Differential regulation of Na,K-ATPase alpha 1, alpha 2, and beta subunit mRNA and protein levels by thyroid hormone

Deficiency of Thyroid Hormone Reduces Voltage-Gated Na+ Currents as Well as Expression of Na+/K+-ATPase in the Mouse Hippocampus

Mice lacking functional thyroid follicular cells, Pax8^−/− mice, die early postnatally, making them suitable models for extreme hypothyroidism. We have previously obtained evidence in postnatal rat neurons, that a down-regulation of Na⁺-current density could explain the reduced excitability of the nervous system in hypothyroidism. If such a mechanism underlies the development of coma and death in severe hypothyroidism, Pax8^−/− mice should show deficits in the expression of Na⁺ currents and potentially also in the expression of Na⁺/K⁺-ATPases, which are necessary to maintain low intracellular Na⁺ levels. We thus compared Na⁺ current densities in postnatal mice using the patch-clamp technique in the whole-cell configuration as well as the expression of three alpha and two beta-subunits of the Na⁺/K⁺-ATPase in wild type versus Pax8^−/− mice. Whereas the Na⁺ current density in hippocampal neurons from wild type mice was upregulated within the first postnatal week, the Na⁺ current density remained at a very low level in hippocampal neurons from Pax8^−/− mice. Pax8^−/− mice also showed significantly decreased protein expression levels of the catalytic α1 and α3 subunits of the Na⁺/K⁺-ATPase as well as decreased levels of the β2 isoform, with no changes in the α2 and β1 subunits.

The change in thyroid hormone signaling by altered training intensity in male rat skeletal muscle

Aerobic (sub lactate threshold; sub-LT) exercise training facilitates oxidative phosphorylation and glycolysis of skeletal muscle. Thyroid hormone (TH) also facilitates such metabolic events. Thus, we studied whether TH signaling pathway is activated by treadmill training. Male adult rats received 30 min/day treadmill training with different exercise intensity for 12 days. Then plasma lactate and thyrotropin (TSH) levels were measured. By lactate levels, rats were divided into stationary control (SC, 0 m/min), sub-LT (15 m/min) and supra lactate threshold (supra-LT; 25 m/min) training groups. Immediately after the last training, the soleus muscles were dissected out to measure TH receptor (TR) mRNA and protein expressions. Other rats received intraperitoneal injection of T3, 24 h after the last training and sacrificed 6 h after the injection to measure TH target gene expression. TSH level was suppressed in both sub-LT and supra-LT groups during the exercise. TRβ1 mRNA and protein levels were increased in sub-LT group. Sensitivity to T3 was altered in several TH-target genes by training. Particularly, induction of Na(+)/K(+)-ATPase β1 expression by T3 was significantly augmented in sub-LT group. These results indicate that sub-LT training alters TH signaling at least in part by increasing TRβ1 expression. Such TH signaling alteration may contribute metabolic adaptation in skeletal muscle during physical training.

Inhibition of Na(+),K(+)-ATPase in the hypothalamus, pons and cerebellum of the offspring rat due to experimentally-induced maternal hypothyroidism

Neurodevelopment is known to be particularly susceptible to thyroid hormone insufficiency and can result in extensive structural and functional deficits within the central nervous system (CNS), subsequently leading to the establishment of cognitive impairment and neuropsychiatric symptomatology. The current study evaluated the effects of gestational and/or lactational maternal exposure to propylthiouracil (PTU)-induced hypothyroidism (as a suggestive multilevel experimental approach to the study of hypothyroidism-induced changes that has been developed and characterized by the authors) on crucial brain enzyme activities of 21-day-old Wistar rat offspring in a CNS region-specific manner. The activities of acetylcholinesterase (AChE), Na(+),K(+)-ATPase and Mg(2+)-ATPase in the offspring hypothalamus, cerebellum and pons were assessed. The study demonstrated that maternal exposure to PTU (0.05% w/v in the drinking water) during the critical periods of neurodevelopment can result in an inhibition of hypothalamic, pontine and cerebellar Na(+),K(+)-ATPase; a major marker of neuronal excitability and metabolic energy production as well as an important regulator of important systems of neurotransmission. On the other hand, no significant changes in the activities of the herein offspring CNS regions' AChE and Mg(2+)-ATPase were recorded. The observed Na(+),K(+)-ATPase inhibition: (i) is region-specific (and non-detectable in whole brain homogenetes), (ii) could constitute a central event in the pathophysiology of clinically-relevant hypothyroidism-associated developmental neurotoxicity, (iii) occurs under all examined experimental schemes, and (iv) certainly deserves further clarification at a molecular and histopathological level. As these findings are analyzed and compared to the available literature, they also underline the need for the adoption and further study of Na(+),K(+)-ATPase activity as a consistent neurochemical marker within the context of a systematic comparative study of existing (and novel) simulation approaches to congenital and early age hypothyroidism.

Multiple digitalis receptors A molecular perspective

The Na,K-ATPase is the only established receptor for cardiac glycosides like digoxin or ouabain. There are now known to be three different isoforms of its principal subunit. These isoforms can differ from one another in their intrinsic affinity for cardiac glycosides. Recent work examines the molecular structure of the binding site. The relative level of expression of the isoforms in cardiac tissue is modified in several developmental, hormonal, and pathological states, contributing to alterations in the digitalis sensitivity of the tissue.

Short-term effects of thyroid hormones during development: Focus on signal transduction

Extranuclear or nongenomic effects of thyroid hormones are mediated by receptors located at the plasma membrane or inside cells, and are independent of protein synthesis. Recently the alphaVbeta3 integrin was identified as a cell membrane receptor for thyroid hormones, and a wide variety of nongenomic effects have now been shown to be induced through binding of thyroid hormones to this receptor. However, also other thyroid hormone receptors can produce nongenomic effects, including the cytoplasmic TRalpha and TRbeta receptors and probably also a G protein-coupled membrane receptor, and increasing importance is now given to thyroid hormone metabolites like 3,5-diiodothyronine and reverse T(3) that can mimick some nongenomic effects of T(3) and T(4). Signal transduction from the alphaVbeta3 integrin may proceed through at least three independent pathways (protein kinase C, Src or mitogen-activated kinases) but the details are still unknown. Thyroid hormones induce nongenomic effects on at least three important Na(+)-dependent transport systems, the Na(+)/K(+)-ATPase, the Na(+)/H(+) exchanger, and amino acid transport System A, leading to a mitogenic response in embryo cells; but modulation of the same transport systems may have different roles in other cells and at different developmental stages. It seems that thyroid hormones in many cases can modulate nongenomically the same targets affected by the nuclear receptors through long-term mechanisms. Recent results on nongenomic effects confirm the old theory that the primary role of thyroid hormones is to keep the steady-state level of functioning of the cell, but more and more mechanisms are discovered by which this goal can be achieved.

Triiodo-L-thyronine rapidly stimulates alveolar fluid clearance in normal and hyperoxia-injured lungs

RATIONALE

Edema fluid resorption is critical for gas exchange and requires active epithelial ion transport by Na, K-ATPase and other ion transport proteins.

OBJECTIVES

In this study, we sought to determine if alveolar fluid clearance (AFC) is stimulated by 3,3',5 triiodo-L-thyronine (T(3)).

METHODS

AFC was measured in in situ ventilated lungs and ex vivo isolated lungs by instilling isosmolar 5% bovine serum albumin solution with fluorescein-labeled albumin tracer and measuring the change in fluorescein isothiocyanate-albumin concentration over time.

MEASUREMENTS AND MAIN RESULTS

Systemic treatment with intraperitoneal injections of T(3) for 3 consecutive days increased AFC by 52.7% compared with phosphate-buffered saline-injected control rats. Membranes prepared from alveolar epithelial cells from T(3)-treated rats had higher Na, K-ATPase hydrolytic activity. T(3) (10(-6) M), but not reverse T(3) (3,3',5' triiodo-L-thyronine), applied to the alveolar space increased AFC by 31.8% within 1.5 hours. A 61.5% increase in AFC also occurred by airspace instillation of T(3) in ex vivo isolated lungs, suggesting a direct effect of T(3) on the alveolar epithelium. Exposure of rats to an oxygen concentration of greater than 95% for 60 hours increased wet-to-dry lung weights and decreased AFC, whereas the expression of thyroid receptor was not markedly changed. Airspace T(3) rapidly restored the AFC in rat lungs with hyperoxia-induced lung injury.

CONCLUSIONS

Airspace T(3) rapidly stimulates AFC by direct effects on the alveolar epithelium in rat lungs with and without lung injury.

