Sperrtonus und Nucleosidtriphosphate

The resting anterior byssal retractor muscle of Mytilus edulis (ABRM) contains 1 -1.2 μmol ATP and 0.6 - 0.8 μmol ADP per g wet weight, traces of AMP and 0.1 - 0.2 μmol GTP plus UTP. The ATP content and the ratio of ADP : ATP remains almost constant during contraction, catch and subsequent serotonin induced relaxation, IMP could not be detected, not even during thaw contracture. Consequently the catch (viscous tone, fused state) is a reversible rigor distinct from rigor in vertebrate skeletal muscle which is caused by ATP deficiency. ATP produces a very much stronger contraction in isolated actomyosin systems of ABRM than UTP, ITP and GTP.

The calmodulin fraction responsible for contraction in an intestinal smooth muscle

Freeze-dried fibers of smooth muscle from Taenia coli were used to determine the concentration of calmodulin responsible for contraction. About 10% of the total intracellular calmodulin (12.6 mumol/kg wet wt) is directly involved in initiation of smooth muscle contraction.

Force responses of skinned molluscan catch muscle following photoliberation of ATP

Isometric force responses following flash photolysis of caged-ATP were measured from skinned preparations of the catch muscle anterior byssus retractor of Mytilus (ABRM). When fibres were transferred from Ca(2+)-free to Ca(2+)-containing rigor solution (pCa < 4) the force remained low, but flash photolysis produced an extended force increase (half-time, 0.30 +/- 0.07 s, n = 6). When Ca(2+)-activated fibres were transferred to a Ca(2+)-free rigor solution, their force remained at a high level. Flash photolysis produced a rapid force decay (half-time, 0.28 +/- 0.06 s, n = 9) to about 19% of the initial Ca(2+)-activated force. In the presence of 0.5 mM MgADP, the force increase was slowed down by a factor of 3 and the force decay by a factor of 5. These effects of MgADP on crossbridge kinetics are comparable to those observed in vertebrate smooth muscle and are thought to cause "latch", a catch-like state (Fuglsang et al. J Muscle Res Cell Motil 14:666-677, 1993). They are consistent with a model implicating competition between MgADP and MgATP for the nucleotide-binding site on crossbridges. Considering the relatively fast force responses induced by caged-ATP photolysis, even in the presence of MgADP, it appears unlikely that the detachment of crossbridges from the rigor state can account for catch-related processes. In view of the low myosin ATPase under maximal activating conditions (0.6 s-1, Butler et al. Biophys J 75:1904-1914, 1998), neither crossbridge attachment nor detachment of rigor crossbridges seems to be the rate-limiting processes of the crossbridge cycle.

Ca2+ sensitivity of stunned myocardium after skinning using retrograde infusion of detergent

Myofilament Ca2+ desensitization contributes to the contractile dysfunction of ischemic/reperfused ("stunned") myocardium. We examined the presence of reduced Ca2+ sensitivity of isometric force in chemically skinned fibers obtained from stunned myocardium using different modes of applying the detergent Triton X-100. Langendorff-perfused rat hearts underwent 20 min ischemia/20 min reperfusion, which caused a 35 +/- 3% decrease in left ventricular developed pressure, compared to continuously perfused control hearts. Stunned and control hearts were randomly allocated to two different permeabilization protocols: In group A, trabeculae were dissected and immersed in skinning solution containing 1% Triton X-100 for 20 min. Group B hearts remained fixed to the aortic cannula and skinning solution was infused retrogradely for 6 min prior to dissection of trabeculae. Extraction of cytosolic marker proteins was more complete in group-B than in group-A preparations. Group-A preparations from stunned hearts exhibited significant Ca2+ desensitization (pCa50 = 5.07 and 5.15 in stunned and control myocardium, respectively). In group B no such difference was observed, all preparations showing higher Ca2+ sensitivity and maximum force than group-A preparations (pCa50 = 5.32 in stunned versus 5.33 in control hearts). Prolonging group-A skinning to 150 min also abolished the difference in Ca2+ sensitivity between stunned and control myocardium. In conclusion, compared to a conventional protocol, skinning by perfusion results in more complete permeabilization and better preservation of myocardial contractile function. Ischemia/reperfusion at this moderate degree of contractile dysfunction induces Ca2+ desensitization at least partially by factors that can be extracted by thorough skinning.

