Annexin 5 overexpression increased articular chondrocyte apoptosis induced by basic calcium phosphate crystals

OBJECTIVES

Basic calcium phosphate (BCP) crystals (octacalcium phosphate (OCP), carbapatite (CA) and hydroxyapatite (HA)) are associated with severe forms of osteoarthritis. In advanced osteoarthritis, cartilage shows chondrocyte apoptosis, overexpression of annexin 5 (A5) and BCP crystal deposition within matrix vesicles. We assessed in vitro whether BCP crystals and overexpression of A5 increased chondrocyte apoptosis.

METHODS

Apoptosis was induced by BCP crystals, tumour necrosis factor (TNF)-alpha (20 ng/ml) and Fas ligand (20 ng/ml) in normal articular chondrocytes (control) and in A5 overexpressed chondrocytes, performed by adenovirus infection. Apoptosis was assessed by caspase 3 (Cas3) activity, and DNA fragmentation.

RESULTS

All BCP crystals, TNF-alpha and Fas ligand induced chondrocyte apoptosis as demonstrated by decreased cell viability and increased Cas3 activity and DNA fragmentation. TUNEL (terminal deoxyribonucleotide transferase-mediated dUTP nick end-labelling)-positive staining chondrocytes were increased by OCP (12.4 (5.2)%), CA (9.6 (2.6)%) and HA (9.2 (3.0)%) crystals and TNF-alpha (9.6 (2.4)%) stimulation compared with control (3.1 (1.9)%). BCP crystals increased Cas3 activity in a dose-dependent fashion. BCP-crystal-induced chondrocyte apoptosis was independent from TNF-alpha and interleukin-1beta pathways but required cell-crystal contact and intralysosomal crystal dissolution. Indeed, preincubation with ammonium chloride, a lysosomal inhibitor of BCP crystal dissolution, significantly decreased BCP-crystal-induced Cas3 activity. Finally, overexpression of A5 enhanced BCP crystal- and TNF-alpha-induced chondrocyte apoptosis.

CONCLUSIONS

Overexpression of A5 and the presence of BCP crystals observed in advanced osteoarthritis contributed to chondrocyte apoptosis. Our results suggest a new pathophysiological mechanism for calcium-containing crystal arthropathies.

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.

Expression of laminin alpha2 chain during normal and pathological growth of myocardium in rat and human

OBJECTIVES

Fibrosis is a classical feature of cardiac hypertrophy. To date changes within the basal lamina during normal and pathological cardiac growth have been poorly investigated. The goal of the present study was to determine if the expression of the muscle specific subunit of merosin (laminin alpha2 chain) together with that of fibronectin (FN) is modified in the diseased human heart. Laminin alpha2 chain expression was also investigated during physiological and pathological cardiac growth in the rat.

METHODS

In ten normal human hearts and ten hearts with idiopathic dilated cardiomyopathy (IDCM), the laminin-alpha2 and FN mRNA levels were quantified by slot-blot using total RNA and the protein distribution was analysed using an immunofluorescence approach. In Wistar rats, laminin alpha2 and FN mRNA expression was analyzed using RNase protection assay (RPA) and slot-blot assays.

RESULTS

The amount of laminin alpha2 mRNA did not vary in normal and pathological human hearts whereas it was significantly decreased in renovascular hypertensive rats (-20%) P<0.05 versus normal tissue). The amount of fibronectin mRNA increased in IDMC patients (x2, P<0.05 versus normal tissue), but was unchanged in hypertensive rats. A negative correlation was found between the cardiac laminin-alpha2 level and the age of the patients whatever the cardiac status. During postnatal development in the rat, a similar decrease in cardiac laminin-alpha2 level was observed between 3 and 30 weeks of age. Finally, the immunofluorescent approach failed to detect any alteration in laminin alpha2 distribution within the human myocardium.

CONCLUSION

These data indicate that an imbalance between myocyte hypertrophy and the level of laminin-alpha2 might contribute to alterations in sarcolemmal properties, which occur during the development of cardiac hypertrophy and its transition to cardiac failure.

Expression and localization of the annexins II, V, and VI in myocardium from patients with end-stage heart failure

Annexins II, V, and VI belong to a family of Ca(2+)-dependent phospholipid-binding proteins that have been involved mainly in signal transduction, differentiation, membrane trafficking events, or binding to the extracellular matrix, or that might be effective as Ca(2+)-channels. They are abundant in the mammalian myocardium and might play a role in ventricular remodeling and altered calcium handling during heart failure. To test this hypothesis, we compared the expression and distribution of these annexins in nonfailing (n = 9) and failing human hearts with idiopathic dilated cardiomyopathy (n = 11). Northern blot and slot blot analysis were used to determine the annexin mRNA levels and Western blots were used to quantify the amounts of annexin proteins. Distribution of annexins was studied by immunohistofluorescence labeling and compared with that of a sarcolemmal marker (Na+/K(+)-ATPase) and of a myofibrillar protein (alpha-actinin). We showed that nonfailing hearts contained a higher amount of annexin VI than of annexin V or II (13.5 +/- 1.8, 3.7 +/- 0.2, and 2.5 +/- 0.5 microg/mg protein, respectively). In failing hearts, there was a parallel increase in both mRNA and protein levels of annexin II (146% and 132%, p < 0.05, respectively) and annexin V (152%, p < 0.01, 147%, p < 0.005, respectively); the protein level of annexin VI was also increased (117%, p < 0.05), whereas the increase of its mRNA level was statistically insignificant. We observed a predominant localization of annexin II in interstitium, and of annexins V and VI in cardiomyocytes at the level of the sarcolemma, T-tubules, and intercalated disks in nonfailing hearts, whereas in failing hearts enlarged interstitium contained all three annexins. Furthermore, annexin V staining at the level of cardiomyocytes almost disappeared. In conclusion, we showed that heart failure is accompanied by marked overexpression of annexins II and V, as well as translocation of annexin V from cardiomyocytes to interstitial tissue. The data suggest that annexins may contribute to ventricular remodeling and annexin V to impaired Ca2+ handling in failing heart.