Thyroid Hormone Treatments Differentially Affect the Temperature Kinetics Properties of FoF1 ATPase and Succinate Oxidase as well as the Lipid/Phospholipid Profiles of Rat Kidney Mitochondria: A Correlative Study

Effect of thyroidectomy (Tx) and subsequent treatment with 3,5,3'-triiodo-L: -thyronine (T(3)) or replacement therapy (T(R)) with T(3)+ L: -thyroxine (T(4)) on the temperature kinetics properties of FoF(1 )adenosine triphosphatase (ATPase, ATP synthase, H(+)-translocating ATP synthase EC 3.6.3.14) and succinate oxidase (SO) and on the lipid/phospholipid makeup of rat kidney mitochondria were examined. Tx lowered ATPase activity, which T(3) treatment restored. SO activity was unchanged in Tx but decreased further by T(3) treatment. T(R )restored both activities. The energies of ATPase activation in the high and low temperature ranges (E (H) and E (L)) increased in the Tx and T(3) animals with decrease in phase transition temperature (Tt). T(R) restored E (H) and E (L) but not Tt to euthyroid levels. E (H) and E (L) of SO decreased in Tx animals. T(3) and T(R) restored E (H) whereas E (L) was restored only in the T(R) group; Tt increased in both groups. Total phospholipid and cholesterol contents decreased significantly in Tx and T(3)-treated animals. In Tx animals, sphingomyelin (SPM) and phosphatidylcholine (PC) components decreased, while phosphatidylserine (PS) and diphosphatidylglycerol components increased. T(3) and T(R) treatments caused decreases in SPM, phosphatidylinositol and PS. PC and phosphatidylethanolamine (PE) increased in the T(3) group. T(R) resulted in increased lysophospolipids and PE. Changes in kinetic parameters of the two enzymes were differently correlated with specific phospholipid components. Both T(3) and T(R) regimens were unable to restore normal membrane structure-function relationships.

Low salinity acclimation and thyroid hormone metabolizing enzymes in gilthead seabream (Sparus auratus)

We investigated the effect of acclimation to low salinity water of gilthead seabream (Sparus auratus), a euryhaline seawater teleost, on the activities of thyroid hormone-metabolizing enzymes in gills, kidney, and liver. Following acclimation to low salinity water, the plasma free thyroxine (T(4)) concentration increases 2.5-fold, and outer ring deiodination activities towards T(4), 3,5,3'-triiodothyronine (T(3)) and 3,3',5'-triiodothyronine (reverse T(3), rT(3)) in the gills are reduced by 20-32%. Conjugation (catalyzed by sulfotransferase and UDP-glucuronyltransferase) and deconjugation pathways (arylsulfatase, beta-glucuronidase) play a role in the biological activity of native and conjugated thyroid hormones. Branchial, renal, and hepatic activities of the enzymes involved in these metabolic pathways respond differentially to low salinity conditions. The results substantiate that thyroid hormones are involved in S. auratus osmoregulation, and that the gills are well equipped to play an important role in the modulation of plasma hormone titers.

Multiple genes for essential-hypertension susceptibility on chromosome 1q

Essential hypertension, defined as elevated levels of blood pressure (BP) without any obvious cause, is a major risk factor for coronary heart disease, stroke, and renal disease. BP levels and susceptibility to development of essential hypertension are partially determined by genetic factors that are poorly understood. Similar to other efforts to understand complex, non-Mendelian phenotypes, genetic dissection of hypertension-related traits employs genomewide linkage analyses of families and association studies of patient cohorts, to uncover rare and common disease alleles, respectively. Family-based mapping studies of elevated BP cover the large intermediate ground for identification of genes with common variants of significant effect. Our genomewide linkage and candidate-gene-based association studies demonstrate that a replicated linkage peak for BP regulation on human chromosome 1q, homologous to mouse and rat quantitative trait loci for BP, contains at least three genes associated with BP levels in multiple samples: ATP1B1, RGS5, and SELE. Individual variants in these three genes account for 2-5-mm Hg differences in mean systolic BP levels, and the cumulative effect reaches 8-10 mm Hg. Because the associated alleles in these genes are relatively common (frequency >5%), these three genes are important contributors to elevated BP in the population at large.

Thyroid hormone increases mRNA and protein expression of Na+-K+-ATPase alpha2 and beta1 subunits in human skeletal muscles

CONTEXT

Thyroid hormone regulates specific Na+-K+-ATPase isoforms in rodent skeletal muscles. No study has examined this relationship in human tissues.

OBJECTIVE

This study investigated the effect of hyperthyroid status on the expression of the alpha- and beta-subunits of the Na+-K+-ATPase.

DESIGN

The vastus lateralis muscles from eight hyperthyroid patients were biopsied before and after treatment. Ten age-matched euthyroid subjects served as controls.

RESULTS

In hyperthyroid patients, the average T3 level was three times higher in pretreatment compared with posttreatment (262 +/- 75 vs. 86 +/- 21 ng/dl, P = 0.001). The relative mRNA expression of the alpha2, but not alpha1 or alpha3, subunit was increased approximately 3-fold in pretreatment (2.98 +/- 0.52 vs. 0.95 +/- 0.40, P < 0.01), whereas that of beta1, not beta2 or beta3, subunit was increased approximately 2.8-fold in pretreatment (2.83 +/- 0.38 vs. 1.10 +/- 0.27, P < 0.01). The relative mRNA expression of the alpha2 and beta1 subunits was positively correlated with the serum T3 (r = 0.75, P = 0.001 and r = 0.66, P = 0.003, respectively). Immunohistochemistry studies revealed an increase in protein abundance of the alpha2 and beta1, but not alpha1 or beta2, subunits in the plasma membrane of muscle fibers of hyperthyroid patients, which decreased after treatment.

CONCLUSIONS

This provides the first evidence that, in human skeletal muscles, thyroid hormone up-regulates the Na+-K+-ATPase protein expression at least, in part, at mRNA level, and the alpha2 and beta1 subunits play the important role in this regulation.

Nonosmotic release of vasopressin and renal aquaporins in impaired urinary dilution in hypothyroidism

The purpose of this study was to examine protein expression of renal aquaporins (AQP) and ion transporters in hypothyroid (HT) rats in response to an oral water load compared with controls (CTL) and HT rats replaced with l-thyroxine (HT+T). Hypothyroidism was induced by aminotriazole administration for 10 wk. Body weight, water intake, urine output, solute and urea excretion, and serum and urine osmolality were comparable among the three groups at the conclusion of the 10-wk treatment period. One hour after oral gavage of water (50 ml/kg body wt), HT rats demonstrated significantly less water excretion, higher minimal urinary osmolality, and decreased serum osmolality compared with CTL and HT+T rats. Despite the hyposmolality, plasma vasopressin concentration was elevated in HT rats. These findings in HT rats were associated with an increase in protein abundance of renal cortex AQP1 and inner medulla AQP2. AQP3, AQP4, and the Na-K-2Cl cotransporter were also increased. Moreover, 1 h following the oral water load, HT rats demonstrated a significant increase in the membrane-to-vesicle fraction of AQP2 by Western blot analysis. The defect in urinary dilution in HT rats was reversed by the V2vasopressin antagonist OPC-31260. In conclusion, impaired urinary dilution in HT rats is primarily compatible with the nonosmotic release of vasopressin and increased protein expression of renal AQP2. The impairment of maximal solute-free water excretion in HT rats, however, appears also to involve diminished distal fluid delivery.

