Modulation of rat distal colonic brush-border membrane Na+-H+ exchange by dexamethasone: role of lipid fluidity

Mammalian NHE2 Na+/H+exchanger mediates efflux of potassium upon heterologous expression in yeast

Na(+)/H+exchangers form a broad family of transporters that mediate opposing fluxes of alkali metal cations and protons across cell membranes. They play multiple roles in different organisms (protection from toxic cations, regulation of cell volume or pH). Rat NHE2 exchanger was expressed in a Saccharomyces cerevisiae mutant strain lacking its own exporters of alkali metal cations. Though most of the overexpressed NHE2 remained entrapped in the secretory pathway, part of it reached the plasma membrane and mediated K+ efflux from the yeast. We demonstrate for the first time that a mammalian Na(+)/H+ exchanger transports alkali metal cations in yeast in the opposite direction than in mammalian cells, and that the substrate specificity of the rat NHE2 exchanger is limited only to potassium cations upon expression in yeast cells.

Corticosteroid effect on Caco-2 cell lipids depends on cell differentiation

Previous studies from our laboratory have indicated that secondary hyperaldosteronism affects phospholipids of rat colonic enterocytes. To assess whether this represents a direct effect of mineralocorticoids on enterocytes, the role of aldosterone and dexamethasone in the regulation of lipid metabolism was examined in Caco-2 cells during development of their enterocyte phenotype. Differentiation of Caco-2 cells was associated with increased levels of triglycerides (TG) and cholesteryl esters (CE), a decreased content of cholesterol and phospholipids and changes in individual phospholipid classes. The phospholipids of differentiated cells had a higher content of n-6 polyunsaturated fatty acids (PUFA) and lower amounts of monounsaturated (MUFA) and saturated fatty acids than subconfluent undifferentiated cells. Differentiated cells exhibited a higher ability to incorporate [3H]arachidonic acid (AA) into cellular phospholipids and a lower ability for incorporation into TG and CE. Incubation of subconfluent undifferentiated cells with aldosterone or dexamethasone was without effect on the content of lipids, their fatty acids and [3H]AA incorporation. In contrast, aldosterone treatment of differentiated cells diminished the content of TG, increased the content of phospholipids and modulated their fatty acid composition. The percentage of n-6 and n-3 PUFA in phospholipids was increased and that of MUFA decreased, whereas no changes in TG were observed. The incorporation of [3H]AA into phospholipids was increased and into TG decreased and these changes were blocked by spironolactone. Treatment of differentiated cells with dexamethasone increased their CE content but no effect was identified upon other lipids, their fatty acid composition and on the incorporation of [3H]AA. As expected for the involvement of corticosteroid hormones the mineralocorticoid and glucocorticoid receptors were identified in Caco-2 cells by RT-PCR. The results suggest that aldosterone had a profound influence on lipid metabolism in enterocytes and that its effect depends on the stage of differentiation. The aldosterone-dependent changes occurring in phospholipids and their fatty acid composition may reflect a physiologically important phenomenon with long-term consequences for membrane structure and function.

1Alpha,25-dihydroxy-3-epi-vitamin D3, a natural metabolite of 1alpha,25-dihydroxy vitamin D3: production and biological activity studies in pulmonary alveolar type II cells

