Identification of two isoforms of the catalytic subunit of Na,K-ATPase in myocytes from adult rat heart

Immunostaining for the α3 isoform of the Na+/K+-ATPase is selective for functionally identified muscle spindle afferents in vivo

Muscle spindle afferent (MSA) neurons can show rapid and sustained firing. Immunostaining for the α3 isoform of the Na⁺/K⁺-ATPase (α3) in some large dorsal root ganglion (DRG) neurons and large intrafusal fibres suggested α3 expression in MSAs (Dobretsov et al. 2003), but not whether α3-immunoreactive DRG neuronal somata were exclusively MSAs. We found that neuronal somata with high α3 immunointensity were neurofilament-rich, suggesting they have A-fibres; we therefore focussed on A-fibre neurons to determine the sensory properties of α3-immunoreactive neurons. We examined α3 immunointensity in 78 dye-injected DRG neurons whose conduction velocities and hindlimb sensory receptive fields were determined in vivo. A dense perimeter or ring of staining in a subpopulation of neurons was clearly overlying the soma membrane and not within satellite cells. Neurons with clear α3 rings (n = 23) were all MSAs (types I and II); all MSAs had darkly stained α3 rings, that tended to be darker in MSA1 than MSA2 units. Of 52 non-MSA A-fibre neurons including nociceptive and cutaneous low-threshold mechanoreceptive (LTM) neurons, 50 had no discernable ring, while 2 (Aα/β cutaneous LTMs) had weakly stained rings. Three of three C-nociceptors had no rings. MSAs with strong ring immunostaining also showed the strongest cytoplasmic staining. These findings suggest that α3 ring staining is a selective marker for MSAs. The α3 isoform of the Na⁺/K⁺-ATPase has previously been shown to be activated by higher Na⁺ levels and to have greater affinity for ATP than the α1 isoform (in all DRG neurons). The high α3 levels in MSAs may enable the greater dynamic firing range in MSAs.

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.

Na(+)-K(+)-ATPase alpha(2)-isoform expression in guinea pig hearts during transition from compensation to decompensation

Disturbance in ionic gradient across sarcolemma may lead to arrhythmias. Because Na(+)-K(+)-ATPase regulates intracellular Na(+) and K(+) concentrations, and therefore intracellular Ca(2+) concentration homeostasis, our aim was to determine whether changes in the Na(+)-K(+)-ATPase alpha-isoforms in guinea pigs during transition from compensated (CLVH) to decompensated left ventricular hypertrophy (DLVH) were concomitant with arrhythmias. After 12- and 20-mo aortic stenosis, CLVH and DLVH were characterized by increased mean arterial pressure (30% and 52.7%, respectively). DLVH differed from CLVH by significantly increased end-diastolic pressure (34%), decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (-75%), and increased Na(+)/Ca(2+) exchanger (25%) mRNA levels and by the occurrence of ventricular arrhythmias. The alpha-isoform (mRNA and protein levels) was significantly lower in DLVH (2.2 +/- 0.2- and 1. 4 +/- 0.15-fold, respectively, vs. control) than in CLVH (3.5 +/- 0. 4- and 2.2 +/- 0.13-fold, respectively) and was present in sarcolemma and T tubules. Changes in the levels of alpha(1)- and alpha(3)-isoform in CLVH and DLVH appear physiologically irrelevant. We suggest that the increased level of alpha(2)-isoform in CLVH may participate in compensation, whereas its relative decrease in DLVH may enhance decompensation and arrhythmias.

Immunologic identification of Na+,K(+)-ATPase isoforms in myocardium. Isoform change in deoxycorticosterone acetate-salt hypertension

There are three isoforms of the catalytic (alpha) subunit of the Na+,K(+)-ATPase, each derived from a different gene, that differ in their sensitivity to inhibition by cardiac glycosides. Antibodies specific for the three isoforms were used to study Na+,K(+)-ATPase isoform expression in ventricular myocardium, where an understanding of digitalis receptor diversity is most important. In the rat heart, there is simultaneous expression of two isoforms in adult ventricle, and immunofluorescence studies demonstrated that both isoforms are expressed uniformly in cardiomyocytes. Hypertension and hypertrophy have been reported to selectively depress alpha 2 isoform mRNA levels, and we show in the present study that alpha 2 protein levels were correspondingly depressed in rats made hypertensive by uninephrectomy and treatment with deoxycorticosterone acetate and a high-salt diet. In the human heart, where mRNA for all three alpha isoforms has been reported, we detected all three isoform proteins (alpha 1, alpha 2, and alpha 3). Two isoforms (alpha 1 and alpha 3) predominated in the macaque heart; dissection of the heart showed uniformity of isoform expression in different ventricular regions but markedly less alpha 3 in the atrium. Finally, isoform-specific antibodies were used to detect which alpha isoforms were expressed in the ventricles of several commonly used experimental animals to test the correlation of isoform expression with cardiac glycoside-response heterogeneity. Two isoforms (alpha 1 and alpha 3) were found in canine myocardium, whereas only one (alpha 1) was found in sheep and guinea pig. Expression of Na+,K(+)-ATPase isoforms can thus be readily followed and related to the physiology of the digitalis receptor.

