Inotropic effect of the cardioprotective agent 2,3-butanedione monoxime in failing and nonfailing human myocardium

The drug 2,3-butanedione monoxime (BDM) was suggested to be a potent cardioprotective agent useful for cardiopreservation. The present study investigated the activity of BDM and its mechanism of action in human myocardium. In electrically driven left ventricular papillary muscle strips and right atrial trabeculae from failing (heart transplants) and nonfailing (donor hearts) human myocardium, isometric force development and the force-frequency relationship were examined. To study the sarcolemmal actions of BDM, competition experiments with 125I-iodocyanopindolol, 3H-ouabain and 3H(+)PN 200-110 were performed. The effect of BDM on the contractile apparatus was tested by investigating its effects on Ca++ sensitivity and on the relaxation parameters of skinned fiber preparations. In papillary muscle strips and in atrial trabeculae, BDM (0.001-30 mM) depressed the isometric force of contraction in a concentration-dependent manner. BDM was more potent in atrial than in ventricular tissue and it shifted the Ca++ concentration-response curve in atrial and ventricular tissue to the right. The potency of BDM to depress force development was significantly lower compared with the L type of Ca++ channel antagonist nifedipine in both atrial and ventricular myocardium. In the presence of 10 mM BDM, the force-frequency relationship becomes positive in failing myocardium but not in the presence of 1 mM BDM, which did not affect the specific binding of 125I-iodocyanopindolol, 3H-ouabain or 3H(+)PN 200-110. This indicated there was no action on beta adrenoceptors, cardiac glycoside receptors and dihydropyridine-type Ca++ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

The calcium sensitizer EMD 53998 antagonizes phosphate-induced increases in energy cost of isometric tension in cardiac skinned fibres

We have investigated whether a Ca(2+)-sensitizing substance, the thiadiazinone derivative EMD 53998, can alter the ratio of ATPase activity to force, i.e. the tension cost in skinned fibres of swine cardiac trabecula in which the tension cost was increased by inorganic phosphate. In the presence of 10 mM inorganic phosphate (Pi) and thapsigargin 20 microM, EMD 53998 reduced the energy cost of isometric tension over the entire range of activating Ca2+ concentrations, resulting in a consistent change in slope (approximately 20% decrease) of the ATPase/force relation. We confirmed that in the absence of added phosphate and at maximal Ca2+ activation EMD 53998 had little if any effect on tension cost. We had previously reported that the effects of EMD 53998 and Pi on calcium sensitivity and maximum isometric tension are mutually antagonistic and our new energy data now support the proposal that EMD 53998 functionally antagonizes the effects of Pi on crossbridges. The decrease in the slope of the relation between ATPase and force caused by EMD 53998 may be interpreted to reflect either a decrease in the rate of 'detachment' (g(app)) of crossbridges or an increase in average force per crossbridge, as predicted by classical crossbridge models. Since the Pi release step of the crossbridge cycle is associated with the rate of 'attachment' (f(app)) rather than g(app), we conclude that the decrease in tension cost with EMD 53998 most likely reflects an increased force per crossbridge.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile properties of skinned preparations from ischaemic canine myocardium and coronary arteries

The influence of prolonged ischaemia on the regulation of contraction in the myocardium and in the smooth muscle of coronary arteries was investigated. Chemically skinned preparations were used which enabled the contraction to be studied with the environment of the contractile filaments controlled. Myocardial ischaemia was produced in anesthetized adult beagle dogs by occlusion of the left anterior descending artery for 3 h and followed by 30 min reperfusion. Myocardial tissue and segments from coronary arteries were obtained from the ischaemic infarcted wall region ("in vivo ischaemic") and compared with control preparations from perfused coronary arteries and from the free wall of the left ventricle. Coronary and myocardial preparations were also obtained from the heart after a 3 h period in vitro under anoxic conditions at 37 degrees C ("in vitro ischaemic") simulating a state of extreme ischaemia. Control myocardial fibres were fully relaxed at pCa (-log-[Ca2+]) 9 and developed 24 +/- 5% (n = 7) of maximum force at intermediate calcium concentration (pCa 5.5). In contrast, the in vivo and in vitro ischaemic preparations produced force at pCa 9 (28 +/- 13 and 39 +/- 8%, respectively, n = 5 and 7) and showed an increased force development at pCa 5.5 (53 +/- 11 and 75 +/- 5%). The in vivo and in vitro ischaemic coronary arteries relaxed more slowly following calcium removal than control vessels. The in vitro ischaemic vascular preparations developed active force at pCa 9 and showed increased levels of myosin light chain phosphorylation and reduced phosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A synthetic peptide mimics troponin I function in the calcium-dependent regulation of muscle contraction