Pulmonary alveolar type II cells have been shown to be a possible target for the secosteroid hormone, 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], during perinatal transition. At present, there is great interest to isolate and identify the metabolites of 1alpha,25(OH)2D3 produced in its target tissues and to determine the contribution of each individual metabolite of 1alpha,25(OH)2D3 to the final expression of the pleiotropic actions attributed to 1alpha,25(OH)2D3. Of all the known metabolites of 1alpha,25(OH)2D3, 1alpha,25(OH)2-3-epi-D3 has gained most attention as it is produced only in specific tissues and possesses significant activity in tissues in which it is produced. Furthermore, in vivo studies indicate that this metabolite when compared to 1alpha,25(OH)2D3 is less calcemic. Therefore, we performed the present study to identify production of 1alpha,25(OH)2-3-epi-D3 in alveolar type II cells, and to evaluate its effect on surfactant synthesis. We incubated NCI-H441 cells, an alveolar type II cell line, with 1alpha,25(OH)2D3 and demonstrated that these cells metabolize 1alpha,25(OH)2D3 to various previously well-characterized polar metabolites, and to a less polar metabolite which was unequivocally identified as 1alpha,25(OH)2-3-epi-D3 by GC/MS and HPLC analysis. Further, biological activity studies in H441 cells indicated that 1alpha,25(OH)2-3-epi-D3 possesses significant activity in terms of its ability: (i) to increase surfactant phospholipid synthesis, (ii) to induce surfactant SP-B mRNA gene expression, and (iii) to increase surfactant SP-B protein synthesis. However, the activity of 1alpha,25(OH)2-3-epi-D3 when compared to 1alpha,25(OH)2D3 in generating VDR-mediated transcriptional activity in ROS 17/2.8 cells transfected with human osteocalcin VDRE/growth hormone gene construct, was significantly reduced. The high metabolic stability of 1alpha,25(OH)2-3-epi-D3, as previously proposed by us, may be a possible explanation for the high in vitro activity in spite of the reduced VDR-mediated transcriptional activity. In summary, we report for the first time the pathways of 1alpha,25(OH)2D3 metabolism in pulmonary alveolar type II cells and indicate that 1alpha,25(OH)2-3-epi-D3, a natural intermediary metabolite of 1alpha,25(OH)2D3 possesses significant activity in stimulating surfactant synthesis in alveolar type II cells.

Glycosphingolipids modulate renal phosphate transport in potassium deficiency

BACKGROUND

Potassium (K) deficiency (KD) and/or hypokalemia have been associated with disturbances of phosphate metabolism. The purpose of the present study was to determine the cellular mechanisms that mediate the impairment of renal proximal tubular Na/Pi cotransport in a model of K deficiency in the rat.

METHODS

K deficiency in the rat was achieved by feeding rats a K-deficient diet for seven days, which resulted in a marked decrease in serum and tissue K content.

RESULTS

K deficiency resulted in a marked increase in urinary Pi excretion and a decrease in the V(max) of brush-border membrane (BBM) Na/Pi cotransport activity (1943 +/- 95 in control vs. 1184 +/- 99 pmol/5 sec/mg BBM protein in K deficiency, P < 0.02). Surprisingly, the decrease in Na/Pi cotransport activity was associated with increases in the abundance of type I (NaPi-1), and type II (NaPi-2) and type III (Glvr-1) Na/Pi protein. The decrease in Na/Pi transport was associated with significant alterations in BBM lipid composition, including increases in sphingomyelin, glucosylceramide, and ganglioside GM3 content and a decrease in BBM lipid fluidity. Inhibition of glucosylceramide synthesis resulted in increases in BBM Na/Pi cotransport activity in control and K-deficient rats. The resultant Na/Pi cotransport activity in K-deficient rats was the same as in control rats (1148 +/- 52 in control + PDMP vs. 1152 +/- 61 pmol/5 sec/mg BBM protein in K deficiency + PDMP). These changes in transport activity occurred independent of further changes in BBM NaPi-2 protein or renal cortical NaPi-2 mRNA abundance.

CONCLUSION

K deficiency in the rat causes inhibition of renal Na/Pi cotransport activity by post-translational mechanisms that are mediated in part through alterations in glucosylceramide content and membrane lipid dynamics.

Age- and tissue-specific induction of NHE3 by glucocorticoids in the rat small intestine

Of the two known apical isoforms of the Na(+)/H(+) exchanger (NHE) family, only the NHE3 gene is regulated by glucocorticoids. The aim of these studies was to investigate the mechanisms underlying the effects of methylprednisolone (MP) on expression of NHE3 in the proximal and distal small intestine of suckling and adult rats. Immunoblots showed that the glucocorticoid responsiveness in the proximal small intestine was greatest in suckling animals (NHE3/beta-actin: 0.43 +/- 0.09 control vs. 1.57 +/- 0.15 MP; P < 0. 001), and responsiveness decreased with age with no effect in adults (0.56 +/- 0.14 vs. 0.64 +/- 0.17). Distal small intestine was responsive only in adult rats (0.49 +/- 0.13 vs. 1.65 +/- 0.09; P < 0.001). This pattern was confirmed at the mRNA level and by (22)Na(+) uptake. Western blot and [(3)H]dexamethasone mesylate binding showed that the responsiveness of NHE3 to glucocorticoids is directly related to the expression of glucocorticoid receptor (GR) in the small intestine. These studies suggest that loss and gain of glucocorticoid responsiveness in the proximal and distal small intestine, respectively, are related to age- and segment-dependent expression of GR.