Localization and quantitation of cardiac annexins II, V, and VI in hypertensive guinea pigs

Annexins are characterized by Ca2+-dependent binding to phospholipids. Annexin II mainly participates in cell-cell adhesion and signal transduction, whereas annexins V and VI also seem to regulate intracellular calcium cycling. Their abundance and localization were determined in left ventricle (LV) and right ventricle (RV) from hypertensive guinea pigs, during the transition from compensatory hypertrophy to heart failure. Immunoblot analysis of annexins II, V, and VI revealed an increased accumulation (2.6-, 1.45-, and 2.3-fold, respectively) in LV from hypertensive guinea pigs and no modification in RV. Immunofluorescent labeling of annexins II, V, and VI; of Na+-K+-ATPase; and of sarcomeric alpha-actinin showed that in control LV and RV, 1) annexin II is present in nonmuscle cells; 2) annexins V and VI are mainly observed in the sarcolemma and intercalated disks of myocytes; 3) annexins II, V, and VI strongly label endothelial cells and adventitia of coronary arteries; and 4) annexin VI is present in the media. At the onset of heart failure, the most striking changes are the increased protein accumulation in LV and the very strong labeling of annexins II, V, and VI in interstitial tissue, suggesting a role in fibrosis development and cardiac remodeling.

Focal cerebral ischaemia induces a decrease in activity and a shift in ouabain affinity of Na+, K+-ATPase isoforms without modifications in mRNA and protein expression

In a mouse model of focal cerebral ischaemia, we observed after 1 h of ischaemia, that the total Na+, K+-ATPase activity was decreased by 39.4%, and then did not vary significantly up to 6 h post-occlusion. In the sham group, the dose-response curves for ouabain disclosed three inhibitory sites of low (LA), high (HA) and very high (VHA) affinity. In ischaemic animals, we detected the presence of only two inhibitory sites for ouabain. After 1 h of permanent occlusion, the first site exhibited a low affinity while the second site presented an affinity intermediate between those of HA and VHA sites, which evolved after 3 h and 6 h of occlusion towards that of the VHA site. The presence of only two ouabain sites for Na+, K+-ATPase after ischaemia could result from a change in ouabain affinity of both HA and VHA sites (alpha2 and alpha3 isoforms, respectively) to form a unique component. Irrespective of the duration of ischaemia, the smaller activity of this second site accounted entirely for the loss in total activity. Surprisingly, no modifications in protein and mRNA expression of any alpha or beta isoforms of the enzyme were observed, thus suggesting that ischaemia could induce intrinsic modifications of the Na+, K+-ATPase.

In vivo left ventricular function and collagen expression in aldosterone/salt-induced hypertension

Cardiac fibrosis is linked to aldosterone-induced hypertension, but the effects on in vivo left ventricular (LV) function are not established. We studied the relations between in vivo LV function and aldosterone/salt cardiac fibrosis. Adult guinea pigs (GPs) were treated for 3 months with an aldosterone infusion and high-salt diet. This treatment induced arterial hypertension (+35%) and moderate LV hypertrophy (LVH; +60%) without right ventricular (RV) hypertrophy. Echo-Doppler LV assessment demonstrated unaltered cardiac output, stroke volume, or LV relaxation. Type I collagen messenger RNA (mRNA) was significantly increased in both ventricles (LV, +48%; RV, +77%) and accompanied by a significant increase in total collagen deposition (LV, from 0.52% in controls to 4.4% in treated GPs; RV, from 0.82 to 5.5% in treated GPs). Plasma norepinephrine levels increased 2.6-fold (p < 0.01) and correlated with the increase in collagen deposition in both ventricles. Collagen content was not correlated with hypertension or LVH. We conclude that aldosterone administration induces cardiac collagen accumulation and a sympathetic stimulation, which might preserve systolic and diastolic function.

Modifications of myocardial Na+,K(+)-ATPase isoforms and Na+/Ca2+ exchanger in aldosterone/salt-induced hypertension in guinea pigs

OBJECTIVE

The aim of this study was to determine whether changes in cardiac Na+,K(+)-ATPase subunits and Na+/Ca2+ exchanger expression are regulated in aldosterone-salt hypertensive guinea pigs.

METHODS

Guinea pigs (GP) were unilaterally nephrectomized and randomized into three groups (aldosterone-salt; control-salt; control). After 90 days of treatment, echocardiographic M-mode assessment and right carotid arterial catheterization were performed in vivo, and plasma hormones and electrolytes were measured. mRNA and protein levels were studied by Northern and Western blot analysis.

RESULTS

Aldosterone-salt treatment induced, (1) arterial hypertension (+40%) and LV hypertrophy (+60%) without altering LV-fractional shortening, (2) an increase in plasma norepinephrine levels (+262%) and suppression of renin activity. Northern blot analysis showed the presence of the mRNA encoding the three alpha isoforms and the beta 1 subunit of Na+,K(+)-ATPase in GP myocardium. In the aldosterone-salt group, levels of alpha 1 and beta 1 mRNAs were unchanged. alpha 2 mRNA was increased in both ventricles, whereas alpha 3 mRNA was increased in hypertrophied LV only. Furthermore, levels of the Na+/Ca2+ exchanger mRNA were decreased in both ventricles. At protein level, the two major transcripts (alpha 1 and alpha 2) were detected but alpha 3 isoform was not. Parallel changes in protein and mRNA accumulation of alpha 1 and alpha 2 isoforms were observed in hypertrophied LV.

CONCLUSION

These results show that alpha 1 and alpha 2 isoforms are expressed in GP heart and that they are independently regulated in aldosterone-salt hypertension. Like the alpha 1 isoform in renal tissue, alpha 2 isoform is the main target of aldosterone-salt. Reciprocal expression of the Na+/Ca2+ exchanger and Na+,K(+)-ATPase suggests an adaptational mechanism which maintains an appropriate sodium gradient and calcium concentration in hypertensive myocardium.

Trophic effect of human pericardial fluid on adult cardiac myocytes. Differential role of fibroblast growth factor-2 and factors related to ventricular hypertrophy

Pericardial fluid (PF) may contain myocardial growth factors that exert paracrine actions on cardiac myocytes. The aims of this study were (1) to investigate the effects of human PF and serum, collected from patients undergoing cardiac surgery, on the growth of cultured adult rat cardiac myocytes and (2) to relate the growth activity of both fluids to the adaptive changes in overloaded human hearts. Both PF and serum increased the rate of protein synthesis, measured by [14C]phenylalanine incorporation in adult rat cardiomyocytes (PF, +71.9 +/- 8.2% [n = 17]; serum, +14.9 +/- 6.5% [n = 13]; both P < .01 versus control medium). The effects of both PF and serum on cardiomyocyte growth correlated positively with the respective left ventricular (LV) mass. However, the magnitude of change with PF was 3-fold greater than with serum (P < .01). These trophic effects of PF were mimicked by exogenous basic fibroblast growth factor (FGF2) and inhibited by anti-FGF2 antibodies and transforming growth factor-beta (TGF-beta), suggesting a relationship to FGF2. In addition, FGF2 concentration in PF was 20 times greater than in serum. On the other hand, the LV mass-dependent trophic effect, present in both fluids, was independent of FGF2 concentration or other factors, such as angiotensin II, atrial natriuretic factor, and TGF-beta. These data suggest that FGF2 in human PF is a major determining factor in normal myocyte growth, whereas unidentified LV mass-dependent factor(s), present in both PF and serum, participates in the development of ventricular hypertrophy.

