Decrease in rat cardiac myosin ATPase with aortic constriction: prevention by thyroxine treatment.

Severe aortic constriction in rats produced cardiac hypertrophy and a chronic decrease in cardiac actomyosin ATPase activity during a six week postoperative period. Two weeks following aortic constriction, Ca²⁺ stimulated cardiac myosin ATPase activity was also depressed; the K_m and V_max were decreased by 86.2% (p < 0.0025) and 84.4% (p < 0.0025), respectively, when compared to sham operated controls. Administration of thyroxine (100 μg/kg/day for 14 days), which was initiated on the same day as aortic constriction, prevented, to a large extent, the decrease in cardiac myosin ATPase activity. The K_m and V_max of myosin from animals with aortic constriction showed substantially smaller decreases as a result of concomitant thyroxine administration (p < 0.0025 for the change from aortic constriction without thyroxine treatment). Thyroxine treatment in rats with aortic stenosis resulted in an additional increment of cardiomegaly when compared to animals with aortic constriction alone. The results of this study indicate that thyroxine, which normally has no effect on Ca²⁺ activated cardiac myosin ATPase in the rat, can prevent the decrease in myosin ATPase activity which results from severe aortic stenosis.     

Heat, mechanics, and myosin ATPase in normal and hypertrophied heart muscle

In this paper we review our previous work on the myothermic economy of isometric force production in compensated cardiac hypertrophy secondary to pulmonary artery constriction (pressure overload) and/or thyrotoxicosis (volume overload). Hypertrophy-induced changes in isotonic and isometric twitch mechanics are correlated with accompanying changes in actin-activated myosin ATPase and heat liberation. Heat measurements were made with rapid, high-sensitivity thermopiles on right ventricular papillary muscles from normal and hypertrophied rabbit hearts. Total activity-related heat was separated into initial and recovery heat. Initial heat was separated into a tension-dependent component (TDH) relating to cross-bridge activity, and a tension-independent component (TIH) relating to excitation-contraction coupling. There were oppositely directed changes in most parameters studied in pressure overload hypertrophy (P) as compared with thyrotoxic hypertrophy (T). Thus, in P there was depression (30-50% in the rate of isometric force production, mechanical Vmax, TDH and TDH rate, myosin ATPase, TIH, and prolongation in time-to-peak twitch tension, whereas in T all parameters were oppositely changed except for no change in TIH. Thyrotoxicosis following pressure overload reversed the P-induced changes in all parameters. There was a direct, linear relation between in vitro actin-activated myosin ATPase and in vivo TDH. However, TDH per unit twitch tension or tension-time integral varied inversely with ATPase, making force production more economical than normal in P muscles and less economical than normal in T muscles. These cellular changes beneficially equip P hearts for slow, high-pressure, economical pumping the T hearts for fast, high-volume, uneconomical pumping. The differences are similar to those between slow and fast skeletal muscle and between neonatal and adult skeletal muscle. The mechanism of these changes is discussed in terms of an enzyme kinetic scheme of chemomechanical coupling in actomyosin interaction.     

Effect of hypertension on hearts of rats trained by swimming

To determine whether a prior chronic swimming program would alter the heart's response to chronic hypertension, female rats were made to swim for 10 wk, and then the left renal artery was stenosed. Heart perfusions were performed 10 wk later. The five groups studied were: control (C), normotensive swimmers (Sw), sedentary hypertensives (H), swimming rats made hypertensive and then allowed to be sedentary (Sw-H-Sd); and swimming animals made hypertensive and continued in a swimming program (Sw-H-Sw). Total heart and left ventricular weights were increased in increasing degrees in the sequence Sw, H, Sw-H-Sd, and Sw-H-Sw. Right ventricular weight was only increased in Sw and Sw-H-Sw. Swimming before the onset of hypertension enhanced total cardiac output and stroke work. Ejection fractions and mean velocity of circumferential fiber shortening (Vcf) were increased in Sw-H-Sd or Sw-H-Sw vs. controls. Myocardial O2 extraction was increased and coronary flow and myocardial O2 consumption were diminished in all hypertensive groups. However, lactate production was similar in all groups. Myosin adenosinetriphosphatase activity was increased in Sw but decreased in the three H groups. The percent of V1 myosin isozyme was greater and the percent of V3 less in Sw than in C; V1 was diminished and V3 increased in H and Sw-H-Sd; isozymes were normal in Sw-H-Sw.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular adaptations to mechanical overload

