In vitro effects of vitamin D3 on the phospholipids of isolated renal brush border membranes

Micronutrients encapsulation in enhanced nanoliposomal carriers by a novel preparative technology

Micronutrients administration by fortification of staple and complementary foods is a followed strategy to fight malnutrition and micronutrient deficiencies and related pathologies. There is a great industrial interest in preparation of formulations for joint administration of vitamin D3 and vitamin K2 for providing bone support, promoting heart health and helping boost immunity. To respond to this topic, in this work, uncoated nanoliposomes loaded with vitamin D3 and K2 were successfully prepared, by using a novel, high-yield and semi continuous technique based on simil-microfluidic principles. By the same technique, to promote and to enhance mucoadhesiveness and stability of the produced liposomal structures, chitosan was tested as covering material. By this way polymer–lipid hybrid nanoparticles, encapsulating vitamin D3 and vitamin K2, with improved features in terms of stability, loading and mucoadhesiveness were produced for potential nutraceutical and pharmaceutical applications.

Micronutrients administration by liposomal vectors is a growing strategy in fortification processes of staple and complementary foods to fight malnutrition and micronutrient deficiencies and related pathologies.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.

Modulation of renal Na-Pi cotransport by hormones acting via genomic mechanism and by metabolic factors

The renal Na-Pi cotransport is subject to multiple regulatory inputs, such as endocrine, paracrine and intracrine. Among lipophilic, long-acting hormones that act via genomic mechanism, thyroid hormones, calcitriol, all-trans-retinoic acid stimulate, whereas glucocorticoids and estradiol inhibit the rate of Na-Pi cotransport across the brush border membrane of proximal tubules in vivo and/or across apical membrane of renal epithelial cells in vitro. Some findings suggest that these hormones may also influence Na-Pi cotransporter by modification of membrane microenvironment. It should be considered that Na-Pi cotransport can be modulated by lipophilic hormones by non-genomic signaling mechanisms such as sphingomyelin-ceramide pathway, NAD-cyclic ADP-ribose-Ca2+i pathway or by Ca2+ influx. Recent studies outline a basis for the putative intracrine signaling mechanisms that utilize Ca(2+)-releasing nucleotides, cyclic ADP-ribose, and nicotinic acid adenosine dinucleotide phosphate (NAADP), as novel second messengers for regulation of Na-Pi cotransport in response to changes of intermediary metabolic processes: gluconeogenesis, pentose phosphate pathway, polyamines and metabolism of fatty acids.

Rapid stimulation of Na+/H+ exchange by 1,25-dihydroxyvitamin D3; interaction with parathyroid-hormone-dependent inhibition

We have examined the rapid effect of 1,25-dihydroxyvitamin-D3 [1,25(OH)2D3] on apical Na+/H+ exchange activity in opossum kidney (OK) cells and in MCT cells (a culture of simian-virus-40-immortalized mouse cortical tubule cells) grown on filter support. Addition of 1,25(OH)2D3 (10 nM) for 1 min increased apical Na+/H+ exchange activity [recovery from an acid load; measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein] in OK cells (by 56%) and in MCT cells (by 36%). The cellular mechanisms involved in 1,25(OH)2D3-dependent stimulation of Na+/H+ exchange were analysed in OK cells; stimulation of Na+/H+ exchange by 1,25(OH)2D3 was not prevented by actinomycin D. Applying parathyroid hormone (PTH) reduced Na+/H+ exchange activity in OK cells (by 34% at 10 nM, 5 min); 1,25(OH)2D3 "reversed" PTH-induced inhibition, either when PTH was added prior to 1,25(OH)2D3 or when the two agonists were applied together. 1,25(OH)2D3 had no effect on basal OK cell cAMP content or on [Ca2+]i (fura-2). 1,25(OH)2D3 attenuated PTH-induced cAMP accumulation and had no effect on the PTH-dependent increase in [Ca2+]i. These data suggest a regulatory control (stimulation) of proximal tubular brush-border Na+/H+ exchange by 1,25(OH)2D3. This effect is non-genomic and might in part be explained by a release from cAMP-dependent control of transport activity.