A new technique for treating skinned cardiac muscle fibers has been developed in which troponin I is extracted, giving rise to unregulated fibers. The effect of the 12-residue troponin I peptide on these fibers indicates that this region of troponin I is solely responsible for muscle relaxation (inhibition of force). Furthermore, troponin I peptide-troponin C reconstituted fibers are stable through several contraction-relaxation cycles indicating the peptide can switch binding sites between actin and troponin C. The troponin I peptide can substitute for the native protein as part of the calcium-sensitive molecular switch that controls muscle regulation.

[Modulation of calcium sensitivity in the heart muscle--physiology, pathophysiology and pharmacology]

Angiotensin II has a plethora of different actions in the heart including, for instance, also slight positive inotropic effects. Thus cardiotonic therapy with angiotensin converting enzyme (ACE) inhibitors should possibly be complemented by positive inotropic drug therapy. With this respect, novel cardiotonic drugs that increase calcium sensitivity of myofilaments such as pimobendan (CAS 118428-36-7) might be particularly useful, as they will increase force with little, if any concomitant increase in myoplasmic free calcium. These drugs mimic mechanistically the Frank-Starling-Mechanism or alpha-adrenergic effects which also involve, at least partly, calcium sensitization of the myofilaments. The latter may therefore be a useful therapeutic principle, in particular in cases where myofibrillar calcium responsiveness is pathologically depressed such as for instance in the stunned myocardium or under hypoxic conditions.

Effect of myosin heavy chain peptides on contractile activation of skinned cardiac muscle fibres

Peptides derived from the sequence of the S1 domain of the myosin heavy chain were tested for their effects on the regulation of cardiac contractility. Basal calcium responsiveness of the contractile apparatus in terms of isometric tension generation and ATPase was determined in chemically demembranated ventricular fibre bundles. Incubation with a series of peptides derived from the peptide sequence around SH thiol group (Cys 707) resulted in a measurable increase in isometric tension and ATPase activity at sub-maximal concentrations of calcium but not at saturating levels of calcium activity, thus demonstrating a "calcium-sensitizing" effect of these peptides. The effects of two of these peptides, S1 687-716 and S1 701-717, are demonstrated to mimic, but importantly were not additive with, the calcium sensitization induced by lowering ATP concentration to 10 microM from 10 mM. This suggests the possibility of a similar mechanism of action underlying both types of sensitization. Because these effects demonstrate tissue specificity, were sensitive with respect to potency to not only amino acid composition but also sequence, and could not be duplicated by a similarly charged, non-homologous peptide, we attribute the effects to be specific to the sequences of these peptides. These data provide further evidence that the sequence between residues 687 and 717 of the S1 domain of the myosin heavy chain influences the calcium responsiveness of the contractile apparatus.

