The effects of thyroid hormones on3H-ouabain binding site concentration,Na, K-contents and86Rb-efflux in rat skeletal muscle

Using a recently developed method based upon vanadate facilitated³H-ouabain binding, the total concentration of³H-ouabain binding sites was determined in biopsies of rat skeletal muscles containing varying proportions of slow-twitch fibres. In extensor digitorum longus, diaphragm, gastrocnemius and soleus muscles from mature (12-week-old) hyperthyroid rats the values obtained were respectively 2.6, 3.5, 5.1 and 9.8 times higher than those found in the same muscles from hypothyroid animals. This indicates that the effect of thyroid hormones is more pronounced on slow-twitch than on fast-twitch fibres. The changes in³H-ouabain binding site concentration with thyroid status could not be accounted for by differences in affinity or the rate of³H-ouabain binding.In young (4–5 week old) rats, where the K-content and the³H-ouabain binding site concentration in muscle had been reduced by K-depletion, T₃-pretreatment produced an even larger relative increase in the³H-ouabain binding site concentration than in age-matched controls, but no increase in K-content. Therefore, the downregulation of³H-ouabain binding sites seen during K-depletion cannot be attributed to a decreased response to thyroid hormones. In normal rats the marked stimulating effect of thyroid hormone on the synthesis of³H-ouabain binding sites was not associated with any significant change in K-content, but clearly preceded by a significant (P<0.001) rise in the efflux of⁸⁶Rb.     

The age-dependent changes in the number of3H-ouabain binding sites in mammalian skeletal muscle

The influence of age on the binding of³H-ouabain in skeletal muscle has been characterized in rats, mice and guinea pigs. Measurements performed using biopsies and intact fibers obtained from different types of rat muscles showed that from birth to the 4th week of life, the number of³H-ouabain binding sites per unit weight increases up to 5-fold, followed by almost the same relative decrease to a plateau around 250 pmol/g wet wt at an age of 22 weeks. These changes were not associated with any major alterations in apparentK _D (1.7–3.1×10^–7M) dissociation rate or heterogeneity in binding characteristics. Measurements of 3-O-methylfluorescein phosphatase activity, an enzyme activity which is closely correlated to the Na–K-ATPase activity, confirmed the³H-ouabain binding data.In mice, the number of³H-ouabain binding sites showed similar, albeit less pronounced changes with age, a maximum being reached at the 4th week of life. In guinea pigs, the number of³H-ouabain binding sites per unit weight decreased by 60% from birth to maturity.The results indicate that the early development and differentiation of individual skeletal muscles is associated with a marked increase in the number of Na–K-pumps (when expressed as pmol/muscle), until at maturity a plateau is reached. However, when expressed as pmol/g wet wt the increase is followed by a decrease to a plateau. This may in part account for the relatively low digitalis sensitivity seen in infants as compared to newborn and mature individuals.     

Relation between extracellular [K+], membrane potential and contraction in rat soleus muscle: modulation by the Na+-K+ pump

An increased extracellular K⁺ concentration ([K⁺]₀) is thought to cause muscle fatigue. We studied the effects of increasing [K⁺]₀ from 4 mM to 8–14 mM on tetanic contractions in isolated bundles of fibres and whole soleus muscles from the rat. Whereas there was little depression of force at a [K⁺]₀ of 8–9 mM, a further small increase in [K⁺]₀ to 11–14 mM resulted in a large reduction of force. Tetanus depression at 11 mM [K⁺]_o was increased when using weaker stimulation pulses and decreased with stronger pulses. Whereas the tetanic force/resting membrane potential (E _M) relation showed only moderate force depression with depolarization from –74 to –62 mV, a large reduction of force occurred whenE _M fell to –53 mV. The implications of these relations to fatigue are discussed. Partial inhibition of the Na⁺-K⁺ pump with ouabain (10^–6 M) caused additional force loss at 11 mM [K⁺]₀. Salbutamol, insulin, or calcitonin gene-related peptide all stimulated the Na⁺-K⁺ pump in muscles exposed to 11 mM [K⁺ ₀] and induced an average 26–33% recovery of tetanic force. When using stimulation pulses of 0.1 ms, instead of the standard 1.0-ms pulses, force recovery with these agents was 41–44% which was significantly greater (P < 0.025). Only salbutamol caused any recovery ofE _M (1.3 mV). The observations suggest that the increased Na⁺ concentration difference across the sarcolemma, following Na⁺-K⁺ pump stimulation, has an important role in restoring excitability and force.     