Cardiac contractility: how calcium activates the myofilaments

In both cardiac and skeletal muscle, the force-generating molecular motors (crossbridges) are turned on by increasing the intracellular free calcium level that regulates the troponin-tropomyosin system. However, calcium activation is a two-way process in the sense that activated crossbridges also affect the troponin-tropomyosin system. Here we review the mechanism of calcium action on myofilament proteins, particularly tropomyosin, that affects both the extent and the rate of force development and hence the contractility of the myocardium. At low myoplasmic Ca2+ concentrations tropomyosin is located at the edge of the thin filament, thereby interfering with the formation of strong actin-myosin linkages (blocked state). An increase in Ca2+ activity causes an azimuthal shift of tropomyosin around the filament (by about 30 degrees), thereby increasing the probability of low-force crossbridge interaction, a process which by cooperative effects induces further tropomyosin movement (by an additional 10 degrees) which results in the open state of the filament characterized by forceful crossbridge interaction. (This mechanism may be analogous to that in ligand-gated ion channels, where ligand binding increases the open probability of the pore.) The extent of activation then depends on the free Ca2+ concentration and on the calcium sensitivity of the thin filament that may be affected by protein phosphorylation, crossbridge attachment, the troponin isoform composition of the filament, and novel calcium-sensitizing drugs that act on the contractile or regulatory proteins and thus increase the force of the heart.

Expression of vascular endothelial growth factor during the development of cardiac hypertrophy in spontaneously hypertensive rats

Left ventricular hypertrophy (LVH) is often associated with an impaired maximal coronary blood flow and increases the vulnerability of the heart tissue to ischaemia. In this study, the correlation between coronary blood flow and expression of the vascular endothelial growth factor (VEGF) mRNA was investigated. Using both haemodynamic measurements and analysis of mRNA, we have demonstrated that during development of LVH, in spontaneously hypertensive rats (SHR), an impaired maximal coronary flow at 12 weeks of age is associated with low levels of VEGF mRNA. However, in older SHR (32 weeks) with stabilised hypertrophy and a normal maximal coronary flow response, VEGF mRNA levels are increased 3-fold. These results suggest that the mechanism for the impaired flow, observed in some types of cardiac hypertrophy, might involve an inadequate growth of the coronary vessels due to insufficient activation of the VEGF gene.

Replacement of troponin-I in slow-twitch skeletal muscle alters the effects of the calcium sensitizer EMD 53998

We extracted troponin-I (TnI) from skinned rat and rabbit soleus muscle fibres using a modification of the method described by Strauss et al. (FEBS Lett 310:229-234, 1992) for replacement of TnI in cardiac preparations. Incubation of soleus muscle fibres with 10 mmol/l vanadate virtually completely abolished the Ca2+dependence of force. Immunoblot analysis revealed that more than 80% of TnI had been extracted from the preparations. The Ca2+dependence of force was restored by incubation with a complex of cardiac TnI (cTnI) and troponin-C (cTnC). We examined the effects of the Ca2+-sensitizing compound EMD 53998 on isometric tension in native porcine cardiac and rabbit soleus skinned fibres as well as soleus in which the endogenous slow skeletal TnI (ssTnI) had been replaced by cTnI (soleus-cTnI). It was found that 10 micromol/l EMD 53998 in native soleus increased maximum Ca2+-activated force to 120+/-1.4% of control. In soleus-cTnI fibres, maximum force was increased to only 105+/-0.9%, which was similar to the effect observed in cardiac muscle (108+/-0.6%). In cardiac muscle, 10 micromol/l EMD 53998 induced a leftward shift of the pCa-tension relation by 0.65 log units. In native soleus, DeltapCa was only 0.40. Again, the effect of EMD 53998 on soleus-cTnI (DeltapCa=0.56) more closely resembled the response found in cardiac muscle than that observed in native soleus muscle. The apparent TnI-isoform dependence of the effects elicited by EMD 53998 suggests that its actions are modulated by the regulatory proteins of the thin filament.