Turnover rates of the canine cardiac Na,K-ATPases

Two functional isoforms alpha (alpha 1) and alpha+ (alpha 3) of the Na,K-ATPase catalytic subunit coexist in canine cardiac myocytes [J. Biol. Chem. (1987) 262, 8941-8943]. The in vitro turnover rates of ATP hydrolysis have been determined in sarcolemma preparations by comparing [3H]ouabain-binding and Na,K-ATPase activity at various doses of ouabain (0.3-300 nM). The correlation between the occupancy of the ouabain-binding sites and the degree of Na,K-ATPase inhibition was not linear. The results showed that the form of low-affinity for ouabain (Kd = 300-700 nM) exhibited a lower turnover rate (88 +/- 10 vs. 147 +/- 15 molecules of ATP hydrolyzed per second per ouabain-binding site) than the high affinity form (Kd = 1-8 nM). Thus our results indicate this specific isoform kinetic difference could contribute to differences in the cardiac cellular function.

Chick heart cells with high intracellular calcium concentration have a higher affinity for cardiac glycosides than those with low intracellular calcium concentration, as revealed by affinity labelling with a digoxigenin derivative

Digital-imaging fluorescence microscopy with fura-2 allows the determination of intracellular calcium concentration ([Ca2+]i) in single cells. At a cell density of 10(5) cells/petri dish 44% of the chick embryo heart cells had a high [Ca2+]i of 99.4 +/- 7.1 nM and 56% of the cells a low [Ca2+]i of 27.8 +/- 4.4 nM (mean +/- SE). This laboratory previously reported that high-[Ca2+]i and low-[Ca2+]i cells from chick embryo hearts differ in their sensitivity to cardiac glycosides, as shown by measuring the increase in [Ca2+]i to reach a new steady state [Ahlemeyer, B., Weintraut, H., Seibold, G. & Schoner, W. (1991) in The sodium pump: recent developments (Kaplan, J. H. & De Weer, P., eds) pp. 653-656, Rockefeller University Press, New York]. This time we used N-hydroxysuccinimidyl digoxigenin-3-O-methylcarbonyl-epsilon-aminocaproate (HDMA) which binds irreversibly to amino groups of the Na+/K(+)-ATPase, and sheep anti-digoxigenin Fab fragments coupled with fluorescein isothiocyanate to identify different cardiac glycoside-binding sites. Half-maximal labelling of high-[Ca2+]i cells was obtained at 0.36 nM HDMA, and at 12.0 nM with the low-[Ca2+]i cells. Specific labelling of the cells by HDMA was 91% and 80% in high-[Ca2+]i and low-[Ca2+]i cells, respectively, as revealed by competition experiments with a 1000-fold excess of ouabain. HDMA half-maximally elevated the [Ca2+]i of high-[Ca2+]i cells at a concentration of 50 pM and that of low-[Ca2+]i cells at 8.0 nM. Concentrations higher than 0.1 microM produced signs of intoxication. When the labelled cells were subjected to a SDS/PAGE, a 100-kDa band was found to contain HDMA. The electrophoretic mobility of a protein labelled at 10 nM HDMA was slightly higher than that of a protein labelled at 1.0 microM. The data suggest that different isoforms of the alpha-subunit of Na+/K(+)-ATPase may exist in low-[Ca2+]i and high-[Ca2+]i cells of chick embryo heart.

Biological adaptation of the myocardium to a permanent change in loading conditions

Cardiac hypertrophy due to permanent mechanical overloading is only one example among thousands of the general process of biological adaptation. The process is randomly governed and results in at least one thermodynamical benefit: to be adaptational and to induce several changes in gene expression. Some of these changes are detrimental, some can even be useless. The cascade of events which finally leads to a permanent modification of the genetic expression involves an initial signal, likely to be the stretch, a pathway which transducts the signal, and a transient change in genetic expression which transmits competence to the cell to be transformed. The permanent modifications occur at all cellular levels including the sarcomere, sarcolemma, energy metabolism, and extra-cellular matrix, but they are species-specific and differ in the ventricles and the atria.