The positive inotropic calcium sensitizer EMD 53998 antagonizes phosphate action on cross-bridges in cardiac skinned fibers

The diazinone derivative EMD 53998 sensitizes skinned myocardial fibers to Ca2+ and enhances maximal calcium-activated force (pCa = 4.5) by approximately 100%; the EC50 is 10 microM in the absence and about 30 microM in the presence of added inorganic phosphate (10 mM). Although concentrations of added phosphate as low as 0.5 mM inhibit force, at high concentrations of EMD 53998 (> or = 50 microM), phosphate only inhibits at concentrations exceeding 20 mM. These data suggest that the effects of EMD 53998 and phosphate are mutually antagonistic. Importantly, both EMD 53998 and phosphate had similar effects on force generation in troponin I-depleted (Ca(2+)-independent) skinned fibers, thus demonstrating that these compounds are likely to affect cross-bridges directly and not via the Ca(2+)-regulatory system.

Troponin replacement in permeabilized cardiac muscle. Reversible extraction of troponin I by incubation with vanadate

Calcium-dependent regulation of tension and ATPase activity in permeabilized porcine ventricular muscle was lost after incubation with 10 mM vanadate. After transfer from vanadate to a vanadate-free, low-Ca2+ solution (pCa greater than 8), the permeabilized muscle produced 84.8% +/- 20.1% (+/- S.D., n = 98) of the isometric force elicited by high Ca2+ (pCa approximately 4.5) prior to incubation with vanadate. Transfer back to a high Ca2+ solution elicited no additional force (83.2% +/- 18.7% of control force). SDS-PAGE and immunoblot analysis of fibers and solutions demonstrated substantial extraction (greater than 90%) of Troponin I (TnI). Calcium dependence was restored after incubation with solutions containing either whole cardiac troponin or a combination of TnI and troponin C subunits. This reversible extraction of troponin directly demonstrates the role of TnI in the regulation of striated muscle contractility and permits specific substitution of the native TnI with exogenously supplied protein.

Counting target molecules by exponential polymerase chain reaction: copy number of mitochondrial DNA in rat tissues

In this report, we show that the actual number of target molecules of the polymerase chain reaction can be determined by measuring the concentration of product accumulating in consecutive cycles. The equation describing product accumulation, log Nn = log eff x n + log N0, can be analyzed by linear regression and the molar concentration of target at cycle zero, N0, is obtained. Using this new approach, the actual content of mitochondrial DNA was determined in rat tissues and ranged from 116 x 10(9) molecules/g in fast-twitch skeletal muscle to 743 x 10(9) molecules/g in liver. Using morphometric data from the literature, mitochondria were found to contain 1 to 3 DNA molecules. There was no relation between the oxidative capacity of a tissue and its content of mitochondrial DNA, indicating that transcriptional and posttranscriptional mechanisms rather than gene dosage, as postulated by others, determine to what extent the mitochondrial genome is expressed.

Peptide competition of actin activation of myosin-subfragment 1 ATPase by an amino terminal actin fragment

The amino-terminal region of actin participates in the binding of myosin subfragment 1 (S1) during cross-bridge cycling, thereby assisting in the activation of the magnesium-dependent myosin ATPase. Effects of three actin fragments on the magnesium-dependent S1 and acto-S1 ATPase activities in solution were studied. One of the peptides, containing residues actin 1-44, mimicked the S1 ATPase-activating properties of actin and in turn inhibited acto-S1 ATPase both in a concentration-dependent manner. This suggests peptide competition for the actin binding site on myosin. The other fragments, residues actin 1-18 and 82-119, respectively, had no detectable effect on S1- and acto-S1 ATPase activity.

Higher Ca2+ sensitivity of triton-skinned guinea pig mesenteric microarteries as compared with large arteries