Effects of immobilization on the rat soleus muscle in relation to age

A hind limb of young adult, adult and old male Wistar rats (4–5, 6–7 and 20–21 months, respectively) was immobilized for 4 weeks by a plaster cast with the knee and ankle joints in a resting position. Enzyme-histochemical, morphometrical and contractile characteristics of the soleus muscle were compared with those in age-matched controls. A pronounced decrease in muscle mass and cross-sectional muscle fibre area was found at all ages. The degree of atrophy after immobilization did not differ between different fibre types in each age group, but the decrease in fibre area was less pronounced in old animals (i.e. the fibre area was decreased by 49–64, 53–66 and 27–38% in young adult, adult and old animals, respectively). The maximum tetanus force was decreased in all age groups (by 73, 78 and 69% in young adult, adult and old rats, respectively) as was the tetanus tension (i.e. tetanus force divided by muscle fibre cross-sectional area). The contraction time of the isometric twitch was significantly altered, i.e. decreased, only in the youngest age group, although it also tended to decrease in old age. A significant increase in the number and proportion of fibre types intermediate to types I and 11 A, was found in the immobilized muscle of 4–5- and 6–7-month-old animals, but not in that of old ones (i.e. the proportion of intermediate fibres increased by 14, 13 and 2% in young adult, adult and old animals, respectively). Thus, in contrast to the atrophic changes, the contractile alterations after immobilization were not markedly different between young and old age. It is further concluded that the age-related fast-to-slow muscle fibre transition that occurs in normal soleus during maturation and growth can be partly reversed by restrictions of the normal muscle activity and that the ability of the soleus to modulate its fibre-type composition in response to a change in activity may be diminished in old age.     

Effects of dipyridamole on adenosine concentration,insulin sensitivity and glucose utilisation in soleus muscle of the rat

Adenosine has been shown to modulate the sensitivity of skeletal muscle to insulin (Budohoski et al. 1984). In an attempt to further characterize the modulatory action of adenosine on insulin sensitivity inskeletal muscle we have investigated the effect of the nucleoside transport inhibitor dipyridamole in isolated incubated soleus muscle strips. At a concentration of 50 M, dipyridamole increased the concentration of adenosine in the soleus muscle by 36% and in the incubation medium by 32%. At this concentration of dipyridamole, the basal rates (in the presence of 1 unit of insulin/ml) of lactate formation, 2-deoxy [2,6-³H]glucose phosphorylation and glucose oxidation were decreased by 48%, 43% and 47% respectively, whilst the rate of glycogen synthesis was unaffected. Insulin-stimulated rates (in the presence of 10000 unit of insulin/ml) of lactate formation, 2-deoxy [2,6-³H] glucose phosphorylation, glycogen synthesis and glucose oxidation were decreased by 70%, 30%, 26% and 20% respectively in the presence of 50 M dipyridamole. Although 50 M dipyridamole was required to exert a significant effect on medium and soleus muscle adenosine concentrations, statistically significant effects on glycolytic rate were observed at concentrations as low as 2 M dipyridamole.It is concluded that the results are not consistent with dipyridamole exerting an effect on skeletal muscle carbohydrate metabolism solely through elevation of the intracellular or interstial adenosine concentration, but strongly suggest that dipyridamole inhibits glucose transport and/or phosphorylation in skeletal muscle.     

Changes in intracellular ion activities induced by adrenaline in human and rat skeletal muscle

To study the stimulating effect of adrenaline (ADR) on active Na⁺/K⁺ transport we used double-barrelled ion-sensitive micro-electrodes to measure the activities of extracellular K⁺ (aK_e) and intracellular Na⁺ (aNa_i) in isolated preparations of rat soleus muscle, normal human intercostal muscle and one case of hyperkalemic periodic paralysis (h.p.p.). In these preparations bath-application of ADR (10⁻⁶ M) resulted in a membrane hyperpolarization and transient decreasesaK_e andaNa_i which could be blocked by ouabain (3×10⁻⁴ M). In the h.p.p. muslce a continuous rise ofaNa_i induced by elevation ofaK_e to 5.2 mM could be stopped by ADR. In addition, the intracellular K⁺ activity (aK_i), the free intracellular Ca²⁺ concentration (pCa_i) and intracellular pH (pH_i) were monitored in rat soleus muscle. During ADRaK_i increased, pH_i remained constant and intracellular Ca²⁺ apparently decreased. In conclusion, our data show that ADR primarily stimulates the Na⁺/K⁺ pump in mammalian skeletal muscle. This stimulating action is not impaired in the h.p.p. muscle. Parts of the results have been presented to the German Physiological Society (Ballanyi and Grafe 1987)     

Changes in membrane ionic conductances and excitability characteristics of rat skeletal muscle during aging

