Dexamethasone prevents transport inhibition by hypoxia in rat lung and alveolar epithelial cells by stimulating activity and expression of Na+-K+-ATPase and epithelial Na+ channels

Hypoxia inhibits Na and lung fluid reabsorption, which contributes to the formation of pulmonary edema. We tested whether dexamethasone prevents hypoxia-induced inhibition of reabsorption by stimulation of alveolar Na transport. Fluid reabsorption, transport activity, and expression of Na transporters were measured in hypoxia-exposed rats and in primary alveolar type II (ATII) cells. Rats were treated with dexamethasone (DEX; 2 mg/kg) on 3 consecutive days and exposed to 10% O(2) on the 2nd and 3rd day of treatment to measure hypoxia effects on reabsorption of fluid instilled into lungs. ATII cells were treated with DEX (1 muM) for 3 days before exposure to hypoxia (1.5% O(2)). In normoxic rats, DEX induced a twofold increase in alveolar fluid clearance. Hypoxia decreased reabsorption (-30%) by decreasing its amiloride-sensitive component; pretreatment with DEX prevented the hypoxia-induced inhibition. DEX increased short-circuit currents (ISC) of ATII monolayers in normoxia and blunted hypoxic transport inhibition by increasing the capacity of Na(+)-K(+)-ATPase and epithelial Na(+) channels (ENaC) and amiloride-sensitive ISC. DEX slightly increased the mRNA of alpha- and gamma-ENaC in whole rat lung. In ATII cells from DEX-treated rats, mRNA of alpha(1)-Na(+)-K(+)-ATPase and alpha-ENaC increased in normoxia and hypoxia, and gamma-ENaC was increased in normoxia only. DEX stimulated the mRNA expression of alpha(1)-Na(+)-K(+)-ATPase and alpha-, beta-, and gamma-ENaC of A549 cells in normoxia and hypoxia (1.5% O(2)) when DEX treatment was begun before or during hypoxic exposure. These results indicate that DEX prevents inhibition of alveolar reabsorption by hypoxia and stimulates the expression of Na transporters even when it is applied in hypoxia.     

Invited review: lung edema clearance: role of Na(+)-K(+)-ATPase.

Acute hypoxemic respiratory failure is a consequence of edema accumulation due to elevation of pulmonary capillary pressures and/or increases in permeability of the alveolocapillary barrier. It has been recognized that lung edema clearance is distinct from edema accumulation and is largely effected by active Na(+) transport out of the alveoli rather than reversal of the Starling forces, which control liquid flux from the pulmonary circulation into the alveolus. The alveolar epithelial Na(+)-K(+)-ATPase has an important role in regulating cell integrity and homeostasis. In the last 15 yr, Na(+)-K(+)-ATPase has been localized to the alveolar epithelium and its contribution to lung edema clearance has been appreciated. The importance of the alveolar epithelial Na(+)-K(+)-ATPase function is reflected in the changes in the lung's ability to clear edema when the Na(+)-K(+)-ATPase is inhibited or increased. An important focus of the ongoing research is the study of the mechanisms of Na(+)-K(+)-ATPase regulation in the alveolar epithelium during lung injury and how to accelerate lung edema clearance by modulating Na(+)-K(+)-ATPase activity.     

Mechanical stretching of alveolar epithelial cells increases Na(+)-K(+)-ATPase activity.