Small mesenteric resistance arteries and the main branch of the mesenteric artery (outer in situ diameter 115 +/- 3 microns [n = 76] and greater than 1,000 microns, respectively) were skinned with 1% Triton X-100. Both preparations were mounted as rings for circumferential force measurement in an EGTA solution (free Ca2+, less than 10 nM; calmodulin, 0.3 microM; pH 6.7). Force-pCa curves were obtained by increasing free Ca2+ (0.05 to 30 microM). The resulting dose-dependent contractions, after normalization to maximal force development (arteries, 21.4 +/- 2.4 [n = 3]; arterioles, 15.3 +/- 2.1 mN/mm2 [n = 5]) were fitted to sigmoidal force-pCa curves. Values of ED50 and of the cooperativity factor h were 6.08 and 2.39 in arterioles and 5.64 and 1.64 in arteries. The higher Ca2+ sensitivity of arteriolar preparations remained at pH 7.0 at higher calmodulin concentrations and after inhibition of smooth muscle phosphatase with okadaic acid. Total myosin light chain kinase activity in crude arteriolar extracts (using [gamma-32P] ATP and isolated gizzard light chains as substrates) was approximately 25% of arterial kinase. Both kinase preparations had identical Ca2+ sensitivities. Likewise, total arteriolar phosphatase activity (using 32P-labeled gizzard light chains) was approximately 25% of the arterial activity; both phosphatases had an identical sensitivity toward okadaic acid. The ratio of kinase/phosphatase activities was identical in both tissues. Extracts of both tissues contained two isozymes of the myosin heavy chain as determined with sodium dodecyl sulfate-polyacrylamide gel electrophoresis.(ABSTRACT TRUNCATED AT 250 WORDS)

What does TnCDANZ fluorescence reveal about the thin filament state?

Fluorescence of skinned psoas fibres reconstituted with the troponin C subunit labelled with the fluorescent probe dansylaziridine (TnCDANZ) increases upon activation with Ca2+. This fluorescence enhancement is due to Ca2+ binding to the Ca(2+)-specific binding sites of TnCDANZ and attachment of cross-bridges to the actin filament. We found that approximately 20% of the enhanced fluorescence signal derived from Ca2+ binding to TnCDANZ and 80% from cross-bridge attachment during maximal activation. Furthermore we studied the effects of different cross-bridge states on TnCDANZ fluorescence. "Weakly" bound, non-force-generating cross-bridge states (pCa 8, low ionic strength) and rigor cross-bridges revealed similar effects on the TnCDANZ fluorescence. "Strongly" attached, force-generating states, however, increased fluorescence to the greatest extent. These results suggests a complex system of reciprocal couplings between TnC and different attached cross-bridge states. Cooling or increase of inorganic phosphate decreased isometric force but hardly decreased fluorescence, suggesting the accumulation of attached cross-bridge states with low tension output.

Contribution of cAMP-phosphodiesterase inhibition and sensitization of the contractile proteins for calcium to the inotropic effect of pimobendan in the failing human myocardium

Previous studies have shown reduced effects of cAMP-dependent positive inotropic agents in the failing human myocardium; thus other cAMP-independent mechanisms of action may be useful to increase force of contraction in this condition. The purpose of this investigation was to determine whether a positive inotropic effect of the cAMP-phosphodiesterase (PDE) inhibitor pimobendan is observed in the failing human myocardium and to study whether other factors, such as an increase in the Ca2+ sensitivity of myofilaments, play a functional role in the increase in force of contraction. Pimobendan produced a positive inotropic effect in isolated preparations from nonfailing donor hearts; however, in moderately (New York Heart Association class II-III, NYHA II-III) and severely (NYHA IV) failing myocardium, this effect was reduced. In addition, in NYHA IV specimens pimobendan inhibited the crude cAMP-PDE (crude PDE) and the isoenzymes I-III (PDE I-III) in a concentration-dependent way. As judged from the IC50 values found in this tissue for the inhibition of PDE III and of crude PDE, the potency of the compound was 18.1 times greater on PDE III. Consistent with a cAMP-PDE-dependent mechanism of action, the positive inotropic effect was potentiated by isoproterenol and inhibited by adenosine in failing myocardium. In failing myocardium, pimobendan also increased the sensitivity of skinned cardiac fibers to Ca2+ and shifted the Ca(2+)-tension relation to the left. This sensitizing effect began at 0.01 mumol/l in NYHA II-III and NYHA IV and rose to about 200% at 300 mumol/l in both groups. In contrast, the demethylated metabolite UD-CG 212 Cl failed to produce positive inotropic effects in failing myocardium alone, but in the presence of isoproterenol, it exerted an increase in force of contraction. The potency of UD-CG 212 Cl for PDE III inhibition in NYHA IV was greater than that of pimobendan. The metabolite pronouncedly decreased the sensitivity of skinned cardiac fibers to Ca2+ at 30-300 mumol/l in NYHA II-III and NYHA IV. It is concluded that in the failing human heart pimobendan inhibited PDE III and sensitized contractile proteins for Ca2+. Both effects appear to be involved in the positive inotropic effect of the compound, because its metabolite, UD-CG 212 Cl, had no effect on force of contraction and on the Ca2+ sensitivity of skinned cardiac fibers but inhibited PDE III even more potently than pimobendan.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of crossbridge kinetics by myosin isoenzymes in skinned human heart fibers

