Ouabain binding and Na+ content in resistance vessels and skeletal muscles of spontaneously hypertensive rats and K+-depleted rats

Quantification of α-subunit isoforms of Na,K-ATPase in rat resistance vessels

AIM

Rat mesenteric resistance vessels (RV) were characterized with respect to concentration of individual alpha-subunit isoforms of Na,K-ATPase.

METHODS

Total vessel homogenates were used to avoid any loss or subfractionation of membranes. They were applied to sodium dodecyl sulphate gels and, for calibration, in parallel lanes were run purified rat Na,K-ATPase preparations with known isoform distribution and content. The capacity per mg protein for Na+-dependent 32P-phosphorylation of Na,K-ATPase isolated from rat kidney was used for alpha1 calibration and that for high-affinity (3H)ouabain binding of Na,K-ATPase isolated from rat brain was used for (alpha2 + alpha3) calibration. Western blots containing homogenate proteins and reference enzyme were incubated with isoform-specific antibodies and radiolabelled secondary antibodies. The signals from adjacent alpha spots were used for qualitative and quantitative characterization of rat vessels.

RESULTS

A concentration of 100.7 +/- 14.4 pmol (n = 11) per g wet weight of the alpha1-isoform containing Na,K-ATPase was found in RV from 12-14-week rats. A much lower and more unreliable content of alpha2- and alpha3-isoforms was found. These ouabain-sensitive isoforms seem to represent a maximum of 5-10% each compared with the ouabain-insensitive rat alpha1-isoform.

CONCLUSIONS

The isoform pattern in RV, in which the isoform with high/intermediate Na+-affinity is the absolutely dominating one representing nearly all sodium pumps in this tissue, is very different from that seen in rat skeletal muscles. Due to the high content of the ouabain-insensitive alpha1-isoform in rat RV this species would seem a less relevant model in studies addressing a role of cardiac glycosides and putative endogenous ouabain-like factors in hypertension.

Myocardial Na,K-ATPase and digoxin therapy in human heart failure

The specific binding of digitalis glycosides to the Na,K-ATPase is used as a tool for Na,K-ATPase quantification with high accuracy and precision. In myocardial biopsies from patients with heart failure, total Na,K-ATPase concentration is decreased, and the decrease in Na,K-ATPase concentration correlates with a decrease in heart function. During digitalization, a fraction of remaining pumps are occupied by digoxin. No evidence for an endogenous digitalis-like factor of any clinical importance was obtained. It is recommended that digoxin be administered to heart failure patients who still have dyspnea after institution of mortality-reducing therapy.

Reduced concentration of myocardial Na+,K(+)-ATPase in human aortic valve disease as well as of Na+,K(+)- and Ca(2+)-ATPase in rodents with hypertrophy

Myocardial Na+,K(+)-ATPase was studied in patients with aortic valve disease, and myocardial Na+,K(+)- and Ca(2+)-ATPase were assessed in spontaneously hypertensive rats (SHR) and hereditary cardiomyopathic hamsters using methods ensuring high enzyme recovery. Na+,K(+)-ATPase was quantified by [3H]ouabain binding to intact myocardial biopsies from patients with aortic valve disease. Aortic stenosis, regurgitation and a combination hereof were compared with normal human heart and were associated with reductions of left ventricular [3H]ouabain binding site concentration (pmol/g wet weight) of 56, 46 and 60%, respectively (p < 0.01). Na+,K(+)- and Ca(2+)-ATPases were quantified by K(+)- and Ca(2+)-dependent p-nitrophenyl phosphatase (pNPPase) activity determinations in crude myocardial homogenates from SHR and hereditary cardiomyopathic hamsters. When SHR were compared to age-matched Wistar Kyoto (WKY) rats an increase in heart-body weight ratio of 75% (p < 0.001) was associated with reductions of K(+)- and Ca(2+)-dependent pNPPase activities (mumol/min/g wet weight) of 42 (p < 0.01) and 27% (p < 0.05), respectively. When hereditary cardiomyopathic hamsters were compared to age-matched Syrian hamsters an increase in heart-body weight ratio of 69% (p < 0.001) was found to be associated with reductions in K(+)- and Ca(2+)-dependent pNPPase activities of 50 (p < 0.001) and 26% (p = 0.05), respectively. The reductions in Na+,K(+)- and Ca(2+)-ATPases were selective in relation to overall protein content and were not merely the outcome of increased myocardial mass relative to Na+,K(+)- and Ca(2+)-pumps. In conclusion, myocardial hypertrophy is in patients associated with reduced Na+,K(+)-ATPase concentration and in rodents with reduced Na+,K(+)- and Ca(2+)-ATPase concentrations. This may be of importance for development of heart failure and arrhythmia in hypertrophic heart disease.