Striational autoantibodies in myasthenia gravis patients recognize I-band titin epitopes

Myasthenia gravis (MG) patients develop autoantibodies primarily against the acetylcholine receptor in the motor endplate, but also against intracellular striated muscle proteins, notably titin, the giant elastic protein of the myofibrillar cytoskeleton. Titin antibodies have previously been shown to be directed against a single epitope on the molecule, located at the A-band/I-band junction and referred to as the main immunogenic region (MIR) of titin. By using immunofluorescence microscopy on stretched single myofibrils, we now report that approximately 40% of the sera from 18 MG/thymoma patients and 8 late-onset MG patients with thymus atrophy contain antibodies that bind to a more central I-band titin region. This region consists of homologous immunoglobulin domains and is known to be differentially spliced dependent on muscle type. All patients with I-band titin antibodies also had antibodies against the MIR. Although a statistically significant correlation between the occurrence of I-band titin antibodies and MG severity was not apparent, the results could hint at an initial immunoreactivity to titin's MIR, followed by reactivity along the titin molecule in the course of the disease.

Exogenous caldesmon promotes relaxation of guinea-pig skinned taenia coli smooth muscles: inhibition of cooperative reattachment of latch bridges?

In smooth muscle, the state of prolonged contraction (latch state) is associated with very slow energy turnover and cycling of crossbridges that are dephosphorylated. A similar state may be reproduced in skinned fibres when the calcium-induced contraction is terminated by calcium removal with ethylenebis(oxonitrilo)tetraacetate (EGTA) and, during the slow relaxation that follows, force is maintained by dephosphorylated crossbridges that cycle slowly or not at all and may cooperatively reattach after detachment (Khromov et al. 1995, Biophys J 69:2611-2622). In guinea-pig skinned taenia coli that has been pretreated by prolonged incubation with caldesmon (5 microM), the rate of relaxation is approximately 1.6 times greater than in untreated controls, with half-times of relaxation being 1.3 and 2.1 min, respectively. In contrast, preloading the fibres with calponin does not accelerate relaxation. Preloading the fibres with caldesmon also accelerates the relaxation of skinned fibres from the state of rigor contraction when the latter is terminated by immersion into an ATP-containing relaxing solution or, in the presence of inorganic phosphate (Pi), also by flash-photolytic release of ATP from caged-ATP. Even in the latter case, relaxation is comparatively slow, possibly because of cooperative reattachment of dephosphorylated crossbridges which delays net crossbridge detachment and hence relaxation. We propose that by inhibition of cooperative reattachment caldesmon accelerates relaxation, even in the presence of Pi, and that the latch-like state of skinned fibres is supported by dephosphorylated cooperatively attaching crossbridges and may be regulated by the activity of caldesmon in the smooth muscle cell.

Actin-titin interaction in cardiac myofibrils: probing a physiological role

The high stiffness of relaxed cardiac myofibrils is explainable mainly by the expression of a short-length titin (connectin), the giant elastic protein of the vertebrate myofibrillar cytoskeleton. However, additional molecular features could account for this high stiffness, such as interaction between titin and actin, which has previously been reported in vitro. To probe this finding for a possible physiological significance, isolated myofibrils from rat heart were subjected to selective removal of actin filaments by a calcium-independent gelsolin fragment, and the "passive" stiffness of the specimens was recorded. Upon actin extraction, stiffness decreased by nearly 60%, and to a similar degree after high-salt extraction of thick filaments. Thus actin-titin association indeed contributes to the stiffness of resting cardiac muscle. To identify possible sites of association, we employed a combination of different techniques. Immunofluorescence microscopy revealed that actin extraction increased the extensibility of the previously stiff Z-disc-flanking titin region. Actin-titin interaction within this region was confirmed in in vitro cosedimentation assays, in which multimodule recombinant titin fragments were tested for their ability to interact with F-actin. By contrast, such assays showed no actin-titin-binding propensity for sarcomeric regions outside the Z-disc comb. Accordingly, the results of mechanical measurements demonstrated that competition with native titin by recombinant titin fragments from Z-disc-remote, I-band or A-band regions did not affect passive myofibril stiffness. These results indicate that it is actin-titin association near the Z-disc, but not along the remainder of the sarcomere, that helps to anchor the titin molecule at its N-terminus and maintain a high stiffness of the relaxed cardiac myofibril.