The highly conserved extracellular peptide, DSYG(893-896), is a critical structure for sodium pump function

The peptide sequence DSYG(893-896) of the sheep sodium pump alpha 1 subunit is highly conserved among all K(+)-transporting P-type ATPases. To obtain information about its function, single mutations were introduced and the mutants were expressed in yeast and analysed for enzymatic activity, ion recognition, and alpha/beta subunit interactions. Mutants of Ser894 or Tyr895 were all active. Conservative phenylalanine and tryptophan mutants of Tyr895 displayed properties that were similar to the properties of the wild-type enzyme. Replacement of the same amino acid by cysteine, however, produced heat-sensitive enzymes, indicating that the aromatic group contributes to the stability of the enzyme. Mutants of the neighbouring Ser894 recognized K(+) with altered apparent affinities. Thus, the Ser894-->Asp mutant displayed a threefold higher apparent affinity for K(+) (EC(50) = 1.4 +/- 0.06 mm) than the wild-type enzyme (EC(50) = 3.8 +/- 0.33 mm). In contrast, the mutant Ser894-->Ile had an almost sixfold lower apparent affinity for K(+) (EC(50) = 21.95 +/- 1.41 mm). Mutation of Asp893 or Gly896 produced inactive proteins. When an anti-beta 1 subunit immunoglobulin was used to co-immunoprecipitate the alpha 1 subunit, neither the Gly896-->Arg nor the Gly896-->Ile mutant could be visualized by subsequent probing with an anti-alpha 1 subunit immunoglobulin. On the other hand, co-immunoprecipitation was obtained with the inactive Asp893-->Arg and Asp893-->Glu mutants. Thus, it might be that Asp893 is involved in enzyme conformational transitions required for ATP hydrolysis and/or ion translocation. The results obtained here demonstrate the importance of the highly conserved peptide DSYG(893-896) for the function of alpha/beta heterodimeric P-type ATPases.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Effect of high-intensity training on exercise-induced gene expression specific to ion homeostasis and metabolism

Changes in gene expression during recovery from high-intensity, intermittent, one-legged exercise were studied before and after 5.5 wk of training. Genes related to metabolism, as well as Na+, K+, and pH homeostasis, were selected for analyses. After the same work was performed before and after the training period, several muscle biopsies were obtained from vastus lateralis muscle. In the untrained state, the Na+-K+-ATPase alpha1-subunit mRNA level was approximately threefold higher (P < 0.01) at 0, 1, and 3 h after exercise, relative to the preexercise resting level. After 3-5 h of recovery in the untrained state, pyruvate dehydrogenase kinase 4 and hexokinase II mRNA levels were elevated 13-fold (P < 0.001) and 6-fold (P < 0.01), respectively. However, after the training period, only pyruvate dehydrogenase kinase 4 mRNA levels were elevated (P < 0.05) during the recovery period. No changes in resting mRNA levels were observed as a result of training. In conclusion, cellular adaptations to high-intensity exercise training may, in part, be induced by transcriptional regulation. After training, the transcriptional response to an exercise bout at a given workload is diminished.

Thyroid hormone stimulates Na-K-ATPase activity and its plasma membrane insertion in rat alveolar epithelial cells

Na-K-ATPase protein is critical for maintaining cellular ion gradients and volume and for transepithelial ion transport in kidney and lung. Thyroid hormone, 3,3',5-triiodo-l-thyronine (T3), given for 2 days to adult rats, increases alveolar fluid resorption by 65%, but the mechanism is undefined. We tested the hypothesis that T3 stimulates Na-K-ATPase in adult rat alveolar epithelial cells (AEC), including primary rat alveolar type II (ATII) cells, and determined mechanisms of the T3 effect on the Na-KATPase enzyme using two adult rat AEC cell lines (MP48 and RLE-6TN). T3 at 10-8 and 10-5 M increased significantly hydrolytic activity of Na-K-ATPase in primary ATII cells and both AEC cell lines. The increased activity was dose dependent in the cell lines (10-9-10-4 M) and was detected within 30 min and peaked at 6 h. Maximal increases in Na-K-ATPase activity were twofold in MP48 and RLE-6TN cells at pharmacological T3 of 10-5 and 10-4 M, respectively, but increases were statistically significant at physiological T3 as low as 10-9 M. This effect was T3 specific, because reverse T3 (3,3',5'-triiodo-l-thyronine) at 10-9-10-4 M had no effect. The T3-induced increase in Na-K-ATPase hydrolytic activity was not blocked by actinomycin D. No significant change in mRNA and total cell protein levels of Na-K-ATPase were detected with 10-9-10-5 M T3 at 6 h. However, T3 increased cell surface expression of Na-K-ATPase alpha1- or beta1-subunit proteins by 1.7- and 2-fold, respectively, and increases in Na-K-ATPase activity and cell surface expression were abolished by brefeldin A. These data indicate that T3 specifically stimulates Na-K-ATPase activity in adult rat AEC. The upregulation involves translocation of Na-K-ATPase to plasma membrane, not increased gene transcription. These results suggest a novel nontranscriptional mechanism for regulation of Na-K-ATPase by thyroid hormone.

Renal expression of sodium transporters and aquaporin-2 in hypothyroid rats

Hypothyroidism is associated with significant abnormalities in the renal handling of salt and water. To address the involvement of tubular transport proteins in these abnormalities, rats were rendered pharmacologically hypothyroid and the abundance of major tubular transport proteins was assessed by immunoblot and immunohistochemistry. Hypothyroidism resulted in a marked reduction in kidney size and creatinine clearance along with decreased or unchanged total kidney abundance of the transport proteins. Whereas the proximal tubular type 3 Na/H exchanger (NHE3) and type 2 Na-phosphate cotransporter (NaPi2) stood out by their disproportionately reduced abundance, the bumetanide-sensitive type 2 Na-K-2Cl cotransporter (NKCC2) and aquaporin-2 (AQP2) were unaltered in their total kidney abundance despite a markedly lower kidney mass. The latter proteins in fact showed enhanced immunostaining. Decreased NHE3 and NaPi2 expression was most likely due to a combination of triiodo-l-thyronine (T(3)) deficiency along with a reduced glomerular filtration rate. The increased abundance of NKCC2 and AQP2 may have been caused by an increased action of vasopressin since urinary excretion of this hormone was elevated. On the other hand, the thiazide-sensitive Na-Cl cotransporter; the alpha-, beta-, and gamma-subunits of the amiloride-sensitive epithelial Na channel; and the alpha(1)-subunit of Na-K-ATPase showed a moderate decrease in total kidney abundance that was largely proportional to the smaller kidney mass. Although the observed expression of transporters was associated with a balanced renal sodium handling, altered transporter abundance may become functionally relevant if the hypothyroid kidney is challenged by an additional destabilization of the milieu interieur that has previously been shown to result in an inadequate natriuresis and clinical symptoms.

Thyroid hormone up-regulates Na+/K+ pump alpha2 mRNA but not alpha2 protein isoform in cultured skeletal muscle

Thyroid hormone (T(3)) is known to up-regulate the physiological expression of the Na(+)/K(+) pump in cultured skeletal muscle. We recently reported that primary cultured rat skeletal muscle expresses only the alpha(1), beta(1) and beta(2) protein isoforms of Na(+)/K(+) pump. Interestingly, alpha(2) mRNA is detectable while the alpha(2) protein isoform is not. We therefore examined whether T(3) might up-regulate the expression of Na(+)/K(+) pump alpha(2) isoform at the protein and mRNA level. We also examined the regulation by this hormone of the other isoforms of the pump. Primary cultures were treated with T3 for 48 h from day 4 to day 6 of differentiation. Protein and mRNA isoforms of Na(+)/K(+) pump were identified by Western blotting and Northern blotting, respectively. T(3) induced a marked increase in the beta(1) protein and a slight increase in the alpha(1) protein. T(3) did not affect expression of the beta(2) protein. The alpha(2) protein was not detected in either untreated or T(3)-treated cells. In contrast, alpha(2) mRNA was highly up-regulated by T(3) treatment compared to the other isoforms. The lack of expression of the alpha(2) protein isoform following T(3) treatment suggests that posttranscriptional events related to this isoform may be dependent on other growth factors or hormones.

Different Na, K-ATPase mRNA(beta1) species exhibit unique translational efficiencies

We have previously identified five Na, K-ATPase beta1-mRNA species that are expressed in the rat heart, kidney, and brain. These mRNAs which are unequal in their abundance have an identical coding region but differ in the length of their 5'- and 3'-untranslated regions (UTRs). In this study we examined the possibility that the beta1-mRNA species exhibit differential translational efficiencies. We constructed expression plasmids encoding each of the five mRNAs and transcribed and translated them in vitro. Using rabbit reticulocyte system we determined the translation of the different mRNAs under conditions optimized for each beta1-cRNA and under an equivalent (standard) condition. The longest beta1-cRNA species (initiating at the first transcription start site and ending at the last [fifth] poly(A) site) exhibited the lowest relative translational efficiency averaging 0.2 +/- 0.05 units/mol of cRNA compared to the shortest beta1-cRNA species initiating at the first transcription start site and ending at the first poly(A) signal (with an assigned relative value of 1.0). These results suggested that the different translation rates of beta1-mRNAs may be due to their 3'-UTRs. To further define the role of beta1-3'-UTR, chimeric luciferase constructs containing different segments of the beta1-3'-UTR were transiently transfected into Clone 9 cells. Compared to the chimeric construct containing the shortest beta1-3'-UTR segment (ending at the first poly(A) site), the construct containing the full-length beta1-3'-UTR exhibited a luciferase expression of 0.23 +/- 0.04. To control for potential changes in the abundance of the expressed chimeric mRNAs which may lead to differences in luciferase expression, luciferase activity was normalized against chimeric luciferase-mRNA content measured in mixtures of cells stably transfected with the above constructs. The ratio of luciferase activity/chimeric luciferase-mRNA content in cells expressing the construct containing the entire beta1-3'-UTR region was 0.17 that in cells expressing chimeric luciferase mRNA containing beta1-3'-UTR up to the first poly(A) signal (P < 0.05). We conclude that the translational efficiency of the different beta1-mRNA species is negatively regulated by the 3'-UTR of the mRNA and that a regulating region appears to be localized between the second and fifth poly(A) signals of beta1-mRNA.

