Decreased expression of phospholamban is not associated with lower beta-adrenergic activation in rat atria.

The aim of the study was to find out whether low phospholamban level in atria as compared with ventricles is associated with differences in sarcoplasmic reticular Ca2+-uptake and contractile performance. Relationship between phospholamban and beta-adrenergic stimulation in rat left atria and papillary muscles were examined by means of contractile measurements, sarcoplasmic reticular oxalate-supported Ca2+-uptake, and Western blotting of phosphorylated phospholamban. Phosphoprotein determination after beta-adrenergic stimulation demonstrated that the levels of Ser16 and Thr17 phosphorylated phospholamban in atria remained at about one-third of that in ventricles. However, comparison of sarcoplasmic reticular Ca2+-uptake in control and isoproterenol perfused preparations demonstrated that the effect of beta-adrenergic stimulation on sarcoplasmic reticular Ca2+-uptake was stronger in atrial preparations. Moreover, atria responded to isoproterenol with much larger increases in developed tension, contractility and relaxation rates than papillary muscles. Thus, despite lower level of phospholamban, the beta-adrenergic activation of sarcoplasmic reticular Ca2+-uptake and contractile indices are higher in atria.

Kettin, a major source of myofibrillar stiffness in Drosophila indirect flight muscle.

Kettin is a high molecular mass protein of insect muscle that in the sarcomeres binds to actin and alpha-actinin. To investigate kettin's functional role, we combined immunolabeling experiments with mechanical and biochemical studies on indirect flight muscle (IFM) myofibrils of Drosophila melanogaster. Micrographs of stretched IFM sarcomeres labeled with kettin antibodies revealed staining of the Z-disc periphery. After extraction of the kettin-associated actin, the A-band edges were also stained. In contrast, the staining pattern of projectin, another IFM-I-band protein, was not altered by actin removal. Force measurements were performed on single IFM myofibrils to establish the passive length-tension relationship and record passive stiffness. Stiffness decreased within seconds during gelsolin incubation and to a similar degree upon kettin digestion with mu-calpain. Immunoblotting demonstrated the presence of kettin isoforms in normal Drosophila IFM myofibrils and in myofibrils from an actin-null mutant. Dotblot analysis revealed binding of COOH-terminal kettin domains to myosin. We conclude that kettin is attached not only to actin but also to the end of the thick filament. Kettin along with projectin may constitute the elastic filament system of insect IFM and determine the muscle's high stiffness necessary for stretch activation. Possibly, the two proteins modulate myofibrillar stiffness by expressing different size isoforms.

Energetic crosstalk between organelles: architectural integration of energy production and utilization.

Cells with high and fluctuating energy demands such as cardiomyocytes need efficient systems to link energy production to energy utilization. This is achieved in part by compartmentalized energy transfer enzymes such as creatine kinase (CK). However, hearts from CK-deficient mice develop normal cardiac function under conditions of moderate workload. We have therefore investigated whether a direct functional interplay exists between mitochondria and sarcoplasmic reticulum or between mitochondria and myofilaments in cardiac cells that catalyzes direct energy and signal channeling between organelles. We used the selective permeabilization of sarcolemmal membranes with saponin to study the functional interactions between organelles within the cellular architecture. We measured contractile kinetics, oxygen consumption, and caffeine-induced tension transients. The results show that in hearts of normal mice, ATP produced by mitochondria (supplied with substrates, oxygen, and adenine nucleotides) was able to sustain calcium uptake and contractile speed. Moreover, direct mitochondrially supplied ATP was nearly as effective as CK-supplied ATP and much more effective than externally supplied ATP, suggesting that a direct ATP/ADP channeling exists between the sites of energy production (mitochondria) and energy utilization (sarcoplasmic reticulum and myofilaments). On the other hand, in cardiac cells of mice deficient in mitochondrial and cytosolic CK, marked cytoarchitectural modifications were observed, and direct adenine nucleotide channeling between mitochondria and organelles was still effective for sarcoplasmic reticulum and myofilaments. Such direct crosstalk between organelles may explain the preserved cardiac function of CK-deficient mice under moderate workloads.

Unfolding of titin domains explains the viscoelastic behavior of skeletal myofibrils.