Dissociation of left ventricular hypertrophy, beta-myosin heavy chain gene expression, and myosin isoform switch in rats after ascending aortic stenosis

BACKGROUND

Reexpression of the fetal beta-myosin heavy chain (beta-MHC) gene was reported to be a marker for phenotypic reprogramming and cardiac hypertrophy in rats. Recent in vitro studies strongly suggested a role of angiotensin II for phenotypic reprogramming. In the present investigation, beta-MHC gene expression was studied in an experimental model of pressure-over-load hypertrophy that is not associated with a concurrent activation of the circulating renin-angiotensin system.

METHODS AND RESULTS

Hypertrophy was induced in rats by ascending aortic banding (n = 40). After 7 days, myosin contained 31% (P < .05) of the beta-MHC isoform in banded but < 5% in sham-operated animals. However, no specific elevation of beta-MHC mRNA levels was found in banded animals. In contrast, hearts of rats with abdominal aortic banding displayed a marked increase in beta-MHC mRNA levels (3-fold to 5-fold, P < .05). Both the left ventricular weight and left ventricular peak systolic pressure were significantly elevated compared with sham-operated animals (abdominal aortic banding, +13% and 164 +/- 7 mm Hg; ascending aortic banding, +27% and 191 +/- 9 mm Hg). Plasma renin activity was elevated in rats with abdominal aortic banding (2.5-fold, P < .05) but not in rats with ascending aortic banding.

CONCLUSIONS

The results of the present work do not support the concept that increased beta-MHC gene expression is a general "stable late marker" of myocardial hypertrophy in rats. Our results suggest that the stimulation of the renin-angiotensin system is crucial for the activation of the beta-MHC gene.

Differential effects of a K+ channel agonist and Ca2+ antagonists on myosin light chain phosphorylation in relaxation of endothelin-1-contracted tracheal smooth muscle

Smooth muscle contraction and relaxation are generally considered to be associated with phosphorylation and dephosphorylation of the 20-kDa regulatory myosin light chain (LC20). Thus, contractions of lamb tracheal smooth muscle induced by Bay K 8644 and relaxed by calcium channel blockers (verapamil, D-600 and nitrendipine) are accompanied by an increase and decrease, respectively, of LC20 phosphorylation. Similarly, endothelin-1 (ET-1) induces a sustained contraction, which is coupled with elevated LC20 phosphorylation and reversed by LC20 dephosphorylation after application of a potassium channel agonist (EMD 52692). In contrast, calcium channel blockers relax ET-1-induced contraction without any dephosphorylation of myosin light chains (MLC), suggesting that MLC phosphatase is inhibited in this case. Obviously, MLC dephosphorylation is not a prerequisite for smooth muscle relaxation. The variable relationship between MLC phosphorylation and force during relaxation suggests that there are mechanisms other than MLC phosphorylation that are important for regulation of contraction and relaxation in smooth muscle.

Cardiac contractility: modulation of myofibrillar calcium sensitivity by beta-adrenergic stimulation

Under conditions of beta-adrenergic receptor stimulation, cardiac performance is enhanced. cAMP-dependent phosphorylation of proteins located in the sarcolemma, in the membrane of the sarcoplasmic reticulum (SR), and in the myofibrils of the cardiomyocytes, mediates the effects of catecholamines on the heart. Altered Ca2+ handling leads to increased levels of intracellular free Ca2+. This is mainly responsible for the enhanced contractility of the myocardium that can be observed following beta-adrenergic receptor stimulation. Phosphorylation of the thin filament regulatory protein troponin I (TnI), on the other hand, decreases the Ca2+ sensitivity of the myofilaments, which means that the Ca2+ concentration necessary for the development of half-maximal force is increased. Cardiac TnI has a 26-33 amino acid N-terminal extension that is not present in fast and slow skeletal muscle TnI isoforms. Within this segment, two adjacent serine residues can be phosphorylated by a cAMP-dependent protein kinase. Replacement of endogenous TnI by different mutants obtained using site-directed mutagenesis of one or both of the serine residues has shown that only the bis-phosphorylated form decreases the Ca2+ sensitivity. This Ca2+ desensitizing effect, together with an increased rate of Ca2+ uptake into the SR due to phosphorylation of the SR membrane protein phospholamban, is responsible for the relaxation-enhancing effect (lusitropic action) of catecholamines. The latter is an important determinant of coronary perfusion and rapid diastolic filling of the ventricles, and is also a prerequisite for the elevation of heart rate that accompanies beta-adrenergic receptor stimulation.