Expression of multiple Na+,K(+)-ATPase genes reveals a gradient of isoforms along the nonpigmented ciliary epithelium: functional implications in aqueous humor secretion

The Na+,K(+)-ATPase alpha 1, alpha 2, and alpha 3 subunit isoforms have been shown to be differentially expressed in the nonpigmented (NPE) and pigmented (PE) cells of the ocular ciliary epithelium (CE) (Martin-Vasallo et al., J. Cell. Physiol., 141:243-252, 1989; Ghosh et al., J. Biol. Chem., 265:2935-2940, 1990). In this study we analyzed and compared the pattern of expression of the multiple Na+,K(+)-ATPase alpha (alpha 1, alpha 2, alpha 3) subunit genes with the pattern of expression of the Na+,K(+)-ATPase beta (beta 1, beta 2) subunit genes along the bovine CE. We have selected three regions in the CE, referred to as 1) the anterior region of the pars plicata, near the iris; 2) the middle region of the pars plicata; and 3) the posterior region of the pars plana, near the ora serrata. Using isoform-specific cDNA probes and antibodies for the Na+,K(+)-ATPase alpha 1, alpha 2, alpha 3, beta 1, and beta 2 subunits on Northern and Western blot analysis, we found that mRNA and polypeptides are expressed in all three CE regions with different abundance. The pattern of expression of alpha and beta isoforms detected along the NPE cell layers suggests a gradient of alpha 1, alpha 2, alpha 3, beta 1, and beta 2 mRNAs and polypeptides that correlates with decreasing Na+,K(+)-ATPase activity from the most anterior region at the pars plicata towards the posterior region at the ora serrata. We also found marked differences in the pattern of immunolocalization of Na+,K(+)-ATPase alpha 1, alpha 2, alpha 3, beta 1, and beta 2 subunit isoforms in different regions of the CE. In the anterior region, NPE cells stained intensely at the basal lateral membrane with specific monoclonal and polyclonal antibodies for each of the alpha (alpha 1, alpha 2, alpha 3) and beta (beta 1, beta 2) Na,K-ATPase isoforms. In the middle and posterior regions of the CE, NPE cells showed lower or absent levels of staining with alpha 1, alpha 2, alpha 3, and beta 1 antibodies, although staining with beta 2 was abundant. In contrast, PE cells throughout the CE were stained at the basal lateral membrane by antibodies to alpha 1 and beta 1, while no staining signals were detected with the rest of the antibodies (i.e. alpha 2, alpha 3, and beta 2). Our results support the conclusion that the three alpha and two beta isoforms of the Na+,K(+)-ATPase are differentially expressed in the two cell layers that make up the CE.(ABSTRACT TRUNCATED AT 250 WORDS)

An organophosphorus compound, Vx, selectively inhibits the rat cardiac Na+,K(+)-ATPase alpha 1 isoform. Biochemical basis of the cardiotoxicity of Vx

Serine-specific reagents, anticholinesterase organophosphorus compounds like Vx provoke, in the micromolar range, digitalis-like ventricular arrhythmias of non-cholinergic origin in rodent hearts. The sensitivities of the two rat cardiac Na+,K(+)-ATPase isoforms (alpha 1 and alpha 2) to Vx (0.1-100 microM) were measured in sarcolemma vesicles. At 1 microM Vx, the inhibition of the total activity averaged 18% but never exceeded 75% with 100 microM. When the alpha 2 isoform activity was inhibited by 0.1 microM ouabain, alpha 1 was 35% inhibited by 1 microM Vx, i.e. a 16 +/- 4% inhibition of the total activity. The cardiac alpha 1 being related to the digitalis-induced toxicity, its selective inhibition by a micromolar dose of Vx fully accounts for the cardiotoxicity of Vx. Inasmuch as Vx had no effect on the rat kidney alpha 1, differentially inactivated the cardiac isozymes and specifically reacted with serine residues, the putative binding-site(s) of the organophosphorus compound on the Na+-K(+)-ATPase molecules has been considered.

