investigated using varying luminal concentrations of Ca (1, 10, 30, 60 and 100 mM). In the blood-space compartment, Ca uptake was saturable at high Ca concentrations in the mid intestine (suggesting mediated transport), while linear uptake was found in the other gut segments. In the mucous-bound and mucosal epithelium compartments, however, saturation kinetics were found for most GIT segments, also suggesting mediated transport. Manipulation of serosal saline osmotic pressure with mannitol demonstrated that Ca uptake was not greatly affected by solvent drag. Elevated mucosal cadmium (Cd) did not appear to inhibit Ca uptake into the blood space in any of the GIT sections, and Ca uptake did not appear to be sodium dependent. Maximum transport capacities for Ca and Cd were found to be comparable between the gills and gut, but affinities were much higher at the gills (up to 3000 times).

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

Effects of isocoumarins isolated from Paepalanthus bromelioides on mitochondria: Uncoupling, and induction/inhibition of mitochondrial permeability transition

Isolated mitochondria may undergo uncoupling, and in presence of Ca(2+) at different conditions, a mitochondrial permeability transition (MPT) linked to protein thiol oxidation, and demonstrated by CsA-sensitive mitochondrial swelling; these processes may cause cell death either by necrosis or by apoptosis. Isocoumarins isolated from the Brazilian plant Paepalanthus bromelioides (Eriocaulaceae) paepalantine (9,10-dihydroxy-5,7-dimethoxy-1H-naptho(2,3c)pyran-1-one), 8,8'-paepalantine dimer, and vioxanthin were assayed at 1-50 microM on isolated rat liver mitochondria, for respiration, MPT, protein thiol oxidation, and interaction with the mitochondrial membrane using 1,6-diphenyl-1,3,5-hexatriene (DPH). The isocoumarins did not significantly affect state 3 respiration of succinate-energized mitochondria; they did however, stimulate 4 respiration, indicating mitochondrial uncoupling. Induction of MPT and protein thiol oxidation were assessed in succinate-energized mitochondria exposed to 10 microM Ca(2+); inhibition of these processes was assessed in non-energized organelles in the presence of 300 microM t-butyl hydroperoxide plus 500 microM Ca(2+). Only paepalantine was an effective MPT/protein thiol oxidation inducer, also releasing cytochrome c from mitochondria; the protein thiol oxidation, unlike mitochondrial swelling, was neither inhibited by CsA nor dependent on the presence of Ca(2+). Vioxanthin was an effective inhibitor of MPT/protein thiol oxidation. All isocoumarins inserted deeply into the mitochondrial membrane, but only paepalantine dimer and vioxantin decreased the membrane's fluidity. A direct reaction with mitochondrial membrane protein thiols, involving an oxidation of these groups, is proposed to account for MPT induction by paepalantine, while a restriction of oxidation of these same thiol groups imposed by the decrease of membrane fluidity, is proposed to account for MPT inhibition by vioxanthin.

The interaction of flavonoids with mitochondria: effects on energetic processes

The study addressed aspects of energetics of isolated rat liver mitochondria exposed to the flavonoids quercetin, taxifolin, catechin and galangin, taking into account influences of the 2,3 double bond/3-OH group and 4-oxo function on the C-ring, and o-di-OH on the B-ring of their structures, as well as mitochondrial mechanisms potentially involved in cell necrosis and apoptosis. The major findings/hypothesis, were: The 2,3 double bond/3-OH group in conjugation with the 4-oxo function on the C-ring in the flavonoid structure seems favour the interaction of these compounds with the mitochondrial membrane, decreasing its fluidity either inhibiting the respiratory chain of mitochondria or causing uncoupling; while the o-di-OH on the B-ring seems favour the respiratory chain inhibition, the absence of this structure seems favour the uncoupling activity. The flavonoids not affecting the respiration of mitochondria, induced MPT. The ability of flavonoids to induce the release of mitochondria-accumulated Ca(2+) correlated well with their ability to affect mitochondrial respiration on the one hand, and their inability to induce MPT, on the other. The flavonoids causing substantial respiratory chain inhibition or mitochondrial uncoupling, quercetin and galangin, respectively, also decreased the mitochondrial ATP levels, thus suggesting an apparent higher potential for necrosis induction in relation to the flavonoids inducing MPT, taxifolin and cathechin, which did not decrease significantly the ATP levels, rather suggesting an apparent higher potential for apoptosis induction.

Thyroid hormones stimulate calcium transport systems in rat intestine

Thyroid hormone status influences calcium metabolism. To elucidate the mechanism of action of thyroid hormones on transcellular transport of calcium in rat intestine, Ca(2+) influx and efflux studies were carried out in brush border membrane vesicles (BBMV) and across the basolateral membrane (BLM) of enterocytes, respectively. Steady-state uptake of Ca(2+) into BBMV as well as Ca(2+) efflux from the BLM enterocytes was significantly increased in hyperthyroid (Hyper-T) rats and decreased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Kinetic studies revealed that increase in steady state Ca(2+) uptake into BBMV from hyper-T rats was fraternized with decrease in Michaelis Menten Constant (K(m)), indicating a conformational change in Ca(2+) transporter. Further, this finding was supported by significant changes in transition temperature and membrane fluidity. Increased Ca(2+) efflux across enterocytes was attributed to sodium-dependent Ca(2+) exchange activity which was significantly higher in Hyper-T rats and lower in Hypo-T rats as compared to Eu-T rats. However, there was no change in Ca(2+)-ATPase activity of BLMs of all groups. Kinetic studies of Na(+)/Ca(2+) exchanger revealed that alteration in Na(+)-dependent Ca(2+) efflux was directly associated with maximal velocity (V(max)) of exchanger among all the groups. cAMP, a potent activator of Na(+)/Ca(2+) exchanger, was found to be significantly higher in intestinal mucosa of Hyper-T rats as compared to Eu-T rats. Therefore, the results of this study suggest that Ca(2+) influx across BBM is possibly modulated by thyroid hormones by mediating changes in membrane fluidity. Thyroid hormones activated the Na(+)/Ca(2+) exchange in enterocytes possibly via cAMP-mediated pathway.