Alveolar epithelial cells effect edema clearance by transporting Na(+) and liquid out of the air spaces. Active Na(+) transport by the basolaterally located Na(+)-K(+)-ATPase is an important contributor to lung edema clearance. Because alveoli undergo cyclic stretch in vivo, we investigated the role of cyclic stretch in the regulation of Na(+)-K(+)-ATPase activity in alveolar epithelial cells. Using the Flexercell Strain Unit, we exposed a cell line of murine lung epithelial cells (MLE-12) to cyclic stretch (30 cycles/min). After 15 min of stretch (10% mean strain), there was no change in Na(+)-K(+)-ATPase activity, as assessed by (86)Rb(+) uptake. By 30 min and after 60 min, Na(+)-K(+)-ATPase activity was significantly increased. When cells were treated with amiloride to block amiloride-sensitive Na(+) entry into cells or when cells were treated with gadolinium to block stretch-activated, nonselective cation channels, there was no stimulation of Na(+)-K(+)-ATPase activity by cyclic stretch. Conversely, cells exposed to Nystatin, which increases Na(+) entry into cells, demonstrated increased Na(+)-K(+)-ATPase activity. The changes in Na(+)-K(+)-ATPase activity were paralleled by increased Na(+)-K(+)-ATPase protein in the basolateral membrane of MLE-12 cells. Thus, in MLE-12 cells, short-term cyclic stretch stimulates Na(+)-K(+)-ATPase activity, most likely by increasing intracellular Na(+) and by recruitment of Na(+)-K(+)-ATPase subunits from intracellular pools to the basolateral membrane.     

Isoproterenol improves ability of lung to clear edema in rats exposed to hyperoxia.

Exposure of adult rats to 100% O(2) results in lung injury and decreases active sodium transport and lung edema clearance. It has been reported that beta-adrenergic agonists increase lung edema clearance in normal rat lungs by upregulating alveolar epithelial Na(+)-K(+)-ATPase function. This study was designed to examine whether isoproterenol (Iso) affects lung edema clearance in rats exposed to 100% O(2) for 64 h. Active Na(+) transport and lung edema clearance decreased by approximately 44% in rats exposed to acute hyperoxia. Iso (10(-6) M) increased the ability of the lung to clear edema in room-air-breathing rats (from 0.50 +/- 0.02 to 0.99 +/- 0. 05 ml/h) and in rats exposed to 100% O(2) (from 0.28 +/- 0.03 to 0. 86 +/- 0.09 ml/h; P < 0.001). Disruption of intracellular microtubular transport of ion-transporting proteins by colchicine (0. 25 mg/100 g body wt) inhibited the stimulatory effects of Iso in hyperoxia-injured rat lungs, whereas the isomer beta-lumicolchicine, which does not affect microtubular transport, did not inhibit active Na(+) transport stimulated by Iso. Accordingly, Iso restored the lung's ability to clear edema after hyperoxic lung injury, probably by stimulation of the recruitment of ion-transporting proteins (Na(+)-K(+)-ATPase) from intracellular pools to the plasma membrane in rat alveolar epithelium.     

Dopamine increases lung liquid clearance during mechanical ventilation

Short-term mechanical ventilation with high tidal volume (HVT) causes mild to moderate lung injury and impairs active Na+ transport and lung liquid clearance in rats. Dopamine (DA) enhances active Na+ transport in normal rat lungs by increasing Na+-K+-ATPase activity in the alveolar epithelium. We examined whether DA would increase alveolar fluid reabsorption in rats ventilated with HVT for 40 min compared with those ventilated with low tidal volume (LVT) and with nonventilated rats. Similar to previous reports, HVT ventilation decreased alveolar fluid reabsorption by ~50% (P < 0.001). DA increased alveolar fluid reabsorption in nonventilated control rats (by ~60%), LVT ventilated rats (by approximately 55%), and HVT ventilated rats (by ~200%). In parallel studies, DA increased Na+-K+-ATPase activity in cultured rat alveolar epithelial type II cells (ATII). Depolymerization of cellular microtubules by colchicine inhibited the effect of DA on HVT ventilated rats as well as on Na+-K+-ATPase activity in ATII cells. Neither DA nor colchicine affected the short-term Na+-K+-ATPase alpha1- and beta1-subunit mRNA steady-state levels or total alpha1- and beta1-subunit protein abundance in ATII cells. Thus we reason that DA improved alveolar fluid reabsorption in rats ventilated with HVT by upregulating the Na+-K+-ATPase function in alveolar epithelial cells.     

Lung edema clearance: 20 years of progress: invited review: alveolar edema fluid clearance in the injured lung.