Cardiac hypertrophy, arrhythmogenicity and the new myocardial phenotype. II. The cellular adaptational process

Ventricular fibrosis is not the only structural determinant of arrhythmias in left ventricular hypertrophy. In an experimental model of compensatory cardiac hypertrophy (CCH) the degree of cardiac hypertrophy is also independently linked to ventricular arrhythmias. Cardiac hypertrophy reflects the level of adaptation, and matches the adaptational modifications of the myocardial phenotype. We suggest that these modifications have detrimental aspects. The increased action potential (AP) and QT duration and the prolonged calcium transient both favour spontaneous calcium oscillations, and both are potentially arrhythmogenic and linked to phenotypic changes in membrane proteins. To date, only two ionic currents have been studied in detail: Ito is depressed (likely the main determinant in AP durations), and If, the pacemaker current, is induced in the overloaded ventricular myocytes. In rat CCH, the two components of the sarcoplasmic reticulum, namely Ca(2+)-ATPase and ryanodine receptors, are down-regulated in parallel. Nevertheless, while the inward calcium current is unchanged, the functionally linked duo composed of the Na+/Ca2+ exchanged and (Na+, K+)-ATPase, is less active. Such an imbalance may explain the prolonged calcium transient. The changes in heart rate variability provide information about the state of the autonomic nervous system and has prognostic value even in CCH. Transgenic studies have demonstrated that the myocardial adrenergic and muscarinic receptor content is also a determining factor. During CCH, several phenotypic membrane changes participate in the slowing of contraction velocity and are thus adaptational. They also have a detrimental counterpart and, together with fibrosis, favour arrhythmias.

Cardiac calcium release channel (ryanodine receptor) in control and cardiomyopathic human hearts: mRNA and protein contents are differentially regulated

Abnormal intracellular calcium handling in cardiomyopathic human hearts has been associated with an impaired function of the sarcoplasmic reticulum, but previous reports on the gene expression of the ryanodine receptors (Ry2) are contradictory. We measured the mRNA levels, the protein levels and the number of high affinity [3H]ryanodine binding sites in the left ventricle of non-failing (n = 9) and failing human hearts [idiopathic dilated (IDCM n = 16), ischemic (ICM n = 7) or mixed (MCM n = 8) cardiomyopathies]. Ry2 mRNA levels were significantly reduced in IDCM (-30%) and unchanged in MCM and ICM and Ry2 protein levels were similar. In contrast, we observed a two-fold increase in the number of high affinity Ry2 (B(max) = 0.43 +/- 0.11 v 0.22 +/- 0.13 pmol/mg protein, respectively; P<0.01) and an unchanged K(d). Furthermore, levels of myosin heavy chain mRNA and protein per g of tissue were similar in failing and non-failing hearts, suggesting that the observed differences in Ry2 are not caused by the increase in fibrosis in failing heart. Therefore, the dissociation between the two-fold increase in the number of high affinity ryanodine receptors observed in all failing hearts and the slightly decreased mRNA level or unchanged protein level suggests that the ryanodine binding properties are affected in failing myocardium and that such modifications rather than a change in gene expression alter the channel activity and could contribute to abnormalities in intracellular Ca2+ handling.

Senescent heart compared with pressure overload-induced hypertrophy

Although systolic left ventricular (LV) function is normal in the elderly, aging is associated in rat papillary muscle with mechanical and sarcoplasmic reticulum Ca2+ ATPase alterations similar to those observed in the hypertrophied heart. However, alterations in the other calcium-regulating proteins implicated in contraction and relaxation are still unknown. To investigate alterations in LV function and calcium-regulating proteins, we measured hemodynamics and Na(+)-Ca2+ exchanger (NCx), ryanodine receptor (RyR2), and sarcoplasmic reticular Ca2+ ATPase (SERCA2) mRNA levels (expressed in densitometric scores normalized to that of poly(A+) mRNA) in left ventricle from 4-month-old (adult, n = 13) and 24-month-old (senescent, n = 15) rats. For ex vivo contractile function, active tension was measured during isolated heart perfusion in adult (n = 11) and senescent (n = 11) rats. For comparison of age-dependent effects of moderate hypertension on both hemodynamics and calcium proteins, renovascular hypertension was induced or a sham operation performed at 2 (n = 11 and n = 6) and 22 (n = 26 and n = 5) months of age. In senescent rats, LV systolic pressure and maximal rates of pressure development were unaltered, although active tension was depressed (4.7 +/- 0.4 versus 8.3 +/- 0.7 g/g heart weight in adults, P < .0001). SERCA2 mRNA levels were decreased in senescent left ventricle (0.98 +/- 0.05 versus 1.18 +/- 0.05 in adults, P < .01), without changes in NCx and RyR2 mRNA accumulation. Renovascular hypertension resulted in 100% mortality in aged rats; in adults, renovascular hypertension resulted, 2 months later, in an increase of LV systolic pressure (170 +/- 7 versus 145 +/- 3 mm Hg in sham-operated rats, P < .05) and in mild LV hypertrophy (+18%, P < .01) associated with a decrease in SERCA2 mRNA levels (1.02 +/- 0.03 versus 1.18 +/- 0.03 in sham-operated rats, P < .001). Contractile dysfunction in senescent isolated heart and decreased SERCA2 mRNA levels were associated with in vivo normal LV function at rest, indicating the existence of in vivo compensatory mechanisms. RyR2 and NCx gene expressions were not implicated in the observed contractile dysfunction. In aged rats, renovascular hypertension resulted in 100% mortality, probably related to elevated levels of circulating angiotensin II, whereas in adult rats, renovascular hypertension induced a mild LV hypertrophy associated with a selective alteration in SERCA2 gene expression.