Calcium transporter 1 and epithelial calcium channel messenger ribonucleic acid are differentially regulated by 1,25 dihydroxyvitamin D3 in the intestine and kidney of mice

We examined the expression of calcium transporter 1 (CaT1) and epithelial calcium channel (ECaC) mRNA in the duodenum and kidney of mice. Intestinal CaT1 mRNA level increased 30-fold at weaning, coincident with the induction of calbindin-D(9k) expression. In contrast, renal CaT1 and ECaC mRNA expression was equal until weaning when ECaC mRNA is induced and CaT1 mRNA levels fall 70%. Long- and short-term adaptation to changes in dietary calcium (Ca) level and 1,25 dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] injection strongly regulated duodenal calbindin D(9k) and CaT1 mRNA. Following a single dose of 1,25(OH)(2)D(3), induction of CaT1 mRNA occurred rapidly (within 3 h, peak at 6 h of 9.6 +/- 0.8-fold) and preceded the induction of intestinal Ca absorption (significantly increased at 6 h, peak at 9 h). Neither renal CaT1 nor ECaC mRNA were strongly regulated by dietary calcium level or 1,25(OH)(2)D(3) injection. Our data indicate that CaT1 and ECaC mRNA levels are differentially regulated by 1,25(OH)(2)D(3) in kidney and intestine and that there may be a specialized role for CaT1 in kidney in fetal and neonatal development. The rapid induction of intestinal CaT1 mRNA expression by 1,25(OH)(2)D(3), and the marked induction at weaning, suggest that CaT1 is critical for 1,25(OH)(2)D(3)-mediated intestinal Ca absorption.

Selenocompounds in plants and animals and their biological significance

There are several selenocompounds in tissues of plants and animals. Selenate is the major inorganic selenocompound found in both animal and plant tissues. Selenocysteine is the predominant selenoamino acid in tissues when inorganic selenium is given to animals. Selenomethionine is the major selenocompound found initially in animals given this selenoamino acid, but is converted with time afterwards to selenocysteine. Selenomethionine is the major selenocompound in cereal grains, grassland legumes and soybeans. Selenomethionine can also be the major selenocompound in selenium enriched yeast, but the amount can vary markedly depending upon the growth conditions. Se-methylselenocysteine is the major selenocompound in selenium enriched plants such as garlic, onions, broccoli florets and sprouts, and wild leeks.

1,25-Dihydroxyvitamin D3 increases the expression of the CaT1 epithelial calcium channel in the Caco-2 human intestinal cell line

BACKGROUND

The active hormonal form of vitamin D (1,25-dihydroxyvitamin D) is the primary regulator of intestinal calcium absorption efficiency. In vitamin D deficiency, intestinal calcium absorption is low leading to an increased risk of developing negative calcium balance and bone loss. 1,25-dihydroxyvitamin D has been shown to stimulate calcium absorption in experimental animals and in human subjects. However, the molecular details of calcium transport across the enterocyte are not fully defined. Recently, two novel epithelial calcium channels (CaT1/ECaC2 and ECaC1/CaT2) have been cloned and suggested to be important in regulating intestinal calcium absorption. However, to date neither gene has been shown to be regulated by vitamin D status. We have previously shown that 1,25-dihydroxyvitamin stimulates transcellular calcium transport in Caco-2 cells, a human intestinal cell line.

RESULTS

In the current study, we have demonstrated that Caco-2 cells express low but detectable levels of CaT1 mRNA in the absence of 1,25-dihydroxyvitamin D treatment. CaT1 mRNA expression is rapidly up regulated (4-fold increase at 4 h and 10-fold at 24 h) by treatment with 1,25-dihydroxyvitamin D (10(-7) moles/L). Moreover, the increase in CaT1 mRNA expression preceded by several hours the vitamin D induction of calbindin D9K, a putative cytosolic calcium transport protein.

CONCLUSION

These observations are the first to demonstrate regulation of CaT1 expression by vitamin D and are consistent with a new model of intestinal calcium absorption wherein vitamin D-mediated changes in brush border membrane CaT1 levels could be the primary gatekeeper regulating homeostatic modulation of intestinal calcium absorption efficiency.

Modeling of transcellular Ca transport in rat duodenum points to coexistence of two mechanisms of apical entry

Employing realistic parameters, we have demonstrated that a relatively simple mathematical model can reproduce key features of steady-state Ca2+ transport with the assumption of two mechanisms of Ca2+ entry: a channel-like flux and a carrier-mediated transport. At low luminal [Ca2+] (1-5 mM), facilitated entry dominates and saturates with Km = 0.4 mM. At luminal [Ca2+] of tens of millimolar, apical permeability is dominated by the channel flux that in turn is regulated by cytosolic Ca2+. The model reproduces the linear relationship between maximum Ca2+ transport rate and intestinal calbindin D9K (CaBP) content. At luminal [Ca2+] > 50 mM, local sensitivity analysis shows transcellular transport to be most sensitive to variations in CaBP. At low luminal [Ca2+], transport becomes sensitive to apical entry regulation. The simulations have been run within the Virtual Cell modeling environment, yielding the time course of external Ca2+ and spatiotemporal distributions of both intracellular Ca2+ and CaBP. Coexistence of two apical entry mechanisms accords with the properties of the duodenal Ca2+ transport protein CaT1 and the epithelial Ca2+ channel ECaC.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Calcium competes with zinc for a channel mechanism on the brush border membrane of piglet intestine

Interactions between Ca(+2) and Zn(+2) at the intestinal brush border membrane occur via unclear mechanisms. We hypothesized that Zn(+2) and Ca(+2) are transported across the brush border membrane via a multidivalent metal channel. Using brush border membrane vesicles (BBMV) prepared from intestines of 8 sow-fed piglets, we sought to determine whether Ca(+2) competes with Zn(+2) for uptake. Extravesicular Zn(+2) was removed with ethylenediamine-tetraacetic acid. Time curves of Zn(+2) and Ca(+2) uptake by BBMV were conducted with increasing concentrations of Ca(+2) and Zn(+2), respectively. Saturation curves compared kinetic parameters of Zn(+2) uptake with and without Ca(+2). In addition, Zn(+2) uptake was measured in the presence of various classical Ca(+2) channel modulators. Over 20 min, a 0.4x concentration of Zn(+2) lowered Ca(+2) uptake by vesicles, whereas a 30x concentration of Ca(+2) was necessary to lower Zn(+2) uptake. These data suggest that Ca(+2) has lower affinity than Zn(+2) for a brush border membrane transport protein. Kinetic parameters showed higher K(m) values with 4 or 15 mM Ca(+2) but unchanged J(max), suggesting competitive inhibition. The Ca(+2) channel blocking agents, La(+3), Ba(+2), verapamil, and diltiazem, inhibited Zn(+2) uptake, whereas calcitriol, trans 1,2 cyclohexanediol, cis/trans 1,3 cyclohexanediol, and the L-type Ca(+2) channel agonist, Bay K8644, induced Zn(+2) uptake. These data were consistent with competition for a common transport mechanism on the brush border membrane, possibly a novel multimetal channel. Copyright 2001 Elsevier Science Inc.

Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption

Calcium is a major component of the mineral phase of bone and serves as a key intracellular second messenger. Postnatally, all bodily calcium must be absorbed from the diet through the intestine. Here we report the properties of a calcium transport protein (CaT1) cloned from rat duodenum using an expression cloning strategy in Xenopus laevis oocytes, which likely plays a key role in the intestinal uptake of calcium. CaT1 shows homology (75% amino acid sequence identity) to the apical calcium channel ECaC recently cloned from vitamin D-responsive cells of rabbit kidney and is structurally related to the capsaicin receptor and the TRP family of ion channels. Based on Northern analysis of rat tissues, a 3-kilobase CaT1 transcript is present in rat duodenum, proximal jejunum, cecum, and colon, and a 6.5-kilobase transcript is present in brain, thymus, and adrenal gland. In situ hybridization revealed strong CaT1 mRNA expression in enterocytes of duodenum, proximal jejunum, and cecum. No signals were detected in kidney, heart, liver, lung, spleen, and skeletal muscle. When expressed in Xenopus oocytes, CaT1 mediates saturable Ca(2+) uptake with a Michaelis constant of 0.44 mM. Transport of Ca(2+) by CaT1 is electrogenic, voltage-dependent, and exhibits a charge/Ca(2+) uptake ratio close to 2:1, indicating that CaT1-mediated Ca(2+) influx is not coupled to other ions. CaT1 activity is pH-sensitive, exhibiting significant inhibition by low pH. CaT1 is also permeant to Sr(2+) and Ba(2+) (but not Mg(2+)), although the currents evoked by Sr(2+) and Ba(2+) are much smaller than those evoked by Ca(2+). The trivalent cations Gd(3+) and La(3+) and the divalent cations Cu(2+), Pb(2+), Cd(2+), Co(2+), and Ni(2+) (each at 100 microM) do not evoke currents themselves, but inhibit CaT1-mediated Ca(2+) transport. Fe(3+), Fe(2+), Mn(2+), and Zn(2+) have no significant effects at 100 microM on CaT1-mediated Ca(2+) transport. CaT1 mRNA levels are not responsive to 1,25-dihydroxyvitamin D(3) administration or to calcium deficiency. Our studies strongly suggest that CaT1 provides the principal mechanism for Ca(2+) entry into enterocytes as part of the transcellular pathway of calcium absorption in the intestine.

Calcium absorption--a paradigm for mineral absorption

Intestinal calcium absorption proceeds by two mechanisms, an active transcellular process that takes place in the duodenum and a passive paracellular process throughout the small intestine. This article characterizes the three steps of transcellular calcium movement-entry, intracellular diffusion and extrusion-and identifies conditions that must be satisfied for other mineral ions to move transcellularly as part of a transepithelial transport process. Passive calcium movement is down a chemical gradient with the amount absorbed by this pathway determined in large measure by the sojourn time, most of which is spent in the ileum. Because transcellular movement of most mineral ions other than calcium, where measured, is either small or negligible, passive transport is likely to be the major route of intestinal absorption, the nature of which, however, has not been well established experimentally.

Intestinal transport of calcium in rat biliary cirrhosis

The characteristics of intestinal calcium transport in chronic cholestasis remain largely unknown. Using an experimental model of biliary cirrhosis in the rat, we aimed to investigate changes in calcium transport at the jejunal and ileal levels. Two methods were used: 1) uptake of 45Ca in brush border membrane vesicles and 2) measurements of transepithelial fluxes of calcium in Ussing chambers. Thirty days postsurgery, cholestatic rats presented biliary cirrhosis, with normal growth, normal daily energy, and calcium intakes, but had depressed circulating levels of 25-(OH)-vitamin D2 and 1,25-(OH)-vitamin D3. Compared with sham-operated controls, 45Ca uptake ([Ca2+] = 0.03 mmol) measured in vesicles from cholestatic rats was decreased by 3-fold in the duodenojejunum, in concordance with a lower content in brush border membrane calmodulin. Other changes in brush border membrane composition included decreases in structural proteins, microvillous enzymes, and in triglyceride content. Transepithelial fluxes of calcium measured in the ileum ([Ca2+] = 1.2 mmol) revealed in controls a net basal secretion flux (Jnet = -30.4 +/- 8.1 mmol.h-1.cm-2) that was reduced by 3-fold (p < 0.05) in vitamin D-deficient rats (Jnet = -10.4 +/- 4.8 mmol.h-1.cm-2). In response to 25-(OH)-vitamin D2 treatment, calcium uptake rates increased by 40% in the jejunum, whereas in the ileum, the secretion flux returned to basal control levels. Oral administration of taurocholate or tauroursodeoxycholate (50 mmol) depressed almost completely calcium uptake capacity in the duodenojejunum. By complexing free calcium, tauroconjugated bile acids inhibited in vitro calcium uptake proportionally to their concentration in the medium (0-40 mmol). Our data indicate that, in rat biliary cirrhosis, transport capacity of calcium in the duodenojejunum is markedly reduced in association with vitamin D deficiency and alterations in brush border membrane composition.