The cardiac changes resulting from mechanical overload of the left ventricle have been well documented and a variety of compensatory mechanisms described. These include a decrease in maximum velocity (V₀) of shortening in the absence of reduction in active tension (P₀), and a reversible decrease in myofibrillar adenosine triphosphatase activity resulting from isoenzymic shift from, predominantly, a form of myosin with high ATPase activity (V₁) to another with low (V₃). The thermodynamic advantage of the transition is the hypertrophied muscle possesses a more energy-efficient form of contraction. These reversible transitions resulted from altered gene expression of isoenzymic forms of myosin heavy chain. It must be borne in mind that the adaptational modifications just described appear to occur only in smaller animals such as the rat, that possesses several myosin isozymes. In large mammals it is mainly the V₃ form of myosin that is present, which does not change with altered contractile state. Responses of the large arteries to hypertension have been poorly studied. This is surprising when one recalls that degenerative disease of such vessels, that include the aorta, carotids and ileo-femoral arteries is almost an obligatory concomitant of hypertension. Such studies as have been carried out indicate that hyperplasia is specific for abdominal aortic stenosis while hypertrophy is found in aortic smooth muscle in rats with systemic hypertension. Mechanically, an increase in V₀ with no change in P₀ have been reported; an increase in myofibrillar ATPase activity was also reported. Though two myosin heavy chain isozymes have been found in aortic smooth muscle densitometry did not reveal any difference in distribution between tissues from control and hypertensive rats. The cause of the increased ATPase activity must be in increased phosphorylation of the muscles' 20,000 dalton light chain.     

Effect of endurance swimming on rat cardiac myofibrillar ATPase with experimental diabetes

Diabetes is characterized by depressed cardiac functional properties attributed to Ca2+-activated ATPase activity. In contrast, endurance swimming enhances the cardiac functional properties and Ca2+-activated myofibril ATPase. Thus, the purpose of this study was to observe if the changes associated with experimental diabetes can be ameliorated with training. Diabetes was induced with a single i.v. injection of streptozotocin (60 mg/kg). Blood and urine glucose concentrations were 802 +/- 44 and 6965 +/- 617 mg/dL, respectively. The training control and training diabetic animals were made to swim (+/- 2% body weight) 4 days/week for 8 weeks. Cardiac myofibril, at 10 microM free Ca2+ concentration was reduced by 54% in the sedentary diabetics compared with sedentary control animals (p less than 0.05). Swim training enhanced the Ca2+-activated myofibril ATPase activities for the normal animals. The diabetic animals, which swam for 8 weeks, had further reduced their Ca2+-activated myofibril ATPase activity when compared with sedentary diabetics (p less than 0.05). Similarly, the Mg2+-stimulated myofibril ATPase activity was depressed by 31% in diabetics following endurance swimming. It is concluded that the depressed Ca2+-activated myofibril ATPase activity of diabetic hearts is not reversible with endurance swimming.     

Protein synthesis, myosin ATPase activity and myofibrillar protein composition in hearts from tumour-bearing rats and mice.