Towards a molecular understanding of the elasticity of titin

Vertebrate striated muscle behaves elastically when stretched and this property is thought to reside primarily within the giant filamentous protein, titin (connectin). The elastic portion of titin comprises two distinct structural motifs, immunoglobulin (Ig) domains and the PEVK titin, which is a novel motif family rich in proline, glutamate, valine and lysine residues. The respective contributions of the titin Ig and the PEVK sequences to the elastic properties of the molecule have been unknown so far. We have measured both the passive tension in single, isolated myofibrils from cardiac and skeletal muscle and the stretch-induced translational movement of I-band titin antibody epitopes following immunofluorescent labelling of sites adjacent to the PEVK and Ig domain regions. We found that with myofibril stretch, I-band titin does not extend homogeneously. The Ig domain region lengthened predominantly during small stretch, but such lengthening did not result in measurable passive tension and might be explained by straightening, rather than by unfolding, of the Ig repeats. At moderate to extreme stretch, the main extensible region was found to be the PEVK segment whose unravelling was correlated with a steady passive tension increase. In turn, PEVK domain transition from a linearly extended to a folded state appears to be principally responsible for the elasticity of muscle fibers. Thus, the length of the PEVK sequence may determine the tissue-specificity of muscle stiffness, whereas the expression of different Ig domain motif lengths may set the characteristic slack sarcomere length of a muscle type.

Recombinant troponin I substitution and calcium responsiveness in skinned cardiac muscle

Using treatment with vanadate solutions, we extracted native cardiac troponin I and troponin C (cTnI and cTnC) from skinned fibers of porcine right ventricles. These proteins were replaced by exogenously supplied TnI and TnC isoforms, thereby restoring Ca2+-dependent regulation. Force then depended on the negative logarithm of Ca2+ concentration (pCa) in a sigmoidal manner, the pCa for 50% force development, pCa50, being about 5.5. For reconstitution we used fast-twitch rabbit skeletal muscle TnI and TnC (sTnI and sTnC), bovine cTnI and cTnC or recombinant sTnIs that were altered by site-directed mutagenesis. Incubation with TnI inhibited isometric tension in TnI-extracted fibers in the absence of Ca2+, but restoration of Ca2+ dependence required incubation with both TnI and TnC. Relaxation at low Ca2+ levels and the steepness of the force/pCa relation depended on the concentration of exogenously supplied TnI in the reconstitution solution (range 20-150 "mu"M), while Ca2+ sensitivity, i.e. the pCa50, was dependent on the isoform, and also on the concentration of TnC in the reconstitution solution. At pH 6.7, skinned fibers reconstituted with optimal concentrations of sTnC and sTnI (120 "mu"M and 150 "mu"M, respectively) were more sensitive to Ca2+ than those reconstituted with cTnC and cTnI (difference in pCa50 approx. 0.2 units). Rabbit sTnI was cloned and expressed in Escherichia coli using a high yield expression plasmid. We introduced point mutations into the TnI inhibitory region comprising the sequence of the minimal common TnC/actin binding site (-G104-K-F-K-R-P-P-L-R-R-V-R115-). The four mutants produced by substitution of T for P110, G for P110, G for L111, and G for K105 were chosen, based on previous work with synthetic peptides showing that single amino acid substitution in this region diminished the capacity of these peptides to inhibit acto-S1 ATPase or contraction of skinned fibers. Therefore, all amino acid residues of the inhibitory region are thought to contribute to biological activity of TnI. However, each of the recombinant TnIs could substitute for endogenous TnI. In combination with exogenous TnC, Ca2+ dependence could be restored when gly110sTnI, thr110sTnI or gly111sTnI was used for reconstitution. The mutant gly105sTnI, on the other hand, reduced the ability of skinned fibers to relax at low Ca2+ concentrations and it caused an increase in Ca2+ sensitivity.