Ion microscopic imaging of calcium during 1,25-dihydroxyvitamin D-mediated intestinal absorption

A combination of ion microscopic and conventional radionuclide techniques was employed to investigate the temporal-spatial dynamics of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-stimulated intestinal calcium (Ca) absorption. At varying times following the administration of a single intravenous dose of 1,25(OH)2D3 to vitamin D-deficient chicks, transepithelial transport and tissue retention of Ca were quantitated in vivo, using the ligated duodenal loop technique and 47Ca as the tracer. The localization of Ca in the intestinal tissue during absorption was monitored by ion microscopy, using the stable Ca isotope, 44Ca, as the absorbed species. There was little transepithelial absorption of Ca in the vitamin D-deficient animals despite a substantial tissue accumulation of luminally derived Ca, the latter localizing predominantly in the brush border region of the enterocyte, as shown by the 44Ca-ion microscopic images. The early (30 min-1 h) response to 1,25(OH)2D3 was an increased tissue uptake of luminal 47Ca, which also primarily associated with the brush border region, again as shown by ion microscopy. At 2-4 h after the 1,25(OH)2D3 dose, there was a progressive redistribution of Ca from the brush border region throughout the cytoplasm and into the lamina propria. At 8-16 h, 47Ca absorption was maximal and 44Ca was sparsely distributed in the intestinal tissue. 47Ca absorption gradually declined and reached pre-dose levels by 72 h. At this time, tissue 44Ca was again largely limited to the brush border region. These results provide support for the multiple actions of 1,25(OH)2D3 on the intestinal Ca absorption process. The ion microscopic images provided unique information on the specific time-dependent changes in the tissue localization of Ca during the process of its intestinal absorption as affected by 1,25(OH)2D3.

Calcium transport mechanism in human colonic apical membrane vesicles

BACKGROUND & AIMS

Recent studies have shown the role of human colon in the absorption of calcium, especially in the presence of severe disease or resection of the small bowel. The aim of the current study was to explore the mechanism(s) of calcium uptake by the purified human colonic apical membrane vesicles.

METHODS

Apical membrane vesicles were purified from mucosal scrapings of colons from organ donors, and 45Ca uptake was measured using a rapid filtration technique.

RESULTS

The majority of the 45Ca associated with vesicles represented uptake into closed intravesicular space, whereas the remaining 45Ca uptake represented binding to the vesicles. Ca2+ uptake was found to be dependent on time, pH, temperature, and ionic strength of the incubation medium and inhibitable by ruthenium red, La3+, and ethylene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid. Experiments of the effects of membrane potential generated by K+/valinomycin or anion substitutions on Ca2+ uptake showed that the Ca2+ uptake process was potential insensitive. Calcium uptake was unaffected by outwardly directed H+, K+, and Na+ gradients. Ca2+ uptake showed saturation kinetics with no significant differences in Michaelis constant and maximum velocity values of this transporter between proximal and distal colonic segments.

CONCLUSIONS

The uptake of Ca2+ by human colonic apical membranes involves predominantly a carrier-mediated transport mechanism.

Binding of lanthanides to cell membranes in the presence of ligands

The effect of a series of ligands on the binding of the lanthanide, europium (Eu), to rabbit intestinal cell membranes was investigated in vitro. When tested as Eu-ligand complexes (ratio of Eu:ligand, 1:2) of intermediate stability (log stability constant, log K1, for the reaction Eu + L = EuL, of about 7-12) such as Eu-citrate and Eu-nitrilotriacetate (NTA), Eu was available for uptake in a soluble form by intestinal brush-border membrane vesicles (BBMV) in phosphate- and bicarbonate-free solutions at pH 7.2. Ligands with lower log K1 did not maintain Eu in solution whilst those of higher affinity did not donate it to membranes. Generally, there was a clear relationship between log K1 of the Eu-ligand complex and the binding of Eu to BBMV. This relationship identifies ligands that can effectively donate Eu to vesicles under these conditions. BBMV uptake of Eu was due to binding at two sites. Binding to the diethylenetriaminepentaacetate (DTPA)-sensitive site predominated at 20 degrees C and uptake by the DTPA-insensitive site was enhanced at 37 degrees C. Only trace amounts of the bound Eu appeared to be internalized within the vesicles. In the presence of physiological concentrations of phosphate and bicarbonate in cell culture medium, Eu was precipitated from most complexes (at 1:2 and 1:5 Eu:ligand ratio) except DTPA and albumin. Eu precipitation could be prevented by increasing the ligand:Eu ratio. When isolated hepatocytes in cell culture medium were incubated with EuCl3, about 60% of Eu was bound to the cells; Eu-albumin was not bound by hepatocytes.

Uptake of selenite, selenomethionine and selenate by brush border membrane vesicles isolated from rat small intestine

The uptake of selenite, selenate and selenomethionine (SeMet) was performed with brush border membrane vesicles (BBMV) prepared from rats fed selenium-deficient and supplemented diets. At equilibrium (60 min), the uptake of 75Se from [75Se]selenite ranged from 16.5 to 18.9 nmol mg-1 protein. There was a curvilinear relationship in the uptake of selenite over a concentration range of 10-1000 microM. About 2 nmol mg-1 protein was obtained with selenomethionine (SeMet) which occurred between 90 and 180 s. In contrast to selenite, there was a linear relationship in the initial uptake of SeMet over a concentration range of 10-1000 microM. The uptake of selenate was approximately 50-fold lower than selenite, reaching 350 pmol mg-1 protein. Dietary selenium level had no effect on the rate of 75Se accumulation by BBMV. Dramatic differences are found in the uptake and binding of selenium by BBMV incubated with different selenocompounds.

Vitamin D and enterocyte brush border membrane calcium transport and fluidity in the rat

Prior studies of vitamin D repletion showed a threefold increase in the maximum rate (Vmax) for calcium uptake by brush border membrane vesicles, but did not differentiate saturable and nonsaturable uptake components. We studied the calcium uptake and fluidity response of intestinal brush border vesicles to vitamin D by treatment with 1 alpha,25-dihydroxy-24,24-difluorocholecalciferol (24,24-F-1,25-(OH)2D3). Treatment responses were measured by effects on (1) saturable and nonsaturable initial uptake rates of calcium by rat proximal small intestinal brush border membrane vesicles; (2) transmucosal calcium transport by everted duodenal sac; and (3) fluorescence anisotropy. Treatment of vitamin D-depleted weanlings increased the Vmax by 50% (P < .05) in vesicles from the proximal 12 cm of small intestine from rats injected with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), but there was no response in rats not injected with EHDP or in vesicles from the proximal 30 cm of small intestine. Vitamin D-depleted weanlings were D-deficient based on serum 25-hydroxycalciferol(25-OH-D) concentration, but to produce 1 alpha,25-dihydroxycalciferol [1,25-(OH)2D] depletion, EHDP injection was required. Treatment of vitamin D-replete adult rats caused a 20% (P < .05) increase in Vmax. Treatment did not affect the calcium concentration at half-Vmax (KT), the rate constant for nonsaturable uptake (KD), or vesicle fluidity measured as fluorescence anisotropy. Contrasting with these minimal effects of treatment on brush border Vmax, treatment increased transmucosal calcium transport by everted duodenal sac almost threefold in vitamin D-depleted weanlings administered EHDP. Thus, vitamin D actions on enterocyte calcium transport (1) at the brush border increase saturable but not nonsaturable uptake, and (2) produce the major transport response distal to the brush border. Despite previously described changes in membrane lipid, brush border fluidity is unaffected by vitamin D treatment.