Increase in membrane fluidity modulates sodium-coupled uptakes and cyclic AMP synthesis by renal proximal tubular cells in primary culture

In order to evaluate the influence of membrane fluidization on three apical transport systems and on a basolateral enzyme, and to analyse the mechanisms involved, we studied, in cultured rabbit proximal tubular cells, the effect of increasing concentrations of the local anesthetic drug benzyl alcohol on Na(+)-dependent uptakes of phosphate (Pi), methyl alpha-D-glucopyranoside (MGP), and L-alanine, as well as on basal and stimulated cyclic AMP content. At 10 mM, benzyl alcohol increased the Vmax of Pi uptake by 31%, decreased that of MGP uptake by 24%, and did not affect alanine uptake. Km values were not affected. Benzyl alcohol, up to 40 mM, increased in a concentration-dependent manner basal, PTH-stimulated, and cholera toxin-stimulated, but not forskolin-stimulated cyclic AMP accumulation. In the presence of 40 mM benzyl alcohol, the magnitude of PTH-induced inhibition of Pi uptake was enhanced from 11% to 24%. It is concluded that: (i) fluidization of apical membranes affected differently Na+/Pi, Na+/MGP, and Na+/alanine cotransports, reflecting differences in the lipidic environments of these transport system; (ii) fluidization of basolateral membranes enhanced PTH-stimulated cyclic AMP generation through improved coupling between the receptor-GS complex and the catalytic subunit of adenylate cyclase; (iii) these variations may result in physiological and pathophysiological modulation of the renal handling of solutes and of the phosphaturic effect of PTH.

Binding and degradation of 125I-insulin by isolated rat renal brush border membranes: evidence for low affinity, high capacity insulin recognition sites

The kidney plays a major role in the handling of circulating insulin in the blood, primarily via reuptake of filtered insulin at the luminal brush border membrane. 125I-insulin associated with rat renal brush border membrane vesicles (BBV) in a time- and temperature-dependent manner accompanied by degradation of the hormone to trichloroacetic acid (TCA)-soluble fragments. Both association and degradation of 125I-insulin were linearly proportional to membrane protein concentration with virtually all of the degradative activity being membrane associated. Insulin, proinsulin and desoctapeptide insulin all inhibited the association and degradation of 125I-insulin by BBV, but these processes were not appreciably affected by the insulin-like growth factors IGF-I and IGF-II or by cytochrome c and lysozyme, low molecular weight, filterable, proteins, which are known to be reabsorbed in the renal tubules by luminal endocytosis. When the interaction of 125I-insulin with BBV was studied at various medium osmolarities (300-1100 mosM) to alter intravesicular space, association of the ligand with the vesicles was unaffected, but degradation of the ligand by the vesicles decreased progressively with increasing medium osmolarity. Therefore, association of 125I-insulin to BBV represented binding of the ligand to the membrane surface and not uptake of the hormone or its degradation products into the vesicles. Attempts to crosslink 125I-insulin to a high-affinity insulin receptor using the bifunctional reagent disuccinimidyl suberate revealed only trace amounts of an 125I-insulin-receptor complex in brush border membrane vesicles in contrast to intact renal tubules where this complex was readily observed. Both binding and degradation of 125I-insulin by brush border membranes did not reach saturation even at concentrations of insulin approaching 10(-5) M. These results indicate the presence of low-affinity, high-capacity binding sites for 125I-insulin on renal brush border membranes which can clearly distinguish insulin from the insulin-like growth factors and other low molecular weight proteins and polypeptides, but which do not differentiate insulin from its analogues as do the biological receptors for the hormone. The properties and location of these binding sites make them attractive candidates for the sites at which insulin is reabsorbed in the renal tubule.