Reconstitution of skinned cardiac fibres with human recombinant cardiac troponin-I mutants and troponin-C

Troponin C (TnC) could be extracted from skinned porcine cardiac muscle fibres and their Ca2+ sensitivity restored by reconstitution with recombinant human cardiac TnC. After extraction of troponin I (TnI) and TnC using the vanadate treatment method of Strauss et al. [Strauss, J. D., Zeugner, C., Van Eyk, J.E., Bletz, C., Troschka, M. and Rüegg, J.C. (1992) FEBS Lett. 310, 229-234], skinned porcine cardiac muscle fibres were reconstituted with wild-type recombinant human cardiac TnC and either wild-type cardiac TnI or several mutant isoforms of human TnI. Reconstitution with wild-type proteins restored the Ca2+ sensitivity of the tissue and phosphorylation of the TnI with the catalytic subunit of protein kinase A reduced the Ca2+ sensitivity (i.e.-log[Ca2+] for 50% of maximal force) as has been shown by others. However, reconstitution with the TnI mutant Ser-23Asp/Ser-24Asp mimicking the phosphorylated form of cardiac TnI, led to a reduced Ca2+ sensitivity compared with reconstitution with wild-type TnI, whereas the mutant Ser-23Ala/Ser-24Ala behaved as the dephosphorylated form of TnI. These data confirm the importance of negative charge in this region of the TnI molecule in altering the Ca2+ responsiveness in this system.

Ca(2+)- and GTP[gamma S]-induced translocation of the glucose transporter, GLUT-4, to the plasma membrane of permeabilized cardiomyocytes determined using a novel immunoprecipitation method

In cardiomyocytes glucose transport is activated not only by insulin but also by contractile activity that causes translocation of the glucose transporter, GLUT-4, from intracellular vesicles to the plasma membrane. The latter effect may possibly be mediated by intracellular Ca2+, as suggested by previous studies. To investigate the role of Ca2+, we permeabilized neonatal rat myocytes with alpha-toxin and incubated them for 1 h either at a pCa (i.e.--log10 [Ca2+]) of 8 (control) or at a pCa of 5 in the presence of adenosine 5'-triphosphate (ATP). Translocation of GLUT-4 was then monitored by a novel immunoprecipitation method using a peptide antibody directed against an exofacial (extracellular) loop of GLUT-4 (residues 58-80). Incorporation of GLUT-4 into the plasmalemma was stimulated 1.8-fold by 10 microM Ca2+ and 1.7-fold by insulin (as in the case of intact cells). The insulin effect was Ca2+ independent, i.e. it was identical in the absence and presence of Ca2+ (10 microM). Guanosine 5'-O-(3-thio-triphosphate) (GTP[gamma S]), which was inactive in intact cells, also caused translocation of GLUT-4 in permeabilized cardiomyocytes. Thus, incorporation of GLUT-4 into the plasma membrane was enhanced 2.5-fold by 200 microM GTP[gamma S] in the virtual absence of Ca2+ (pCa 8) and even 3.5-fold at 10 microM free Ca2+. We conclude that an increase in intracellular Ca2+ concentration increases GLUT-4 translocation of (permeabilized) cardiomyocytes to a similar extent as do insulin and GTP[gamma S] in the absence of Ca2+, but that the effects of Ca2+ and GTP[gamma S] may be additive.