Resolution of pulmonary edema involved active transepithelial sodium transport. Although several of the cellular and molecular mechanisms involved are relatively well understood, it is only recently that the regulation of these mechanisms in injured lung are being evaluated. Interestingly, in mild-to-moderate lung injury, alveolar edema fluid clearance is often preserved. This preserved or enhanced alveolar fluid clearance is mediated by catecholamine-dependent or -independent mechanisms. This stimulation of alveolar liquid clearance is related to activation or increased expression of sodium transport molecules such as the epithelial sodium channel or the Na(+)-K(+)-ATPase pump and may also involve the cystic fibrosis transmembrane conductance regulator. When severe lung injury occurs, the decrease in alveolar liquid clearance may be related to changes in alveolar permeability or to changes in activity or expression of sodium or chloride transport molecules. Multiple pharmacological tools such as beta-adrenergic agonists, vasoactive drugs, or gene therapy may prove effective in stimulating the resolution of alveolar edema in the injured lung.     

Catecholamines increase lung edema clearance in rats with increased left atrial pressure.

During hydrostatic pulmonary edema, active Na(+) transport and alveolar fluid reabsorption are decreased. Dopamine (DA) and isoproterenol (ISO) have been shown to increase active Na(+) transport in rat lungs by upregulating Na(+)-K(+)-ATPase in the alveolar epithelium. We studied the effects of DA and ISO in isolated rat lungs with increased left atrial pressure (Pla = 15 cmH(2)O) compared with control rats with normal Pla (Pla = 0). Alveolar fluid reabsorption decreased from control value of 0.51 +/- 0.02 to 0.27 +/- 0.02 ml/h when Pla was increased to 15 cmH(2)O (P < 0.001). DA and ISO increased the alveolar fluid reabsorption back to control levels. Treatment with the D(1) antagonist SCH-23390 inhibited the stimulatory effects of DA (0.30 +/- 0.02 ml/h), whereas fenoldopam, a specific D(1)-receptor agonist, increased alveolar fluid reabsorption in rats exposed to Pla of 15 cmH(2)O (0.47 +/- 0.04 ml/h). Propranolol, a beta-adrenergic-receptor antagonist, blocked the stimulatory effects of ISO; however, it did not affect alveolar fluid reabsorption in control or DA-treated rats. Amiloride (a Na(+) channel blocker) and ouabain (a Na(+)-K(+)-ATPase inhibitor), either alone or together, inhibited the stimulatory effects of DA. Colchicine, which disrupts the cellular microtubular transport of ion-transporting proteins to the plasma membrane, inhibited the stimulatory effects of DA, whereas the isomer beta-lumicolchicine did not block the stimulatory effects of DA. These data suggest that DA and ISO increase alveolar fluid reabsorption in a model of increased Pla by regulating active Na(+) transport in rat alveolar epithelium. The effects of DA and ISO are mediated by the activation of dopaminergic D(1) receptors and the beta-adrenergic receptors, respectively.     

Intact alveolar epithelial permeability and transalveolar fluid absorption after thoracic irradiation in rats.

We have addressed the question of how the alveolar space stays relatively free of fluid when thoracic irradiation injures the pulmonary capillary endothelium and plasma fluid leaks into the interstitium. A single dose of 15 Gy to the thorax of rats significantly increased the pulmonary capillary filtration coefficient and the lung wet/dry weight ratio 2 h after irradiation. However, there was no significant increase in the release of lactose dehydrogenase or leaking of Evans blue dye into the alveolar space, indicating that alveolar epithelial permeability remained intact. We found no significant difference in the basal alveolar fluid clearance between control and irradiated animals. There was also no significant difference in blockage of alveolar fluid clearance by amiloride. This indicates that the function of the alveolar epithelial Na(+) channels is not impaired and that alveolar epithelium absorbs fluid normally. Examination of lung tissue by light microscopy demonstrated accumulation of fluid in the perivascular region but not in the alveolar space. Our data appear to indicate that the alveolar epithelial barrier function is more resistant to radiation than that of the pulmonary capillary endothelium. We conclude that intact alveolar epithelial permeability and normal transalveolar epithelial fluid absorption ability are of critical importance in keeping the alveolar space relatively free of fluid during acute radiation lung injury.     