Two functional Na+/K(+)-ATPase isoforms in the left ventricle of guinea pig heart

Guinea pig left ventricular muscle contains two distinct molecular forms of the Na+/K(+)-ATPase catalytic alpha subunit. Sarcolemmal vesicles highly enriched in Na+/K(+)-ATPase were isolated by a new procedure that yielded specific activities of 60-100 mumol Pi.h-1.mg-1. SDS/PAGE of isolated sarcolemma after reduction and alkylation of the sulfhydryl groups and identification on immunoblots with specific anti-(alpha subunit) antibodies indicated the presence of two major polypeptides of 100 kDa and 103 kDa, respectively. The two alpha subunits were functional: the dose/response curves of Na+/K(+)-ATPase activity with ouabain, dihydroouabain and digitoxigenin were biphasic, revealing the presence of high-affinity [concentration of drug causing 50% inhibition (IC50) = 10 nM] and low-affinity (IC50 = 2 microM) forms with proportional contributions of 55% and 45%, respectively. The involvement of the high-affinity form in the positive inotropic effect of digitalis and of the low-affinity sites in both inotropy and toxicity are consistent with the literature data on rodents.

The effect of micromolar Ca2+ on the activities of the different Na+/K(+)-ATPase isozymes in the rat myometrium

In the present work we show the existence of two Na+/K(+)-ATPase isozymes in rat myometrial microsomes and suggest that they have different Ca2+ sensitivities. The catalytic subunits (alpha 1, alpha 2) of Na+/K(+)-ATPase were labelled by fluorescein-isothiocyanate and separated by SDS gel electrophoresis. The two isozyme Ca2(+)-sensitivities were studied by comparing the kinetics of Ca2+, strophantidin, ouabain and N-ethylmaleimide inhibitions. Our results indicate that the activity of the high ouabain-sensitive part (alpha 2 type) of Na+/K(+)-ATPase enzyme could only be inhibited by micromolar Ca2+. Furthermore, treatment of the microsomal preparation with 1mM N-ethylmaleimide selectively inactivated the high Ca2+ sensitive isoform of myometrial Na+/K(+)-ATPase.

Thyroid hormone regulation of Na,K-ATPase subunit-mRNA expression in neonatal rat myocardium

Regulation of Na,K-ATPase mRNA alpha isoform and mRNA beta expression by thyroid hormone (T3) in neonatal rat myocardium was examined. In euthyroid neonates between ages of 2 and 5 days, mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 abundances were nearly constant while mRNA alpha 2 was undetectable. During the interval between postnatal days 5 and 15, mRNA alpha 3 decreased to negligible levels and mRNA alpha 2 became expressed and increased in abundance to account for approximately 20% of the mRNA alpha pool by the 15th postnatal day. To examine the effect of T3 on this developmental program, neonates were injected with 75 micrograms T3/100 g body weight or diluent alone on the second and third postnatal days and myocardial Na,K-ATPase subunit-mRNA abundances were determined on the third and fourth postnatal days. Because T3 treatment increased the RNA/DNA ratios of myocardial tissue, the subunit-mRNA abundances were normalized per unit DNA. Following 24 and 48 hr of T3 treatment, the abundances of mRNA alpha 1, mRNA alpha 3, and mRNA beta 1 increased, while mRNA alpha 2 continued to remain undetectable during the 2-day interval between the second to fourth postnatal days. It is concluded that T3 augments the abundance of Na,K-ATPase subunit mRNAs that are already being expressed in the neonatal rat myocardium. The results further suggest that T3 does not act as a "molecular switch" in the developmental expression of the mRNA alpha isoforms in rat myocardium during the first four postnatal days.

Tissue localization of active Na,K-ATPase isoenzymes by determination of their profile of inhibition with ouabain, digoxin, digitoxigenin and LND 796, a new aminosteroid cardiotonic

Recently, Na,K-ATPase isoforms with differential affinities for digitalis have been identified that may contribute to different toxicity profiles. Our objectives were to localize them and to define tissue receptor patterns by examining the effect of different glycosides on the Na,K-ATPase activity. The digitalis derivatives used exhibit variation in lipophilicity and rate of enzyme inhibition. Membrane fractions enriched in Na,K-ATPase were prepared from canine heart, brain, aorta and peripheral nerves. The inhibition of enzyme activities indicates a pattern of differential sensitivities with IC50 values starting from 3 nM in heart and 30 nM in brain. Therefore, high and low affinity active forms of the Na,K-ATPase enzyme coexist in these tissues. The data also suggest the existence of two Na,K-ATPase isoforms in aorta and peripheral nerves as identified by the action of digitoxigenin and LND 796 where the predominant expression is that of a high affinity form. The comparison of the patterns of digitalis sensitivities in these different tissues, suggests a more complex molecular interaction than that which can be explained by the presence of only two forms.