Skinned fibers from the normal human heart with the beta-myosin heavy chain (ventricular fibers) revealed both a higher force generation per cross section and a higher Ca2+ sensitivity than skinned fibers with the alpha-myosin heavy chain (atrial fibers). The relation between isometric ATPase activity and isometric tension of atrial fibers was higher than that of ventricular fibers. Since the ATPase-tension relation equals the rate constant for the transition from force-generating into non-force-generating crossbridge states (g(app)), myosin heavy chain isoenzymes seem to have different crossbridge turnover kinetics. Modulation of g(app) by myosin heavy chain isoenzymes could explain the different contractile behavior of atrial and ventricular fibers. g(app) was independent of Ca2+.

Contractile protein interactions in smooth muscle

Smooth muscle tone and 'holding economy' depend on the rate constants governing the cross-bridge cycle. Thus, calcium activation via calmodulin-dependent myosin light chain phosphorylation may determine the apparent rate constant ('f') at which cross-bridges enter the force-generating state, forming actin-attached, strongly bound cross-bridges. This phosphorylation of the light chain may be inhibited in skinned fibers by a peptide mimic of the calmodulin recognition site of the myosin light chain kinase (RS 20) that relaxes smooth muscle. In smooth muscle, the apparent cross-bridge detachment rate constant ('g') also seems to be variable, a low constant allowing for a high holding economy and low shortening velocity in the 'latch state'. It may also account for force maintenance at low levels of myosin phosphorylation. Additionally, cross-bridge attachment may, however, be also controlled by other regulatory proteins such as calponin and caldesmon.

Calcium sensitivity and myosin light chain pattern of atrial and ventricular skinned cardiac fibers from patients with various kinds of cardiac disease

In the present study, the Ca2(+)-sensitivity and myosin light chain patterns of skinned fibers of right atrium and left papillary muscles of 27 patients suffering from mitral valve disease (MVD, moderate heart failure), ischemic cardiomyopathy (ICM, severe heart failure), dilated cardiomyopathy (DCM, severe heart failure), and coronary heart disease (CHD, no heart failure, no atrial hypertrophy) were investigated. Myosin light chains of both chemically skinned and intact samples were studied by two-dimensional gel electrophoresis (2D-PAGE). Ca2(+)-sensitivity of ventricular fibers was about 0.14 pCa-units higher than that of atrial fibers in all groups except dilated cardiomyopathy where this difference was markedly diminished (only 0.06 pCa-units). Generally, Ca2(+)-sensitivity of skinned ventricular fibers was the same among the different heart diseases. Skinned atrial fibers from patients with dilated cardiomyopathy, however, were significantly (about 0.08 pCa-units) more sensitive for Ca2+ than those of the other groups (coronary heart disease, mitral valve disease or ischemic cardiomyopathy) which showed similar Ca2(+)-tension relationships. Ventricle-specific P-light chain forms could be observed in atrial samples from patients of all groups, whereas no atrium-specific light chain forms were detectable in any ventricular sample. It is concluded that there is no difference in Ca2(+)-sensitivity of the ventricular contractile elements of the human heart in different heart diseases. In atrial myocardium, there is an increased Ca2(+)-sensitivity of skinned fibers from hearts with dilated cardiomyopathy which is probably related to an elevation of right atrial pressure.