Effects of cardiogenic edema fluid on ion and fluid transport in the adult lung

We have previously shown that cardiogenic pulmonary edema fluid (EF) increases Na(+) and fluid transport by fetal distal lung epithelia (FDLE) (Rafii B, Gillie DJ, Sulowski C, Hannam V, Cheung T, Otulakowski G, Barker PM and O'Brodovich H. J Physiol 544: 537-548, 2002). We now report the effect of EF on Na(+) and fluid transport by the adult lung. We first studied primary cultures of adult type II (ATII) epithelium and found that overnight exposure to EF increased Na(+) transport, and this effect was mainly due to factors other than catecholamines. Plasma did not stimulate Na(+) transport in ATII. Purification of EF demonstrated that at least some agent(s) responsible for the amiloride-insensitive component resided within the globulin fraction. ATII exposed to globulins demonstrated a conversion of amiloride-sensitive short-circuit current (I(sc)) to amiloride-insensitive I(sc) with no increase in total I(sc). Patch-clamp studies showed that ATII exposed to EF for 18 h had increased the number of highly selective Na(+) channels in their apical membrane. In situ acute exposure to EF increased the open probability of Na(+)-permeant ion channels in ATII within rat lung slices. EF did increase, by amiloride-sensitive pathways, the alveolar fluid clearance from the lungs of adult rats. We conclude that cardiogenic EF increases Na(+) transport by adult lung epithelia in primary cell culture, in situ and in vivo.     

Effect of increased calcium intake on cardiac and vascular Na(+)-K(+)-ATPase activity in oral contraceptive-treated female Sprague-Dawley rats

The present study aimed at investigating the influence of increased dietary calcium on Na(+)-K(+)-ATPase activity in heart and aorta of female Sprague-Dawley rats treated with oral contraceptive (OC) steroids. Rats were grouped as control (CR), OC-treated and OC+calcium-treated. OC-treated and OC+calcium-treated received a combination of OC steriods (ethinyloestradiol and norgestrel; ig). OC+calcium-treated rats were fed with 2.5% calcium diet, while OC-treated and CR groups were fed on 0.9% calcium diet. The activity of Na(+)-K(+)-ATPase in heart and aorta was significantly lower in OC-treated rats than those in the other groups. OC treatment caused significant increase in plasma glucose and significant decrease in plasma K+ as compared to control group. Decrease in Na(+)-K(+)-ATPase activity and plasma K+ was abrogated by increased calcium intake, while increase in plasma glucose was not normalized by calcium supplementation. Plasma levels of Na+, lipid peroxidation index and ascorbic acid were comparable among the three groups. These results showed that OC treatment could lead to impaired activity of cardiac and vascular Na(+)-K(+)-ATPase, possibly due to reduced plasma K+ level and these effects could be abolished by high calcium diet.     

Alveolar fluid reabsorption is increased in rats with compensated heart failure

Alveolar fluid reabsorption (AFR) is important in keeping the air spaces free of edema. This process is accomplished via active transport of Na(+) across the alveolo-capillary barrier mostly by apical Na(+) channels and basolateral Na(+)-K(+)-ATPases. Recently, we have reported that acute elevation of left atrial pressures is associated with decreased AFR in isolated rat lungs. However, the effect of chronic elevation of pulmonary capillary pressure, such as seen in patients with congestive heart failure (CHF), on AFR is unknown. CHF was induced by creating an aorto-caval fistula (ACF) in Sprague-Dawley male rats. Seven days after the placement of the fistula, AFR was studied in the isolated perfused rat lung model. AFR in control rats was 0.49 +/- 0.02 ml/h (all values are means +/- SE) and increased by approximately 40% (0.69 +/- 0.03 ml/h) in rats with chronic CHF (P < 0.001). The albumin flux from the pulmonary circulation into the air spaces did not increase in the experimental groups, indicating that lung permeability for large solutes was not increased. Na(+)-K(+)-ATPase activity and protein abundance at the plasma membrane of distal alveolar epithelial tissue were significantly increased in CHF rats compared with controls. These changes were associated with increased plasma norepinephrine levels in CHF rats compared with controls. We provide evidence that in a rat model of chronic compensated CHF, AFR is increased, possibly due to increased endogenous norepinephrine upregulating active sodium transport and protecting against alveolar flooding.     