Growing rats and adult weight-stable mice bearing a transplantable methylcholanthrene-induced sarcoma were compared with animals with various states of malnutrition. Heart protein synthesis was measured in vivo. Myocardial RNA, myofibrillar protein composition and the Ca2+-activated ATPase activity in heavy chains of native myosin were measured. 'Fingerprints' were made from myosin by trypsin treatment to evaluate possible structural changes in the protein. Cardiac protein-synthesis rate was decreased by 20% in growing tumour-bearing rats, by 35% in protein-malnourished (rats) and by 47% in starved rats, compared with freely fed controls (P less than 0.05). Adult tumour-bearing mice showed no significant decrease in myocardial protein synthesis. Pair-weighed control mice had significantly depressed heart protein synthesis. Protein translational efficiency was maintained in both tumour-bearing rats and mice, but was decreased in several groups of malnourished control animals. The Ca2+-activated myosin ATPase activity was decreased in all groups of malnourished animals, including tumour-bearing mice and rats, without any evidence of a change in cardiac isomyosin composition. We conclude that loss of cardiac muscle mass in tumour disease is communicated by both depressed synthesis and increased degradation largely owing to anorexia and host malnutrition. Increased adrenergic sensitivity in hearts from tumour-bearing and malnourished animals is not communicated by increased Ca2+-activated ATPase activity. This may be down-regulated in all groups with malnutrition, without any observable alterations in the isomyosin profile.     

Chronic swimming reverses cardiac dysfunction and myosin abnormalities in hypertensive rats

The purpose of this study was to determine whether a chronic swimming program could reverse the decreased cardiac function and altered myosin biochemistry found in hearts of rats with established renal hypertension. Ten wk after the onset of hypertension [midpoint (m)], hearts from normotensive controls (C) and hypertensives (H) were studied in an isolated working heart apparatus, and myosin biochemistry was analyzed. Half of the control and hypertensive animals were then subjected to a 10-wk swimming program (Sw) and their hearts were compared with those from age-matched sedentary rats. Body weight was no different at the midpoint of the study between Cm and Hm or at the end point (e) of the study among Ce, Swe, He, or H-Swe. Swimming had no effect on blood pressure in either normotensive or hypertensive rats. Dry heart weight was increased by 46% in Hm compared with Cm and by 36% in He, 21% in Swe, and 61% in H-Swe when compared with Ce. Hypertension was associated in both the mid- and end-point studies, with decreases in coronary flow, stroke work (both per gram left ventricle), ejection fraction, and midwall fractional shortening. In addition, actin-activated myosin adenosinetriphosphatase (ATPase) activity was decreased in Hm and He associated with an increase in the content of the V3 myosin isoenzyme. Although the coronary deficit was not corrected in H-Swe, stroke work, ejection fraction, and fractional midwall shortening were normalized compared with control hearts. Myosin ATPase activity and the myosin isoenzyme distribution were similarly restored in H-Swe.(ABSTRACT TRUNCATED AT 250 WORDS)     

Minimally invasive aortic banding in mice: effects of altered cardiomyocyte insulin signaling during pressure overload

We developed a minimally invasive method for producing left ventricular (LV) pressure overload in mice. With the use of this technique, we quickly and reproducibly banded the transverse aorta with low surgical morbidity and mortality. Minimally invasive transverse aortic banding (MTAB) acutely and chronically increased LV systolic pressure, increased heart weight-to-body weight ratio, and induced myocardial fibrosis. We used this technique to determine whether reduced insulin signaling in the heart altered the cardiac response to pressure overload. Mice with cardiac myocyte-restricted knockout of the insulin receptor (CIRKO) have smaller hearts than wild-type (WT) controls. Four weeks after MTAB, WT and CIRKO mice had comparably increased LV systolic pressure, increased cardiac mass, and induction of mRNA for beta-myosin heavy chain and atrial natriuretic factor. However, CIRKO hearts were more dilated, had depressed LV systolic function by echocardiography, and had greater interstitial fibrosis than WT mice. Expression of connective tissue growth factor was increased in banded CIRKO hearts compared with WT hearts. Thus lack of insulin signaling in the heart accelerates the transition to a more decompensated state during cardiac pressure overload. The use of the MTAB approach should facilitate the study of the pathophysiology and treatment of pressure-overload hypertrophy.     