Ca2+ sensitizing effects of EMD 53998 after troponin replacement in skinned fibres from porcine atria and ventricles

Skinned fibres from porcine ventricles exhibited a higher Ca2+ sensitivity (pCa50, i.e. -log10 Ca2+ concentration required for half-maximal activation, for force generation) than atrial fibres. The thiadiazinone derivative EMD 53998 increased Ca2+ sensitivity and Ca2+ efficacy in both preparations. The drug effect depended on the isoform of troponin (Tn). Using the vanadate method TnI and TnC could be partly extracted and replaced by foreign tropin or by the TnI subunit of added foreign troponins. We investigated the relationship between pCa and force development before and after replacement of TnI with foreign troponin (bovine ventricular troponin, cTn, or rabbit skeletal muscle troponin, sTn) in the presence and absence of EMD 53998. Substitution with bovine cTn increased Ca2+ sensitivity to a value characteristic of bovine ventricular skinned fibres (pCa50 = 5.4) and was further increased by EMD 53998. Substitution with sTn also increased Ca2+ sensitivity, but subsequent addition of EMD 53998 caused little further increase in Ca2+ sensitivity. Following extraction of TnI with vanadate, skinned fibres contracted in a Ca(2+)-independent manner and failed to relax at a pCa of 8. Relaxation could be induced, however, by bovine ventricular TnI and rabbit skeletal muscle recombinant TnI. This relaxation could be reversed by EMD 53998 (100 microM). The Ca(2+)-independent force of contracted fibres could also be depressed by a TnI inhibitory peptide, (cTnI 137-148) and, in addition, this effect was antagonized by EMD 53998.(ABSTRACT TRUNCATED AT 250 WORDS)

Myofibrillar Ca2+ sensitivity of cardiomyopathic hamster hearts

We studied the Ca2+ responsiveness of skinned muscle fibre preparations from the right and left ventricles of normal (FIB) and genetically cardiomyopathic (Bio-To-2) Syrian hamsters. Thus, we compared the Ca2+/force relationships of preparations from myopathic hamsters to those of age-matched (11-16 months old) normal animals. The pCa (i.e. -log10 [Ca2+]) required for 50% force activation (Ca2+ sensitivity) was higher in the myopathic hamsters than in controls (pCa50 values of 5.3 +/- 0.03 and 5.17 +/- 0.04, respectively); this difference might be due to an alteration in regulatory proteins. Indeed, after extraction (with vanadate) and replacement of troponin I with bovine cardiac troponin the pCa50 values were similar (pCa 5.35) to those of bovine ventricular fibres. The Ca2+ sensitizer EMD 53998 (10 microM) increased Ca2+ sensitivity in preparations from normal and cardiomyopathic hamsters equally, by 0.4 pCa units. Incubation of fibre bundles with the catalytic subunit of cyclic-adenosine-monophosphate-dependent protein kinase decreased Ca2+ sensitivity, thereby "normalizing" the enhanced Ca2+ responsiveness of fibres from cardiomyopathic hamsters. It is not clear, however, whether the pathologically increased Ca2+ sensitivity of the hearts of aged myopathic hamsters reflects a maladaptation, or a compensatory mechanism of the failing heart.

Effects of troponin-I extraction with vanadate and of the calcium sensitizer EMD 53998 on the rate of force generation in skinned cardiac muscle

The rate of tension development following release of ATP from caged-ATP in the presence of calcium was studied in skinned cardiac fibres from swine. A low-force rigor state was obtained by using butanedione monoxime (BDM) during the induction of rigor. BDM was washed out and following release of ATP in the presence of Ca2+ (pCa 4.3), the muscles contracted with an apparent rate of about 2 s-1 at 22 degrees C. After treatment with 10 mM vanadate to extract troponins I and C the fibres contracted independently of calcium. The rate of contraction upon release of ATP was slower than prior to extraction and was independent of [Ca2+]. Since treatment with vanadate has been shown to extract about 90% of troponin-I the results suggest that the muscles under these conditions are partially activated by removal of an inhibition of cross-bridge interaction by troponin I. A partial recovery of force was obtained by prolonged incubation in DTT containing solutions possibly reflecting reconstitution with troponin I still present in the fibre bundle. Treatment with a solution containing whole troponin caused almost complete recovery of calcium sensitivity and rate of force development. The calcium sensitizer EMD 53998 increased rates of contraction in a dose dependent manner, suggesting that this compound increases force and calcium sensitivity by increasing the cross-bridge attachment rates.