Na(+)-K(+)-ATPase activity in alveolar epithelial cells increases with cyclic stretch

Na(+)-K(+)-ATPase pumps (Na(+) pumps) in the alveolar epithelium create a transepithelial Na(+) gradient crucial to keeping fluid from the pulmonary air space. We hypothesized that alveolar epithelial stretch stimulates Na(+) pump trafficking to the basolateral membrane (BLM) and, thereby, increases overall Na(+) pump activity. Alveolar type II cells were isolated from Sprague-Dawley rats and seeded onto elastic membranes coated with fibronectin or 5-day-conditioned extracellular matrix. After 2 days in culture, cells were uniformly stretched for 1 h in a custom-made device. Na(+) pump activity was subsequently assessed by ouabain-inhibitable uptake of (86)Rb(+), a K(+) tracer, and BLM Na(+) pump abundance was measured. In support of our hypothesis, cells increased Na(+) pump activity in a "dose-dependent" manner when stretched to 12, 25, or 37% change in surface area (DeltaSA), and cells stretched to 25% DeltaSA more than doubled Na(+) pump abundance in the BLM. Cells on 5-day matrix tolerated higher strain than cells on fibronectin before the onset of Na(+) pump upregulation. Treatment with Gd(3+), a stretch-activated channel blocker, amiloride, a Na(+) channel blocker, or both reduced but did not abolish stretch-induced effects. Sustained tonic stretch, unlike cyclic stretch, elicited no significant Na(+) pump response.     

Estradiol-induced expression of N(+)-K(+)-ATPase catalytic isoforms in rat arteries: gender differences in activity mediated by nitric oxide donors

We tested the hypothesis that previously demonstrated gender differences in ACh-induced vascular relaxation could involve diverse Na(+)-K(+)-ATPase functions. We determined Na(+)-K(+)-ATPase by measuring arterial ouabain-sensitive 86Rb uptake in response to ACh. We found a significant increase of Na+ pump activity only in aortic rings from female rats (control 206 +/- 11 vs. 367 +/- 29 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.01). Ovariectomy eliminated sex differences in Na(+)-K(+)-ATPase function, and chronic in vivo hormone replacement with 17beta-estradiol restored the ACh effect on Na(+)-K(+)-ATPase. Because ACh acts by enhancing production of NO, we examined whether the NO donor sodium nitroprusside (SNP) mimics the action of ACh on Na(+)-K(+)-ATPase activity. SNP increased ouabain-sensitive 86Rb uptake in denuded female arteries (control 123 +/- 7 vs. 197 +/- 12 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.05). Methylene blue (an inhibitor of guanylate cyclase) and KT-5823 (a cGMP-dependent kinase inhibitor) blocked the stimulatory action of SNP. Exposure of female thoracic aorta to the Na+/K+ pump inhibitor ouabain significantly decreased SNP-induced and ACh-mediated relaxation of aortic rings. At the molecular level, Western blot analysis of arterial tissue revealed significant gender differences in the relative abundance of catalytic isoforms of Na(+)-K(+)-ATPase. Female-derived aortas exhibited a greater proportion of alpha2-isoform (44%) compared with male-derived aortas. Furthermore, estradiol upregulated the expression of alpha2 mRNA in male arterial explants. Our results demonstrate that enhancement of ACh-induced relaxation observed in female rats may be in part explained by 1) NO-dependent increased Na(+)-K(+)-ATPase activity in female vascular tissue and 2) greater abundance of Na(+)-K(+)-ATPase alpha2-isoform in females.     

Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid clearance in adult rats.