Expression of the cardiac ryanodine receptor in the compensated phase of hypertrophy in rat heart

OBJECTIVES

Abnormal calcium handling is a general feature of cardiac hypertrophy and alteration in the expression of SR proteins has been suggested to be involved in this alteration. To determine the expression of the cardiac ryanodine receptor (Ry2) gene during compensatory hypertrophy, we studied the mRNA and protein accumulation in left ventricles from rats with 30 to 100% hypertrophy.

METHODS

Cardiac hypertrophy was obtained after 1 month of aortic constriction. Ry2 mRNA was analyzed by RNase protection assay, Northern and slot blots, and Ry2 protein by high-affinity [3H]ryanodine binding and Western blot.

RESULTS

We demonstrate that: (1) the cardiac Ry2 mRNA concentration is decreased by 50% in severe hypertrophy; (2) both the density of the high-affinity sites and the Ry2 protein level are decreased by 25%; (3) the decrease in the mRNA and protein levels and the number of high-affinity sites are highly correlated to the severity of hypertrophy.

CONCLUSION

Our results suggest that, as for SR Ca(2+)-ATPase, there is either a downregulation or a lack of upregulation of the gene coding for the Ry2 in compensatory hypertrophy. The decreased density of Ry2 may alter SR Ca2+ transport and contribute to the impaired Ca2+ handling by slowing the Ca2+ movements.

Biological determinants of aldosterone-induced cardiac fibrosis in rats

To determine the events leading to cardiac fibrosis in aldosterone-salt hypertensive rats, we studied protein and mRNA accumulation of procollagens I and III for 60 days. After 3 and 7 days of treatment systolic pressure was normal, and no histological or biochemical changes were seen in rat hearts. At day 15 arterial pressure was raised (+40%) and left ventricular hypertrophy was +15%. Cardiac examination after hemalun-eosin staining and immunolabeling with anticollagen I and III antibodies showed no structural alterations, but an 83% increase in right ventricular type III procollagen mRNA levels was found. At 30 and 60 days we found progressive cardiac fibrosis, with inflammatory cells, myocyte necrosis, and elevation of both types I and III procollagen mRNA levels in both ventricles. To determine whether aldosterone had effects on Na,K-ATPase that might lead to ionic disturbances and induce myocyte necrosis, we studied the major cardiac Na,K-ATPase isoform genes. Although Na,K-ATPase alpha 1- and beta 1-subunit mRNA levels were elevated in kidney at day 1, neither of these cardiac transcripts nor the specific alpha 2 isoform was altered between 1 and 15 days. These results show that accumulation of procollagen mRNAs occurs before collagen deposition. Cardiac alterations are late and not preceded by changes in Na,K-ATPase cardiac gene expression, precluding a direct modulation of cardiac collagen synthesis and Na,K-ATPase by aldosterone.

Molecular and cellular biology of the senescent hypertrophied and failing heart

During aging, experimental studies have revealed various cellular changes, principal among which is myocyte hypertrophy, which compensates for the loss of myocytes and is associated with fibrosis. The expression of alpha-myosin heavy chain is replaced by that of the isogene beta-myosin, which leads to decreased myosin adenosine triphosphatase (ATPase) activity. In consequence, contraction is slower and more energetically economical. The Ca(2+)-ATPase of the sarcoplasmic reticulum and Na+/Ca2+ exchange activity are decreased, which probably explains the reduced velocity of relaxation. Membrane receptors are also modified, since the density of both the total beta-adrenergic and muscarinic receptors is decreased. The senescent heart is able to hypertrophy in response to overload and to adapt to the new requirements. Similar alterations are observed both in the senescent heart and in the overloaded heart, in clinical as well as in experimental studies; however, differences do exist, especially in terms of fibrosis and arrhythmias.

Myocardial phenotypic changes in Na+, K+ ATPase in left ventricular hypertrophy: pharmacological consequences

Cardiac adaptation to permanent overload induces several phenotypic changes which finally result in a system which works more economically, together with a slower Vmax. The molecular target of digitalis is the NA+, K+ ATPase, which is a polymorphic molecule. We have recently demonstrated that during cardiac hypertrophy this target is modified and that a shift occurs in the alpha 1 subunit, from the normally present alpha 2 isosubunit to alpha 3, which is a fetal isoform with a lower affinity for sodium and a higher affinity for ouabain. Such a shift explains why, in rat cardiac hypertrophy ouabain is less toxic than normal and is released from its target more slowly. It may also explain at least in part the discrepancies observed in clinical trials on the efficacy of digitalis.

The effects of compensated cardiac hypertrophy on dihydropyridine and ryanodine receptors in rat, ferret and guinea-pig hearts

The number of dihydropyridine and ryanodine receptors (DHP-R and RyR) has been measured in control and hypertrophied ventricles from rats, guinea pigs and ferrets to determine whether these two channels contribute to the alterations in excitation-contraction coupling (ECC), and in Ca2+ transient during compensated cardiac hypertrophy. We found that ventricular hypertrophy did not change the density of DHP-R. Mild hypertrophy did not alter the density of RyR in the rat but decreased it in the guinea-pig and in the ferret (30% and 36%, respectively). Severe hypertrophy decreased the density of RyR by 20% in the rat and by 34% in the guinea-pig. Therefore, the decrease is greater in ferret and guinea-pig hearts than in rat heart. We conclude that the sarcoplasmic reticulum (SR) Ca2+ release channels but not the L-type Ca2+ channels could contribute to the slowing of intracellular Ca2+ movements and to the reduced velocity of shortening of the hypertrophied hearts. We suggest that, in the guinea pig and ferret hearts which express only the beta myosin heavy chain (MHC) isoform, the reduced velocity of shortening during hypertrophy is related to the decrease in RyR density, whereas in the rat, it is regulated primarily via a shift in the MHC isoform, except in severe hypertrophy in which the moderate decrease in RyR would also be involved.