Membrane electrical properties,component ionic conductances and excitability characteristics of extensor digitorum longus muscle from 3–4, 16 and 29 months old rats were measured “in vitro”. Fiber diameter, membrane resistance(Rm) and membrane capacitance, increased with aging,and the increase was significant at 29 months. The increase of Rm was mostly due to a decrease of chloride conductance(GCl),whereas potassium conductance(GK) increased only slightly, at 16 and 29 months. Due to the lowered GCl, the latency of action potential increased at both ages with a consequent prolongation of the duration of action potential. Nevertheless, a decrease in the firing capability was recorded in the aged fibers. Our results indicate,that during aging, the most affected parameter of skeletal muscle fibers is GCl, although changes of this passive conductance alone cannot entirely account for the changes in the excitability characteristics recorded.     

Slow-to-fast transformation of denervated soleus muscle of the rat,in the presence of an antifibrillatory drug

The myofibrillar changes of rat denervated soleus muscle were studied in the presence and in the absence of an antifibrillatory drug. After bilateral sciaticotomy, a concentrated solution of procainamide hydrochloride was steadily released, by way of a miniosmotic pump, in the space between the soleus and the gastrocnemius muscles of one leg. Fibrillation activity of soleus muscles was checked electromyografically at 3- to 5-day intervals. On the 21st day following denervation the muscles were excised, stained for adenosine triphosphatase activity and analysed for myosin heavy chain (MHC) isoforms. In the denervated-procainamidetreated muscles fibrillation was consistently (–75% on average) depressed in comparison to the contralateral denervated muscles. Type 1 (slow) fibres and MHC isoform were also significantly reduced, to the advantage of type 2A (fast) fibres and MHC isoform. The results support the view that denervation inactivity, like other kinds of muscle inactivity, favours the expression of fast type myofibrillar isoforms, and that this effect is counteracted, at least partially, by the spontaneous activity of the denervated muscle.     

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La³⁺, Gd³⁺) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05–0.5 mM). By contrast in the presence of phospholipase D (1–5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]_o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]_o and membrane potential and their binding properties modified during and/or following variation in membrane potential.     

Effects of Mg2+ on Ca2+ handling by the sarcoplasmic reticulum in skinned skeletal and cardiac muscle fibres

The influence of myoplasmic Mg²⁺ (0.05–10 mM) on Ca²⁺ accumulation (net Ca²⁺ flux) and Ca²⁺ uptake (pump-driven Ca²⁺ influx) by the intact sarcoplasmic reticulum (SR) was studied in skinned fibres from the toad iliofibularis muscle (twitch portion), rat extensor digitorum longus (EDL) muscle (fast twitch), rat soleus muscle (slow twitch) and rat cardiac trabeculae. Ca²⁺ accumulation was optimal between 1 and 3 mM Mg²⁺ in toad fibres and reached a plateau between 1 and 10 mM Mg²⁺ in the rat EDL fibres and between 3 and 10 mM Mg²⁺ in the rat cardiac fibres. In soleus fibres, optimal Ca²⁺ accumulation occurred at 10 mM Mg²⁺. The same trend was obtained with all preparations at 0.3 and 1 M Ca²⁺. Experiments with 2,5-di-(tert-butyl)-1,4-benzohydroquinone, a specific inhibitor of the Ca²⁺ pump, revealed a marked Ca²⁺ efflux from the SR of toad iliofibularis fibres in the presence of 0.2 M Ca²⁺ and 1 mM Mg²⁺. Further experiments indicated that the SR Ca²⁺ leak could be blocked by 10 M ruthenium red without affecting the SR Ca²⁺ pump and this allowed separation between SR Ca²⁺ uptake and SR Ca²⁺ accumulation. At 0.3 M Ca²⁺, Ca²⁺ uptake was optimal with 1 mM Mg²⁺ in the toad iliofibularis and rat EDL fibres and between 1 and 10 mM Mg²⁺ in the rat soleus and trabeculae preparations. At higher [Ca²⁺] (1 M), Ca²⁺ uptake was optimal with 1 mM Mg²⁺ in the iliofibularis fibres and between 1 and 3 mM Mg²⁺ in the EDL fibres. In the soleus and cardiac preparations Ca²⁺ uptake was optimal between 1 and 10 mM Mg²⁺. The results of this study demonstrate that SR Ca²⁺ accumulation is different from SR Ca²⁺ uptake and that these two important determinants of muscle function are differently affected by Mg²⁺ in different muscle fibre types.     