The in vivo effect of 48-h glucocorticoid and thyroid hormone 3,3', 5-triiodine-L-thyronine (T(3)) pretreatment on alveolar epithelial fluid transport was studied in adult rats. An isosmolar 5% albumin solution was instilled, and alveolar fluid clearance was studied for 1 h. Compared with controls, dexamethasone pretreatment increased alveolar fluid clearance by 80%. T(3) pretreatment stimulated alveolar fluid clearance by 65%, and dexamethasone and T(3) had additive effects (132%). Propranolol did not inhibit alveolar fluid clearance in either group, indicating that stimulation was not secondary to endogenous beta-adrenergic stimulation. With the use of bromodeoxyuridine in vivo labeling, there was no evidence of cell proliferation. Alveolar fluid clearance was partially inhibited by amiloride in all groups. Fractional amiloride inhibition was greater in dexamethasone- and dexamethasone-plus-T(3)-pretreated rats than in control animals, but less in T(3)-pretreated rats. In summary, pretreatment with dexamethasone, T(3), or both in combination upregulate in vivo alveolar fluid clearance similarly to short-term beta-adrenergic stimulation. The effects are mediated partly by increased amiloride-sensitive Na(+) transport, because the stimulated alveolar fluid clearance was more amiloride sensitive than in control rats. These observations may have clinical relevance because glucocorticoid therapy is commonly used with acute lung injury.     

Hypoxia decreases proteins involved in epithelial electrolyte transport in A549 cells and rat lung

Fluid reabsorption from alveolar space is driven by active Na reabsorption via epithelial Na channels (ENaCs) and Na-K-ATPase. Both are inhibited by hypoxia. Here we tested whether hypoxia decreases Na transport by decreasing the number of copies of transporters in alveolar epithelial cells and in lungs of hypoxic rats. Membrane fractions were prepared from A549 cells exposed to hypoxia (3% O(2)) as well as from whole lung tissue and alveolar type II cells from rats exposed to hypoxia. Transport proteins were measured by Western blot analysis. In A549 cells, alpha(1)- and beta(1)-Na-K-ATPase, Na/K/2Cl cotransport, and ENaC proteins decreased during hypoxia. In whole lung tissue, alpha(1)-Na-K-ATPase and Na/K/2Cl cotransport decreased. alpha- and beta-ENaC mRNAs also decreased in hypoxic lungs. Similar results were seen in alveolar type II cells from hypoxic rats. These results indicate a slow decrease in the amount of Na-transporting proteins in alveolar epithelial cells during exposure to hypoxia that also occurs in vivo in lungs from hypoxic animals. The reduced number of transporters might account for the decreased transport activity and impaired edema clearance in hypoxic lungs.     

Postischemic Na(+)-K(+)-ATPase reactivation is delayed in the absence of glycolytic ATP in isolated rat hearts

Normalization of intracellular sodium (Na) after postischemic reperfusion depends on reactivation of the sarcolemmal Na(+)-K(+)-ATPase. To evaluate the requirement of glycolytic ATP for Na(+)-K(+)-ATPase function during postischemic reperfusion, 5-s time-resolution 23Na NMR was performed in isolated perfused rat hearts. During 20 min of ischemia, Na increased approximately twofold. In glucose-reperfused hearts with or without prior preischemic glycogen depletion, Na decreased immediately upon postischemic reperfusion. In glycogen-depleted pyruvate-reperfused hearts, however, the decrease of Na was delayed by approximately 25 s, and application of the pyruvate dehydrogenase (PDH) activator dichloroacetate (DA) did not shorten this delay. After 30 min of reperfusion, Na had almost normalized in all groups and contractile recovery was highest in the DA-treated hearts. In conclusion, some degree of functional coupling of glycolytic ATP and Na(+)-K(+)-ATPase activity exists, but glycolysis is not essential for recovery of Na homeostasis and contractility after prolonged reperfusion. Furthermore, the delayed Na(+)-K(+)-ATPase reactivation observed in pyruvate-reperfused hearts is not due to inhibition of PDH.     