Coordination of nuclear and mitochondrial gene expression during the development of cardiac hypertrophy in rats

We studied the coordination of nuclear and mitochondrial gene expression during cardiac hypertrophy following aortic stenosis or thyroid hormone treatment in rats. We measured mRNA levels for representative subunits of cytochrome-c oxidase, two encoded by mitochondrial DNA and two encoded by the nucleus, as well as the levels of one mitochondrial rRNA. In both models of hypertrophy, an increase of total tissue RNA, reflecting mainly cytosolic ribosomes, accompanied the increase in ventricular weight. Relative levels of mitochondrial rRNA remained unchanged, indicating a net synthesis of mitochondrial ribosomes as well. In both models, cytochrome-c oxidase activity and nuclear-encoded mRNAs remained fairly constant, whereas levels of mitochondrial mRNAs were transiently decreased 24 h after the growth stimulus. We conclude that, in the initial phase of hypertrophy, the signal regulating the synthesis of mitochondrial rRNA is synchronized with nuclear gene expression, whereas the signal regulating mitochondrial mRNA synthesis is not. We postulate that differential regulation of mitochondrial transcription and premature termination of the polycistronic transcript (the latter giving rise to the mitochondrial rRNAs) account for the observed results.

Infusion of oxidized glutathione enhances postischemic segment-shortening in dog hearts

Oxidized glutathione (GSSG) but not its reduced form (GSH) is taken up by intact myocardial cells, and is rapidly converted into GSH. Reduced glutathione is an important intracellular defense against oxygen-derived free radicals and has been found to enhance calcium sensitivity in skinned cardiac fibers. We have investigated the effects of intravenous GSSG on left ventricular systolic pressure, maximal rate of rise of pressure and regional segment-shortening in dogs subjected to occlusion of the left anterior descending artery for 30 minutes, followed by 45 minutes reperfusion. Starting 10 minutes before reperfusion, the dogs were randomly treated with either GSSG (100 mM, 5 ml/min, n = 5) or Ringer's solution (5 ml/min, n = 5) until 30 minutes of reperfusion. Myocardial blood flow was measured by radioactive microspheres. Infusion of GSSG increased total glutathione content in both ischemic (47 +/- 16 mumol/g protein) and nonischemic myocardium (71 +/- 17 mumol/g protein) as compared to controls (23 +/- 2 mumol/g protein, p < 0.05). In both groups paradoxical wall motion occurred in the ischemic region during occlusion. On reperfusion, regional dyskinesia persisted in controls; while, in glutathione-treated dogs, systolic segment-shortening reached half the baseline values (p < 0.05, treated vs controls, at 15, 30, 45 minutes reperfusion). During ischemia the area of pressure-length loops, obtained from simultaneous recordings of left ventricular pressure and regional segment length, decreased to 30 +/- 7% of baseline in controls and to 40 +/- 18% of baseline in GSSG-treated animals. After 45 minutes reperfusion it was restored to 78 +/- 22% baseline in treated hearts but was still 36 +/- 16 of baseline in controls (p < 0.05). We conclude that infusion of GSSG increases the intracellular stores of glutathione and improves the contractile state of postischemic myocardium.

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.

Glutathione alters calcium responsiveness of cardiac skinned fibers

The glutathione status of cardiac muscle, that is the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) changes in certain forms of cardiomyopathy and during reperfusion of ischemic heart. Here we show that it also affects the sensitivity of contractile proteins to calcium. GSH (4 mM) increased the pCa50 for force development in skinned fibers by 0.2 pCa units and increased force by 44% +/- 5.4% at pCa 5.6 whereas GSSG (4 mM) decreased it by 54% +/- 17.8% at pCa 5.6. Half maximal activations and inhibitions were seen with 4 mM GSH or GSSG, respectively. In contrast to GSH, the reducing agent dithiothreitol at 5 mM had no activating effect. Our results suggest that the loss of contractility observed after a reperfusion of the ischemic heart may, at least in part, be due to a decreased responsiveness of the contractile proteins due to an altered glutathione status.