Ca2+ transport across intestinal brush border membranes of the cichlid teleost Oreochromis mossambicus

Brush border membranes were isolated from tilapia (Oreochromis mossambicus) intestine by the use of magnesium precipitation and differential centrifugation. The membrane preparation was enriched 17-fold in alkaline phosphatase. The membranes were 99% right-side-out oriented as indicated by the unmasking of latent glyceraldehyde-3-phosphate dehydrogenase and acetylcholine esterase activity by detergent treatment. The transport of Ca2+ in brush border membrane vesicles was analyzed. A saturable and a nonsaturable component in the uptake of Ca2+ was resolved. The saturable component is characterized by a Km much lower than the Ca2+ concentrations predicted to occur in the intestinal lumen. The nonsaturable component displays a Ca2+ permeability too high to be explained by simple diffusion. We discuss the role of the saturable component as the rate-limiting step in transmembrane Ca2+ movement, and suggest that the nonsaturable component reflects a transport mechanism operating well below its level of saturation.

Zinc binding in intestinal brush-border membrane isolated from pig

Zinc binding to brush-border membrane vesicles isolated from pig jejunum was investigated by a rapid filtration method, for long incubation periods (up to 180 min). Zn2+ influx revealed a large accumulation of the metal, reaching an apparent intravesicular volume of 160 microliters/mg protein at equilibrium, a volume 45-times that of an osmotically reactive sugar, sorbitol (3.6 microliters/mg protein). Changes in medium osmolarity had no effect on zinc accumulation. These results suggested a large degree of zinc binding to vesicular components (membrane or core). 65Zn efflux measurements led to the conclusion that two vesicular pools of zinc existed: a small external pool, accessible to different chelators (EGTA) or competitive cations, and a large intravesicular pool. Accumulated 65Zn was quickly removed from its internal sites only after the membrane had been permeabilized by the cation ionophore A23187 in association with an exchange molecule or a chelator. Scatchard plot analyses revealed, on one hand a first class of high-affinity extravesicular zinc binding sites (Ka = 8.6.10(3) M-1, n = 0.455 nmol Zn2+/mg protein) and a second class of extravesicular sites having a very low affinity (Ka = 22 M-1, n = 25.35 nmol Zn2+/mg protein) and, on the other hand one type of intravesicular sites (Ka = 3.3.10(4) M-1, n = 550 nmol Zn2+/mg protein). The intravesicular sites have a high affinity for zinc and are specific, since only nonlabelled zinc (or cadmium) but not calcium present in the bathing medium is exchanged with the internally accumulated labelled cation.

The influence of zinc, magnesium, and iron on calcium uptake in brush border membrane vesicles

The effects of adding increasing concentrations of magnesium (Mg), zinc (Zn), and iron (Fe) on calcium (Ca) uptake into rat brush border membrane vesicles was measured. At ratios of 1:1, based on the RDA, none of these minerals were found to significantly decrease calcium uptake. Mg, however, inhibited Ca uptake at an RDA ratio of 3:1 (Mg:Ca). At higher RDA ratios (Zn:Ca 10:1), Zn also decreased Ca uptake. In a comparison of the molar ratios of the above cations, Zn inhibited Ca uptake the most (0.12:1), possibly by competing directly at high-affinity Ca binding sites. Fe did not decrease Ca uptake but rather demonstrated an enhancing effect at high concentrations (20:1). When mixtures of the above divalent cations were studied, the effects on Ca uptake could largely be explained by the actions of the ions individually.

Studies of zinc transport into brush-border membrane vesicles isolated from pig small intestine

Zinc transport into brush-border membrane vesicles was investigated by measuring uptake rates at a very short incubation time (2 seconds), during the initial linear uptake. A divalent cation chelator (EGTA) was added to the stop and washout solutions in order to remove the zinc bound to the external surface of the vesicles. Under these conditions, we showed that zinc enters the vesicles by (1) a saturable carrier-mediated process, and (2) an unsaturable pathway. The kinetic parameters we calculated were an affinity of 0.215 +/- 0.039 mM, a Jmax of 17.2 +/- 1.7 nmol.min-1.(mg protein)-1 and an unsaturable constant of 0.025 +/- 0.006 (n = 6). The imposition of an outwardly directed K+ gradient (negative inside) did not affect the Jmax value of the zinc uptake but increased the Km value significantly. This suggests that, at least a portion of zinc which crosses the membrane does not do so in a cationic form. Zinc uptake was decreased or increased according to the nature of accompanying anions (Cl-, SO4(2)-, SCN-) in the absence of any membrane potential. With highly permeant anions such as thiocyanates, zinc uptake was considerably augmented, suggesting a movement of zinc in a complexed form involving the presence of negative species. We also showed that cadmium competitively inhibited the zinc uptake; we measured a Ki value of 0.21 mM, indicating a similar affinity of cadmium for the carrier as zinc itself. By contrast, the presence of calcium had little effect on zinc entry into vesicles. The calcium ionophore A23187 had only a slight stimulating effect on zinc uptake. These results indicate that zinc and calcium transports are probably independent of each other.