Time course adaptations in rat skeletal muscle isomyosins during compensatory growth and regression

The purpose of this study was to ascertain the time course of change during both compensatory growth (hypertrophy) and subsequent growth regression on myosin isoform expression in rodent fast-twitch plantaris muscle in response to functional overload (induced by removal of synergists). Peak hypertrophy of the plantaris muscle (92%) occurred after 9 wk of overload. After 7 wk of overload regression (induced by a model of hindlimb unweighting), muscle weight returned to within 30% of control values. Myofibril protein content (mg/g muscle) remained relatively constant throughout the overload period but became significantly depressed relative to control values after 7 wk of regression. However, when expressed on a per muscle basis (mg/muscle) no differences existed at this time point (t = 7 wk regression). The distribution of native myosin isoforms in the myofibril protein pool of the overloaded plantaris muscle reflected a progressive increase (23% at t = 9 wk; P less than 0.001) in the relative proportion of slow myosin (Sm). This change was also accompanied by increases in intermediate myosin (Im) as well as the repression of the fast myosin one (Fm1) isoform (P less than 0.001). These shifts in Sm and Fm1 isoform expression were gradually reversed during the regression period, whereas Im remained elevated relative to control values. These adaptive changes in myosin isoform expression during both hypertrophy and regression were further supported by concomitant shifts in both myosin adenosinetriphosphatase (ATPase) activity (decreased during overload) and slow myosin light chain (SLC) expression. However, during regression the changes in myosin isoform expression and myosin ATPase were not as synchronous as they were during overload. Estimation of the mixed myosin heavy chain (MHC) half-life (t 1/2), using a linear model that assumes zero-order synthesis and first-order degradation kinetics, revealed t 1/2 values of approximately 19 and 10 days for the overload and regression periods, respectively. Collectively these data suggest that 1) skeletal muscle myosin isoforms and corresponding ATPase activity are in a dynamic state of change, although not completely synchronous, in response to altered muscle stress, and 2) the kinetics of change in the mixed MHC protein pool are slower during compensatory growth compared with regression of growth.     

Hypertrophic and dilated cardiomyopathy mutations differentially affect the molecular force generation of mouse alpha-cardiac myosin in the laser trap assay

Point mutations in cardiac myosin, the heart's molecular motor, produce distinct clinical phenotypes: hypertrophic (HCM) and dilated (DCM) cardiomyopathy. Do mutations alter myosin's molecular mechanics in a manner that is predictive of the clinical outcome? We have directly characterized the maximal force-generating capacity (F(max)) of two HCM (R403Q, R453C) and two DCM (S532P, F764L) mutant myosins isolated from homozygous mouse models using a novel load-clamped laser trap assay. F(max) was 50% (R403Q) and 80% (R453C) greater for the HCM mutants compared with the wild type, whereas F(max) was severely depressed for one of the DCM mutants (65% S532P). Although F(max) was normal for the F764L DCM mutant, its actin-activated ATPase activity and actin filament velocity (V(actin)) in a motility assay were significantly reduced (Schmitt JP, Debold EP, Ahmad F, Armstrong A, Frederico A, Conner DA, Mende U, Lohse MJ, Warshaw D, Seidman CE, Seidman JG. Proc Natl Acad Sci USA 103: 14525-14530, 2006.). These F(max) data combined with previous V(actin) measurements suggest that HCM and DCM result from alterations to one or more of myosin's fundamental mechanical properties, with HCM-causing mutations leading to enhanced but DCM-causing mutations leading to depressed function. These mutation-specific changes in mechanical properties must initiate distinct signaling cascades that ultimately lead to the disparate phenotypic responses observed in HCM and DCM.     

Effect of swimming training on cardiac function and myosin ATPase activity in SHR

Male spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) were subjected to swimming training 6 times/wk, commencing at 4 wk of age, to determine whether this type of endurance exercise might alter contractile proteins and cardiac function in young adult SHR. The total duration of exercise was 190 h. Myofibrillar adenosinetriphosphatase (ATPase) activity was assayed at various free [Ca2+] ranging from 10(-7) to 10(-5) M. Ca2+-stimulated ATPase activity of actomyosin and purified myosin was determined at various Ca2+ concentrations both in the low and high ionic strength buffers. Actin-activated myosin ATPase activity of purified myosin was assayed at several concentrations of actin purified from rabbit skeletal muscle. Under all these conditions the contractile protein ATPase activity was comparable between trained and untrained WKY and SHR. Analysis of myosin isoenzymes on pyrophosphate gels showed a single band corresponding to V1 isoenzyme, and there were no differences between swimming-trained and nontrained WKY and SHR. Ventricular performance was assessed by measuring cardiac output and stroke volume after rapid intravenous volume overloading. Both cardiac index and stroke index were comparable in nontrained WKY and SHR but were significantly increased in the trained groups compared with their respective nontrained controls. These results suggest that myosin ATPase activity and distribution of myosin isoenzymes are not altered in the moderately hypertrophied left ventricle whether the hypertrophy is due to genetic hypertension (SHR) or to exercise training (trained WKY). Moreover, the data indicate that SHR, despite the persistence of a pressure overload, undergo similar increases in left ventricular mass and peak cardiac index after training, as do normotensive WKY.     

