Modulating factors of calcium-free contraction at low [MgATP]: a physiological study on the steady states of skinned fibres of frog skeletal muscle

Dynamic localization of αB-crystallin at the microtubule cytoskeleton network in beating heart cells

αB-crystallin is highly expressed in the heart and slow skeletal muscle; however, the roles of αB-crystallin in the muscle are obscure. Previously, we showed that αB-crystallin localizes at the sarcomere Z-bands, corresponding to the focal adhesions of cultured cells. In myoblast cells, αB-crystallin completely colocalizes with microtubules and maintains cell shape and adhesion. In this study, we show that in beating cardiomyocytes α-tubulin and αB-crystallin colocalize at the I- and Z-bands of the myocardium, where it may function as a molecular chaperone for tubulin/microtubules. Fluorescence recovery after photobleaching (FRAP) analysis revealed that the striated patterns of GFP-αB-crystallin fluorescence recovered quickly at 37°C. FRAP mobility assay also showed αB-crystallin to be associated with nocodazole-treated free tubulin dimers but not with taxol-treated microtubules. The interaction of αB-crystallin and free tubulin was further confirmed by immunoprecipitation and microtubule sedimentation assay in the presence of 1-100 μM calcium, which destabilizes microtubules. Förster resonance energy transfer analysis showed that αB-crystallin and tubulin were at 1-10 nm apart from each other in the presence of colchicine. These results suggested that αB-crystallin may play an essential role in microtubule dynamics by maintaining free tubulin in striated muscles, such as the soleus or cardiac muscles.

Activating efficiency of Ca2+ and cross-bridges as measured by phosphate analog release

To assess the activating efficiency of Ca2+ and cross-bridges, the release rates of phosphate analogs from skinned fibers were estimated from the recovery of contractility and that of stiffness. Estimations were performed based on the assumptions that contractility was indicative of the population of analog-free myosin heads and that stiffness reflected the population of formed cross-bridges. Aluminofluoride (AlFx) and orthovanadate (Vi) were used as phosphate analogs with mechanically skinned fibers from rabbit psoas muscle. The use of the analogs enabled the functional assessment of activation level in the total absence of ATP. Fibers loaded with the analogs gradually recovered contractility and stiffness in normal plain rigor solution. The addition of Ca2+ to the plain rigor solution significantly accelerated their recovery, whereas ADP had no appreciable effect. ATP plus Ca2+(contracting condition) accelerated the recovery by several tens of times. These results indicate that the cross-bridges formed during contraction have prominent activating efficiency, which is indispensable to attain full activation. A comparison between the activating efficiency evaluated from stiffness and that from contractility suggested that Ca2+ is more potent in accelerating the binding of actin to analog-bound myosin heads whereas cross-bridges mainly accelerate the subsequent analog-releasing step.

Influence of ADP on cross-bridge-dependent activation of myofibrillar thin filaments

Contraction of skeletal muscle is regulated by calcium at the level of the thin filament via troponin and tropomyosin. Studies have indicated that strong cross-bridge binding is also involved in activation of the thin filament. To further test this, myofibrils were incubated with a wide range of fluorescent myosin subfragment 1(fS1) at pCa 9 or pCa 4 with or without ADP. Sarcomere fluorescence intensity and the fluorescence intensity ratio (non-overlap region/overlap region) were measured to determine the amount and location of bound fS1 in the myofibril. There was lower sarcomere fluorescence intensity with ADP compared to without ADP for both calcium levels. Similar data were obtained from biochemical measures of bound fS1, validating the fluorescence microscopy measurements. The intensity ratio, which is related to activation of the thin filament, increased with increasing [fS1] with or without ADP. At pCa 9, the fluorescence intensity ratio was constant until 80-160 nM fS1 without ADP conditions, then it went up dramatically and finally attained saturation. The dramatic shift of the ratio demonstrated the cooperative character of strong cross-bridge binding, and this was not observed at high calcium. A similar pattern was observed with ADP in that the ratio was right-shifted with respect to total [fS1]. Saturation was obtained with both the fluorescence intensity and ratio data. Plots of intensity ratio as a function of normalized sarcomere intensity (bound fS1) showed little difference between with and without ADP. This suggests that the amount of strongly bound fS1, not fS1 state (with or without ADP) is related to activation of the thin filament.

Tropomyosin modulates pH dependence of isometric tension

We investigated the effect of pH on isometric tension in actin filament-reconstituted and thin filament-reconstituted bovine cardiac muscle fibers in the pH range of 6.0-7.4. Thin filament was reconstituted from purified G-actin with either bovine cardiac tropomyosin (Tm) or rabbit skeletal Tm in conjunction with cardiac or skeletal troponin (Tn). Results showed that isometric tension decreased linearly with a decrease in pH. The slope of the pH-tension relation, DeltaF/DeltapH (Deltarelative tension/Deltaunit pH), was 0.28 and 0.44 in control cardiac fibers and skeletal fibers, respectively. In actin filament-reconstituted fibers without regulatory proteins, DeltaF/DeltapH was 0.62, namely larger than that in cardiac or skeletal fibers. When reconstituted with cardiac Tm-Tn complex (nTm), DeltaF/DeltapH recovered to 0.32, close to the value obtained in control cardiac fibers. When reconstituted with skeletal nTm, DeltaF/DeltapH recovered to 0.48, close to the value for control skeletal fibers. To determine whether Tm or Tn is responsible for the inhibitory effects of nTm on the tension decrease caused by reduced pH, thin filament was reconstituted with cardiac Tm and skeletal Tn, or with skeletal Tm and cardiac Tn. When cardiac Tm was used, pH dependence of isometric tension coincided with that of control cardiac fibers. When skeletal Tm was used, the pH dependence coincided with that of control skeletal fibers. Furthermore, closely similar results were obtained in fibers reconstituted with actin and either cardiac or skeletal Tm without Tn. These results demonstrate that Tm but not Tn modulates the pH dependence of active tension.