Intracellular pH alterations induced by tacrine in a rat liver biliary epithelial cell line

1. The effects of tacrine (THA) on intracellular pH (pH(i)) were examined in a rat liver biliary epithelial cell line (RLEC) in HEPES-buffered medium. pH(i) was recorded using the pH-sensitive fluoroprobe, carboxy-SNARF-1 (carboxy-seminaphtorhodafluor). 2. In the steady state, short-term exposures to THA resulted in alkalinization and re-acidification at 0.1 and 0.25 mM. Following a 24 h-treatment, no significant difference in pH(i) could be detected at 0.1 and 0.25 mM THA, whereas at 0.05 mM, pH(i) was slightly more acid (7.17+/-0. 02, n=16 versus 7.21+/-0.02, n=24 [control]). 3. In control and short-term treated cells, intracellular intrinsic buffering power (beta(i)) increased roughly linearly as pH(i) decreased. This dependence was not seen following long-term treatment. In all cases, beta(i) was increased by THA (by 1.6 to 3.5 fold). 4. Following an acid load (induced by 20 mM NH(4)Cl removal), pH(i) recovery in RLEC relied upon Na(+)/H(+) exchange. A short-term treatment (0.25 mM THA) did not affect total acid extrusion. In contrast, a 24 h-treatment with 0.05 mM THA reduced it (by approximately 36% at a pH(i) of 6.73) while at 0.25 mM, a large increase was detected (by approximately 109% at a pH(i) of 6.75). In Na(+)-free medium, THA (0. 25 mM) still induced an alkalinization in the steady state. Following an acid load, THA stimulated a Na(+)-independent acid efflux in a dose-dependent manner, inhibitable by alpha-cyano-4-hydroxy cinnamate (CHC, 4 mM) but not by quercetin (0. 125 mM). 6. In conclusion, this work demonstrates that THA affects pH(i) in RLEC, through a decrease in Na(+)/H(+) exchange and an increase in beta(i). Stimulation of a CHC-inhibitable, Na(+)-independent acid efflux is also detected.

Modulation by pH0 and intracellular Ca2+ of Na(+)-H+ exchange in diabetic rat isolated ventricular myocytes

We have previously shown that diabetes is associated with a decrease in Na(+)-H+ exchange activity in rat cardiac papillary muscle. The present work has been carried out in order to elucidate the factors responsible for such an alteration. Thus, we have studied the effects of pH0 and intracellular Ca2+ on Na(+)-H+ exchange in ventricular myocytes isolated from streptozotocin-induced diabetic rat hearts. pH1 was recorded using carboxy-seminaphthorhodafluor (SNARF-1). The NH4+ (10 mmol/L) prepulse method was used to induce an acid load in order to activate Na(+)-H+ exchange in HEPES-buffered Tyrode's solution. Whereas diabetes did not change intracellular buffering power, it significantly decreased acid efflux through Na(+)-H+ exchange (acid efflux, 4.32 +/- 0.4 [n = 32, normal cells] versus 2.5 +/- 0.2 [n = 43, diabetic cells] meq/L per minute at pHi 6.9; P < .02). Upon changes of pH0 (at a range of 8.0 to 6.8), acid efflux similarly varied in normal and diabetic cells, thus pointing to an unchanged pH0 sensitivity of Na(+)-H+ exchange. Buffering of intracellular Ca2+ by pretreatment of the cells with BAPTA-AM (25 mumol/L Ca2(+)-chelator) resulted in a decrease by approximately 58% of acid efflux in the diabetic group. This decrease was even more marked in normal cells (by approximately 74%). Interestingly, the pH1 dependence of the acid efflux carried by Na(+)-H+ exchange then became identical in both groups of cells, thus pointing to a role for intracellular Ca2+ in the diabetes-related alterations of the exchange. Inhibition of calmodulin (by 1.5 mumol/L calmidazolium) and of Ca2+/calmodulin-dependent protein kinase II (by 2 mumol/L 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazin e [KN-62]) significantly slowed down pH1 recovery in both normal and diabetic cells. However, the effect of KN-62 was significantly lower in diabetic cells (efflux decreased by approximately 17%) compared with normal cells (decrease by 45%). In conclusion, these data, in light of recent observations showing a decreased [Ca2+]i associated with diabetes in isolated ventricular myocytes, suggest that changes in intracellular Ca2+ may play an important role in altering Na(+)-H+ exchange activity in diabetic ventricular myocytes. They also point to diabetes-related alterations in the Ca2+/calmodulin protein kinase II-dependent phosphorylation of Na(+)-H+ exchange.