The elastic section of the giant muscle protein titin contains many immunoglobulin-like domains, which have been shown by single-molecule mechanical studies to unfold and refold upon stretch-release. Here we asked whether the mechanical properties of Ig domains and/or other titin regions could be responsible for the viscoelasticity of nonactivated skeletal-muscle sarcomeres, particularly for stress relaxation and force hysteresis. We show that isolated psoas myofibrils respond to a stretch-hold protocol with a characteristic force decay that becomes more pronounced following stretch to above 2.6-microm sarcomere length. The force decay was readily reproducible by a Monte Carlo simulation taking into account both the kinetics of Ig-domain unfolding and the worm-like-chain model of entropic elasticity used to describe titin's elastic behavior. The modeling indicated that the force decay is explainable by the unfolding of only a very small number of Ig domains per titin molecule. The simulation also predicted that a unique sequence in titin, the PEVK domain, may undergo minor structural changes during sarcomere extension. Myofibrils subjected to 1-Hz cycles of stretch-release exhibited distinct hysteresis that persisted during repetitive measurements. Quick stretch-release protocols, in which variable pauses were introduced after the release, revealed a two-exponential time course of hysteresis recovery. The rate constants of recovery compared well with the refolding rates of Ig-like or fibronectin-like domains measured by single-protein mechanical analysis. These findings suggest that in the sarcomere, titin's Ig-domain regions may act as entropic springs capable of adjusting their contour length in response to a stretch.

Subcellular creatine kinase alterations. Implications in heart failure.

We have tested the hypothesis that decreased functioning of creatine kinase (CK) at sites of energy production and utilization may contribute to alterations in energy fluxes and calcium homeostasis in congestive heart failure (CHF). Heart failure was induced by aortic banding in 3-week-old rats. Myofilaments, sarcoplasmic reticulum (SR), mitochondrial functions, and CK compartmentation were studied in situ using selective membrane permeabilization of left ventricular fibers with detergents (saponin for mitochondria and SR and Triton X-100 for myofibrils). Seven months after surgery, animals were in CHF. A decrease in total CK activity could be accounted for by a 4-fold decrease in activity and content (Western blots) of mitochondrial CK and a 30% decrease in M isoform of CK (MM-CK) activity. In myofibrils, maximal force, crossbridge kinetics, and alpha-myosin heavy-chain expression decreased, whereas calcium sensitivity of tension development remained unaltered. Myofibrillar CK efficacy was unchanged. Calcium uptake capacities of SR were estimated from the surface of caffeine-induced tension transient (SCa) after loading with different substrates. In CHF, SCa decreased by 23%, and phosphocreatine was 2 times less efficient in enhancing calcium uptake. Oxidative capacities of the failing myocardium measured as oxygen consumption per gram of fiber dry weight decreased by 28%. Moreover, the control of respiration by creatine, ADP, and AMP was severely impaired. Our observations provide evidence that alterations in CK compartmentation may contribute to alterations of energy fluxes and calcium homeostasis in CHF.

Nitric oxide inhibits cardiac energy production via inhibition of mitochondrial creatine kinase.

Nitric oxide biosynthesis in cardiac muscle leads to a decreased oxygen consumption and lower ATP synthesis. It is suggested that this effect of nitric oxide is mainly due to the inhibition of the mitochondrial respiratory chain enzyme, cytochrome c oxidase. However, this work demonstrates that nitric oxide is able to inhibit soluble mitochondrial creatine kinase (CK), mitochondrial CK bound in purified mitochondria, CK in situ in skinned fibres as well as the functional activity of mitochondrial CK in situ in skinned fibres. Since mitochondrial isoenzyme is functionally coupled to oxidative phosphorylation, its inhibition also leads to decreased sensitivity of mitochondrial respiration to ADP and thus decreases ATP synthesis and oxygen consumption under physiological ADP concentrations.

Mechanisms of thyroid hormone control over sensitivity and maximal contractile responsiveness to beta-adrenergic agonists in atria.

This paper discusses the mechanisms of two basic effects of thyroid hormones on atrial responses to beta-adrenergic agonists, i.e. increased inotropic sensitivity and decreased maximal contractile responsiveness. The increased sensitivity of atria to beta-adrenergic agonists under thyroid hormones appears to be related to increases in beta-adrenoceptor density and Gs/Gi protein ratio, leading to activation of Gs-mediated pathway, but suppression of Gi-mediated pathway of adenylate cyclase regulation. Therefore, the i/c concentrations of cAMP and corresponding inotropic responses achieve their maximums at lower doses of beta-adrenergic agonist. Thyroid hormones also decrease the expression of phospholamban, but increase the expression of sarcoplasmic reticulum Ca2+-pump. As a result, the basal activity of sarcoplasmic reticulum Ca2+-pump increases, but its beta-adrenergic activation through phosphorylation of phospholamban decreases. It is suggested that these changes are causal for decreased maximal inotropic and lusitropic responses of atria to beta-adrenergic agonists.

Thyroid hormones differentially affect sarcoplasmic reticulum function in rat atria and ventricles.