Sulfate transport in apical membrane vesicles isolated from tracheal epithelium

Sulfate uptake in apical membrane vesicles isolated from bovine tracheal epithelium is shown to occur into an osmotically sensitive intravesicular space, via a carrier-mediated system. This conclusion is based on three lines of evidence: 1) saturation kinetics; 2) substrate specificity; and 3) inhibition by the anion transport inhibitors SITS and DIDS. The affinity of the transport system is highest in low ionic strength media (apparent Km = 0.13 mM) and decreases in the presence of gluconate (apparent Km = 0.68 mM). Chloride appears to cis-inhibit sulfate uptake and to trans-stimulate sulfate efflux. Cis-inhibition and trans-stimulation studies with a variety of anions indicate that this exchange system may be shared by HCO3-, S2O3(2-), SeO4(2-), and MoO4(2-) but not by H2PO4- or HAsO4(2-). Studies indicate that protons may play two distinct roles in sulfate transport in this system. 1) Their possible modifier role is suggested by the fact that protons affect SO2-4 transport in an uncompetitive manner. 2) The possibility that the proton gradient may act as an energy source for a secondary active transport is indicated by the fact that the imposition of a proton gradient stimulates a transient movement of sulfate in to the tracheal apical membrane vesicle, against its concentration gradient, causing an "overshoot" phenomenon. Our studies show that the carrier-mediated system can function in the absence of chloride. The overshoot observed in the presence of a proton gradient (OH- gradient) indicates that under those conditions the mechanism of transport may be a SO4(2-)-OH- exchange. The fact that chloride cis-inhibits and trans-stimulates SO4(2-) transport indicates that SO2-4 uptake may also occur via a SO4(2-)-Cl- exchange. Studies carried out so far do not enable us to conclude unequivocally whether the tracheal apical membrane system displays two distinct carrier activities (SO4(2-)-Cl-; SO4(2-)-OH-) or one anion exchanger, which like the erythrocyte anion exchanger, may interact with SO4(2-), Cl-, and H+. The fact that the anion transport inhibitors DIDS and SITS inhibit SO4(2-) transport in the presence or absence of chloride suggests that the latter possibility may be the case.

1,25-Dihydroxyvitamin D-3 alters membrane phospholipid composition and enhances calcium efflux in HL-60 cells

1,25-Dihydroxyvitamin D-3 (1,25(OH)2D3) had direct effects on the HL-60 cell membrane. Treatment of HL-60 cells with 1,25(OH)2D3 for short time periods (2-4 hours) caused an increase in calcium efflux. This phenomenon was found to be unrelated to new protein synthesis since it was not inhibited in the presence of RNA and protein synthesis inhibitors. The treatment of the HL-60 cells with 1,25(OH)2D3 for four hours caused changes in their membrane phospholipid composition. The phosphatidylcholine:phosphatidylethanolamine ratio increased from 1.2 to 1.5. Thus the alteration in the phospholipid composition in the membrane induced by 1,25(OH)2D3 may be responsible for the changes in the permeability of the membrane to calcium ions.

Phosphatidylcholine metabolism in neonatal mouse calvaria

Phosphatidylcholine metabolism was examined in neonatal mouse calvaria in vitro. Incorporation of choline into phosphatidylcholine was slow in this tissue. At 2 h after a pulse of [methyl-3H]choline only 30% of the tissue radioactivity was in the organic phase. Chromatography of the aqueous phase of the tissue extract revealed that more than half of the radioactivity was present as choline at this time. There was no accumulation of phosphocholine, which would have been expected if the cytidylyltransferase were the rate-limiting step in the CDP-choline pathway in the tissue. Choline kinase activity in calvarial cytosol was lower than choline kinase activity in liver cytosol of the same animals. No evidence for significant phosphatidylcholine synthesis through the methylation pathway was found in the calvarial tissue. Although rates of choline-phosphatidylcholine base exchange were higher in bone microsomes than in microsomes from liver, the rate of phosphatidylcholine production through this pathway appeared to be too slow to account for the phosphatidylcholine produced by the calvaria. Phosphatidylcholine synthesis in the calvaria was unaffected by 2 h of treatment with 10 nM-parathyroid hormone, 0.1 nM-0.1 microM-1 alpha,25-dihydroxycholecalciferol, 5 microM-prostaglandin E1 or 2.5 nM-salmon calcitonin, or by 17 h of treatment with 10 nM-parathyroid hormone or 0.1 nM-1 alpha,25-dihydroxycholecalciferol.