Effects of trimetazidine on pHi regulation in the rat isolated ventricular myocyte

1. We have examined the effects of trimetazidine (TMZ) on intracellular pH (pHi) regulation in rat isolated ventricular myocytes. pHi was recorded ratiometrically by use of the pH-sensitive fluoroprobe, carboxy-SNARF-1 (carboxy-seminaphtorhodafluor). 2. Following an intracellular acid load (induced by 10 mM NH4Cl removal), pHi recovery in HEPES-buffered Tyrode solution was significantly slowed down upon application of 0.3 mM TMZ only when myocytes were pretreated for 5 h 30 min (slowing by approximately 50%; P < 0.01). This effect of TMZ on pHi recovery was shown to be not only time- but also dose-dependent with a large, quickly reversible, effect obtained with 1 mM TMZ applied for 2-3 h (slowing by approximately 64%; P < 0.001). This slowing of pHi recovery was also associated with a decrease of the NH4+ removal-induced acidification. 3. Relationship between intracellular intrinsic buffering power (beta i) and pHi was assessed in absence or presence of TMZ (0.3 mM or 1 mM). As expected, beta i increased roughly linearly with a decrease in pHi in all cases. However, both concentrations of TMZ significantly increased beta i (by approximately 55 and 65% at pHi 7.1, respectively). 4. When Na+/H+ exchange was inhibited by dimethyl amiloride (DMA; 40 microM), trimetazidine (1 mM) did not change the H+ flux estimated at pHi 7.1 (0.31 +/- 0.03 mequiv l-1 min-1, n = 5, control, versus 0.30 +/- 0.025 mequiv l-1 min-1, n = 5, TMZ), ruling out any effect of TMZ on background acid loading. 5. Acid efflux carried by Na+/H+ exchange was significantly decreased only when myocytes were pretreated with 1 mM TMZ, for 2-3 h (JeH = 2.86 +/- 0.38 mequiv l-1 min-1, n = 26, control, versus 1.66 +/- 0.26 mequiv l-1 min-1, n = 10, TMZ, estimated at pHi 7.1; P < 0.05). 6. In conclusion, the present work demonstrates that, following an intracellular acid load in HEPES-buffered medium, trimetazidine slows down pHi recovery in rat isolated ventricular myocytes, primarily through an increase of beta i. An effect on Na+/H+ exchange is also detected but only after long-term incubation of the myocytes with TMZ.

Dexamethasone modulates rat renal brush border membrane phosphate transporter mRNA and protein abundance and glycosphingolipid composition

Glucocorticoids are important regulators of renal phosphate transport. This study investigates the role of alterations in renal brush border membrane (BBM) sodium gradient-dependent phosphate transport (Na-Pi cotransporter) mRNA and protein abundance in the dexamethasone induced inhibition of Na-Pi cotransport in the rat. Dexamethasone administration for 4 d caused a 1.5-fold increase in the Vmax of Na-Pi cotransport (1785 +/- 119 vs. 2759 +/- 375 pmol/5 s per mg BBM protein in control, P < 0.01), which was paralleled by a 2.5-fold decrease in the abundance of Na-Pi mRNA and Na-Pi protein. There was also a 1.7-fold increase in BBM glucosylceramide content (528 +/- 63 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02). To determine whether the alteration in glucosylceramide content per se played a functional role in the decrease in Na-Pi cotransport, control rats were treated with the glucosylceramide synthase inhibitor, D-threo-1-phenyl-2-decanoyl-amino-3-morpholino-1-propanol (PDMP). The resultant 1.5-fold decrease in BBM glucosylceramide content (199 +/- 19 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02) was associated with a 1.4-fold increase in Na-Pi cotransport activity (1422 +/- 73 vs. 1048 +/- 85 pmol/5 s per mg BBM protein in control, P < 0.01), and a 1.5-fold increase in BBM Na-Pi protein abundance. Thus, dexamethasone-induced inhibition of Na-Pi cotransport is associated with a decrease in BBM Na-Pi cotransporter abundance, and an increase in glucosylceramide. Since primary alteration in BBM glucosylceramide content per se directly and selectively modulates BBM Na-Pi cotransport activity and Na-Pi protein abundance, we propose that the increase in BBM glucosylceramide content plays an important role in mediating the inhibitory effect of dexamethasone on Na-Pi cotransport activity.