Anomalies in the electrophoretic resolution of Na+/K(+)-ATPase catalytic subunit isoforms reveal unusual protein--detergent interactions

Three different isozymes of the Na+/K(+)-ATPase have slightly different different electrophoretic mobilities in sodium dodecyl sulfate (SDS). Certain procedures (reduction and alkylation, heating, and the use of sodium tetradecyl sulfate) have been reported either to improve the electrophoretic separation of isoforms or to reveal the presence of new isoforms. The variables affecting gel electrophoretic mobility were investigated here. Reduction and alkylation decreased the mobility of all three isozymes, and slightly improved the separation of alpha 1 from alpha 2 and alpha 3 without causing a qualitative change in the alpha isoforms detected. Heating the enzyme in SDS caused splitting into two bands. Both bands were intact polypeptides but migrated differently in 5% and 15% polyacrylamide, disclosing an anomalous conformation in detergent. The use of sodium tetradecyl or decyl sulfate instead of dodecyl sulfate altered the relative mobilities of the isozymes, revealing differences in detergent affinity, but no new isoforms were found. In conclusion, Na+/K(+)-ATPase alpha-subunit mobility reflects complex detergent-protein interaction that can be affected by experimental conditions. The existence of more than one band on gels may reflect different conformations in detergent, but should not be accepted alone as evidence for subunit structural heterogeneity.

Inotropic effect of ouabain in hypertrophied rat heart

The contribution of the heterogeneous digitalis receptors to the inotropic effect of ouabain was studied in hypertrophied rat hearts (aortic stenosis) by using isolated Langendorff heart preparations. Development and washout of the biological effects as well as the dose/response curves revealed two inotropic components of high and low drug sensitivity. Maximal inotropy was observed with 100 microM ouabain in both control and hypertrophied rat hearts. The high-sensitivity component accounted for only one-third of the response in control hearts but for two-thirds in hypertrophied hearts. The respective apparent affinities (10-20 nM and 10-20 microM) of the two inotropic components found in isolated hearts were similar to those of the high- and low-affinity Na+,K(+)-ATPase activities detected in isolated cardiac sarcolemma. Onset and reversion of the pharmacological effects of ouabain occurred at respective rates that were similar to those of the association and dissociation of ouabain to the Na+,K(+)-ATPase level. In hypertrophied heart, the high- and low-sensitivity components (or receptors) reacted seven- and threefold, respectively, more slowly than the corresponding sites in normal hearts. These alterations in inotropic responsiveness and properties of the digitalis receptors in cardiac hypertrophy suggest that new regulations should be taken into account in the adaptation to pressure overload.

Quantification of the total Na,K-ATPase concentration in atria and ventricles from mammalian species by measuring 3H-ouabain binding to intact myocardial samples. Stability to short term ischemia reperfusion

Na,K-ATPase concentration was measured by vanadate facilitated 3H-ouabain binding to intact samples taken from various parts of porcine and canine myocardium. In porcine and canine heart 3H-ouabain binding site concentration in ventricles was 1.4-2.5 times larger than in atria. Evaluation of 3H-ouabain binding kinetics revealed no major difference between atria and ventricles: Equilibrium was obtained after the same incubation time in right atrium (RA) as in left ventricle (LV), both in porcine and canine heart. Unspecific uptake and retention of 3H-ouabain was for porcine heart RA and LV 1.5 and 1.4, respectively, and for canine heart RA and LV, both 1.2% filling (i.e., volume (ml) of incubation medium 3H-radioactivity taken up per mass unit (g wet wt.) of tissue multiplied by 100). The apparent dissociation constant (KD) was 1.4 x 10(-8) and 1.9 x 10(-8) in porcine RA and LV and 2.6 x 10(-8) and 6.1 x 10(-8) mol/l in canine RA and LV. Loss of specifically bound 3H-ouabain during the washout procedure occurred with a half-life time (T1/2) of 16.7 and 28.6 in RA and LV of porcine heart and 91.2 and 151.6 h in RA and LV of canine heart. Duly corrected for these errors of the method--factor 1.16 and 1.13, respectively, for porcine RA and LV, and factor 1.11 and 1.13 for canine RA and LV, total 3H-ouabain binding site concentration was found to be 553 +/- 74 and 1037 +/- 45 pmol/g wet wt. (means +/- SEM, n = 5) in porcine RA and LV, and 569 +/- 37 and 1410 +/- 40 pmol/g wet wt. (means +/- SEM, n = 5) in the canine RA and LV. These values were confirmed by measurements of 3H-digoxin binding to the porcine heart. The present quantification of myocardial Na,K-ATPase gives values up to 154 times higher than measurements based upon Na,K-ATPase activities in membrane fractions where the recovery of Na,K-ATPase may be less than 1% due to loss during purification. A higher Na,K-ATPase concentration is found in small animals than in large animals. A relationship between higher concentration of Na,K-ATPase and larger pressure work in ventricles compared to atria is suggested. Myocardial 3H-ouabain binding sites were found to be stable for 20 min of ischemia, followed by 1 h of reperfusion, supporting the concept that myocyte injury induced by short term ischemia may be reversible and that reperfusion may result in normalization.