Translational regulation of Na-K-ATPase subunit mRNAs by glucocorticoids

Glucocorticoids (GC) regulate Na-K-ATPase-subunit mRNA transcription. However, GC-induced increases in Na-K-ATPase activity are not always paralleled by changes in subunit mRNA abundance. We therefore examined posttranscriptional mechanisms of subunit gene regulation by GC. cDNA-derived mRNAs encoding alpha 1-, alpha 3-, and beta 1-subunits were tested for stability and translation efficiency in a cell-free lysate, in the presence of hydrocortisone (HC) or dexamethasone (Dex). No effect of HC on subunit mRNA stability was noted. Translation efficiency of alpha1- and alpha 3-mRNAs, but not of beta 1-mRNA, was significantly increased by HC and Dex. Deletion of the 5'untranslated region (5'UT) of alpha 1-mRNA abolished this effect. Translation of a chimeric beta 1-mRNA, constructed by transposing the 5'UT of alpha 1 onto the coding region of beta1, was enhanced by HC. Transposition of a putative steroid-modulatory element conserved in the 5'UT of all alpha isoforms (ACAGGACCC) onto the coding region of beta 1-mRNA rendered it responsive to HC. A synthetic primer containing the ACAGGACCC sequence abolished the effect of HC on alpha 1- and chimeric beta 1-mRNAs. Our results indicate that GC can directly enhance Na-K-ATPase translation in vitro in a subunit-specific manner, via a putative GC-modulatory element situated in a predicted loop structure within the 5'UT of alpha-mRNAs.

Thyroid hormone regulation of myocardial Na/K-ATPase gene expression

Employing published methods for isolation of cardiac myocyte nuclei from adult rat ventricular myocardium with the use of mechanical disruption without digestive enzymes, we obtained transcriptionally active cardiac myocyte nuclei with sufficient yield and purity. The relative content of Na/K-ATPase subunit mRNAs (alpha 1, alpha 2, and beta 1) in ventricular myocardium of euthyroid rats closely matched the relative rates of transcription of the respective subunit genes determined by nuclear run-on assay. Treatment of hypothyroid rats with T(3)to elicit hyperthyroidism was associated with 2.9-, 7.5-, and seven-fold increases in the contents of alpha 1-, alpha 2, beta 1-mRNAs, respectively. In contrast, rates of transcription of the subunit genes were not changed significantly by T(3), while transcription of the 18 S ribosomal gene was stimulated identical with three-fold by the treatment. A quantitative reverse transcription-polymerase chain reaction assay for measurement of primary RNA transcripts of the beta 1 gene was developed employing a rat genomic DNA fragment that contains the first exon and part of the first intron of the beta 1 gene. The relative abundance of beta 1 primary transcripts did not change in RNA isolated from hypothyroid, euthyroid, and hyperthyroid rats. It is concluded that: (1) The relative contents of Na/K-ATPase subunit mRNAs in euthyroid adult myocardium is primarily controlled at the transcriptional level, and (2) T(3)-induced increases in the contents of Na/K-ATPase subunit mRNAs in the heart is not associated with increased rates of transcription of the subunit genes, and the effect is mediated at the post-transcriptional level.

Thyroid hormone stimulates Na, K-ATPase gene expression in the hemodynamically unloaded heterotopically transplanted rat heart

Regulation of myocardial Na, K-ATPase gene expression by thyroid hormone was investigated in the heterotopically transplanted rat heart to distinguish the direct effects of the hormone on the heart from effects secondary to increased hemodynamic workload. In this model, the transplanted heart is histologically normal and spontaneously beating, but hemodynamically unloaded. Three days after transplantation, relative contents of ventricular Na, K-ATPase alpha2- and beta1-mRNAs and alpha1- and alpha2-proteins were increased twofold to threefold in the transplanted heart, but these changes were transient. We next determined the maximal triiodothyronine (T3)-induced changes that are observed in various parameters of Na, K-ATPase expression in the heart: treatment of nontransplanted euthyroid rats with T3 to reach hyperthyroid steady state resulted in significant increases in heart weight, RNA and RNA/protein ratio, Na, K-ATPase activity, Na, K-ATPase alpha2-protein and enzyme activity, and approximately threefold increase in both alpha2- and beta1-mRNA content. The effect of treatment with thyroxine (T4) on the heterotopically transplanted and the in situ heart was then examined. T4 treatment (of the host) resulted in a significant increase in Na, K-ATPase alpha1-, alpha2-, and beta1-mRNAs in transplanted hearts (1.6 +/- 0.1-, 2.4 +/- 0.2-, and 1.7 +/- 0.1-fold, respectively), that was associated with a 2.2 +/- 0.2-fold increase in alpha2 protein as compared to transplanted hearts in diluent-treated euthyroid hosts (p < 0.05 for all changes). In addition, T4-induced increments in transplanted hearts were similar to those observed in the corresponding in situ hearts of host rats treated with T4. We conclude that the increase in Na, K-ATPase expression by thyroid hormone largely occurs independently of increased cardiac work elicited by the hormone and reflects a direct action of the hormone on Na, K-ATPase gene expression.

Temporal responses of oxidative vs. glycolytic skeletal muscles to K+ deprivation: Na+ pumps and cell cations

When K+ output exceeds input, skeletal muscle releases intracellular fluid K+ to buffer the fall in extracellular fluid (ECF) K+. To investigate the mechanisms and muscle specificity of the K+ shift, rats were fed K+-deficient chow for 2-10 days, and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gastrocnemius (WG). After 2 days of low-K+ chow, plasma K+ concentration ([K+]) fell from 4.6 to 3.7 mM, and Na+-K+-ATPase alpha2 (not alpha1) protein levels in both muscles, measured by immunoblotting, decreased 36%. Cell [K+] decreased from 116 to 106 mM in soleus and insignificantly in WG, indicating that alpha2 can decrease before cell [K+]. After 5 days, there were further decreases in alpha2 (70%) and beta2 (22%) in WG, not in soleus, whereas cell [K+] decreased and cell [Na+] increased by 10 mM in both muscles. By 10 days, plasma [K+] fell to 2.9 mM, with further decreases in WG alpha2 (94%) and beta2 (70%); cell [K+] fell 19 mM in soleus and 24 mM in WG compared with the control, and cell [Na+] increased 9 mM in soleus and 15 mM in WG; total homogenate Na+-K+-ATPase activity decreased 19% in WG and insignificantly in soleus. Levels of alpha2, beta1, and beta2 mRNA were unchanged over 10 days. The ratios of alpha2 to alpha1 protein levels in both control muscles were found to be nearly 1 by using the relative changes in alpha-isoforms vs. beta1- (soleus) or beta2-isoforms (WG). We conclude that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta2 are distinct and that decreases in alpha2-isoform pools can occur early enough in both muscles to account for the shift of K+ to the ECF.