Myosin from fast and slow skeletal and cardiac muscles of mammals of different size.

The ATPase activity of myosin and contraction time in extensor digitorum longus muscle, soleus muscle and cardiac muscle was compared in mammals differing in size. It was shown that the myosin ATPase activity of homologous muscles decreases and contraction time increases with increasing size of animals. The rate of tryptic digestion of myosin, the electrophoretic pattern of light chains of myosin and the effect of p-chloromercuribenzoate on ATPase activity of myosin were also studied. All these three myosin properties are very characteristic when the myosin from a fast muscle is compared with the myosin from a slow muscle of the same animal, but no relationship between these three myosin properties and ATPase activity of myosin was found, when homologous muscles of various mammals were compared.     

Modification of subcellular organelles in pressure-overloaded heart by etomoxir, a carnitine palmitoyltransferase I inhibitor.

To examine the signals regulating cardiac growth and molecular structure of subcellular organelles, cardiac hypertrophy was induced in rats by constriction of the abdominal aorta for 12-13 wk or by treatment with a carnitine palmitoyltransferase I inhibitor, etomoxir (12-15 mg/kg body wt) for 12-13 wk. In contrast to pressure overload, etomoxir redistributed the myosin isozyme population from V3 to V1 and increased the sarcoplasmic reticulum (SR) Ca(2+)-stimulated ATPase activity. When rats with pressure-overloaded hearts were treated with etomoxir, the cardiac hypertrophy was increased whereas the shift in myosin isozymes from V1 to V3 was prevented and the depression in SR Ca(2+)-stimulated ATPase activity was reversed. Plasma thyroid hormone and insulin concentrations were not altered but triglyceride concentrations were reduced in etomoxir-treated rats with pressure overload. The data demonstrate a dissociation between cardiac muscle growth and changes in subcellular organelles and indicate that a shift in myocardial substrate utilization may represent an important signal for molecular remodeling of the heart.     

Temporal changes in cardiac force- and flow-generation capacity, loading conditions, and mechanical efficiency in streptozotocin-induced diabetic rats

Diabetes mellitus may result in impaired cardiac contractility, but the underlying mechanisms remain unclear. We aimed to investigate the temporal alterations in cardiac force- and flow-generation capacity and loading conditions as well as mechanical efficiency in the evolution of systolic dysfunction in streptozotocin (STZ)-induced diabetic rats. Adult male Wistar rats were randomized into control and STZ-induced diabetic groups. Invasive hemodynamic studies were done at 8, 16, and 22 wk post-STZ injection. Maximal systolic elastance (E(max)) and maximum theoretical flow (Q(max)) were assessed by curve-fitting techniques, and ventriculoarterial coupling and mechanical efficiency were assessed by a single-beat estimation technique. In contrast to early occurring and persistently depressed E(max), Q(max) progressively increased with time but was decreased at 22 wk post-STZ injection, which temporally correlated with the changes in cardiac output. The favorable loading conditions enhanced stroke volume and Q(max), whereas ventriculoarterial uncoupling attenuated the cardiac mechanical efficiency in diabetic animals. The changes in E(max) and Q(max) are discordant during the progression of contractile dysfunction in the diabetic heart. In conclusion, our study showed that depressed Q(max) and cardiac mechanical efficiency, occurring preceding overt systolic heart failure, are two major determinants of deteriorating cardiac performance in diabetic rats.     