Thyroidal enhancement of rat myocardial Na,K-ATPase: preferential expression of alpha 2 activity and mRNA abundance

In hypothyroid rat myocardium, the low-ouabain-sensitivity Na,K-ATPase activity had a KI = 10(-4) M and accounted for approximately 95% of the enzyme activity, while the high-ouabain-sensitivity activity contributed approximately 5% to the total activity, with a KI = 3 x 10(-7) M. mRNA alpha 1 was 7.2- and 5.5-fold more abundant than mRNA alpha 2 and mRNA beta, respectively, in hypothyroid ventricles while mRNA alpha 3 was undetectable. Administration of T3 increased total Na,K-ATPase activity 1.6-fold; the low-ouabain-sensitivity activity increased 1.5-fold while high-ouabain-sensitivity activity was stimulated 3.2-fold. T3 increased the number of high-affinity ouabain-binding sites 2.9-fold with no change in Kd (approximately 2 x 10(-7) M). The abundances of mRNA alpha 1, mRNA alpha 2, and mRNA beta (per unit RNA) following T3 treatment increased 3.6-, 10.6-, and 12.7-fold, respectively. The larger increments in subunit mRNA abundances than in Na,K-ATPase activity suggests the involvement of translational and/or post-translational regulatory steps in Na,K-ATPase biogenesis in response to T3. It is concluded that T3 enhances myocardial Na,K-ATPase subunit mRNA abundances and Na,K-ATPase activity, and that the expression of the high- and low-ouabain-sensitivity activities are probably a reflection of the abundances of the alpha 2 and alpha 1 isoforms, respectively. The physiological role played by the beta subunit remains uncertain.

Two active Na+/K+-ATPases of high affinity for ouabain in adult rat brain membranes

The degree of heterogeneity of active Na+/K(+)-ATPases has been investigated in terms of ouabain sensitivity. A mathematical analysis of the dose-response curves (inhibition of Na+/K(+)-ATPase) at equilibrium is consistent with the putative existence of three inhibitory states for ouabain two of high (very high plus high) and one of low affinity. The computed IC50 values are: 23.0 +/- 0.15 nM, 460 +/- 4.0 nM and 320 +/- 4.6 microM, respectively. The relative abundance of the three inhibitory states was estimated as: 39%, 36% and 20%, respectively. Direct measurements of [3H]ouabain-binding at equilibrium carried out on membrane preparations with ATP, Mg2+ and Na+ also revealed two distinct high affinity-binding sites, the apparent Kd values of which were 17.0 +/- 0.2 nM (very high) and 80 +/- 1 nM (high), respectively. Dissociation processes were studied at different ouabain concentrations according to both reversal of enzyme inhibition and [3H]ouabain release. The reversal of enzyme inhibition occurred at three different rates, depending upon the ouabain doses used (10 nM, 2 and 100 microM). When the high-affinity sites were involved (ouabain doses lower than 2 microM) the dissociation process was biphasic. A similar biphasic pattern was also detected by [3H]ouabain-release. The time-course of [3H]ouabain dissociation (0.1 microM) was also biphasic. These data indicate that the three catalytic subunits of rat brain Na+/K(+)-ATPase alpha 1, alpha 2 and alpha 3 (Hsu, Y.-M. and Guidotti, G. (1989) Biochemistry 28, 569-573) are able to hydrolyse ATP and exhibit different affinities for cardiac glycosides.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Determination of total (Na+ + K+)-ATPase activity of isolated or cultured cells

The aim of this work was to determine if the total (Na+ + K+)-ATPase of the plasma membrane of a cell population could be assayed without cell homogenization and partial purification of the enzyme. Several types of intact cells that were placed in an assay medium containing MgATP, Na+, and K+ hydrolyzed little or none of the added ATP. When the cells were pretreated with the ionophore alamethicin and then placed in the assay medium, they exhibited an ouabain-sensitive (Na+ + K+)-ATPase activity that increased and reached a limiting value with increasing alamethicin concentration. Since alamethicin did not increase the activity of the purified membrane-bound (Na+ + K+)-ATPase, its effects on the intact cells are probably due to the formation of large channels within the plasma membrane that allow the free access of the components of the assay medium to the intracellular domains of (Na+ + K+)-ATPase. Utilizing whole cells treated with alamethicin, total (Na+ + K+)-ATPase activity was determined in clonal pheochromocytoma cells (PC12), neuroblastoma x glioma hybrid cells (NG108-15), and myocytes isolated from adult and neonatal rat hearts. With the use of this whole-cell assay, the ouabain sensitivities of the enzymes in adult and neonatal rat heart myocytes were determined and found to be the same as those that have been determined with the use of partially purified enzymes.