Towards a molecular understanding of contractility

The contractile performance of the heart depends on the molecular force generators, the myosin crossbridges between thick and thin filaments. These are oscillating between two states, strong and weak, or force-generating and non-force-generating. Calcium activation of the contractile machinery increases the probability of forming force-generating crossbridges, whereas the speed of contraction depends on the probability of returning force-generating crossbridges into weak ones. It is inversely related to the energetic cost of contraction. The probabilities or rate constants describing the transitional states of the force generators (and denoted as g and f) describe not only the kinetic properties of the contractile system but also its responsiveness to the calcium activator. The development of force depends not only on the free calcium ion concentration in the myoplasm, but also on calcium responsiveness which is dependent both on crossbridge kinetics and the calcium affinity of the regulatory protein, troponin. In future, it will be important to find out how all these molecular properties of the force-generators are affected in diseased states as well by pharmacological interventions, such as the application of novel cardiotonic drugs.

Inorganic phosphate inhibits contractility and ATPase activity in skinned fibers from human myocardium

During hypoxic heart failure, inorganic phosphate (Pi) accumulates. We report the effects of Pi on force development and on myofibrillar ATPase-activity of human skinned atrial fibers, both at normal and at reduced levels of Mg-ATP. Pi (10 mM) depressed force production at maximal calcium activation (pCa 4.3) by about 40%. At higher pCa values (pCa 5.6), force inhibition was even more pronounced, but at low concentrations of Mg-ATP (10 microM), Pi was less effective. In contrast to contractile force, myofibrillar ATPase was only inhibited by about 10% at pCa 4.3, whereas it could be inhibited by 40-50% at submaximal calcium activation (pCa 5.6). As Pi inhibited contractile force more than ATPase activity, the ratio of ATPase-activity to force (tension cost) was increased by inorganic phosphate. ATPase-activity and tension cost were significantly reduced by lowering Mg-ATP concentration to 10 microM, whereas contractile force was less affected. Pi did not affect ATPase under these conditions at 10 mM Mg-ATP. Pi also shifted the calcium-force relationship towards higher Ca++ concentrations, that is, it decreased calcium sensitivity. In contrast, the calcium sensitivity of myofibrillar ATPase was less affected. These findings suggest that inorganic phosphate may affect the myocardium by altering crossbridge kinetics rather than the calcium affinity of troponin-C. Because of its inhibitory effect on myofibrillar ATPase, inorganic phosphate may be partly cardioprotective in the hypoxic myocardium. However, this "energy sparing' effect is probably offset by the greater "tension cost' that decreases the "efficiency' of tension maintenance in the presence of inorganic phosphate.

Phosphorylation and thiophosphorylation by myosin light chain kinase: different effects on mechanical properties of chemically skinned ventricular fibers from the pig

The influence of myosin light chain phosphorylation (treatment with myosin light chain kinase = MLCK, calmodulin and ATP) and thiophosphorylation (incubation with MLCK, calmodulin and ATP gamma S) on the maximal shortening velocity (Vmax) and Ca2+ sensitivity of chemically-skinned ventricular fibers from the pig has been studied. Vmax was determined by the slack-test method and by extrapolation of the force-velocity relation by the isotonic quick release method. Vmax was 1.53 muscle length/s (L/s) and 1.94 L/s using the force-velocity relation and the slack-test, respectively. Phosphorylation increased the Ca2+ sensitivity for isometric force development of skinned fibers but had no influence on Vmax. Thiophosphorylation decreased Vmax but had no influence on Ca2+ sensitivity. Phosphorylation pattern of the myosin light chains of the skinned fibers was studied using [gamma-32P]ATP or [gamma-P35S]ATP (250 muCi each) and autoradiography. Incubation of skinned fibers with labeled ATP led to a phosphate incorporation into the 18-kDa myosin light chain (MPLC or regulatory light chain) while incubation with labeled ATP gamma S led to an incorporation of thiophosphate into the 28-kDa myosin light chain (alkali light chain) and tropomyosin. We suggest that the difference in mechanical behavior between phosphorylated and thiophosphorylated skinned fibers are due to differences in the phosphorylation profiles of myofibrillar regulatory proteins.