Regulation of intracellular pH in periportal and perivenular hepatocytes isolated from ethanol-treated rats

The aim of this study was to gain information on intracellular pH (pHi) regulation in periportal (PP) and perivenular (PV) hepatocytes isolated from rats pair-fed liquid diets with either ethanol (T rats) or isocaloric carbohydrates (C rats). pHi was analyzed by the pH-sensitive dye BCECF in perfused subconfluent hepatocyte monolayers. Cells were acid-loaded by pulse exposure to NH4Cl and were alkali-loaded by suddenly reducing external CO2 and HCO3- (from 10% and 50 mM, respectively, to 5% and 25 mM) at constant pHout. In cells from C rats: (a) steady-state pHi was higher in PP than in PV hepatocytes in the presence, but not in the absence, of bicarbonate; (b) pHi recovery from an acid load was 35% higher in PP than in PV cells in the presence of HCO3-, whereas it was similar in HCO3(-)-free experiments; and, on the contrary, (c) pHi recovery from an alkaline load was 30% higher in PV than in PP cells. In cells from T rats: (a) steady-state pHi was always lower than in cells isolated from pair-fed animals; (b) steady-state pHi was similar in PP and PV hepatocytes either in the presence or absence of bicarbonate in the perfusate; (c) pHi recovery from an acid load was not significantly different in PP and PV cells either in the presence of HCO3- or in HCO3(-)-free experiments; and (d) pHi recovery from an alkaline load was similar in PP and PV cells. Our data suggest that chronic ethanol treatment selectively modifies pHi by affecting the activity of ion transport mechanisms regulating pHi in PP and PV hepatocytes isolated from rat liver.

Glucocorticoids regulate Na+/H+ exchange expression and activity in region- and tissue-specific manner

Na+/H+ exchangers (NHEs) are integral membrane proteins that exchange Na+ for H+ across membranes. Four isoforms have been cloned (NHE-1-4). NHE-3 localizes to the apical domain, and its expression is increased in dexamethasone-treated rats by Northern analysis. This stimulatory effect on expression is region and tissue specific, being present in ileum and proximal colon, but not in jejunum, distal colon, or kidney. The increase in transcript expression in ileum correlates with an increase in protein expression by immunoblotting. Changes in apical Na+/H+ exchange activity, as measured by 22Na uptake into brush-border membrane vesicles, correlate with expression differences, with significant increases observed in ileum and proximal colon. In situ hybridization showed NHE-3 mRNA only in villus and absorptive cells of control rats, the pattern not being altered by dexamethasone treatment. This suggests that dexamethasone does not increase expression by inducing crypt cells to express NHE-3 prematurely. We conclude that glucocorticoids selectively increase intestinal NHE-3 activity in a region-specific manner and that this effect also appears to be tissue specific.

Na(+)-H+ antiporter activity in relation to membrane fatty acid composition and cell proliferation

Na(+)-H+ exchange activity was studied in a human breast cancer cell line. At steady state, the intracellular pH (pHi) of the cells was 7.23 +/- 0.01, and intracellular buffering capacity (beta i) was 44 +/- 4 mM/pH unit. pHi was controlled by a Na(+)-H+ antiporter that was reversible, electroneutral, inhibited by 5-(N-methyl-N-isobutyl)amiloride, and dependent on extracellular Na+. The exchanger function depended on internal H+ concentration, according to an allosteric activation mechanism obeying the model of Hill. The exchanger was inactive at pHi > or = 7.22, and its maximal activity was reached at pHi < 6.60. The exchanger was stimulated by osmotic shrinking but was unaffected by growth factors (epidermal growth factor, insulin-like growth factor I) or by serum. When cells were grown in a medium supplemented with linoleic or alpha-linolenic acids, large quantities of the additional fatty acid accumulated in membranes, saturated fatty acids increased, and monounsaturated fatty acids decreased. These changes reduced cell proliferation but had no effect on the steady-state value of pHi, on beta i, or on the kinetic parameters of the Na(+)-H+ exchanger. Therefore, in this system, cell proliferation is not directly related to the activation of the Na(+)-H+ exchanger.