Reduction of cerebral cortical [3H]ouabain binding site (Na+,K(+)-ATPase) density in dementia as evaluated in fresh human cerebral cortical biopsies

Na+,K(+)-ATPase density in human cerebral cortex was for the first time studied by vanadate facilitated [3H]ouabain binding to intact samples. Fresh human cerebral cortical biopsies were obtained as a result of diagnostic frontal lobe biopsy from patients with normal pressure hydrocephalus (NPH) syndrome and associated dementia. For control measurements post-mortem samples were obtained from patients without clinically observed dementia. [3H]ouabain binding kinetics were evaluated: when incubating samples in 1 microM [3H]ouabain binding equilibrium was obtained after 6 h of incubation, non-specific uptake and retention amounted to only 2.3% of total uptake and retention of [3H]ouabain and release of specifically bound [3H]ouabain during washout in the cold occurred only slowly (T1/2 = 37 h). Evaluation of receptor affinity for ouabain was in agreement with a heterogeneous population of [3H]ouabain binding sites. [3H]Ouabain binding was significantly reduced after frozen storage of samples before measurements. Post-mortem degradation of cerebral [3H]ouabain binding sites occurred only slowly (T1/2 = 75 h). No significant variation in [3H]ouabain binding site density was observed between the cerebral lobes with occipital, parietal and temporal values (means +/- S.E.M., n = 5) amounting to 10281 +/- 649, 11267 +/- 1011 and 9263 +/- 615 pmol/g wet wt., respectively. [3H]Ouabain binding measured in frontal cortical samples gave values of (means +/- S.E.M., n = 5) 4274 +/- 1020 and 11397 +/- 976 pmol/g wet wt. delta % = 62; P < 0.05) in patients with dementia and controls, respectively. Human cerebral cortical capacity for active K+ uptake was around 37- and 16-fold greater than in skeletal muscular and myocardial tissue, respectively.

Quantification in crude homogenates of rat myocardial Na+, K(+)- and Ca(2+)-ATPase by K+ and Ca(2+)-dependent pNPPase. Age-dependent changes

Assays for complete quantification of Na+, K(+)- and Ca(2+)-ATPase in crude homogenates of rat ventricular myocardium by determination of K(+)- and Ca(2+)-dependent p-nitrophenyl phosphatase (pNPPase) activities were evaluated and optimized. Using these assays the total K(+)- and Ca(2+)-dependent pNPPase activities in ventricular myocardium of 11-12 week-old rats were found to be 2.98 +/- 0.10 and 0.29 +/- 0.02 mumol x min-1 x g-1 wet wt. (mean +/- SEM) (n = 5), respectively. Coefficient of variance of interindividual determinations was 7 and 12%, respectively. The total Na+, K(+)- and Ca(2+)-ATPase concentrations were estimated to 2 and 10 nmol x g-1 wet wt., respectively. Evaluation of a putative developmental variation revealed a biphasic age-related change in the rat myocardial Ca(2+)-dependent pNPPase activity with an increase from birth to around the third week of life followed by a decrease. By contrast, the K(+)-dependent pNPPase activity of the rat myocardium showed a decrease from birth to adulthood. It was excluded that the changes were simple outcome of variations in water and protein content of myocardium. Expressed per heart, the K(+)- and Ca(2+)-dependent pNPPase activity gradually increased to a plateau. The present assay for Na+, K(+)-ATPase quantification has the advantage over [3H] ouabain binding of being applicable on the ouabain-resistant rat myocardium, and is more simple and rapid than measurements of K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase) in crude tissue homogenates. Furthermore, with few modifications the pNPPase assay allows quantification of Ca(2+)-ATPase on crude myocardial homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased Na(+)-K+ pump number and decreased pump activity in soleus muscles in SHR