3H-Ouabain binding sites in porcine skeletal muscle as influenced by environmental temperature and energy intake

The influence of environmental temperature and energy intake on³H-ouabain binding sites in skeletal muscle has been investigated in young growing pigs at 8 weeks of age. Animals lived for several weeks at 35 or 10°C on a high (H) or low (L) level of energy intake. The four treatment groups were thus: 35H, 35L, 10H and 10L. The total number of³H-ouabain binding sites (B _max) in longissimus dorsi muscle (mean values ± SEM) were 221±66, 214±61, 350±76 and 486±114 pmol/g wet weight for the 35H, 35L, 10H and 10L groups respectively.B _max was significantly greater in those living in the cold than the warm (P<0.001). Moreover, at 10°C energy intake had a significant effect, withB _max being greater in the 10L than the 10H group (P<0.005). Level of energy intake had no influence onB _max at 35°C. The apparent dissociation constant was not affected by either temperature or intake. The elevatedB _max and hence the increase in number of Na⁺, K⁺-pumping sites in the cold is probably related to increased muscular activity associated with shivering. However, thyroid status also influences the number of Na⁺, K⁺-pumping sites and this may have been a contributory factor in the present study. In addition, the elevatedB _max suggests a greater potential for non-shivering thermogenesis associated with increased Na⁺, K⁺-ATPase concentration in the cold. Differences in relative stage of development between the four groups may help to explain the results forB _max in relation to level of energy intake.     

Stiffness changes and fibre type transitions in rat soleus muscle produced by jumping training

Rat soleus muscles were overloaded with intent to induce a relative increase in fast fibres and modifications in muscular stiffness. The overloading technique was a training period consisting of an 11-week vertical jump programme. The method of controlled releases was used to obtain tension/extension curves characterizing the elastic behaviour of the soleus. Fibre typing was made by myofibrillar adenosine 5-triphosphatase staining. With regard to a control group, training resulted in a relative decrease in type I fibres for the benefit of type II fibres. Training also induced a decrease in muscle stiffness as attested notably by significant differences in maximal extension. These results are interpreted in terms of modifications occurring in the active fraction of the so-called series elastic component.     

模拟失重对大鼠比目鱼肌肌重和形态结构的影响

目的 研究模拟失重对大鼠抗重力肌—比目鱼肌的湿重、干重及形态结构的影响.方法 采用健康雌性Sprague-Dawlwy大鼠117只,按照体重配对原则随机分悬吊各组及其各自的对照组.采用组织学方法和透射电镜观察尾部悬吊法模拟失重状态下,比目鱼肌干、湿重体重比的变化,以及肌纤维的光镜结构与超微结构的变化.结果 与对照组相比,比目鱼肌干重体重比、湿重体重比在悬吊7d组即明显下降,悬吊14d组达峰值,此后有所回升.悬吊14d后的比目鱼肌,光镜下肌束结构紊乱,肌纤维面积和面积构成减少,肌纤维间间隙加大,结缔组织增生.电镜下可见肌原纤维分解,局部溶解,Z线排列紊乱、中断,肥厚层破坏,部分的纹带中出现坏死区.以上变化在解除悬吊7d时不能完全恢复.结论 尾部悬吊可致大鼠比目鱼肌萎缩,光镜结构和超微结构出现异常改变.     

Effects of isoproterenol on rubidium transport in slow- and fast-twitch muscles from euthyroid and hyperthyroid rats

The influence of experimental hyperthyroidism on the catecholamine induced stimulation of rubidium ion transport in the soleus (SOL), a slow-twitch muscle and the extensor digitorum longus (EDL), a fast-twitch skeletal muscle of rats was studied. Thyroxine administration (800 g/kg/day), for ten days induced a rise of ouabainsensitive⁸⁶Rb uptake in SOL muscle, without affecting the ouabain-insensitive uptake, whereas both fractions of⁸⁶Rb uptake were increased in EDL muscle from hyperthyroid rats. Isoproterenol (5 mol/l) caused a two-fold rise in ouabain-sensitive Rb uptake of euthyroid SOL muscle, while in hyperthyroid SOL it could stimulate only the ouabain-insensitive fraction of⁸⁶Rb influx. On the other hand, the stimulating action of isoproterenol on euthyroid EDL muscle was due to an enhancement of ouabain-insensitive Rb uptake, but in hyperthyroid EDL it failed to stimulate the ouabain-insensitive transport and caused a marked rise in ouabain-sensitive Rb uptake.The changes in catecholamine mediated transport properties in SOL muscle may be related to fibre type trans-formation induced by thyroid hormone, although in EDL the changes of catecholamine stimulation are unlikely due to fibre type conversion.Basal and isoproterenol stimulated cAMP levels were significantly reduced in both EDL and SOL muscles from hyperthyroid rats, in contrast with an insignificant decrease in net rubidium uptake caused by isoproterenol at the same concentration.The work was supported by a grant from the Hungarian Ministry of Health (No 17/2-06/072)     