Na,K-ATPase mRNA beta 1 expression in rat myocardium--effect of thyroid status

The abundance of Na,K-ATPase and its alpha and beta subunit mRNAs is upregulated in cardiac and other target tissue by thyroid hormone (T3). Multiple Na,K-ATPase mRNA beta 1 species encoding an identical beta 1 polypeptide are expressed in the heart. The different mRNA beta 1 species result from utilization of two transcription start-sites in the first exon and multiple (five) poly(A) signals in the terminal exon of the beta 1 gene. In the present study we identify the mRNA beta 1 species that are expressed in rat ventricular myocardium under basal conditions, and determine whether they are differentially regulated by T3. mRNA beta 1 species were identified by 3'-RACE followed by DNA sequencing, and by Northern blotting using probes derived from different regions of rat cDNA beta 1. Five mRNA beta 1 species are expressed in rat heart: mRNA beta 1 species that are initiated at the first transcription start-site and end at the first, second and fifth poly(A) sites (resulting in mRNAs of 1630, 1810, and 2780 nucleotides), and mRNA beta 1 species initiated at the second transcription start-site and ending at the second and fifth poly(A) sites (resulting in mRNAs of 1500 and 2490 nucleotides); in order of increasing length, the five mRNAs constitute 0.04, 0.15, 0.38, 0.11 and 0.32 of total mRNA beta 1 content. In hypothyroid rats (induced by addition of propyl-thiouracil to the drinking water for 3 weeks), total mRNA beta 1 content decreased to 0.18 euthyroid levels, which was associated with a disproportionate 7.5-fold decrease in the abundance of the longest transcript (P < 0.05); transcripts initiating at the first transcription start-site and ending at the second poly(A) signal in hypothyroid hearts were 0.26 euthyroid levels (P < 0.05). Hyperthyroidism induced by injection of normal rats with three doses of 100 micrograms T3/100 g body weight every 48 h resulted in an overall approximately 2-fold increase in mRNA beta 1 content with no change in the fractional contribution of any of the mRNA beta 1 species. The results indicate a complex heterogeneity in the expression of mRNA beta 1 in myocardium.

Na(+)-K(+)-ATPase expression in alveolar epithelial cells: upregulation of active ion transport by KGF

We evaluated the effects of keratinocyte growth factor (KGF) on alveolar epithelial cell (AEC) active ion transport and on rat epithelial Na channel (rENaC) subunit and Na(+)-K(+)-adenosinetriphosphatase (ATPase) subunit isoform expression using monolayers of AEC grown in primary culture. Rat alveolar type II cells were plated on polycarbonate filters in serum-free medium, and KGF (10 ng/ml) was added to confluent AEC monolayers on day 4 in culture. Exposure of AEC monolayers to KGF on day 4 resulted in dose-dependent increases in short-circuit current (Isc) compared with controls by day 5, with further increases occurring through day 8. Relative Na(+)-K(+)-ATPase alpha 1-subunit mRNA abundance was increased by 41% on days 6 and 8 after exposure to KGF, whereas alpha 2-subunit mRNA remained only marginally detectable in both the absence and presence of KGF. Levels of mRNA for the beta 1-subunit of Na(+)-K(+)-ATPase did not increase, whereas cellular alpha 1- and beta 1-subunit protein increased 70 and 31%, respectively, on day 6. mRNA for alpha-, beta-, and gamma-rENaC all decreased in abundance after treatment with KGF. These results indicate that KGF upregulates active ion transport across AEC monolayers via a KGF-induced increase in Na pumps, primarily due to increased Na(+)-K(+)-ATPase alpha 1-subunit mRNA expression. We conclude that KGF may enhance alveolar fluid clearance after acute lung injury by upregulating Na pump expression and transepithelial Na transport across the alveolar epithelium.

Changes in ouabain affinity of Na+, K+-ATPase during focal cerebral ischaemia in the mouse

We investigated the effect of focal cerebral ischaemia on the activity and the affinity of the ouabain sites of Na+,K+-ATPase in the mouse. The Na+,K+-ATPase activity was decreased by 38% as early as 30 min following ischaemia. In the sham group, the dose-response curves for ouabain disclosed three inhibitory states which contribute, respectively, 24.9 +/- 6.7%, 39.1 +/- 7.5% and 36.0% of the total activity (low affinity, LA; high affinity, HA and very high affinity, VHA, respectively). Their computed IC50 values are, respectively: 1.3 X 10(-3) M, 4.5 X 10(-6) M and 2.9 X 10(-9) M. Surprisingly, in ischaemic cortices, only two sites for ouabain were detected. The first site exhibits a LA (IC50 = 2.0 X 10[-4] M) but its relative contribution to the total activity (46.1 +/- 5.2%) is twice that noted for the LA site in non-ischaemic tissues. The second site presents an affinity intermediate between those of HA and VHA sites of the sham group (IC50 = 1.7 X 10[-7] M) and contributes 53.9% to the total activity. Loss in the specific activity of the second site explains that of the total activity. The most likely explanation in the presence of only two ouabain sites of Na+,K+-ATPase following ischaemia may be a change in ouabain affinity of alpha2 and/or alpha3 isoforms, as the presence of all three alpha isoforms has been observed by Western blotting. These results suggest that ischaemia induces intrinsic modifications in Na+,K+-ATPase which result from perturbations in membrane integrity and/or association of the alpha isoforms of this enzyme.

Changes in the expression of Na+/K+-ATPase isoenzymes in the left ventricle of diabetic rat hearts: effect of insulin treatment

Na+/K+-ATPase related strophanthidin sensitive 3-O-methylfluorescein-phosphatase activity, [3H]ouabain binding and expression of Na+/K+-ATPase subunit isoforms were measured in the left ventricle of the heart of normal and streptozotocin-diabetic rats with and without insulin treatment. Compared to control animals, the enzyme activity was 0.75 +/- 0.09 and 0.62 +/- 0.06 times lower in rats diabetic for 2 and for 4 weeks, respectively. This was associated with a proportional decrease of the [3H]ouabain binding sites. Immunoblots indicated a 0.76 +/- 0.08 and 0.61 +/- 0.08-fold decrease of alpha1, a 0.68 +/- 0.09 and 0.41 +/- 0.04-fold decrease of alpha2 subunit in 2- and 4-week diabetic rats, respectively relative to controls. Beta1 subunit decreased proportionally 0.71 +/- 0.07 and 0.38 +/- 0.06-fold, and beta2 decreased 0.75 +/- 0.08 and 0.31 +/- 0.06-fold, respectively. Northern blot analysis revealed a significant reduction in mRNA level of Na+/K+-ATPase subunit isoforms after 2 and 4 weeks of diabetes (for alpha1 66.2 +/- 8.2 and 55.9 +/- 7.8% of controls for alpha2 91.7 +/- 12.1 and 41.1 +/- 7.1% of controls and for beta subunit 93.4 +/- 11.1 and 49.8 +/- 6.8% of controls, respectively). Although, mRNA levels of isoform reverted to even higher levels than the control values after insulin treatment, insulin caused only a partial recovery of enzyme activity, [3H]ouabain binding capacity and protein expression. We have obtained evidence that in cardiac left ventricle there are more than one type of Na+/K+-ATPase alpha and beta subunit isoforms which are affected in diabetes and by insulin treatment. The time course of diabetes induced changes and the degree of involvement suggest that the Na+/K+-ATPase isoforms are altered individually.

Developmental changes in regulation of the Na+, K(+)-ATPase alpha 3 isoform by thyroid hormone in ferret heart

Ferret heart expresses the alpha 1- as well as the alpha 3-isoform of the Na+, K(+)-ATPase. We have shown previously that the alpha 3 isoform is differentially upregulated during postnatal cardiac development and that in adult ferrets expression of alpha 3 is not responsive to regulation by thyroid hormone (TH). Since developmental-stage dependent effects of TH have been reported previously, the present study examined whether effects of TH on expression of the Na+, K(+)-ATPase isoforms in ferret heart is modulated during development and possible mechanisms were examined. Ferrets of different age groups were treated with TH and the relative abundance of Na+, K(+)-ATPase isoforms in ferret myocardium was determined by immunoblotting. Thyroid hormone (T3; 50 micrograms/100 g body weight on 3 alternating days, s.c.) increased protein levels of the alpha 3 isoform, but not that of alpha 1 or beta 1, in myocardium of 5-day-old and 3-week-old ferrets. By contrast, in myocardium of 6- and 8-week-old ferrets T3 failed to increase protein levels of alpha 1 and alpha 3. To determine whether elevated plasma levels of TH during development plays a role in the transition, mature ferrets were first made hypothyroid before TH treatment. In these hypothyroid ferrets expression of the alpha 3 isoform remained unresponsive to TH (T4, 0.5 mg/kg for 7 days, s.c.). The transition from TH-responsive to TH-unresponsive appears to be isoform-specific because in skeletal muscle of 8-week-old ferrets and in hypothyroid ferrets the alpha 2 isoform is upregulated by TH. Finally, there appears to be functional thyroid hormone receptors throughout development because in each age group TH effectively induced expression of alpha-MHC in the myocardium. In conclusion, these findings demonstrate that expression of alpha 3 isoform in the myocardium of newborn ferret is responsive to TH; however, the responsiveness terminates between 3- and 6-weeks of age. Neither elevated endogenous TH level nor a lack of functional thyroid hormone receptor appears to be responsible for the transition from TH-responsive to TH-unresponsive.