We have previously demonstrated electrophysiological and contractile abnormalities in soleus muscles of the spontaneously hypertensive rat (SHR). The age-related decrease in force and fatigue resistance observed in SHR muscles may be produced by alterations in sarcolemmal ion conductance and/or Na+ pump function. The experiments reported in the present paper were designed to assess the functional capacity of the Na+ pump in 6- to 8- and 24- to 28-wk-old SHR and Wistar-Kyoto (WKY) soleus muscles and to correlate pump activity with Na+ pump number and binding affinity ([3H]ouabain binding). Functional capacity was determined by measuring the change in resting membrane potential (RMP) of soleus muscle fibers in response to agents that stimulate (epinephrine and insulin) or inhibit (ouabain) the pump and by measuring maximum ouabain-suppressible 86Rb+ uptake in Na(+)-loaded muscles. Na+ pump number and affinity were quantified by determining the specific binding of [3H]ouabain in soleus muscle slices. SHR soleus muscles contain a greater number of Na+ pump sites (ouabain binding sites) than are present in age-matched WKY muscles but also experience a significant decrease in pump activity with age. SHR may upregulate pump number in response to the significantly higher intracellular Na+ concentration found in soleus muscles at all the ages examined. The apparent reduction in pump capacity with age may play a major role in the observed age-related decrease in SHR soleus force and fatigue resistance.

Quantification of rat cerebral cortex Na+,K(+)-ATPase: effect of age and potassium depletion

Na+,K(+)-ATPase concentration in rat cerebral cortex was studied by vanadate-facilitated [3H]ouabain binding to intact samples and by K(+)-dependent 3-O-methylfluorescein phosphatase activity determinations in crude homogenates. Methodological errors of both methods were evaluated. [3H]Ouabain binding to cerebral cortex obtained from 12-week-old rats measured incubating samples in buffer containing [3H]ouabain, and ouabain at a final concentration of 1 x 10(-6) mol/L gave a value of 11,351 +/- 177 (n = 5) pmol/g wet weight (mean +/- SEM) without any significant variation between the lobes. Evaluation of affinity for ouabain was in agreement with a heterogeneous population of [3H]ouabain binding sites. K(+)-dependent 3-O-methylfluorescein phosphatase activity in crude cerebral homogenates of age-matched rats was 7.24 +/- 0.14 (n = 5) mumol/min/g wet weight, corresponding to a Na+,K(+)-ATPase concentration of 12,209 +/- 236 pmol/g wet weight. It was concluded that the present methods were suitable for quantitative studies of cerebral cortex Na+,K(+)-ATPase. The concentration of rat cerebral cortex Na+,K(+)-ATPase showed approximately 10-fold increase within the first 4 weeks of life to reach a plateau of approximately 11,000-12,000 pmol/g wet weight, indicating a larger synthesis of Na+,K+ pumps than tissue mass in rat cerebral cortex during the first 4 weeks of development. K+ depletion induced by K(+)-deficient fodder for 2 weeks resulted in a slight tendency toward a reduction in K+ content (6%, p > 0.5) and Na+,K(+)-ATPase concentration (3%, p > 0.4) in cerebral cortex, whereas soleus muscle K+ content and Na+,K(+)-ATPase concentration were decreased by 30 (p < 0.02) and 32% (p < 0.001), respectively. Hence, during K+ depletion, cerebral cortex can maintain almost normal K+ homeostasis, whereas K+ as well as Na+,K+ pumps are lost from skeletal muscles.

Na,K-ATPase expression in normal and failing human left ventricle

The expression of the Na,K-ATPase was studied in both normal and failing human myocardium which was collected within 5 min of cardiac explantation in preparation for orthotopic transplantation or at the time of organ harvest. Abundance of mRNA for all three catalytic alpha subunits of the Na,K-ATPase was analyzed in samples from patients with end-stage heart failure due to either ischemic or dilated cardiomyopathy, as well as from normal controls. Vanadate facilitated 3H-ouabain binding before and after a Digibind wash was analyzed on tissue from a subset of these patients. mRNA analysis demonstrated that all three catalytic Na,K-ATPase alpha subunits were expressed in human heart and that there was no evidence for change in relative expression or abundance induced by disease. The specific digitalis receptor concentration was 760 +/- 58 and 614 +/- 47 pmol/g wet weight in the samples from normal and failing hearts, respectively (p = NS). From these studies it can be concluded that, whereas there is a tendency for a decrease in the number of ouabain receptors in heart failure, there is no significant alteration in the expression of Na,K-ATPase message or protein caused by chronic heart failure.