Expression of Na,K-ATPase alpha subunit isoforms in the human ciliary body and cultured ciliary epithelial cells

We have analyzed the expression of Na,K-ATPase alpha subunit isoforms in the transporting ciliary processes of the human eye and in cultured cells derived from non-pigmented (NPE) and pigmented (PE) ciliary epithelium. Northern hybridization analysis shows that the mRNAs encoding all the three distinct forms of Na,K-ATPase alpha subunit [alpha 1, alpha 2, and alpha 3] are expressed in the human ciliary processes in vivo. Immunohistochemical analysis using antibodies specific for each of the three alpha subunit isoforms confirms that these polypeptides are present in the microsomal fraction from the human ciliary processes. The monoclonal antibody McB2, which is specific to the Na,K-ATPase alpha 2 subunit isoform, has been found to decorate specifically the basolateral membrane domains of NPE cells but not of the PE cells, suggesting its expression in vivo only in the ocular NPE ciliary epithelium. However, cultured cells derived from the NPE and PE layers exhibit a different pattern of expression of mRNA and protein for the Na,K-ATPase alpha subunit isoforms when compared to the tissue. Both the NPE and PE cells express alpha 1 and alpha 3 mRNA and polypeptide, whereas alpha 2 mRNA and polypeptide are undetectable in these cells. The established cell lines derived from the NPE layer express comparable levels of the alpha 1 and alpha 3 isoforms of Na,K-ATPase as detected in the primary culture. However, the established NPE cell lines are also distinguishable from the normal PE cells when analyzed by Western blot analysis with A x 2 antibodies. The results presented here clearly show that the NPE and PE cells in the ciliary body have a distinct expression of Na,K-ATPase alpha subunit isoforms as compared to cultured cells.

Cell-free synthesis of rat and rabbit gastric proton pump

Ribonucleic acid was isolated from the fundic gastric mucosae of rats and rabbits by cesium chloride centrifugation of guanidine isothiocyanate-denatured mucosal homogenates, and poly A+ RNA was recovered from the pellets by oligodeoxythymidine column selection. When added to rabbit reticulocyte lysates, this poly A+ RNA stimulated [35S]methionine incorporation into trichloroacetic acid-precipitable material. Fluorographic analysis of the lysates showed protein synthesis to be dominated by polypeptides with molecular weights from 40,000 to 50,000, presumably prepepsinogen isoforms. Immune precipitation of the lysates with monoclonal antibodies directed against the gastric H+,K+-adenosine triphosphatase yielded bands at 94 kilodaltons and more diffuse banding at 180 kilodaltons. Further purification of the poly A+ RNA on sucrose gradients eliminated prepepsinogen messenger RNA; nascent H+,K+-adenosine triphosphatase synthesized by purified messenger RNA consisted of polypeptides with molecular weights between 88,000 and 94,000. The study indicates that cell-free translation of gastric mucosal messenger RNA may provide a useful model for analysis of gastric H+,K+-adenosine triphosphatase biosynthesis and processing.

Diminished toxicity of ouabain in the hypertrophied rat heart

The responsiveness to ouabain of hypertrophied rat hearts has been investigated either in vivo using an isolated Langendorff rat heart perfused at various external calcium concentrations, or in vitro on purified sarcolemma vesicles. (i) The physiological study shows that at 0.25 mM CaCl2, the positive inotropic effect of 10(-5) M ouabain was diminished in hypertrophied hearts (p less than 0.02). At 0.5 mM CaCl2, the drug has no effect in controls, but it has a slight positive inotropic effect in hypertrophied hearts. At 2.50 mM CaCl2, ouabain has a negative inotropic effect accompanied by extrasystoles in controls, but in hypertrophied hearts it still has a positive inotropic effect and is not arrhythmogenic. (ii) After the pretreatment of the hearts with 2.5 mM CaCl2, the responsiveness of the (Na+, K+)-ATPase activity to ouabain was studied: the sarcolemma from hypertrophied heart contains half as many low affinity forms of (Na+, K+)-ATPase for ouabain (35% +/- 6) than in controls (80% +/- 2). Assuming that the low affinity forms are responsible for the toxic effect, these data correlate well with some of the physiological findings and suggest that the diminished toxicity for ouabain in hypertrophied hearts rather reflects a modification of the properties of the (Na+, K+)-ATPases than a change in the myocardial calcium metabolism.