Alterations in alpha subunit expression of cardiac Na+,K+-ATPase in spontaneously hypertensive rats: effect of antihypertensive therapy

The alpha-2 subunit abundance of Na+,K(+)-ATPase in the rat heart has been reported to be reduced in several induced hypertensive models. To determine whether this reduction also occurs in a genetic model of hypertension, we studied expression of the alpha subunits in left ventricles of spontaneously hypertensive rats (SHR), and normotensive Wistar-Kyoto (WKY) and Sprague-Dawley rats using Western blotting and quantitative dot-blotting analysis with monoclonal antibodies. While the alpha-1 subunit was not affected in any of the strains, a significant reduction of the alpha-2 subunit expression was noted in 19-week-old SHRs, but not in age-matched WKY and Sprague-Dawley rats, supporting the hypothesis that elevated arterial pressure may differentially downregulate the alpha-2 subunit in the rat heart. To further test this hypothesis we designed experiments in which hypertensive rats were treated with the antihypertensive agents hydralazine and nifedipine. Both agents effectively normalized the blood pressure in the SHRs with no significant effect on the blood pressure in the WKY and Sprague-Dawley rats. The alpha-2 subunit in SHRs treated with hydralazine and nifedipine showed a 63.3% (n = 6, P < 0.05, analysis of variance and Fischer's test) and a 27.4% increase, respectively, over the hypertensive SHR controls, although the reversal effect of nifedipine did not quite reach significance. The alpha-1 subunit expression was not affected by any of the drug treatments. No effect of either of the drugs on the alpha-1 or alpha-2 subunit was observed in the WKY or Sprague-Dawley rat groups. These data support our hypothesis that the alpha-2 subunit may be a pressure-sensitive isoform of the cardiac Na+,K(+)-ATPase and that high blood pressure is, directly or indirectly, responsible for the reduction of the alpha-2 subunit protein expression.

Effects of ouabain on vascular reactivity

Ouabain is an endogenous substance occurring in the plasma in the nanomolar range, that has been proposed to increase vascular resistance and induce hypertension. This substance acts on the alpha-subunit of Na+,K(+)-ATPase inhibiting the Na(+)-pump activity. In the vascular smooth muscle this effect leads to intracellular Na+ accumulation that reduces the activity of the Na+/Ca2+ exchanger and to an increased vascular tone. It was also suggested that circulating ouabain, even in the nanomolar range, sensitizes the vascular smooth muscle to vasopressor substances. We tested the latter hypothesis by studying the effects of ouabain in the micromolar and nanomolar range on phenylephrine (PE)-evoked pressor responses. The experiments were performed in normotensive and hypertensive rats in vivo, under anesthesia, and in perfused rat tail vascular beds. The results showed that ouabain pretreatment increased the vasopressor responses to PE in vitro and in vivo. This sensitization after ouabain treatment was also observed in hypertensive animals which presented an enhanced vasopressor response to PE in comparison to normotensive animals. It is suggested that ouabain at nanomolar concentrations can sensitize vascular smooth muscle to vasopressor stimuli possibly contributing to increased tone in hypertension.

Ouabain sensitivity and expression of Na/K-ATPase alpha- and beta-subunit isoform genes during bovine early development

The fluid movements that arise during blastocyst formation (cavitation) are, at least in part, driven by the Na/K-ATPase. In this study, the reverse transcriptase-polymerase chain reaction (RT-PCR) was used to survey bovine pre-attachment embryos for transcripts encoding known isoforms of the Na/K-ATPase alpha- and beta-subunits, including isoforms not previously detected during the first week of mammalian development. Transcripts encoding the Na-K-ATPase alpha 1, alpha 2, alpha 3 and beta 2 isoforms were detected throughout bovine preattachment development. This is the first indication that alpha 2, alpha 3 and beta 2 mRNAs are expressed during this early developmental interval. As in the mouse, beta 1-subunit transcripts were not detected until the morula stage and were also present in blastocysts. Thus, in two mammalian species an increase in abundance of beta 1 isoform transcripts in the morula stage is coincident with the onset of cavitation. Transcripts encoding the recently characterized alpha 4 isoform were not detected. The sensitivity of bovine blastocysts to ouabain (a potent inhibitor of Na/K-ATPase) was determined by assessing the ability of bovine blastocysts to recover in ouabain supplemental culture medium following cytochalasin-induced blastocyst collapse. Re-expansion of bovine blastocysts was inhibited in all ouabain concentrations down to 10(-9) M. Mouse blastocysts, in contrast, were sensitive to ouabain at or above 10(-3)M. These results have established that transcripts encoding multiple isoforms of both the alpha and beta subunits of the Na/K-ATPase are expressed throughout early bovine development and that bovine blastocysts display a greater sensitivity to ouabain than murine blastocysts. Future analysis will determine the possible individual and collective roles of these isoforms during blastocyst formation.

Thyroid hormone represses protein kinase C isoform expression and activity in rat cardiac myocytes

We have previously demonstrated that at least four isoforms of protein kinase C (PKC; alpha, delta, epsilon, and zeta) are expressed in neonatal rat ventricular myocytes and that development is associated with a decline in their expression. The mechanism(s) regulating PKC isoform expression in ventricular myocytes is completely unknown. The developmental decline in PKC expression occurs, in large part, during the first 2 weeks of postnatal life, while thyroid hormone levels are known to be progressively increasing. Accordingly, this study examined the influence of thyroid hormone on PKC isoform expression to determine whether thyroid hormone can be implicated as a potential physiological regulator of PKC gene expression during normal cardiac development. Hypothyroidism was induced in adult rats by surgical thyroidectomy; thyroid status was manipulated in cultured neonatal ventricular myocytes by growth in serum-free medium with varying triiodothyronine (T3) levels. In each case, hypothyroidism was verified by a 10- to 50-fold increase in steady state mRNA for beta-myosin heavy chain. In hypothyroid adult ventricular myocardium, there was a selective 60% increase in the expression of PKC epsilon protein that corresponded to an increase in maximally stimulated PKC enzyme activity with PKC epsilon substrate peptide (epsilon pep) but not with histone as substrate. Northern blot analysis revealed a 70% increase in PKC epsilon mRNA, indicating that the regulatory effects of thyroid hormone are mediated, at least in part, at the message level. In neonatal ventricular myocytes, there was a T3-dependent reduction in immunoreactivity for both PKC alpha and PKC epsilon that was associated with significant reductions in both histone- and epsilon pep-kinase activities. The concentration of T3 that half-maximally repressed PKC alpha and PKC epsilon expression was approximately 0.5 nmol/L. Thyroid hormone had no effect on PKC delta and PKC zeta expression in neonatal or adult ventricular myocytes. PKC isoform expression in cardiac fibroblasts was not influenced by variations in the thyroid hormone concentration during culture. These results provide evidence that thyroid hormone specifically represses PKC alpha and PKC epsilon in the neonatal heart and PKC epsilon in the adult heart. Thyroid hormone-induced changes in PKC may play an important permissive role in the modulation of autonomic responsiveness in ventricular cardiomyocytes.

Responsiveness of alpha 1 and beta 1 cochlear Na, K-ATPase isoforms to thyroid hormone

The effects of thyroid hormone on Na, K-ATPase subunit isoforms under euthyroid (EUTH), hypothyroid (HYPO) and hyperthyroid (HYPER) states were investigated via immunocytochemistry and the use of polyclonal antibodies specific to each isoform (alpha 1, alpha 2, alpha 3 and beta 1, beta 2). In HYPO animals, there was a distinct decrease in Na, K-ATPase alpha 1 isoform immunoreactivity in the stria vascularis (SV), spiral ganglion (SG) cells, spiral limbus (SLi) and cochlear nerve (CN) as compared with that in EUTH animals by the 17th day of the experiment. Immunostaining of the alpha 1 isoform increased in HYPER animals as compared with that in HYPO animals, and reached a level comparable to that in EUTH animals after 2 days of triiodothyronine (T3) treatment. Levels of alpha 2, alpha 3 and beta 2 isoforms did not appear to be affected by T3 administration. By the 19th day of a low I2 diet, the immunoreactive intensity of the beta 1 isoform was reduced in cochlear tissues of HYPO animals as compared with that in EUTH animals. The immunoreactivity of the beta 1 isoform increased after treatment with T3 for 4 days and was comparable with levels in EUTH animals. These data indicate that alpha 1 and beta 1 isoforms within specific cochlear regions of the adult rat are responsive to thyroid hormone.