Mechanisms of linoleic acid uptake by rabbit small intestinal brush border membrane vesicles

We examined the initial transport of a long-chain unsaturated fatty acid, linoleic acid, by brush border membrane vesicles isolated from rabbit small intestine. This preparation allowed us to examine the transport of linoleic acid across the brush border membrane without the effect of the unstirred water layer or cytosol binding proteins. Linoleic acid was solubilized in a 2 mM taurocholate solution which did not compromise the functional integrity of the vesicles. Linoleic acid uptake in the range of 1 to 100 microM followed passive diffusion kinetics. Time course study showed that linoleic acid uptake reached maximal levels during the initial 15 seconds. Although the amount of linoleic acid accumulated in the vesicles diminished over the next 30 minutes, the molar quantity was still twentyfold higher than that of D-glucose (6.5 vs 0.33 nmol/mg protein). Uptake of D-glucose by the vesicles demonstrated typical osmotic responsiveness. We found no osmotic effect on linoleic acid uptake. Hypotonic lysis of membrane vesicles loaded with linoleic acid released 40% of the fatty acid. We concluded that a major portion of the accumulated fatty acid was bound to or incorporated into the membrane itself while ca. 40% did traverse the membrane and accumulated in the intravesicular space as nonmicellar aggregates. The known inhibitors of anion transport, diisothiocyanatostilbene and isothiocyanatostilbene did not change the transport of linoleic acid. We conclude that, in the absence of an unstirred layer or cytosol proteins, linoleic acid transport at up to 100 microM concentration is passive with rapid accumulation both by the cell membrane and the lumen of vesicles.

Characterization of calcium uptake by brush border membrane vesicles of human small intestine

Calcium uptake was characterized in human duodenal, jejunal, and ileal brush border membrane vesicles. Calcium uptake into human intestinal brush border membrane vesicles represented uptake into intravesicular space as evidenced by studies of osmolality, temperature dependence, calcium ionophore A23187-induced efflux and influx, and lanthanum displacement. Calcium uptake into membrane vesicles was sodium-independent. Negative membrane potential induced by valinomycin and anion substitution studies indicated an electroneutral process. Initial rate of uptake of calcium was linear up to 30 s (Y = 0.11 + 0.02x, r = 0.99). Kinetic parameters were determined from uptake measurements at 7 s, well within the linear phase of uptake. Calcium uptake represented mediated and nonmediated components. These components showed changes along the intestinal tract. Km values of the mediated component increased aborally, being lowest in the duodenum and highest in the terminal ileum. Vmax was highest in the duodenum, followed by, in descending order, the ileum, terminal ileum, and jejunum. The nonmediated component was greatest in the duodenum and decreased aborally. The duodenum appears to have a high-affinity, high-capacity system for the transport of calcium in humans. These studies are the first to characterize calcium transport by brush border membranes of the human small intestine.

Equilibrium uptake of D-glucose by osmotically stressed unilamellar phospholipid vesicles

The assumptions inherent in the use of osmotic manipulation to determine the extent of solute binding to brush border membrane vesicles (the ideal osmotic responsiveness of the vesicles and the independence of solute binding from the incubation medium osmotic pressure) were examined in a model system (large unilamellar lipid vesicles). The equilibrium uptake of D-glucose by unilamellar vesicles composed of egg lecithin (PC), phosphatidic acid (PA), and cholesterol (Chol) was measured as a function of the osmotic concentration of the incubation medium. The variation of the encapsulated aqueous volume of PC:PA and PC:PA:Chol vesicles with the osmotic stress was directly determined by a fluorescence self-quenching technique. Encapsulated volume changes of both PC:PA and PC:PA:Chol vesicles were found to be resistant to the osmotic stress, exhibiting positive deviations from ideal behavior. Equilibrium uptake experiments with these vesicles showed that glucose was taken up in excess of that amount predicted on the basis of the encapsulated volume when the vesicles were subjected to osmotic stress less than 0.25 osmol/kg. At osmotic stresses greater than 0.75 osmol/kg, equilibrium uptake could be predicted solely on the basis of the encapsulated volume. These results, based on a model vesicle system, strongly suggest that osmotic manipulation may be an inappropriate method to assess the extent of solute binding to natural membrane vesicle preparations, such as brush border membrane vesicles, without more direct evidence.

Effect of chlorpromazine on the permeability of beta-lactam antibiotics across rat intestinal brush border membrane vesicles

The effect of chlorpromazine on the membrane permeability of beta-lactam antibiotics (benzylpenicillin, ampicillin, cephradine and cephalexin) and actively transported substances (glycylglycine and D-glucose) has been studied using rat intestinal brush border membrane vesicles. Except for cephalexin, the initial uptakes at 25 degrees C of these antibiotics were significantly enhanced in the presence of chlorpromazine. In contrast, the transport of glycylglycine and D-glucose was significantly inhibited. These results suggest that the two groups, drugs and actively transported substances, have a different permeation process. The effect of chlorpromazine concentration on membrane lipid fluidity, as assessed by the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-anilino-8-naphthalene sulphonate (ANS), was also examined. The fluorescence polarization of ANS decreased with increasing concentration of chlorpromazine, while that of DPH increased suggesting an increase of membrane surface fluidity might affect the permeation of beta-lactam antibiotics and actively transported substances in a different manner.

Vitamin D-dependent active calcium transport: the role of CaBP

Transepithelial calcium transport in the intestine involves an active and a passive route. The active route is totally vitamin D-dependent, transcellular, and is largely expressed in the proximal intestine. Of the three steps involved in transcellular transport--entry into the mucosal cell, intracellular movement, and extrusion at the basolateral pole of the cell--neither entry nor extrusion appears rate-limiting in the absence of vitamin D, even though both are enhanced as a result of the action of the vitamin D. However, intracellular calcium movement inside the mucosal cell can match the experimental Vm of transcellular transport only in the presence of the vitamin D-dependent calcium-binding protein (CaBP, Mr = 8.8kDa). CaBP is thought to act as the equivalent of a calcium ferry by amplifying the intracellular movement of calcium. Thus, the major action of vitamin D on cellular calcium transport is via its hormonal product, CaBP, which amplifies intracellular calcium movement by raising total and free calcium levels in the transporting cell.