Alteration of Na,K-ATPase subunit mRNA and protein levels in hypertrophied rat heart

To determine if an altered expression of the Na,K-ATPase alpha isoform genes is responsible for an observed increase in cardiac glycoside sensitivity in compensatory hypertrophy, we performed Northern and slot blot analyses of RNA and specific immunological detection of Na,K-ATPase isoforms in rat hearts from normal and pressure overload-treated animals induced by abdominal aortic constriction. During the early phase of hypertrophy, the only alteration is a decrease in the alpha 2 mRNA isoform. In the compensated hypertrophied heart, the levels of the predominant alpha 1 isoform (mRNA and protein) and the beta 1 subunit mRNA are unchanged. In contrast, the alpha 2 isoform (mRNA and protein) is decreased by 35% and up to 61-64% in mild (< 55%) and severe (> 55%) hypertrophy, respectively. The alpha 3 isoform (mRNA and protein), which is extremely low in adult heart, is increased up to 2-fold during hypertrophy but accounts for only approximately equal to 5% of the total alpha isoform mRNA. These findings demonstrate that, in cardiac hypertrophy, the three alpha isoforms of the Na,K-ATPase are independently regulated and that regulation occurs at a pretranslational level. The pattern of expression in hypertrophied adult heart is similar to that of the neonatal heart where the inverse regulation between the alpha 2 and alpha 3 ouabain high affinity isoforms has been reported. This suggests that distinct regulatory mechanisms controlling Na,K-ATPase isoform expression may, at least in part, be involved in the sensitivity to cardiac glycosides.

Arrhythmogenicity of the hypertrophied and senescent heart and relationship to membrane proteins involved in the altered calcium handling

The high incidence of arrhythmias in human left ventricular hypertrophy has been well established but the mechanisms of arrhythmias are not well defined. In attempt to clarify these mechanisms, we tried to determine if a relationship might exist in the hypertrophied or senescent hearts between the incidence of arrhythmias and alterations in the gene expression of the main membrane proteins involved in the regulation of calcium movements. Holter monitoring was used in young and senescent rats where hypertrophy had been induced by aortic stenosis and hyperthyroidism (young rats) or by DOCA-salt treatment (senescent rats). Different types of spontaneous arrhythmias were detected. In the aortic stenosis group, the heart rate and the number of supraventricular premature beats were increased significantly, whereas the number of ventricular premature beats was increased in some animals but not in all. In senescent rats, the numbers of ventricular and supraventricular premature beats and the incidence of atrioventricular block were very high. At the cellular level, the density of calcium channels from the sarcolemma and of the alpha 1 subunit of the Na+/K(+)-ATPase were unchanged in the hypertrophied and senescent hearts but most of the proteins involved in the regulation of calcium movements (calcium release channel and Ca(2+)-ATPase from the sarcoplasmic reticulum, Na+/Ca2+ exchange, and beta adrenergic and muscarinic receptors from the sarcolemma) have a decreased density or activity. These changes might account for the slowing of the maximum shortening velocity and the impaired contractility of the hypertrophied and senescent hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac hypertrophy and failure--a disease of adaptation. Modifications in membrane proteins provide a molecular basis for arrhythmogenicity

Cardiac hypertrophy is the physiological adaptation of the heart to chronic mechanical overload. Cardiac failure indicates the limits of the process. Cardiac hypertrophy is only one example of biological adaptation and results from the induction of several changes in gene expression, mostly of the fetal type, including those coding for the myosin heavy chain or the alpha-subunit of the Na+,K(+)-ATPase. From a thermodynamic point of view, the decrease in Vmax allows the heart to produce a normal tension at a lower cost. This process results from changes both in the sarcomere and in the expression of certain membrane proteins. The decrease in calcium transient is determined by several changes in membrane proteins that result in a rather fragile equilibrium in terms of calcium homeostasis. Any abnormal input in calcium will have exaggerated detrimental consequences on a hypertrophied myocyte and may cause automaticity and arrhythmias or an exaggerated response to anoxia in terms of compliance.

Molecular and cellular level of action of digitalis

The pharmacological receptor of cardiac glycosides is the Na+/K(+)-ATPase which consists of a catalytic alpha (M(r) = 112,000) and glycosylated beta (M(r) = 35,000) subunit. The enzyme is responsible for the vectorial transport across the sarcolemma of three Na+ ions outward and two K+ ions inward against their electrochemical gradient. Specific inhibition of the Na+ pump by digitalis induces a positive inotropic effect by increasing the intracellular Na+ concentration which in turn induces an increase in the intracellular Ca2+ concentration by the Na+/Ca2+ exchange and an increase in the Ca2+ pool of the sarcoplasmic reticulum; toxic effects are observed at higher doses of cardiac glycosides leading to spontaneous calcium release from the sarcoplasmic reticulum. Three isoforms of the alpha catalytic subunit have been identified by molecular cloning. They share a high homology in the deduced amino acid sequence with eight transmembrane domains. The ouabain binding domain is located on the extracellular side and ouabain sensitivity depends mainly on the two residues at the border of the first extracellular domain. The isoforms differed by their ouabain sensitivity, are expressed in a tissue-specific and hormonally-regulated manner. Moreover, expression of the isoforms and their ouabain sensitivity vary from species to species with an alpha 1 isoform of very low affinity being the major isoform (80%) in the adult rat heart and an alpha 1 isoform of high affinity representing 50% of total alpha mRNA abundance in the human heart. Therefore the effect of digitalis on the heart depends mainly on the isoform which is expressed and on the regulation of their expression according to age, hormonal influence and pathology.

Cell and membrane lipid analysis by proton magnetic resonance spectroscopy in five breast cancer cell lines

The lipid composition of five human breast cancer cell lines (MCF-7, T47D, ZR-75-1, SKBR3 and MDA-MB231) was assessed by proton magnetic resonance spectroscopy (MRS) in whole cells and membrane-enriched fractions. The proportions of the three main lipid resonances in 1D spectra were different for each cell line. These resonances included mobile methyl and methylene functions from fatty acids of triglycerides and phospholipids and N-trimethyl from choline of phospholipids. T47D and ZR-75-1 cells presented a high methylene/methyl ratio (6.02 +/- 0.35 and 6.28 +/- 0.90). This ratio was significantly lower for SKBR3, MCF-7 and MDA-MB231 cells (2.76 +/- 0.22, 2.27 +/- 0.57 and 1.39 +/- 0.39). The N-trimethyl/methyl ratio was high for MDA-MB231 and SKBR3 cells (1.38 +/- 0.54 and 0.86 +/- 0.32), but lower for MCF-7, T47D and ZR-75-1 cells (0.49 +/- 0.11, 0.16 +/- 0.07 and 0.07 +/- 0.03). 2D COSY spectra confirmed these different proportions in mobile lipids. From 1D spectra obtained on membrane preparations, T47D and ZR-75-1 were the only cell lines to retain a signal from mobile methylene functions. These differences might be related to the heterogeneity found for several parameters of these cells (tumorigenicity, growth rate, hormone receptors); an extended number of cases from fresh samples might enable clinical correlations.