Increased calcium absorption in prehypertensive spontaneously hypertensive rat. Role of serum 1,25-dihydroxyvitamin D3 levels and intestinal brush border membrane fluidity

Changes in Ca absorption have been described in the spontaneously hypertensive rat (SHR) compared with Wistar-Kyoto (WKy) rats. In 3.5-wk-old SHR and age-matched WKy controls, we measured direct arterial blood pressure, Ca absorption, and serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels and small intestine brush border membrane (BBM) fluidity and lipid composition. The two objectives were (a) to define the nature of the absorptive changes before detectable hypertension and (b) to evaluate the potential mechanism(s). We found that even at this normotensive stage (106 +/- 4 vs. 107 +/- 2 torr for the female and 109 +/- 3 vs. 104 +/- 3 torr for the male), the SHR (a) absorbed more Ca (1.46 +/- 0.06 vs. 1.14 +/- 0.08 mmol/d and 1.53 +/- 0.06 vs. 1.28 +/- 0.06 mmol/d, respectively) and retained more Ca, (b) had higher serum 1,25(OH)2D3 levels (340 +/- 36 vs. 160 +/- 18 pg/ml and 230 +/- 25 vs. 150 +/- 16 pg/ml, respectively), and (c) possessed BBM with increased fluidity and with reduced fatty acyl saturation index owing to decreased stearic (32.2 +/- 2.6% vs. 38.2 +/- 0.9%) but increased linoleic acids (12.2 +/- 2.0% vs. 7.6 +/- 1.6%). These results demonstrate increased Ca absorption in prehypertensive SHR associated with increased serum 1,25(OH)2D3 levels, increased intestinal BBM fluidity, and reduced saturation index, which singly or in combination could produce the changes in intestinal Ca transport.

Effects of benzyl alcohol on enzyme activities and D-glucose transport in kidney brush-border membranes

Addition of increasing amounts of benzyl alcohol progressively reduced the steady-state anisotropies of diphenylhexatriene and trimethylammoniumdiphenylhexatriene in brush-border membranes from rat kidney. The decrease in order of membrane lipids, equivalent for 50 mM benzyl alcohol to that produced by a rise in temperature of approx. 6 degrees C, had no effect on the activities of alkaline phosphatase or gamma-glutamyltranspeptidase. On the other hand, benzyl alcohol markedly inhibited the D-glucose uptakes measured in the presence of a 100 mM sodium gradient. For concentrations less than 30 mM, benzyl alcohol reduced the Jmax without significant effects on Km, 22Na+ uptake or the vesicular volume of brush-border preparations. Comparable results were obtained substituting octanol for benzyl alcohol. Our data strongly suggest that, at constant temperature, the D-glucose carrier present in renal brush-border membranes is extremely sensitive to variations in membrane physical state.

Mechanism of stimulation of renal phosphate transport by 1,25-dihydroxycholecalciferol

Vitamin D has been shown to stimulate renal phosphate transport and to alter membrane phospholipid composition. The present studies examine the possibility that the effects of 1,25(OH)2D3 on phosphate transport are related to its effects on membrane lipids. Arrhenius plots, which relate maximum rates of sodium dependent phosphate uptake into brush-border membrane vesicles to temperature were constructed. Phosphate transport was studied using brush-border membrane vesicles from normal, vitamin D-deficient, and physiologically replete (15 pmol/100 g body weight per 24 h) rats. These plots were triphasic with characteristic, lipid-dependent, slopes (M1,M2,M3) representing activation energies and transition temperatures (T1,T2). Physiologic 1,25(OH)2D3 repletion normalized these plots by stimulating phosphate transport at all temperatures, increasing T2 from 18 +/- 0.7 to 23.5 +/- 0.9 degrees C and decreasing M2 and M3 from -5.8 +/- 0.2 and -10.2 +/- 0.4 to -4.5 +/- 0.4 and -7.7 +/- 0.3, respectively. Pharmacologic (1.2 nmol/100 g per 3 h) 1,25(OH)2D3 treatment resulted in a change in the Arrhenius plot of phosphate transport to a biphasic one with a transition temperature of 30 degrees C. This effect was not blocked by cycloheximide. The Arrhenius plots of glucose transport were triphasic and unchanged with vitamin D repletion. These data support a liponomic mechanism of action for 1,25(OH)2D3 on phosphate transport.