Subcellular distribution of sodium pump isoform subunits in mammalian cardiac myocytes

The cardiac Na+ pump (Na+ -K+ -ATPase) provides the driving force for the Na+/Ca2+ exchanger, a determinant of intracellular Ca2+ stores. Three Na+ pump alpha-catalytic subunit isoforms are expressed in human heart, alpha1 and alpha2 are expressed in rat heart, and only alpha1 is expressed in guinea pig heart. The objective of this study was to determine whether there are isoform-specific patterns of expression in the transverse tubules (T tubules) vs. the peripheral sarcolemma. In adult rat cardiomyocytes, anti-alpha1-specific antibodies labeled the T tubules more intensely than the peripheral sarcolemma, in which labeling was patchy, the same pattern reported for distribution of the Na+/Ca2+ exchanger (J. S. Frank, G. Mottino, D. Reid, R. S. Molday, and K. D. Philipson, J. Cell Biol. 117: 337-345, 1992), whereas anti-alpha2- and anti-beta1-antibodies uniformly labeled T tubules and peripheral sarcolemma. In guinea pig cardiomyocytes, an anti-alpha-antibody against an extracellular epitope evenly labeled the peripheral sarcolemma and T tubules, and immunogold labeling demonstrated coincidence of alpha-subunits and intramembranous particles in sarcolemma. In summary, Na+ pumps are located in both peripheral sarcolemma and T tubules of cardiomyocytes expressing either multiple or single Na+ pump isoforms.

Regulation of Na+,K+-ATPase alpha-subunit expression by mechanical strain in aortic smooth muscle cells

We have previously demonstrated that vascular sodium pump activity is stimulated in several rat models of hypertension. In addition, others have reported an upregulation of mRNA for the Na+,K+-ATPase alpha1-subunit in hypertension. To test the effect of sustained, cyclic, stretch-relaxation stimuli on the expression of alpha1- and alpha2-subunits of Na+,K+-ATPase in vascular smooth muscle cells, we used the Flexercell strain unit to stretch rat aortic smooth muscle cells for several days on a collagen-coated silicone elastomer substratum. Six-second cycles of stretch-relaxation were applied to obtain 10% average surface elongation (22% maximum) for 4 days. Control cells were not stretched but were grown on a similar surface. The effect of Gd3+, a blocker of stretch-activated channels, was also investigated. At the end of 4 days, protein expression of alpha1- and alpha2-subunits was determined by Western blot analysis. Intensity of the bands for alpha1- and alpha2-subunits was quantified with the use of a computerized image analyzer. In the stretched cells, both the alpha1- and the alpha2-subunit protein-band intensities were significantly increased compared with those of the non-stretched cells. Treatment with 50 micromol/L Gd3+ during the application of stretch prevented the upregulation of alpha2-expression but not that of alpha1-expression. Sodium pump activity, the functional counterpart of Na+,K+-ATPase, was inhibited as a result of stretch; Gd3+ had no effect on this variable. Our results suggest that in vascular smooth muscle, stretch may be a signal for the upregulation of both the alpha1- and alpha2-isoforms. However, a differential response of the two isoforms to the blocker of stretch-activated channels implies involvement of different mechanisms. This alteration in protein expression is not reflected in the function of the enzyme.

Renal Na+/H+ exchanger isoforms and their regulation by thyroid hormone

Na+ crosses the luminal membrane of the proximal tubule primarily via Na+/H+ exchange (NHE), and NHE activity is influenced by thyroid status. Pharmacological, immunological, and kinetic studies indicate multiple isoforms of NHE, and four full-length cDNAs have been cloned to date. The aims of this study were to determine which NHE mRNAs (NHE1, -2, -3, and -4) were expressed in the rat proximal tubule, the relative abundance of each in the renal cortex, and the effect of thyroid status on their expression. By blot hybridization of poly(A)+ RNA, all NHE isoform mRNAs were detected in the rat renal cortex; NHE1, -2, and -3 in the proximal tubule; and NHE1 and -3 in LLC-PK1 cells. NHE3 mRNA abundance was fourfold higher than the other three isoforms in renal cortex. The effect of thyroid status was assessed in renal cortex from euthyroid, hypothyroid, and hyperthyroid rats. Although none of the NHE mRNA levels was altered in the transition from euthyroid to hypothyroid states, both NHE2 and NHE3 mRNA levels increased 1.5-fold in the transition from hypo- to hyperthyroidism. NHE3 protein, measured by immunoblot with the use of an NHE3-specific antibody, was detected at 83-85 kDa in renal cortex and codistributed on sorbitol gradients with the brush-border marker alkaline phosphatase. No significant difference in NHE3 protein abundance was detected between hypothyroid and hyperthyroid rats. In conclusion, in the renal cortex, the NHE3 isoform predominates at the mRNA level, is expressed in apical membranes, and increases at the mRNA but not the protein levels in response to thyroid hormone treatment, suggesting parallel changes in synthesis and turnover of NHE3 by thyroid hormone.

Thyroid hormone stimulation of Na/Pi-cotransport in opossum kidney cells

Thyroid hormone (T3), a known stimulator of renal proximal tubular brush border membrane Na-dependent phosphate (Pi) uptake (Na/Pi-cotransport), stimulated Na-dependent Pi transport in opossum kidney (OK) cells. Na/Pi-cotransport was stimulated in a time- and dose-dependent manner with maximal effects (57%) at 24 h and 10(-10) M T3. This stimulation was related to an increase in the apparent capacity (Vmax) of Na/Pi-cotransport. Treatment with T3 had no effect on Na-independent transport of Pi or of L-arginine. The stimulation of Na/Pi-cotransport was paralleled by an increase in the messenger ribonucleic acid (mRNA) encoding the OK cell apical Na/Pi-cotransporter (termed NaPi-4); the mRNA levels related to the activity of Na-independent L-arginine transport (rBAT) were unaffected by T3. Actinomycin D (10(-7) M) completely prevented the stimulatory effect of T3 on OK cell Na/Pi-cotransport and on NaPi-4 mRNA content. In conclusion, T3 stimulates apical Na/Pi-cotransport in OK cells most likely by enhancing its transcription.

Decrease in beta 1-adrenergic and M2-muscarinic receptor mRNA levels and unchanged accumulation of mRNAs coding for G alpha i-2 and G alpha s proteins in rat cardiac hypertrophy

During compensatory cardiac hypertrophy in the rat, hemodynamic overload induces a parallel decrease in the densities of both beta 1-adrenergic (beta 1-AR) and M2-muscarinic (M2-MR) receptors in the left ventricle, but the total number of receptors remains unchanged. It is not known whether this reduction is transcriptionally or translationally regulated, or if the functionally closely linked alpha-subunits of G protein (G alpha s and G alpha i-2) partake in this regulation. In order to resolve these questions, the absolute concentrations of mRNAs for both receptors and for G alpha s and G alpha i-2 were quantified by slot blot analysis of the left ventricles of adult rats 5 weeks after aortic banding. The results showed a significant decrease of both receptor mRNA levels in hypertrophied left ventricle (beta 1-AR: -48%; M2-MR: -42%) that paralleled the reduction in receptor protein densities and was negatively correlated with the left ventricular weight/body weight ratio (LVW/BW). By contrast, the relative levels of G alpha s and G alpha i-2 mRNAs remained unchanged, and both accumulated proportionally to the increase in LVW/BW. These results show that the beta 1-AR and the M2-MR were pretranslationally regulated. This suggests the hypothesis that the corresponding genes do not follow the general increase in transcriptional activity. By contrast, the genes coding for G alpha s and G alpha i-2 may follow the general pattern of activation during hypertrophy. Receptors and coupling proteins belong to two different groups of genes that are controlled by distinct mechanisms of regulation.