Beta-receptor properties in soleus muscles from spontaneously hypertensive rats

We have compared the properties of beta-adrenergic receptors in slow-twitch, oxidative skeletal muscles (soleus) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at three different ages. The investigation was based on the hypothesis that the increase in Na+ content and decrease in fatigue resistance observed previously in the soleus of SHR might be the result of a down regulation of muscle beta-receptors. Activation of beta-adrenergic receptors in skeletal muscle stimulates sarcolemmal sodium-potassium adenosine triphosphatase, which produces an efflux of Na+ and an influx of K+. Receptor down-regulation would be expected to reduce hormonal stimulation of Na+ pump activity, particularly during exercise. The results of receptor binding studies, however, and an investigation of cyclic adenosine monophosphate (cAMP) production in response to applied epinephrine indicated that there were no significant differences in receptor properties in the soleus muscles of SHR and WKY rats. Receptor number and affinity were the same in the two strains, and the rate, magnitude, and duration of the increase in cAMP in response to 10(-6) M epinephrine were also similar. beta-Adrenergic receptor down-regulation does not appear to be a generalized phenomenon in tissues of SHR, despite the appearance of other physiological changes in the tissue.

Quantitative determination of Na+-K+-ATPase and other sarcolemmal components in muscle cells

A recurring problem in the characterization of plasma membrane enzymes in tissues and cells is whether the samples tested are representative for the entire population of enzyme molecules present in the starting material. Measurements of [3H]-ouabain binding, enzyme activity, and maximum transport capacity all indicate that the concentration of Na+-K+ pumps in mammalian skeletal muscle is high (300-800 pmol/g wet wt). Studies on Na+-K+-ATPase activity in isolated sarcolemma, however, generally give little or no information on total cellular enzyme concentration. Due to the low and variable enzyme recovery (0.2-8.9%), such subcellular preparations may, therefore, give misleading data on factors regulating Na+-K+-ATPase in heart and skeletal muscle cells. As the same isolation and purification procedures are used for the study of other sarcolemmal components (lipids, hormone receptors, enzymes, and other transport systems), this inadequate recovery has general implications for statements on regulatory changes in the sarcolemmal composition of muscle cells. On the other hand, complete quantification of Na+-K+-ATPase in muscle tissue can now be achieved using simple procedures and the entire material (intact muscle fibers, biopsies, and whole homogenates). Recent studies have shown that regulatory changes in the entire population of Na+-K+ pumps in muscle can be quantified in measurements of [3H]-ouabain binding, K+-activated 3-O-methylfluorescein phosphatase activity, as well as maximum ouabain suppressible Na+-K+ transport capacity.

Ouabain binding to cultured vascular smooth muscle cells of the spontaneously hypertensive rat

The binding of ouabain and K+ to the Na+ pump were analyzed in serially passed cultured vascular smooth muscle cells (VSMCs) originating from spontaneously hypertensive (SH), Wistar-Kyoto (WKY), and American Wistar (W) rats. Our techniques have utilized analyses of displacement of [3H]ouabain by both unlabeled ouabain and K+ from specific binding sites on the VSMCs. We have found that each of the VSMC preparations from the three rat strains appeared to demonstrate one population of specific ouabain receptors (Na+ pumps); the number of Na+ pump units (mean +/- SE, expressed as 10(5) units/cell; number of observations indicated in parentheses) of both the SH and WKY rats was significantly lower than the number of Na+ pump units of W rat VSMCs [SH: 3.00 +/- 0.02 (231), WKY: 2.87 +/- 0.05 (245), and W: 3.62 +/- 0.04 (225)]; the equilibrium dissociation constant values (microM) for ouabain in VSMCs of SH and WKY rats were similar but were significantly higher than that of VSMCs derived from W rats [SH: 4.69 +/- 0.09 (231), WKY: 4.57 +/- 0.12 (245), and W: 3.69 +/- 0.17 (225)]; and among the VSMCs originating from the three rat strains, the apparent equilibrium dissociation constant value for K+ (mM) was the lowest in those of the SH rat [1.04 +/- 0.003 (143), compared with VSMCs of the WKY rat [1.54 +/- 0.006 (135)] and W rat [1.19 +/- 0.003 (136)]. Our previous studies have demonstrated increased passive Na+ and K+ transport rate constants of SH rat VSMCs compared with either W or WKY rat cells. These findings suggest the possibility of higher permeabilities of the SH cells.(ABSTRACT TRUNCATED AT 250 WORDS)