Glucocorticoids and the renal Na-H antiporter: role in respiratory acidosis

We examined the role of glucocorticoids in the activity of the renal brush border Na-H antiporter under baseline conditions (5% CO2 gassing) and during respiratory acidosis (10% CO2 gassing) in cultured monolayers of a proximal tubule suspension (primary cultures of the proximal tubule). Primary cultures of the proximal tubule showed an adaptive increase in renal brush border Na-H antiporter activity in response to respiratory acidosis in presence but not in the absence of physiologic concentrations of hydrocortisone in the medium. The effect of hydrocortisone to increase the activity of the renal brush border Na-H antiporter in respiratory acidosis could also be elicited by dexamethasone. Deletion of hydrocortisone from the medium also impaired the baseline activity of the Na-H antiporter. The effect of hydrocortisone to increase the activity of the Na-H antiporter under baseline conditions and during respiratory acidosis was elicited by physiologic concentrations of the hormone and 100-fold increase in concentration did not further increase the activity of the Na-H antiporter. These results demonstrate that the presence of physiologic concentrations of glucocorticoids are necessary for the baseline activity of the renal brush border Na-H antiporter and its adaptive increase in response to respiratory acidosis.

Phenotypic effects of aldosterone and dexamethasone in a SV40-transformed mammalian cortical ascending limb cell line exhibiting mineralocorticoid receptors

We have analyzed the functional and morphological effects of corticosteroid hormones in a SV40-transformed rabbit cortical-ascending-limb (CAL) cell line (RC.SV2, Vandewalle et al., 1989) having mineralocorticoid (MR) and glucocorticoid (GR) receptors (Rafestin-Oblin et al., 1993). Both aldosterone and dexamethasone (5 x 10(-8) M) induced a marked increase in (3H)ouabain binding (used to quantify membrane Na(+)-K+ ATPase) detectable as early as 6 hours and maximal at 24 hours (+56-57%) (due to a 1.6-1.8-fold increase in cell membrane binding sites without Kd alteration), and significantly augmented the ouabain-sensitive component of Rb+ influx. Triiodothyronine (T3, 10(-9) M) also stimulated ouabain binding by 21% but was not permissive for steroid action, whereas 5 micrograms/ml insulin had no effect. Both steroid hormones, T3 and insulin induced the formation of domes that was tightly correlated with ouabain binding (r = 0.949) except for insulin. The effects of aldosterone and dexamethasone on cell monolayers and cell ultrastructure were, however, strikingly different as aldosterone induced a marked amplification of basolateral areas with appearance of large intercellular spaces, reminiscent of the changes observed in deoxycorticosterone-treated rats, whereas dexamethasone predominantly influenced cell height. This discrepancy might be due to specific occupancy of MR and GR by aldosterone and dexamethasone, respectively, and/or to nongenomic effects of dexamethasone. We have thus characterized a cell culture model making it possible to analyze the actions of mineralocorticoid and glucocorticoid hormones in the mammalian kidney.

Glucocorticoid-mediated alterations in fluidity of rabbit cardiac muscle microvessel endothelial cell membranes: influences on eicosanoid release

Experiments were conducted to determine effects of the synthetic glucocorticoid, dexamethasone, on the lipid fluidity of cultured rabbit cardiac muscle microvessel endothelial cells and the possible role(s) for altered fluidity in the steroid inhibition of cellular eicosanoid production. Following a sixteen hour exposure to 10(-7) M dexamethasone, membranes prepared from treated cells exhibited a decreased fluidity compared to their control counterparts, as assessed by steady-state fluorescence polarization techniques using 1,6-diphenyl-1,3,5-hexatriene (DPH). Examination of the effects of temperature on the anisotropy values of DPH using Arrhenius plots revealed consistent differences in the steroid treated cells over the entire temperature range (40-5 degrees C). These dexamethasone-dependent fluidity changes were associated with increases in the cholesterol/phospholipid ratio of membrane lipids. Restoration of membrane fluidity to control values with the fluidizing agent, 2-(2-methoxyethoxy)ethyl-8-(cis- 2-n-octylcyclopropyl)octanoate (A2C), partially reversed dexamethasone induced inhibition of A23187-stimulated eicosanoid release. These observations suggest that at least part of dexamethasone's inhibitory actions on eicosanoid generation in microvessel endothelial cells are mediated by alterations in membrane composition and fluidity.