Remodeling of the heart (membrane proteins and collagen) in hypertensive cardiopathy

The basis for impaired left ventricular function of hearts in moderate to severe stages of hypertrophy and congestive heart failure remains uncertain. At the cellular level, the mechanisms governing the movements of calcium in the myocardium are actually depressed and might at least in part account for the slowing of the maximum shortening velocity and the impaired relaxation. These alterations of membrane proteins seem particularly important in species where the slowing of Vmax cannot be a consequence of the myosin heavy chain shift. They lead to an unstable equilibrium of calcium homeostasis and to calcium overload in heart failure. On the other hand, the enhanced density and remodeling of collagen in the hypertrophied heart, which would depend on elevation in circulating aldosterone, impair myocardial stiffness with diastolic dysfunction and lead to altered pumping capacity of the heart. Disturbances of calcium metabolism and matrix collagen remodeling enhance early afterdepolarizations and arrhythmias.

The membrane proteins of the overloaded and senescent heart

Cardiac hypertrophy which occurs during chronic mechanical overload is one of the numerous examples of biological adaptation to environmental requirements. As such, it is obtained at random by trial and error, and adaptation represents the sum of various modifications in gene expression, including the shift in isoform of myosin or in iso Na+, K+ ATPase, the decrease in beta-adrenergic and muscarinic receptors, ryanodine channels or SR Ca2+ ATPase densities and the unchanged density in Ca2+ current. Some of these changes are beneficial at the cellular level, but are finally detrimental for the organism as a whole, as is the slowing of Vmax. It was suggested that the calcium homeostasis of the hypertrophied cardiocyte was fragile and that this modified cell was less able to buffer the changes in the intracellular calcium, thus providing a biological basis for the arrhythmogenicity of the hypertrophied heart. These various modifications may provide a new key for future pharmaceutical research.

Effect of pressure overload on cardiac Ca2+ antagonist binding sites of guinea pig. Comparison with the adaptational response of the hypertrophied rat heart

STUDY OBJECTIVE

The aim was to determine if the adaptational process of the cardiac calcium channel to pressure overload observed in rat heart also occurs in species characterised by a higher sensitivity to external calcium than in the rat. This adaptation occurs via a maintained density of dihydropyridine receptors and calcium current in hypertrophied rat heart.

DESIGN

The guinea pig was chosen and the dissociation constant (Kd), association and dissociation rate constants (k+1,k-1), and maximal number (Bmax) of the dihydropyridine receptors were measured through binding of [3H]PN 200-110 to crude sarcolemma fractions from control and hypertrophied guinea pig left ventricle.

EXPERIMENTAL MATERIAL

Hypertrophy of the left ventricle was obtained by stenosis of the abdominal aorta in guinea pigs.

MEASUREMENTS AND MAIN RESULTS

Hypertrophy reached at least 50% in 15% of the surviving animals. No significant differences in the binding of [3H]PN 200-110 to the dihydropyridine receptor were observed between control and hypertrophied left ventricle microsomal preparations: Kd = 1.59(SEM 0.22) and 1.17(0.36) nM; Bmax = 225(18) and 213(4) fmol.mg-1 of protein; k-1 = 2.30(0.26) and 2.00(0.13) min-1 x 10(-2); k+1 = 3.8(0.7) and 3.5(0.3) nM-1.min-1 x 10(-2) respectively.

CONCLUSIONS

In guinea pig as in rat, the total number of dihydropyridine receptors per left ventricle increased proportionately to the hypertrophy. This is consistent with an unchanged density of the cardiac Ca2+ channels in the hypertrophied guinea pig heart as previously shown in hypertrophied rat heart.

The density of ryanodine receptors decreases with pressure overload-induced rat cardiac hypertrophy

We investigate the possibility that alterations in the calcium movements of the hypertrophied rat heart might involve sarcoplasmic reticulum (SR) ryanodine receptors. A decreased receptor density was observed with severe hypertrophy (0.26 +/- 0.05 and 0.35 +/- 0.06 pmol/mg protein and 170 and 366 receptors/micron2 of SR in 50-80% hypertrophy and control, respectively); however, the total number of receptors per left ventricle was unchanged. The dissociation constant (0.7 nM) was similar in both hypertrophied and control left ventricles. Thus the decreased density of the ryanodine receptors may participate in altered calcium movements in hypertrophied rat heart.

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.

Calcium current in single cells isolated from normal and hypertrophied rat heart. Effects of beta-adrenergic stimulation

The L-type calcium current was investigated in normal and hypertrophied rat ventricular myocytes as a possible cause of the action potential lengthening that has been reported during hypertrophy. Regulation of the calcium current (ICa) by a beta-adrenergic agonist (isoproterenol) was also analyzed since beta-agonist-induced positive inotropy is less marked in hypertrophied heart. Left ventricular hypertrophy was induced by stenosis of the abdominal aorta. For recording ICa, the whole-cell patch-clamp technique was used. Potassium currents were suppressed by replacing K+ ions with Cs+ ions in both the extracellular and intracellular media, and sodium current was blocked by 50 microM tetrodotoxin. The Ca2+ current was larger in hypertrophied cells (2.2 +/- 0.6 nA [n= 31]) than in normal cells (1.2 +/- 0.5 nA [n = 33]). However, if one relates ICa amplitude to the cell membrane area, as estimated by membrane capacitance measurement, no significant difference was observed in current density (8.5 +/- 2.5 pA/pF [n = 31] and 8.3 +/- 2.1 pA/pF [n = 33] in hypertrophied and in normal cells, respectively). In both cell types, ICa displayed the same voltage and time dependence. When expressed as a percentage, the maximal increase in ICa amplitude that was obtained with 100 nM isoproterenol was less in hypertrophied cells (+78%) than in normal cells (+120%). The sensitivity of ICa to beta-adrenergic stimulation was not modified: EC50 was 3.8 nM for hypertrophied cells and 4.8 nM for normal cells. Forskolin and cyclic AMP were as effective in both cell types. Stimulation of ICa by beta-adrenergic agonist was decreased in agreement with a reduced number of binding sites of beta-agonists and/or an altered coupling of the G-proteins.