In vitro ethanol effects on the transport properties of isolated renal brush-border membrane vesicles

The in vitro effect of ethanol on membrane structure and transport properties was studied in isolated renal brush border membrane vesicles. 31P-NMR studies showed a dose-dependent increase in the quantity of an isotropic, possibly inverted-micellar component of the renal brush-border membrane as a result of treatment with ethanol. Such structures have been shown to be instrumental in the translocation of material across membrane bilayers. A 23Na-NMR study of Na+ exchange in artificial phosphatidylcholine liposomes indicated that ethanol (0.1%) was capable of rendering the otherwise inert vesicles permeable to sodium, supporting the idea that ethanol may exert its action via a direct effect on the structure of the phospholipid bilayer. In the isolated renal brush-border membrane vesicles, like in the artificial liposomes, amiloride-insensitive pathways of Na+ transport were shown to be markedly activated by ethanol. These results were consistent with the inhibitory effect ethanol had on Na+ gradient-dependent transport systems such as the Na+ gradient-dependent D-glucose transport and Na+/H+ exchange. In conclusion, our results indicate that ethanol exerts its effect on the renal brush-border membrane by causing a structural change in the phospholipid bilayer which activates sodium intake. The inhibitory effect of ethanol on glucose uptake and Na+/H+ exchange is secondary, as a result of the dissipation of the energy-producing Na+ gradient.

Evidence from 23Na NMR studies for the existence of sodium-channels in the brush border membrane of the renal proximal tubule

A fast Na+-exchange is shown to be present in isolated renal brush border membranes. The lower limit of the rate constant for this process, calculated from the 23Na-NMR spectrum is 580 sec-1. The actual exchange rate may be higher. A fast 7Li exchange is also shown to be present in the isolated membrane vesicles. The characteristic overshoot of the Na+ dependent D-glucose cotransport and Na+/H+ antiport can be demonstrated. The fact that neither treatment with papain, nor lowering of the temperature to 5 degrees C affected the 23Na-NMR spectra obtained in the renal brush border membrane vesicles is consistent with the possibility that the fast Na+-exchange occurs through a channel mechanism.

Spin probe clustering in human erythrocyte ghosts

A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37 degrees C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membrane-bound clusters of variable size. No 'low' probe remains at high P/L where all I(12,3) clusters in a 'concentrated' phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes.

Evidence for the presence of an ATP transport system in brush-border membrane vesicles isolated from the kidney cortex

Results of 31P-NMR studies and transport experiments using the radioactive tracer technique are presented. They point to the conclusion that ATP is taken up into isolated renal brush-border membrane vesicles, possibly by a carrier-mediated mechanism.

Effects of aging on the lipid order and composition of rat adipocyte ghosts

An analysis of the cholesterol/phospholipid ratio of adipocyte ghosts from rat epididymal fat pads shows a significant increase with age (P less than 0.005). An attempt to correlate these changes with the order of the lipid matrix was made using the stearic acid spin label 2-(3-carboxypropyl)-4, 4-dimethyl-2-tridecyl-3-oxazolidinyloxyl [I(12,3)]. Although order was negatively correlated with temperature in preparations from both 6- and 24-month-old rats, no effect of age could be detected.