Correction of enhanced Na(+)-H+ exchange of rat small intestinal brush-border membranes in streptozotocin-induced diabetes by insulin or 1,25-dihydroxycholecalciferol

Diabetes was induced in rats by administration of a single i.p. injection of streptozotocin (50 mg/kg body wt). After 7 d, diabetic rats were further treated with insulin or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] for an additional 5-7 d. Control, diabetic, diabetic + insulin, and diabetic + 1,25(OH)2D3 rats were then killed, their proximal small intestines were removed, and villus-tip epithelial cells were isolated and used to prepare brush-border membrane vesicles. Preparations from each of these groups were then analyzed and compared with respect to their amiloride-sensitive, electroneutral Na(+)-H+ exchange activity, using 22Na uptake as well as acridine orange techniques. The results of these experiments demonstrated that (a) H+ gradient-dependent 22Na uptake as well as Na+ gradient-dependent transmembrane H+ fluxes were significantly increased in diabetic vesicles compared to their control counterparts, (b) kinetic studies demonstrated that this enhanced 22Na uptake in diabetes was a result of increased maximal velocity (Vmax) of this exchanger with no change in apparent affinity (Km) for Na+, (c) serum levels of 1,25(OH)2D3 were significantly lower in diabetic animals compared with their control counterparts; and (d) insulin or 1,25(OH)2D3 treatment restored the Vmax alterations to control values, without any significant changes in Km, concomitant with significantly increasing the serum levels of 1,25(OH)2D3 in diabetic animals. These results indicate that Na(+)-H+ activity is significantly increased in proximal small intestinal luminal membranes of streptozotocin-induced diabetic rats. Moreover, alterations in the serum levels of 1,25(OH)2D3 may, at least in part, explain this enhanced antiporter activity and its correction by insulin.

Ion transport in epithelial cells from the bovine trachea in culture: effects of dexamethasone and aldosterone

The ion transport properties of epithelial cells from the bovine trachea grown in culture were investigated. The cellular uptake of 22Na+, 36Cl- and 86Rb+ was measured and the sensitivity of ion uptake to amiloride and bumetanide revealed the presence of the transport pathways that exist in the native tissue. The action of corticosteroids, dexamethasone and aldosterone (0.1 or 1.0 microM), was studied by measuring the short-circuit current and the uptake of Na+ and Cl- following a treatment of 18 h, 2 or 8 days. The results showed that, in each situation, dexamethasone enhanced markedly the amiloride-sensitive current and Na+ uptake. In contrast, aldosterone exerted no detectable effect on the ion transport and electrical properties.

Characterization of basolateral membrane Na/H antiport in rat jejunum

Na uptake studies were performed in order to examine the activity of a Na/H exchanger in basolateral membrane vesicles isolated from rat jejunum. Experiments were carried out under voltage-clamped conditions in order to avoid electrodiffusional ionic movements. 1 mM Na uptake was found to be enhanced by an outward proton gradient and its initial rate was further increased by the presence of monensin or nigericin. The pH gradient-driven Na uptake was inhibited by 2 mM amiloride and unaffected by 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The initial rate of the proton gradient-induced Na uptake was saturable with respect to external Na, with a Km of 13.6 +/- 1.4 mM and a Vmax of 35.4 +/- 2.2 nmol/mg protein per min. Li competed with Na for the exchange process, whereas K, Rb, Cs, tetramethylammonium had no effect. We conclude that rat jejunal basolateral membrane contains a Na/H exchanger whose properties are similar to those of the antiporter identified in the brush-border membrane.

Apical plasma membrane vesicles formed from organ donor colon demonstrate Na+ and H+ conductances and Na+/H+ exchange

Apical plasma membrane vesicles were prepared from human organ donor colon mucosal scrapings. These vesicles were enriched 10-fold in cysteine-sensitive alkaline phosphatase activity compared to starting homogenates, and showed minimal contamination of microsomal, mitochondrial or basolateral membranes. Transport studies using [22Na] uptake into proximal colonic vesicles demonstrated Na+ and H+ conductances, Na+/H+ exchange and amiloride inhibition of Na+ uptake. The isolation of these apical vesicles will permit detailed study of human colonic transport processes.