Normal responsiveness to external Ca and to Ca-channel modifying agents in hypertrophied rat heart

In rat heart, the maximum velocity of shortening is decreased in response to chronic pressure overload. This is in part explained by an isomyosin shift, but several arguments suggest changes in membrane proteins. Inotropic response to calcium channel modifiers and to external calcium were simultaneously determined to explore this possibility. Left ventricular hypertrophy was induced by abdominal aortic stenosis and after 4-5 wk the left ventricular-to-body weight ratio increased by 61%. The effects of BAY K 8644 (10(-9) to 10(-6) M), a calcium channel activator, nifedipine (10(-9) to 10(-7) M), and external calcium (0.25-2.50 mM) were studied on isolated hearts at a coronary flow of 20 ml.min-1.g of left ventricle-1. The inotropic response (in percent changes in developed pressure and in dP/dtmax) was unmodified in the hypertrophied hearts. This work is in agreement with previous findings that both the total number of dihydropyridine binding sites and the peak magnitude of calcium current increase in proportion to the degree of hypertrophy. It suggests that the slowing of velocity could not be explained by a decreased number of Ca2+ channels but may more likely reflect modifications of the sarcomeres or sarcoplasmic reticulum.

Membrane proteins of the myocytes in cardiac overload

1. Hypertrophy of the cardiac myocytes resulting from a mechanical overload may be responsible for major membraneous modifications, either at the sarcolemmal or at the sarcoplasmic level. In this study several sarcolemmal markers such as beta-adrenoceptors, muscarinic receptors or (Na+, K+)-ATPase were investigated in an experimental model of cardiac hypertrophy, the chronic aortic stenosis in adult rats. 2. Left ventricular beta-adrenoceptor density (expressed in fmol mg-1 protein) was decreased in the aortic stenosis group by about 30%; however, when expressed in number of receptors per cardiac cell beta-adrenoceptor number in the hypertrophied myocytes was unchanged. 3. Similarly, the number of muscarinic receptors in the hypertrophied cells, expressed as number of receptors per cardiac cell, was unchanged. 4. The number of (Na+, K+)-ATPase molecules with high affinity for ouabain was markedly increased in the hypertrophied myocytes, while those with low affinity for ouabain were not. 5. These results indicate the necessity in chronic hypertrophy to calculate receptors not only in density (fmol mg-1 protein) but also in number per cardiac cell. The unchanged number of beta-adrenergic and muscarinic receptors present on the hypertrophied myocytes suggests a non-regulation for the genes coding for these receptors.

Signal and adaptational changes in gene expression during cardiac overload

Chronic cardiac overload stimulates various quantitative and qualitative mechanisms of adaptation, some of them being species-specific. The signals responsible for these changes in gene expression are still speculative, nevertheless early modifications of the microtubular network have been reported. Soon after overload an increased expression of various genes coding for regulatory proteins has also been observed, this includes various oncogenes and the genes of several heat-shock proteins. Hypertrophy only, is non species-specific and is adaptational because it both multiples the number of contractile units and it lowers wall stress. The slowing of the shortening velocity allows the heart to produce normal tension, at a lower cost, and has different biological explanations depending on the species. In small rodent ventricles, the main but probably not the unique, determinant of this physiological parameter is an isomyosin shift from a high ATPase activity form V1 to a low activity form V3, discovered in our laboratory in 1979. This shift has a transcriptional origin and also occurs in atria in every mammalian including humans; nevertheless it has not been evidenced in the ventricles of humans, dog, cat or guinea-pig. In these species it is necessary to take into account other mechanisms, namely those involved intracellular calcium movements. The number of total, and possibly active, calcium channels is normal in rat overloaded heart suggesting that their synthesis is activated commensurate to the development of hypertrophy. The situation is more complex for other sarcolemma proteins such as the beta-adrenergic system and the Na+, K(+)-ATPase. For the latter there is presently some evidence that an isoenzymatic shift is likely to occur, at least in rats.

Adaptational process of the cardiac Ca2+ channels to pressure overload: biochemical and physiological properties of the dihydropyridine receptors in normal and hypertrophied rat hearts

Inotropic responsiveness to dihydropyridines (DHP) and characterization of DHP receptors were studied during the onset of hypertrophy in rat hearts. The inotropic responsiveness of isolated hearts to external Ca2+ (0.25-2.50 mM) and nifedipine (10(-9)-10(-7) M), as expressed in percent change in dP/dtmax, was unchanged by the process of hypertrophy. Characterization of DHP receptors by Scatchard plots (Kd = 0.45 and 0.47 nM for nitrendipine), displacement curves (Kd = 0.44 and 0.42 nM for PN 200-110), and dissociation kinetics (k-1 = 4.82 X 10(-2) X min-1 and 4.85 X 10(-2) X min-1) revealed the similarity of the Ca2+ antagonist binding sites in hypertrophied and control hearts, respectively. These results on crude or purified sarcolemmal preparations from left ventricle were consistent with the presence of only one type of binding site of high affinity for DHP. The total number of Ca2+ channels was increased in hypertrophied left ventricle (LV) as compared with left ventricle from sham-operated animals (15,000 fmol/LV and 8900 fmol/LV), respectively. This increased synthesis of Ca2+ channels was observed as early as 5 days after the aortic stenosis and was related to the increase in ventricular mass. Results are in favor of an adaptational process of regulation of the total number of Ca2+ channels as an answer to pressure overload.

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

The present study demonstrates that two forms of the alpha catalytic subunit of the Na,K-ATPase are present in rat heart and originate from cardiomyocytes. They were resolved on sodium dodecyl sulfate-polyacrylamide gel electrophoresis after reduction and alkylation of the sulfhydryl groups. The two forms were identified on immunoblots using two specific antisera against either the alpha subunit from Bufo marinus kidney and the alpha and beta subunits from lamb kidney. Comparison of the two forms to the alkylated Na,K-ATPase from rat kidney (containing one catalytic subunit) and from rat brain (containing alpha and alpha + subunits) suggested that, in rat cardiac myocytes, the form with a fast migration rate (alpha F) corresponds to the alpha subunit of low ouabain affinity and the one with a slow migration rate (alpha S), to a subunit of high ouabain affinity. Thus, the existence of two isoforms of the catalytic subunit in cardiac myocytes accounts well for the biphasic ouabain inhibition of the Na,K-ATPase activity and for the biphasic inotropic responsiveness to cardiac glycosides of the rat heart.