Downregulation of cardiac myocyte Na(+)-K(+)-ATPase by adenovirus-mediated expression of an alpha-subunit fragment

Cultured rat cardiac myocytes and A7r5 cells were transfected with an adenoviral vector used earlier for in vivo expression of functional alpha(2)-isoform of the catalytic subunit of rat Na(+)-K(+)-ATPase. Expressions of truncated forms of alpha(2), but little or no intact alpha(2), were detected, suggesting the rapid degradation of alpha(2) in these cultured cells. In neonatal myocytes normally containing the alpha(1)- and the alpha(3)-isoforms, expression of the alpha(2)-fragment led to 1) a significant decrease in the level of endogenous alpha(1)-protein and a modest decrease in alpha(3)-protein, 2) decreases in mRNAs of alpha(1) and alpha(3), 3) decrease in Na(+)-K(+)-ATPase function measured as ouabain-sensitive Rb(+) uptake, 4) increase in intracellular Ca(2+) concentration similar to that induced by ouabain, and 5) eventual loss of cell viability. These findings indicate that the alpha(2)-fragment downregulates endogenous Na(+)-K(+)- ATPase most likely by dominant negative interference either with folding and/or assembly of the predominant housekeeping alpha(1)-isoform or with signal transducing function of the enzyme. Demonstration of rise in intracellular Ca(2+) resulting from alpha(1)-downregulation 1) does not support the previously suggested special roles of less abundant alpha(2)- and alpha(3)-isoforms in the regulation of cardiac Ca(2+), 2) lends indirect support to proposals that observed decrease in total Na(+)-K(+)-ATPase of the failing heart may be a mechanism to compensate for impaired cardiac contractility, and 3) suggests the potential therapeutic utility of dominant negative inhibition of Na(+)-K(+)-ATPase.     

Alterations in jejunal transport and (Na+-K+)-ATPase in an experimental model of hypoxia in rats

Hypoxia was induced by exposing rats to an atmosphere of 93% N2, 7% O2 for 4-48 hr. The animals became hypoxic as indicated by a decreased blood PaO2 (mean +/- SEM: 48 +/- 10 mm Hg). Hypoxia was accompanied by metabolic acidosis (pH 7.22 +/- 0.02) and decreased serum bicarbonate levels (9.0 +/- 4.0 meq/liter). Hypoxic rats also showed evidence of tissue hypoxia; liver tryptophan oxygenase levels were increased to 21 +/- 2 nmole/min/mg protein. In the hypoxic animals there was decreased jejunal mucosal (Na+-K+)-ATPase activity and an inhibition of active intestinal transport of sodium, glucose, 3-O-methylglucose, galactose, tyrosine, phenylalanine, and glycine as determined by in vivo perfusion studies. Jejunal fructose transport, which has a large passive component, was unaffected by hypoxia. The electrolyte, carbohydrate, and amino acid transport alterations produced by hypoxia were seen in the absence of an effect on jejunal cell number, DNA synthesis, or cell turnover. There was also no evidence of histological or ultrastructural damage. Furthermore, studies with a luminal macromolecular tracer, horseradish peroxidase, indicated that the jejunal lumen-to-blood barrier to macromolecules was also unaltered in these hypoxic animals. In vitro local oxygenation of the jejunum, by bubbling of 95% O2:5% CO2, markedly improved sodium and glucose (but not 3-O-methylglucose) absorption in hypoxic rats and control rats. The (Na+-K+)-ATPase activity of the jejunal mucosa of hypoxic rats was significantly enhanced by the local bubbling of 95% O2:5% CO2. Overall, our data indicate that during relatively mild conditions of hypoxia there is an inhibition of jejunal (Na+-K+)-ATPase activity and related transport processes that is prevented by in situ oxygenation.     

Characteristics of the K(+)-ion transport in the rat intestine following the use of natural zeolites as food additives

Effects of zeolites as a food supplement have been studied on Wistar rats both in vivo perfusion experiments in the jejunum and distal colon and Rb fluxes through intestinal wall in the Ussing chamber. It has been found that zeolites decrease the K+ absorption and stimulate K+ secretion in the gut. This effect was due to inhibition of the apical N(+)-K(+)-ATPase and ouabain-sensitive Na(+)-independent K(+)-ATPase as well as the activation of the basolateral N(+)-K(+)-ATPase.