Vitamin D3 stimulates calcium-45 uptake by isolated mouse islets in vitro

Islets isolated from ob/ob mice which had been fed a vitamin D-deficient diet released significantly less insulin in response to glucose than did vitamin D-replete islets but showed normal net 45Ca2+ uptake. To determine whether vitamin D3 has a direct effect on the pancreatic B cell, islets from ob/ob mice on a normal diet were exposed to vitamin D3 in vitro for 1 week or only 3 h, and then glucose-stimulated 45Ca2+ uptake and insulin release were measured. Exposure to 1 nM or 1 microM vitamin D3 for 1 week stimulated 45Ca2+ uptake in the presence of 3 mM, but not 20 mM glucose, and did not affect insulin release. Exposure to vitamin D3 for 3 h did not significantly increase net 45Ca2+ uptake although there was a tendency to such an effect (P = 0.10). In conclusion, vitamin D-deficiency in vivo suppressed subsequent glucose-stimulated insulin release in vitro and this effect may be due to a direct effect of the sterol (or one of its metabolites) on calcium handling by the B cell.

Lipid changes in the bones of the healing vitamin D-deficient phosphate-deficient rat

The bones of vitamin D-deficient, phosphate-deficient rats have a lipid composition that is significantly different from that of normal bones. Specifically, these bones have elevated cholesterol and reduced lysophosphatide and free fatty acid contents. Treatment of these animals with a single dose of vitamin D and phosphate produces healing within 72 h and causes rapid corrections of alterations in growth plate and cancellous bone lipid composition. Healing of the rachitic/osteomalacic state in these animals was demonstrated radiographically and histologically. Histomorphometric measurements showed that the relative osteoid volume of the cancellous bone rapidly approached the 7% value of normal controls, decreasing from 29% in the rachitic animals to 16% by 12 h and 8.5% by 72 h. Significant changes in ash weight, Ca:P ratio, and crystal-lite size and perfection were detectable at 12 h, with these parameters approaching values found in normal animals within 72 h. Calcium-acidic phospholipid-phosphate complexes, which are known to promote hydroxyapatite formation, peaked in concentration at 12 h in epiphysis, cancellous, and cortical bone, returning rapidly to normal values after that time. In untreated animals the complexed acidic phospholipid content of the nonmineralized epiphysis was comparable to that in normal mineralizing epiphysis, whereas the content of the complexes was reduced in the cancellous bones of the untreated animals.

Polyamines stimulate D-glucose transport in isolated renal brush-border membrane vesicles

Polyamines are natural constituents of most living organisms. However, their function in normal or pathological conditions is not fully understood. We have investigated in vitro effects of polyamines on characteristic properties of isolated renal brush-border membrane vesicles in order to determine whether polyamines have a regulatory role in membrane transport processes. The polyamines putrescine, spermidine and spermine were found to stimulate D-glucose uptake. Diffusional L-glucose uptake was not altered, indicating that the polyamines affected the active transport of D-glucose, rather than inducing nonspecific changes in membrane lipid properties. The amiloride-sensitive Na+ /H + exchange was slightly inhibited by polyamines while Mg2+ -ATPase activity was stimulated. The polyamine effects could not be explained solely by the polycationic properties of these agents, since polycationic polypeptides had an opposite effect. For example, lysozyme was found to inhibit D-glucose transport. Spermine was incorporated into the trichloroacetic acid-insoluble fraction of brush-border membrane proteins. Results indicated that this incorporation process consisted of at least two components: a Ca2+ -independent component and a Ca2+ -dependent component, possibly as a result of transglutaminase activity which was present in the isolated renal brush-border membranes. By using SDS-polyacrylamide gel electrophoresis in conjunction with fluorography, [3H]spermine was shown to be incorporated into several brush-border membrane proteins, mainly the 57 kDa, 74 kDa, 100 kDa, a heavy molecular weight band (greater than 200 kDa) and a low molecular weight band (less than 10 kDa). Our results suggest that the polyamine effects on membrane function may be due to a covalent modification of membrane proteins, possibly via a transglutaminase-mediated incorporation of polyamines or to the crosslinking of membrane proteins.