NMR measurements of intra- and extravesicular sodium in renal microvilli

Previous attempts to separate the nuclear magnetic resonances of intra- and extravesicular Na+ in brush-border membrane vesicles (BBMV) were unsuccessful and led to the proposal of rapid exchange of Na+ via sodium channels in BBMV. However, passive conductance of Na+ in this membrane has been found to be relatively small. This inconsistency prompted us to use a different shift reagent to reassess the issue. In guinea pig renal BBMV (15-30 mg protein/ml) equilibrated with Na+ (130 mequiv. 1), using the impermeant Na+ shift reagent dysprosium tripolyphosphate (3 mM), the resonances of intra- (3.3%) and extravesicular (96.7%) Na+ were resolved by 6 ppm. Increases in Na+ conductance induced by gramicidin D did not alter the characteristics of intra- and extravesicular Na+ resonances. By contrast, addition of glucose caused a transient increase in the area of the intravesicular Na+ resonance. The clear separation between the intra- and the extravesicular Na+ resonances allowed us to measure the relaxation times of Na+, which depend on its interactions with its immediate environment. The longitudinal relaxation time of intravesicular Na+ (13 +/- 1 ms) was much shorter than that of the extravesicular Na+ (44.0 +/- 0.4 ms). Thus, in intact renal BBMV, as well as in membranes treated with the cationophore gramicidin D, the exchange of Na+ between the intra- and the extravesicular compartments is slow on the NMR time scale, consistent with the low Na+ channel density of this membrane. In contrast, the increase in intravesicular Na+ induced by glucose, is consistent with a significant contribution of the glucose cotransport pathway to Na+ flux across these membranes. The short longitudinal relaxation time of Na+ in the intravesicular space indicates interaction of Na+ with BBMV binding sites or ordering of these ions in the intravesicular compartment.

Effect of the somatostatin analogue SMS 201-995 on ATP-dependent calcium transport of basolateral vesicles from human duodenum

Brush border membrane vesicles (BBMV) and basolateral membrane vesicles (BLMV) were simultaneously prepared from surgically resected pieces of morphologically intact human duodenum with a modified Percoll gradient centrifugation method. Alkaline phosphatase was enriched 20-fold in BBMV, whereas (Na+ + K+)-stimulated adenosine triphosphatase was enriched 15-fold in BLMV. BBMV and BLMV were preincubated with 3 microM synthetic somatostatin-14 or 3 microM SMS 201-995 for 10 min at 5 degrees C. In BBMV calcium, sodium, D-glucose, L-alanine, and D-mannitol uptake was unaffected by somatostatin-14 and SMS 201-995. In BLMV we found two Ca++ transport systems: Na+/Ca++ exchange and ATP-driven Ca++ transport. Somatostatin-14 had no effect on either of the two transport mechanisms. SMS 201-995 had no effect on Na+/Ca++ exchange but significantly inhibited basolateral ATP-dependent Ca++ transport (-40% +/- 5%, p less than 0.005).

Effect of two potent calmodulin antagonists on calcium transport of brush border and basolateral vesicles from human duodenum

In the present in-vitro study we investigated the possible role of the calmodulin-antagonistic drugs loperamide and calmidazolium in the regulation of transepithelial Ca2+ transport of human duodenum. Brush border membrane vesicles and basolateral membrane vesicles were simultaneously prepared from surgically resected pieces of morphologically intact human duodenum with a modified Percoll-gradient centrifugation method. Brush border and basolateral membrane vesicles were characterized using enzyme marker analysis and electron microscopy: alkaline phosphatase was enriched 20-fold in brush border membrane vesicles, whereas [Na+ + K+]-stimulated adenosine triphosphatase was enriched 15-fold in basolateral membrane vesicles. Calmodulin activity was determined by a specific radioimmunoassay after solubilizing brush border and basolateral membrane vesicles in 1% Triton X-100. In basolateral membrane vesicles, we found no calmodulin activity. In brush border membrane vesicles calmodulin activity was impaired by 50% after pre-incubation with loperamide or calmidazolium. We measured calcium, sodium, D-glucose and D-mannitol uptake with a rapid filtration technique. Before the transport experiments, brush border and basolateral membrane vesicles were pre-incubated with 5 microM loperamide or 5 microM calmidazolium for 60 min at 5 degrees C. In drug-pretreated, brush border membrane vesicles calcium uptake was significantly reduced after 1 min incubation (-25% +/- 5%, P less than 0.05); this effect was completely reversed in the presence of 5 microM calmodulin. In basolateral membrane vesicles, we found two Ca2+ transport systems: (1) Na+/Ca2+ exchange and (2) ATP-dependent Ca2+ transport. In basolateral membrane vesicles loperamide had no effect. Calmidazolium had no effect on Na+/Ca2+ exchange, but significantly inhibited ATP-dependent Ca2+ transport. This effect could not be reversed by calmodulin.

Calcium uptake into rat small intestinal brush border membrane vesicles: characterization of transmembrane calcium transport at short initial incubation times

Calcium transport into brush border vesicles from rat small intestine was investigated by determining uptake rates at very short incubation periods. At incubation times up to 1 second a linear relationship between calcium uptake and time was observed at free calcium concentrations ranging from 1 microM to 5 mM. At time points above 1 second calcium uptake deviates progressively from linearity. Several lines of evidences (EGTA-wash, dependency on membrane potential, temperature sensitivity and effect of the calcium ionophore A23187) suggest transmembrane transport rather than extravesicular binding of calcium as being responsible for calcium uptake. Saturation experiments performed under initial linear and curvilinear uptake conditions show a saturable transport component in the mu molar and only a tendency to saturate in the molar concentration range. It is concluded that uptake values far from equilibrium are characteristic for transmembrane flux of calcium. Transmembrane flux of calcium is mediated by multiple and potential-sensitive mechanisms.

Relation between Ca2+ uptake and fluidity of brush-border membranes isolated from rabbit small intestine and incubated with fatty acids and methyl oleate

The rate of incorporation of oleic acid into isolated brush-border membranes was found to be considerably faster than methyl oleate incorporation under similar experimental conditions. The effects of fatty acids and methyl oleate incorporation on Ca2+ uptake and fluidity were monitored. Whereas treatment with 0.01-0.05 mM oleic acid corresponding to incorporations smaller than 90 nmol/mg protein enhanced Ca2+ transport, exposures to higher concentrations of this fatty acid corresponding to incorporations larger than 150 nmol/mg protein, decreased uptake of this cation. On the other hand, treatment with 0.01-0.2 mM methyl oleate corresponding to incorporations of up to 220 nmol/mg protein had only a stimulatory effect on the Ca2+ uptake. Oleic acid, linoleic acid and methyl oleate decreased the fluorescence anisotropy of membranes labelled with diphenylhexatriene in a dose-dependent manner. In contrast, palmitic acid had little or no effect on the diphenylhexatriene-reportable order of the membrane within the range of concentrations used. Monitored as a function of temperature, the anisotropy values showed a gradual melting for both the control and lipid-treated membranes. The results support the concept that saturated and cis-unsaturated fatty acids dissolve in different lipid domains and this in itself appears to be an important factor defining whether the biological function of the membrane is affected by the uptake. Incorporation of cis-unsaturated fatty acids in domains harboring the Ca2+ uptake process increases Ca2+ uptake in concert with increased diphenylhexatriene-monitored fluidity. However, when concentrations of such fatty acids in these domains become sufficiently great, the presence of a largely increased number of free carboxyl groups at the membrane surface causes inhibition of Ca2+ uptake.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