Temporal relationship between force, ATPase activity, and myosin phosphorylation during a contraction/relaxation cycle in a skinned smooth muscle

The temporal relationship between myosin phosphorylation, contractile force and ATPase activity was studied in skinned preparations from the guinea-pig Taenia coli. When free Calcium concentration [( Ca2+]) was increased from pCa (-log[Ca2+]) 9 to pCa 4.5 at low calmodulin concentration (0.05 microM), ATPase activity and myosin light-chain phosphorylation rose quickly, while the increase in force and stiffness was delayed. The time-course of tension increase was faster at higher calmodulin concentrations (5 microM), although the maximal level of phosphorylation was unchanged. Lowering the calcium concentration from pCa 4.5 to pCa 9 at the plateau of contraction caused a rapid decrease in ATPase activity and in myosin phosphorylation, while force and stiffness decayed more slowly. The force decay could be accelerated by inorganic phosphate. These results suggest that, during contraction, force may be produced actively by phosphorylated and ATP-splitting crossbridges, but may be maintained by dephosphorylated crossbridges which cycle slowly. However, force could also be modulated by calmodulin and inorganic phosphate in a manner not involving an alteration in the extent of myosin phosphorylation.

Regulation of myosin heavy chain expression in the hearts of hypertensive rats by testosterone

Stroke-prone spontaneously hypertensive rats were used for our investigation of the influence of prepubertal gonadectomy and testosterone substitution on blood pressure, cardiac hypertrophy, and the expression of ventricular myosin heavy chain (MHC) isoenzymes at different developmental stages. Blood pressure and the degree of cardiac hypertrophy were decreased by castration and increased by testosterone substitution. We found the same relative distributions of MHC isoforms on the protein level (investigated by pyrophosphate electrophoresis) and on the messenger RNA level (investigated by the polymerase chain reaction). Castration favored the expression of the beta-MHC form, and testosterone substitution enhanced the expression of the alpha-MHC form. These effects were more pronounced in 8-week-old than in 14-16-week-old animals. We conclude that testosterone regulates cardiac MHC expression on a pretranslational level. This regulation is independent of hemodynamic load or cardiac hypertrophy.

Stretch-induced increase in the Ca2+ sensitivity of myofibrillar ATPase activity in skinned fibres from pig ventricles

The increase in force development in the heart with increase in end-diastolic pressure (Frank-Starling mechanism) has been ascribed to an increase in contractile responsiveness of the myofibrils to calcium. We now show that this calcium sensitization is also associated with an increase in calcium responsiveness of the myofibrillar ATPase. Thus, at submaximal Ca activation (pCa 6.0), the ATPase activity of skinned fibres from pig right ventricles is increased from 57.9 +/- 4.4% to 70.6 +/- 4.4% of the maximal Ca2+ activation of ATPase by stretching (by 15% lo). At maximal Ca2+ activation, ATPase was barely altered by stretching. The relationship between ATPase activity of skinned trabecula of pig right ventricle and ATPase-Ca2+ concentrations is shifted (by 0.1 pCa unit) to higher pCa values after a stretch-induced increase of the sarcomere length from 2.1 microns to 2.4 microns. The relationship between force and pCa was affected in a similar way by extension. This increased calcium sensitivity is, however, not associated with an alteration in the relationship between ATPase activity and force development (tension cost). In accordance with Brenner's hypothesis, we propose therefore that stretch activation of ATPase is associated with an increase in the apparent rate constant of crossbridge attachment rather than with a decrease in the apparent rate constant of crossbridge detachment.