The present study was undertaken to compare the effects of hypothyroidism and hyperthyroidism on sarcoplasmic reticulum (SR) Ca(2+)-pump activity, together with assessment of the functional role of SR in providing activator Ca2+ under these altered thyroid states. In response to a shift from hypothyroid to hyperthyroid state, a 10 fold and 2 fold increase in SR Ca(2+)-pump activity in atria and ventricles, respectively, were observed. This was associated with the 8-9 fold increases in atrial contractility (+dT/dt) and relaxation (-dT/dt), but only with a 3-4 fold increase in their ventricular counterparts. Also, the recirculation fraction of activator Ca2+ (RFA) increased to a far greater extent in atria (4 fold) than in papillary muscles, and the relative increment in inhibition of developed tension by ryanodine became 3 times larger in atria than in papillary muscles. A positive force-frequency relationship (FFR) was observed in hypothyroid atria, whereas the hyperthyroid atria, hypothyroid and hyperthyroid papillary muscles showed a negative FFR. These results suggest the greater role of transsarcolemmal (SL) Ca2+ and smaller role of SR Ca2+ in activating contraction in hypothyroid atria compared to other preparations. Thyroid hormones decrease the contribution of SL and increase that of SR in providing activator Ca2+ to the greater extent in atria than in ventricles. This effect of thyroid hormones is based on larger stimulation of SR Ca(2+)-pump in atria compared to ventricles.

Sarcoplasmic reticulum function in determining atrioventricular contractile differences in rat heart.

The relationships between the contractile characteristics and the sarcoplasmic reticulum (SR) function of rat atrial and ventricular trabeculae were compared. The isometric developed tension (DT) and the rates of contraction (+ dT/dt) and relaxation (-dT/dt) normalized to cross-sectional area were 3.7, 2.2, and 1.8 times lower, respectively, in intact atrial strips compared with ventricular strips, whereas + dT/dt and -dT/dt (normalized to DT) were 2.3 and 2.8 times higher, respectively, in atria. Atria exhibited a maximal potentiation of DT after shorter rest periods than ventricles and a lower reversal for prolonged rest periods. Caffeine-induced tension transients in saponin-permeabilized fibers suggested that the Ca2+ concentration released in atrial myofibrils reached a lower maximum and decayed more slowly than in ventricular preparations. However, the tension-time integrals indicated an equivalent capacity of sequestrable Ca2+ in SR from both tissues. In atrial, as in ventricular myocardium, the SR Ca2+ uptake was more efficiently supported by ATP produced by the SR-bound MM form of creatine kinase (CK; MM-CK) than by externally added ATP, suggesting a tight functional coupling between the SR Ca2+ adenosinetriphosphatase (ATPase) and MM-CK. The maximal rate of oxalate-supported Ca2+ uptake was two times higher in atrial than in ventricular tissue homogenates. The SR Ca(2+)-ATPase 2a mRNA content normalized to 18S RNA was 38% higher in atria than in ventricles, whereas the amount of mRNA encoding the alpha-myosin heavy chain, calsequestrin, and the ryanodine receptor was similar in both tissues. Thus a lower amount of readily releasable Ca2+ together with a faster uptake rate may partly account for the shorter time course and lower tension development in intact atrial myocardium compared with ventricular myocardium.

Ca2+ uptake by cardiac sarcoplasmic reticulum ATPase in situ strongly depends on bound creatine kinase.

The role of creatine kinase (CK) bound to sarcoplasmic reticulum (SR), in the energy supply of SR ATPase in situ, was studied in saponin-permeabilised rat ventricular fibres by loading SR at pCa 6. 5 for different times and under different energy supply conditions. Release of Ca2+ was induced by 5 mM caffeine and the peak of relative tension (T/Tmax) and the area under isometric tension curves, ST, were measured. Taking advantage of close localisation of myofibrils and SR, free [Ca2+] in the fibres during the release was estimated using steady state [Ca2+]/tension relationship. Peak [Ca2+] and integral of free Ca2+ transients (S[Ca2+]f) were then calculated. At all times, loading with 0.25 mM adenosine diphosphate, Mg2+ salt (MgADP) and 12 mM phosphocreatine (PCr) [when adenosine triphosphate (ATP) was generated via bound CK] was as efficient as loading with both 3.16 mM MgATP and 12 mM PCr (control conditions). However, when loading was supported by MgATP alone (3.16 mM), T/Tmax was only 40% and S[Ca2+]f 31% of control (P < 0.001). Under these conditions, addition of a soluble ATP-regenerating system (pyruvate kinase and phosphoenolpyruvate), did not increase loading substantially. Both ST and S[Ca2+]f were more sensitive to the loading conditions than T/Tmax and peak [Ca2+]. The data suggest that Ca2+ uptake by the SR in situ depends on local ATP/ADP ratio which is effectively controlled by bound CK.