The search for the endogenous digitalis: an alternative hypothesis

The universal presence of a binding site for cardiac glycosides on Na+-K+-ATPase has engendered speculation as to whether it also serves as a receptor for an endogenous digitalis-like hormone or autacoid. If such a hormone were to exist, it could play a role in sodium homeostasis and in the pathophysiology of primary hypertension and uremia. However, we believe that this hypothesis rests on unproven assumptions. Although typical of many toxins and drugs, binding to a single protein that acts as both its receptor and effector mechanism at the cell membrane, thereby directly affecting transmembrane ion flux, would be unusual for a hormone or autacoid. As an alternative hypothesis for the evolutionary conservation of the cardiac glycoside binding site, we suggest that its endogenous ligand may exist within the cell. After cotranslational insertion of the alpha- and beta-subunits into the membrane of the rough endoplasmic reticulum, Na+-K+-ATPase, like most integral membrane proteins, 1) must be targeted through a complex network of intracellular organelles to the correct plasmalemmal domain, 2) must be monitored for appropriate protein conformation and subunit assembly, and perhaps 3) could have its catalytic function regulated before insertion in the cell membrane. Because the lumina of the endoplasmic reticulum, Golgi, and other organelles and vesicles are topologically equivalent to the outside of the cell, all three functions could be subserved by an intraorganellar ligand for the cardiac glycoside binding site.

Na,K-ATPase: comparison of the cellular localization of alpha-subunit mRNA and polypeptide in mouse cerebellum, retina, and kidney

A clone encoding mouse brain Na,K-ATPase alpha-subunit was isolated from a mouse brain lambda gt11 cDNA library by using antisera to mouse and bovine brain alpha-subunit. A comparison of the nucleotide sequence of this clone with published sequences of rat brain alpha-subunit isoform clones showed it to be most similar to rat brain alpha 1. An RNA antisense probe prepared from the cDNA insert of the mouse clone detected a single mRNA of approximately 4.5 kb in Northern blots of mouse brain and kidney RNAs. This probe hybridized only to an alpha 1-cDNA insert from rat brain under high stringency conditions on Northern blots. The RNA antisense probe was used for in situ hybridization to sections of mouse kidney, cerebellum, and retina, and the cellular distribution of alpha-subunit mRNA (alpha-mRNA) was compared with that of alpha-subunit polypeptide (alpha-subunit) detected by immunofluorescence in similar sections. In kidney, alpha-mRNA distribution closely paralleled that of the polypeptide with abundant expression in ascending thick limbs and cortical distal tubules and weaker labeling in cortical proximal tubules. The co-distribution of alpha-mRNA and polypeptide in kidney where Na,K-ATPase localization is well established is consistent with the specificity of these probes. In the retina, prominent labeling with both probes was seen in photoreceptor inner segments, inner nuclear layer, and ganglion cell bodies. Plexiform layers and optic fibers expressed abundant alpha-subunit but little mRNA. Light labeling for both was seen in the outer nuclear layer. In cerebellum, alpha-mRNA and alpha-subunit were associated with soma of granule cells, basket cells, and stellate cells. Glomeruli and basket terminals contained abundant alpha-subunit but exhibited little reactivity with the riboprobe. In Purkinje cell bodies, in contrast, the antibody used to identify the cDNA clone did not resolve significant polypeptide in the somal plasmalemma despite abundant somal mRNA expression. Comparison of distribution of the two probes in cerebellum and retina indicates that message accumulation is primarily in cell bodies, while alpha-subunit epitopes may be co-expressed in cell bodies and/or transported to distant sites in cell-specific patterns.

Purification of cardiac (Na+,K+)-activated adenosine triphosphatase from rat

A procedure is described for preparation of highly active (Na+,K+)-ATPase from rat heart which has a specific activity of 200-600 mumol Pi/mg/h. The procedure is simple and can be applied to small amounts of heart muscle (approximately 1 g). The ATPase activity was more than 90% sensitive to ouabain (at concentrations up to 1 mM). The ouabain sensitivity is biphasic with about 20% of the ATPase activity being inhibited at approximately 3 X 10(-7) M ouabain.