Two Na,K-ATPase isoenzymes in canine cardiac myocytes. Molecular basis of inotropic and toxic effects of digitalis

Canine cardiac myocytes contain two distinct molecular forms of the Na,K-ATPase catalytic subunit. They are resolved by gel electrophoresis and identified using immunological techniques. The apparent molecular weights of the catalytic subunits are 95,000 (alpha) and 98,000 (alpha +). As judged by [3H]ouabain-binding measurements and Na,K-ATPase assays, the two forms are active and differ by a factor of 150 in their respective affinity for digitalis (ouabain and digitoxigenin). The dissociation constant of the high affinity form (alpha +) is KD, 2 nM, and that of the low affinity molecular form (alpha) is KD, 300 nM. According to both enzymatic and binding assays, up to 70% of maximum inhibition is caused by occupation of the high affinity sites (alpha +). Inasmuch as the pharmacological and toxic concentrations of digitalis in dog are 1 and 200 nM, respectively, and as maximum inhibition of Na+ pump in vivo should not exceed 80% to avoid toxicity (Akera, T. and Brody, T. (1982) Annu. Rev. Physiol. 44, 375-388), it appears that the high affinity molecular form (alpha +) is the pharmacological receptor exclusively related to positive inotropy, whereas the low affinity form (alpha) is mainly associated with toxicity.

Can changes in sarcolemmal membranes account for the altered inotropic responsiveness in hypertrophied heart?

Hypertrophy is an adaptive mechanism of the heart subjected to pressure overload. Ultrastructural, electrophysiological and mechanical changes occur during this adaptation. A decrease in the inotropic responsiveness of the hypertrophied heart has often been observed as compared to the normal heart. Four sarcolemmal mechanisms that could account for this modification have been described. The mechanism of action of each system (calcium channel, alpha-and beta-adrenergic systems, (Na+,K+)-ATPase) of the hypertrophied heart has been compared to that of the normal heart. In spite of the paucity of results available relating to the calcium channel, the lengthening of the action potential in every case of compensatory hypertrophy could be explained by an altered functioning of the calcium channel. alpha- and beta-adrenergic systems in the hypertrophied heart could be modified at the receptor level itself, or at another level in the cascade of events under their control. For example, two different models of hypertrophy showed a decreased inotropic responsiveness correlated to a defect in the GS regulatory protein. The modification of the ouabain-receptor (Na+,K+)-ATPase mediates a decrease and a prolongation of the inotropic response. According to the modifications of each system, a direct relationship does not seem to exist between the stimulated membrane system and the inotropic responsiveness of the hypertrophied heart.

Respective involvements of high- and low-affinity digitalis receptors in the inotropic response of isolated rat heart to ouabain

High- and low-affinity digitalis receptors coexist in rat cardiac sarcolemma. In this study, their relative involvement in the inotropic effect of ouabain was evaluated on an isolated Langendorff rat heart preparation working under isovolumic conditions at a low external calcium concentration (0.25 mM). This involvement was estimated according to both the development of the inotropic response to ouabain (10(-8)-10(-4)M) and the time course of the washing out of the biological effect. In each phenomenon considered, and whatever the index of inotropy chosen, the high-affinity digitalis receptor (EC50: 1-2 X 10(-8) M) contributed to 25-40% of the maximal inotropy (evoked by 10(-4) M ouabain). Low-affinity receptors (EC50: 1-2 X 10(-5) M) accounted for 60-75%. These apparent affinities were identical to those previously determined in sarcolemma isolated from rat heart perfused with 0.25 mM Ca2+. The biphasic effect of ouabain was related to both the inhibition of high- and low-sensitivity Na+, K+-ATPase forms and the corresponding number of ouabain-binding sites occupied. These results support the concept that the Na+, K+-ATPase highly sensitive to ouabain as revealed by lowering calcium is the in vivo manifestation of the high-sensitivity inotropic component.

Prolonged responsiveness to ouabain in hypertrophied rat heart: physiological and biochemical evidence

The inotropic effect of ouabain on cardiac hypertrophy was evaluated on an isolated Langendorff rat heart preparation with performances registrated by means of an intraventricular balloon. These effects were compared with the drug action on the sarcolemma-bound Na+-K+-ATPase activity. On both normal and pressure-overload induced hypertrophied rat hearts (ventricular wt-to-body wt ratios of 2.1 and 3.3, respectively) the inotropic effect of ouabain (10(-9)-10(-4) M) was evaluated at 0.25 mM external Ca2+. Compared with normal hearts, the recovery of a normal contractile function after the inotropic response was significantly slower in hypertrophied hearts. This was valid with the two protocols applied. During a 30-min washout period, the inotropic response remained nearly unchanged in hypertrophied hearts, whereas it was almost completely reversed in control groups. Sarcolemmal vesicles from both heart groups exhibited high Na+-K+-ATPase activities (sp. act.: 105 +/- 16 mumol X h-1 X mg-1). In both normal and hypertrophied cardiac sarcolemmal preparations, the Na+-K+-ATPase was heterogeneous, with high- and low-sensitivity forms. Their relative proportion was two-to-one. In both heart groups, their respective apparent affinities for ouabain were similar (inhibitory concentration of 50% = 10(-8) and 10(-6) M, respectively). The release of ouabain from these two sites was measured, in washout experiments, by the rates of enzyme relief from inhibition. High- and low-sensitivity forms in hypertrophied heart preparations released ouabain at seven- and threefold lower rates, respectively, than the corresponding forms present in normal cardiac sarcolemmal vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain binding sites and (Na+,K+)-ATPase activity in rat cardiac hypertrophy. Expression of the neonatal forms

The adaptation of the myocardium to mechanical overload which results in cardiac hypertrophy involves several membrane functions. The digitalis receptor in sarcolemma vesicles from hypertrophied rat hearts is characterized by binding of [3H]ouabain and ouabain-induced inhibition of (Na+,K+)-ATPase. The results show the existence of two families of ouabain binding sites with apparent dissociation constants (Kd) of 1.8-3.2 X 10(-8) M and 1-8 X 10(-6) M, respectively, which are similar to those found in normal hearts. The presence of the high affinity receptor in hypertrophied rat heart is correlated to a detectable inhibition of the (Na+,K+)-ATPase (IC50 = 1-3 X 10(-8) M). However, the high and low affinity sites in hypertrophied hearts bind and release ouabain at 4-5-fold slower rates than the corresponding sites in normal hearts. These properties are similar to that we observed in newborn rat cardiac preparations. Taken together with the expression of myosin isoforms (Schwartz, K., Lompre, A.M., Bouveret, P., Wisnewsky, C., and Whalen, R.G. (1982) J. Biol. Chem. 23, 14412-14418), our data show that the physiological adaptation of the heart also involves the resurgence of the neonatal forms of the digitalis receptor.