Studies on calcium binding to brush-border membranes from rabbit small intestine

A study was made of the uptake of Ca2+ by brush-border membrane vesicles prepared from rabbit small intestine. The process was found to be time, temperature and substrate concentration dependent, displayed saturability, did not depend on added energy sources and occurred optimally in a pH range of 7.5-8.0. Although the transport of D-glucose by these membrane vesicles responded to changes in osmotic pressure as modified by adding cellobiose to the medium, the uptake of Ca2+ was found not to be osmotically-sensitive. Moreover, the equilibrium uptake value obtained when vesicles were exposed to 0.36 mM Ca2+ was some 60-fold higher than the amount that could have been accommodated by the intravesicular space, calculated from the equilibrium uptake of D-glucose. It was concluded from these results that the uptake involved complete binding of the Ca2+ to the membrane. The ionophore A23187 enhanced the rates of uptake and efflux of Ca2+ without affecting equilibrium values, which suggests that the binding of Ca2+ measured under our conditions was to interior sites of the membrane. The binding capacity was decreased in the presence of 10 mM lidocaine as indicated by a diminution of the equilibrium binding values. Generating an electrochemical potential (negative inside) by addition of valinomycin to vesicles pre-equilibrated with K2SO4, enhanced the rate of uptake of Ca2+. Addition of metal ions, on the other hand, inhibited the uptake, La3+ and Tb3+ being most effective followed by Mn2+, Ba2+ and Mg2+. Na+ and K+ were the least inhibitory. The properties of the Ca2+ uptake process found in rabbit brush-border membranes were compared to those of similar processes occurring in other species.

Preparation of subcellular membranes from rat intestinal scrapings or isolated cells. Different Ca2+ binding, nonesterified fatty acid levels, and lipolytic activity

Basolateral, brush-border, and Golgi-enriched subcellular membrane fractions, prepared from homogenates of rat small intestinal mucosa obtained by scraping, had unusually high concentrations of nonesterified fatty acids. These fatty acids appear to be responsible for the large amount of calcium binding, an effect that previously was shown to be reduced in vitamin D deficiency. In contrast, basolateral and Golgi membranes prepared from isolated cells had low levels of nonesterified fatty acids and calcium binding. Intermediate levels were found with isolated cells that were not put through the usual washing procedures. Addition to homogenates of scrapings of a lipase inhibitor, diethyl-p-nitrophenyl phosphate, reduced calcium binding and nonesterified fatty acids to levels similar to those in membranes prepared from isolated cells. Phospholipase A activity was low in homogenates of isolated cells and high in scrapings; this was reduced in intestinal scrapings of vitamin D-deficient rats. Ileal membranes had more calcium binding than duodenal membranes, and ileal homogenates also had greater phospholipase A activity. Preparation of subcellular membranes from rat intestinal scrapings can result in altered lipid composition, probably due to lipolytic enzyme activity; in addition to increasing cation-binding, these high levels of fatty acids may affect other membrane properties and enzyme function.

Effects of exogenous fatty acids on calcium uptake by brush-border membrane vesicles from rabbit small intestine

The uptake of a variety of fatty acids by isolated brush-border membranes from rabbit small intestine was studied. This uptake increased with acyl chain-length and was not diminished by washing of the lipid-treated membranes with 0.25 M CsBr. The binding of fatty acid was not accompanied by a decrease in endogenous acyl groups or of cholesterol and therefore corresponded to a net uptake accountable qualitatively and quantitatively by the fatty acid added to the membranes. The uptake of Ca2+ was stimulated by treatment of the membranes with low concentrations of unsaturated fatty acids (0.05 mM) as well as with various concentrations of caprylic acid (0.10-3.00 mM) and inhibited by treatment with higher concentrations of unsaturated fatty acids (0.20-0.60 mM). Saturated fatty acids had no marked effects on Ca2+ uptake. The stimulatory concentrations of unsaturated fatty acids did not change the Ca2+-binding characteristics of the membranes, whereas the higher concentrations decreased equilibrium binding of Ca2+ and very probably the number of high-affinity binding sites. The results of this study are assessed in terms of the effects of normal fatty acids found in the diet on the absorptive properties of the brush-border membranes.

1,25-Dihydroxyvitamin D increases calmodulin binding to specific proteins in the chick duodenal brush border membrane

In previous studies we demonstrated that the biologically active vitamin D metabolite 1,25-dihydroxyvitamin D [1,25(OH)2D] increased the calmodulin (CaM) content of chick duodenal brush border membranes (BBM) without increasing the total cellular CaM content. Therefore, we evaluated the binding of CaM to discrete proteins in the BBM and determined whether 1,25(OH)2D could influence such binding. We observed one major and several minor CaM-binding bands on autoradiograms of sodium dodecyl sulfate polyacrylamide gels incubated with [125I]CaM. The major band had a molecular weight of 102,000-105,000. It bound CaM even in the presence of EGTA, but not in the presence of trifluoperazine or excess nonradioactive CaM. The administration of 1,25(OH)2D increased the apparent binding of CaM to this protein as assessed by densitometry of the autoradiogram. This increase in CaM binding coincided with the increased ability of the same BBM vesicles to accumulate calcium. Cycloheximide in doses that markedly reduced the incorporation of [35S]methionine into BBM proteins did not reduce the ability of 1,25-dihydroxyvitamin D3 to stimulate either calcium uptake by the BBM vesicles or CaM binding to the 102,000-105,000-mol-wt protein. These results suggest that 1,25(OH)2D administration increases the CaM content of duodenal BBM by increasing the ability of a 102,000-105,000-mol-wt protein to bind CaM. This mechanism may underlie the ability of 1,25(OH)2D to stimulate calcium movement across the intestinal BBM.