Immunological, electrophoretic and kinetic properties of cardiac myosins from various species.

1. In a homologous radioimmunoassay for canine ventricular myosin light chains, the following percentages of cross-reactivities were obtained using the dog as a reference: human, 28%; sheep, 21%; cat, 8%; guinea-pig, 7%; rabbit, 5%; and rat, 4%. 2. In a homologous double diffusion immunoassay using specific gamma G to canine cardiac myosin heavy chains, dog cardiac myosin showed immunological identity with human and sheep cardiac myosin but partial identity with myosins of other species. 3. On a 5-20% polyacrylamide gradient, light chain C1 was electrophoretically distinct in some species; light chain C2 was electrophoretically identical in all species. 4. The K+-activated myosin ATPase of small animals was higher than that of larger animals at an alkaline pH; the same was true for Ca2+-activated myosin when assayed at pH 6.3.     

Troponin subunits contribute to altered myosin ATPase activity in diabetic cardiomyopathy

Our group has documented that myocardial performance is impaired in the hearts of chronically diabetic rats and rabbits. Abnormalities in the contractile proteins and regulatory proteins may be responsible for the mechanical defects in the streptozotocin (STZ)-diabetic hearts. Previously, the major focus of our research on contractile proteins in abnormal states has concentrated on myosin ATPase and its isoenzymes. Our present study is based on the overall hypothesis that regulatory proteins, in addition to contractile protein, myosin contribute to altered cardiac contractile performance in the rat model of diabetic cardiomyopathy. The purpose of our research was to define the role of cardiac regulatory proteins (troponin-tropomyosin) in the regulation of actomyosin system in diabetic cardiomyopathy.For baseline data, myofibrillar ATPase studies were conducted in the myofibrils from control and diabetic rats. To focus on the regulatory proteins (troponin and tropomyosin), individual proteins of the cardiac system were reconstituted under controlled conditions. By this approach, myosin plus actin and troponin-tropomyosin from the normal and diabetic animals could be studied enzymatically. The proteins were isolated from the cardiac muscle of control and STZ-diabetic (4 weeks) rats. Sodium dodecyl sulfate gel electrophoretic patterns demonstrate differences in the cardiac TnT and TnI regions of diabetic animals suggesting the different amounts of TnT and/or TnI or possibly different cardiac isozymes in the regulatory protein complex. Myofibrils probed with a monoclonal antibody TnI-1 (specific for adult cardiac TnI) show a downregulation of cardiac TnI in diabetics when compared to its controls. Enzymatic data confirm a diminished calcium sensitivity in the regulation of the cardiac actomyosin system when regulatory protein(s) complex was recombined from diabetic hearts. Actomyosin ATPase activity in the hearts of diabetic animals was partially reversed when myosin from diabetic rats was regulated with the regulatory protein complex isolated from control hearts. To our knowledge, this is the first study which demonstrates that the regulatory proteins from normal hearts can upregulate cardiac myosin isolated from a pathologic rat model of diabetes. This diminished calcium sensitivity along with shifts in cardiac myosin heavy chain (V1V3) may be partially responsible for the impaired cardiac function in the hearts of chronic diabetic rats. (Mol Cell Biochem151: 165–172, 1995)     

Calcium requirements of cardiac myofibril ATPase activity following exhaustive exercise

Myocardial contractility is reduced in rats following strenuous activity. Thus, the purpose of this study was to determine some of the cellular mechanisms that may contribute to the depressed contractile function. Myofibril ATPase activity was determined with varying free calcium and monomeric vanadate (Vi) concentrations. The Mg2+ stimulated myofibril ATPase activities were significantly reduced in the activity group (E). Myofibril ATPase activity from control animals increased from 0.056 +/- 0.021 to 0.216 +/- 0.030 mumol X Pi X mg-1 X min-1 with 0.1-10.0 microM Ca2+. The addition of 15.0 microM Vi resulted in a 37% decrease in ATPase activity of C animals. With regard to the experimental group, the myofibril ATPase activity at 0.1 and 1.0 microM Ca2+ were depressed (P less than 0.05) with the values at 5.0 and 10.0 microM Ca2+ being similar to the control group (P greater than 0.05). Incubations with Vi resulted in an enhanced myofibril ATPase activity for E compared to C animals. The ATPase activities were increased by 17, 10, 10 and 15% at 3.0, 5.0, 10.0 and 15.0 microM Vi. The results suggest that the exhaustive exercise raises the CA2+ requirement for half-maximal activation of cardiac myofibril ATPase activity and that the contracto-regulatory mechanism of cardiac muscle is similarly altered.     