Reciprocal regulation of cardiac Na-K-ATPase and Na/Ca exchanger: hypertension, thyroid hormone, development

Inhibiting cardiac Na pump activity decreases the driving force for the Na/Ca exchanger transport that increases cellular Ca stores and contractility. Decreased abundance of Na pumps would be expected to have the same effect as decreased activity unless there was reciprocal regulation of Na/Ca exchanger expression to oppose the effects of depressed Na pump activity on intracellular Ca stores. The aim of this study was to test the hypothesis that cardiac Na/Ca exchanger abundance is regulated in a reciprocal fashion to Na-K-ATPase abundance in a number of models known to have altered Na-K-ATPase abundance. In renovascular hypertension, cardiac ventricular Na-K-ATPase alpha 1-levels are unaltered, alpha 2-isoform subunit mRNA and protein levels decrease to 0.76 +/- 0.06 and 0.56 +/- 0.07 of control, respectively, and the Na/Ca exchanger protein (not mRNA) increased 1.35 +/- 0.11-fold. In the transition from hypothyroid to hyperthyroid cardiac alpha 1 doubles, alpha 2-protein increases 8.83 +/- 1.06-fold, and the Na/Ca exchanger protein decreases to 0.64 +/- 0.11. A similar pattern was seen during cardiac development in the preweaning rat heart. Treatment with the antiarrhythymic amiodarone has no effect on alpha 1, decreases alpha 2-protein expression to 0.51 +/- 0.08 of control, and increases exchanger expression 1.42 +/- 0.16-fold. In conclusion, the reciprocal regulation of the Na/Ca exchanger and of Na-K-ATPase alpha 2-expression provides evidence for a homeostatic mechanism that would oppose the changes in cellular Ca stores driven by the changes in Na-K-ATPase activity.

Regulation of Na+,K(+)-ATPase alpha-subunit isoforms in rat tissues during hypertension

We investigated the regulation of the protein expression of the alpha isozymes of Na+,K(+)-ATPase in reference to the enzyme activity in the heart, brain and skeletal muscle of rats during deoxycorticosterone acetate (DOCA)-salt hypertension. Treatment of rats with DOCA and salt for 28 days produced a significant increase in systolic blood pressure compared to the control groups which remained normotensive. Rats treated with DOCA expressed greater amounts of the immunoreactive alpha-1 isoform than untreated controls in whole heart membranes. However, the DOCA-induced increase in the alpha-1 isoform did not occur during DOCA-salt hypertension. There was a parallel change in the enzyme activity of the Na+,K(+)-ATPase and the protein expression of the alpha-1 isoform as a result of these treatments. We have also demonstrated that the hearts of DOCA-salt hypertensive rats expressed less of the alpha-2 isoform compared to the controls. We could not detect any alteration in the alpha-1 and alpha-2 isoforms of the skeletal muscle and alpha-1, alpha-2 and alpha-3 isoforms of the whole brain Na+,K(+)-ATPase during salt or DOCA treatments alone or DOCA-salt hypertension. Furthermore, the Na+,K(+)-ATPase activity was unaltered in these tissues during these treatments. In conclusion, cardiac Na+,K(+)-ATPase alpha-subunit protein expression appears to be regulated during DOCA-salt hypertension. In the skeletal muscle and brain, tissues not subjected directly to increased pressure, this regulation of the Na+,K(+)-ATPase was not apparent.

Na,K-ATPase beta subunit isoform expression in the peripheral nervous system of the rat

Using the RNase protection assay (RPA) to study the distribution of isoforms of the non-catalytic (beta) subunit of Na,K-ATPase in the peripheral nervous system, we found both beta 1 and beta 2 isoform mRNAs in dorsal root ganglion (DRG), but only beta 2 mRNA in sciatic nerve. Using Western blot to measure accumulation of the polypeptides at a ligature on the nerve we found that beta 1 but not beta 2 polypeptide is carried by rapid axonal transport in the sciatic nerve. These results imply that beta 1 is the prominent isoform of Na,K-ATPase in neurons and beta 2 the prominent isoform in Schwann cells.

Developmental cell-specific regulation of Na(+)-K(+)-ATPase alpha 1-, alpha 2-, and alpha 3-isoform gene expression

Na(+)-K(+)-activated adenosine triphosphatase (Na(+)-K(+)-ATPase) is the integral membrane protein that maintains the Na(+)-K(+) electrochemical gradient across the plasma membrane. Because of the importance of the Na(+)-K(+) electrochemical gradient to fundamental and specialized cell functions, we investigated the cell-specific modulation of Na(+)-K(+)-ATPase alpha-subunit isoform (alpha 1, alpha 2, and alpha 3) gene expression in different stages of postimplantation mouse embryos and neonatal rat tissues by in situ hybridization with use of isoform-specific rat-derived antisense RNA probes. At early organogenesis (9.5-10.5 days postcoitus), we demonstrated generalized coexpression of alpha 1- and alpha 2-isoforms throughout the mouse embryo with greater levels in the developing but already functional heart, in contrast to the distinct spatially restricted alpha 3-isoform gene expression in the early developing neural tube. At midorganogenesis (15.5-16.5 days postcoitus), differential spatial variation in alpha 1-, alpha 2-, and alpha 3-isoform gene expression was already evident in all organs. Interestingly, region-specific expression patterns within single cell types were noted throughout development and were exemplified by 1) alpha 3-isoform gene expression in marginal cells of the 10.5-day-postcoitus developing neural tube; 2) alpha 1-, alpha 2-, and alpha 3-isoform gene expression in cerebellar granular cells of the 4-day-old rat brain; and 3) alpha 1- and alpha 3-isoform gene expression in 4-day-old rat ventricular cardiomyocytes. These isoform-specific changes in cellular and regional Na(+)-K(+)-ATPase alpha-isoform gene expression may play an active role in development and specialized cell functions.

Thyroid hormones and the creatine kinase system in cardiac cells

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

Isoform-specific regulation of myocardial Na,K-ATPase alpha-subunit in congestive heart failure. Role of norepinephrine

BACKGROUND

Myocardial ouabain-binding sites and Na,K-ATPase activity are reduced in congestive heart failure (CHF), but the mechanisms by which CHF reduces the Na,K-ATPase remain unknown. We proposed to investigate whether the changes are accompanied by isoform-specific reductions of the Na,K-ATPase alpha-subunit proteins in CHF and whether similar changes could be produced by exogenous norepinephrine administration.

METHODS AND RESULTS

CHF was induced in dogs by rapid ventricular pacing at a rate of 225 beats per minute for 8 weeks (protocol 1). A second group of dogs were paced at 100 beats per minute and served as controls. In protocol 2, norepinephrine was infused in normal dogs using a subcutaneous osmotic minipump for 8 weeks. The control dogs received normal saline through the pump. Animals were studied after 8 weeks of pacing or norepinephrine infusion. After the baseline hemodynamics and interstitial norepinephrine concentration had been obtained, the hearts were removed for measuring [3H]ouabain-binding sites and Na,K-ATPase alpha-subunit proteins using isoform-specific monoclonal antibodies.

RESULTS

Myocardial [3H]ouabain-binding sites were reduced in dogs with CHF and chronic norepinephrine infusion. The Western blot analysis showed that adult canine hearts possess both alpha 1 and alpha 3 isoforms of the Na,K-ATPase alpha-subunit but not the alpha 2 isoform protein. CHF and NE infusion had no effect on the Na,K-ATPase alpha 1-subunit protein but did reduce the alpha 3 isoform protein significantly. In addition, there was a significant inverse correlation between the amount of myocardial alpha 3 isoform protein and interstitial norepinephrine content in the dogs. In contrast, the specific activity of the sarcolemmal marker 5'-nucleotidase did not differ among the groups of animals.

CONCLUSIONS

The reduction of myocardial Na,K-ATPase in CHF is limited to the alpha 3 isoform. Furthermore, because similar changes in myocardial ouabain-binding sites and Na,K-ATPase alpha 3 isoform were produced by chronic norepinephrine infusion, the decrease in the Na,K-ATPase in CHF is most likely mediated via excess sympathetic stimulation.

Surplus Na+ pumps: how low-K(+)-incubated LLC-PK1 cells respond to K+ restoration

We have previously shown that a pig kidney cell line (LLC-PK1/Cl4) responds to chronic exposure to 0.25 mM extracellular K+ by increasing the beta-, not alpha-, subunit mRNA levels and both alpha- and beta-abundance twofold over control. Our objective in the present study was to determine how the LLC-PK1/Cl4 cells respond when returned to control (5.5 mM) medium. A 1.8-fold increase in ouabain binding established that the induced pumps were expressed at the cell surface following 24-h incubation in low K+. On restoration to 5.5 mM K+, intracellular Na+ and K+ concentrations ([Na+]i and [K+]i, respectively) rapidly returned to control levels within 15 min. The doubled pool size of pumps in the chronic low K+ cells had no significant influence on the rate of ion restoration when compared with the rate in cells acutely exposed to low K+. Despite the rapid return of ions to control values, beta-mRNA levels remained elevated for 2 h, then sharply declined to control levels by 6 h of K+ restoration. From these data, we estimate that the half-life of beta-mRNA is 2-3 h during restoration. alpha-Subunit mRNA remained essentially unchanged from control after return of K+ to the medium and restoration of intracellular ions.(ABSTRACT TRUNCATED AT 250 WORDS)