Dietary triacylglycerol modulates sodium-dependent D-glucose transport, fluidity and fatty acid composition of rat small intestinal brush-border membrane

Rats were maintained on nutritionally complete diets enriched in unsaturated (menhaden fish oil) or saturated (butter fat) triacylglycerols. After 4 weeks, the animals were killed, proximal small intestinal brush-border membranes were prepared, and examined and compared with respect to their lipid composition, molecular species of phosphatidylcholine, lipid fluidity and sodium-dependent D-glucose transport. Membranes prepared from the two dietary groups were found to possess similar ratios of cholesterol/phospholipid (mol/mol), sphingomyelin/phosphatidylcholine (mol/mol), and protein/lipid (w/w). In contrast to these findings, however, striking differences were noted in the total fatty acid compositions of these membranes. Plasma membranes prepared from animals fed the fish oil diet possessed higher percentages of saturated fatty acids as well as (n - 3) unsaturated fatty acids and lower percentages of monounsaturated and (n - 6) unsaturated fatty acids than those prepared from animals fed the butter fat diet. Analysis of the molecular species of phosphatidylcholine by HPLC, moreover, revealed that membranes from rats fed fish oil had higher levels of 16:0-20:5, 16:0-22:6 and 18:0-20:5 and lower levels of 18:0-18:2 and 16:0-18:1 than their butter fat counterparts. As assessed by steady-state fluorescence polarization, differential polarized phase fluorometric and excimer/monomer fluorescence intensity techniques using various fluorophores, the lipid fluidity of membranes from rats fed fish oil was also found to be significantly lower compared to membranes from rats fed butter fat. Finally, comparison of the kinetic parameters of Na+-dependent D-glucose transport revealed that fish oil-membrane vesicles had a higher maximum velocity (Vmax) than butter fat membrane vesicles but a similar Km for glucose.

Lipid permeability of rat jejunum and ileum: correlation with physical properties of the microvillus membrane

It is now generally accepted that dietary lipids permeate through the cholesterol-phospholipid bilayer of the intestinal microvillus membrane during the process of intestinal absorption. Therefore, it has been assumed that rates of lipid permeation depend upon the physical properties of the microvillus membrane. In this study the lipid permeability properties of the microvillus membrane were compared in two regions of the intestine, jejunum and ileum. Since the jejunum is exposed to the majority of dietary lipid it would be reasonable to suppose that it would be more efficient at lipid absorption. The ileum was found to be less permeable to all fatty acids examined and this could be correlated with increased rigidity of ileal microvillus membrane vesicles measured with multiple fluorescent probes. Differences in membrane fluidity were found in both the outer third and central regions of the bilayer. When measurements of membrane fluidity were performed either in the presence or the absence of fatty acids, it could be demonstrated that these acids perturb the physical properties of the outer region or the membrane. Therefore, this suggests that the rate-limiting step in fatty acid permeation may be localized to the outer third of the bilayer. Pharmacologic or dietary manipulations attempting to alter rates of lipid permeability should, therefore, be directed towards altering the physical properties of this region of the microvillus membrane.

Dexamethasone influences the lipid fluidity, lipid composition and glycosphingolipid glycosyltransferase activities of rat proximal-small-intestinal Golgi membranes

Experiments were performed to examine the effects of subcutaneous administration of the synthetic glucocorticoid dexamethasone (100 micrograms/day per 100 g body wt.) on the lipid fluidity, lipid composition and glycosphingolipid glycosyltransferase activities of rat proximal-small-intestinal Golgi membranes. After 4 days of treatment, Golgi membranes and liposomes prepared from treated rats were found to possess a greater fluidity than their control (diluent or 0.9% NaCl) counterpart, as assessed by steady-state fluorescence-polarization techniques using three different fluorophores. Moreover, analysis of the effects of temperature on the anisotropy values of 1,6-diphenylhexa-1,3,5-triene, using Arrhenius plots, demonstrated that the mean break-point temperatures of treated preparations were 4-5 degrees C lower than those of control preparations. Changes in the fatty acyl saturation index and double-bond index of treated membranes, secondary to alterations in stearic acid, linoleic acid and arachidonic acid, at least in part, appeared to be responsible for the differences in fluidity noted between treated and control Golgi membranes. Concomitant with these fluidity and lipid-compositional alterations, treated membranes possessed higher specific activities of UDP-galactosyl-lactosylceramide galactosyltransferase and CMP-N-acetylneuraminic acid:lactosylceramide sialyltransferase than their control counterparts. Experiments utilizing benzyl alcohol, a known fluidizer, furthermore suggested that the fluidity alteration induced by dexamethasone may be responsible for the increased activity of the former, but not the latter, glycosphingolipid glycosyltransferase.