Effects of N-ethylmaleimide on the ATPase activities of cardiac myosin from thyrotoxic rabbits.

The ATPase activities of cardiac myosin from thyrotoxic and euthyroid rabbits have been compared. The Ca²⁺-ATPase activity of myosin from thyrotoxic animals was elevated by 200%, while the K⁺(EDTA)-ATPase activity was the same as in euthyroid animals. Modification by N-ethyl-maleimide of the most rapidly reacting class of sulfhydryls (SH₁) in myosin from euthyroid animals increased Ca²⁺-ATPase activity about 177% over the unreacted value. Modification of the SH₁ groups in myosin from thyrotoxic animals had no effect on CA²⁺-ATPase activity. We conclude that thyroxin may increase cardiac myosin ATPase activity by a conformational change in the same region as the SH₁ thiols.     

Exercise alters cardiac myosin isozyme distribution in obese Zucker and Wistar rats

Recent evidence suggests that exercise training may significantly increase the expression of the cardiac myosin isozyme V1 in the diabetic heart, a change associated with improved cardiac functional capacity. To test this hypothesis, cardiac myofibrillar adenosinetriphosphatase (ATPase) activity and myosin isozyme profiles were determined in trained and sedentary male hyperinsulinemic obese Zucker (OZT, OZS) and obese Wistar (OWT, OWS) rats. Lean sedentary (LZS, LWS) animals served as age-matched controls. Myofibrillar ATPase activity and the relative quantity of the high-ATPase isozyme V1 was significantly lower in both strains of sedentary obese rats than in the respective lean sedentary controls (P less than 0.05). Both 5 (OZT) and 10 wk (OWT) of moderate treadmill training increased these markers of cardiac myosin biochemistry in the obese animals (P less than 0.05). Thus, endurance exercise training remodels the cardiac isomyosin profile of hyperinsulinemic rats and, in doing so, may enhance cardiac contractility and functional capacity. Such changes may reflect an improvement in glucose availability and utilization in these hearts.     

Distribution and structure-function relationship of myosin heavy chain isoforms in the adult mouse heart

The two cardiac myosin heavy chain isoforms, alpha and beta, exhibit distinct functional characteristics and therefore may be distributed regionally within the heart to match the functional demands of a specific region. In adult mouse hearts, which predominantly express alpha-myosin heavy chain, we observed high concentrations of beta-myosin in distinct areas such as at the tip of papillary muscles and at the base close to the valvular annulus. In light of these distinct distribution patterns of the myosin isoforms, we subsequently explored the isoform-specific structure-function relationships of the myosins. The alpha- and beta-isoforms are 93% identical in amino acid sequence, but it remains unclear which of the nonidentical residues determines isoform functionality. We hypothesized that residues situated within or close to the actin-binding interface of the myosin head influence actin binding and thereby modulate actin-activated ATPase activity. A chimeric myosin was created containing beta-sequence from amino acid 417 to 682 within the alpha-backbone. In mice, approximately 70% of the endogenous cardiac protein was replaced with the chimeric myosin. Myofibrils containing chimeric myosin exhibited ATPase activities that were depressed to the levels observed in hearts expressing approximately 70% beta-myosin. In vitro motility assays showed that the actin filament sliding velocity generated by chimeric myosin was similar to that of alpha-myosin, almost twice the velocities observed with beta-myosin. These data indicate that this large domain sequence switch conferred beta-like actin-activated ATPase activities to the chimeric myosin, suggesting that this region is responsible for the distinct hydrolytic properties of these myosin isoforms.