Age-related changes in power output during repetitive contractions of rat medial gastrocnemius muscle

Changes in isometric force, power output and relaxation rate have been measured during repetitive tetanic contractions in 2 groups of rats of different ages. During the first 5 contractions there were no differences between a young and mature group. In contrast to isometric force production, which decreased about 3% per contraction, power output initially increased to 108% of the power output in the first contraction. A greater reduction in power output and relaxation rate after the 5th contraction indicated a greater reduction of the cross-bridge cycling rate in the younger rats. ATP, phosphocreatine and lactate concentrations after the last contraction were not different between the age-groups. In contrast IMP production, which has been suggested may play a regulatory role during fatigue was twice as high in the young rats. Judged by isometric force production there is no age-related difference in fatiguability. However, profound differences were observed in power output, which indicates that quantification of fatigue as a loss of isometric force may be seriously misleading when considering the functional status of the muscle for normal dynamic contractions.     

Functional characterization of heterotrimeric G-proteins in rat diaphragm muscle

Seven-transmembrane receptors mediate diverse skeletal muscle responses for a wide variety of stimuli, via activation of heterotrimeric G-proteins. Herein we evaluate the expression and activation of rat diaphragm or cultured skeletal muscle G-proteins using [(35)S]GTPγS. Total membrane Gα subunit content was 4-7 times higher in rat primary cultured myotubes and L6 cell line than in diaphragm (32.6±1.2fmol/mg protein) and 7-27% of them were in the active conformational state. Immunoprecipitation assay showed equal expression of diaphragm Gαs, Gαq and Gαi/o. Addition of GDP allowed the measurement of G-protein activation by different GPCR, including adrenoceptor, adenosine, melatonin and muscarinic receptors. Diaphragm denervation resulted in a marked increase in both total and active state G-protein levels. Together, the results show that [(35)S]GTPγS binding assay is a sensitive and valuable method to evaluate GPCR activity in skeletal muscle cells, which is of particular interest for pharmacological analysis of drugs with potential use in the management of respiratory muscle failure.     

Use of selective toxins to separate surface and tubular sodium currents in frog skeletal muscle fibers

The interaction between toxin from the venom of the scorpionTityus serrulatus and sodium channels in skeletal muscle membranes from the frogCaudiverbera caudiverbera was studied. Sodium current from cut sartorius muscle fibers is a complex signal in which early and late components are difficult to separate. External application of Tityus toxin initially blocked the early component in a voltage-dependent manner. Longer exposure to the toxin induced a complete blockade of the two components of the inward current. Application of tetrodotoxin to fibers pretreated with Tityus toxin at submaximal concentrations allowed the observation of the two distinct components of the inward current. Binding of¹²⁵I-labelled toxin to highly purified membrane fractions from the same muscle was used to establish the presence of high affinit receptors both in the transverse-tubular and in the surface membrane.     

Effects of aerobic exercise training on antioxidant enzyme activities and mRNA levels in soleus muscle from young and aged rats

The aim of this study was to investigate the effect of aerobic exercise training on activities and mRNA levels of catalase (CAT), glutathione peroxidase (GPX), Cu,Zn- and Mn-superoxide dismutases (SOD), TBARS content, and xanthine oxidase (XO) activity, in soleus muscle from young and aged rats. The antioxidant enzyme activities and mRNA levels were markedly increased in soleus muscle with aging. TBARS content of soleus muscle from the aged group was 8.3-fold higher as compared with that of young rats. In young rats, exercise training induced an increase of all antioxidant enzyme activities, except for Cu,Zn-SOD. XO also did not change. The TBARS content was also increased (2.9-fold) due to exercise training in soleus muscle from young rats. In aged rats, the activities of CAT, GPX and Cu,Zn-SOD in the soleus muscle did not change with the exercise training, whereas the activities of Mn-SOD (40%) and XO (27%) were decreased. The mRNA levels of Mn-SOD and CAT were decreased by 42% and 24%, respectively, in the trained group. Exercise training induced a significant decrease of TBARS content (81%) in the soleus muscle from aged rats. These findings support the proposition that exercise training presents an antioxidant stress effect on skeletal muscle from both young and aged rats.     

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<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.Supported by a grant from Caisse Nationale d'Assurances Maladie des Travailleurs Salariés (1986)     

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 M 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 M) 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 propertries of the digitalis receptors in cardiac hypertrophy suggest that new regulations should be taken into account in the adaptation to pressure overload.