Model-Independent Fluorescence Depolarization Studies Of Labeled Myosin Cross-Bridges In Muscle Fibers

The effects of 1,2-bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid (BAPTA) on force and intracellular Ca2+ transient were studied during isometric twitches and tetanuses in single frog muscle fibers. BAPTA was added to the bathing solution in its permeant AM form (50 and 100 μM). There was no clear correlation between the changes in force and the changes in Ca2+ transient. Thus during twitch stimulation BAPTA did not suppress the Ca2+ transient until the force had been reduced to <50% of its control value. At the same time, the peak myoplasmic free Ca2+concentration reached during tetanic stimulation was markedly increased, whereas the force was slightly reduced by BAPTA. The effects of BAPTA were not duplicated by using another Ca2+ chelator, EGTA, indicating that BAPTA may act differently as a Ca2+ chelator. Stiffness measurements suggest that the decrease in mechanical performance in the presence of BAPTA is attributable to a reduced number of active cross bridges. The results could mean that BAPTA, under the conditions used, inhibits the binding of Ca2+ to troponin C resulting in a reduced state of activation of the contractile system.

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Effect of Pi on unloaded shortening velocity of slow and fast mammalian muscle fibers

AJP Cell Physiology ◽

10.1152/ajpcell.00186.2001 ◽

2002 ◽

Vol 282 (4) ◽

pp. C647-C653 ◽

Cited By ~ 19

Author(s):

Jeffrey J. Widrick

Keyword(s):

Fiber Type ◽

Muscle Fibers ◽

Medial Gastrocnemius ◽

Type I ◽

Shortening Velocity ◽

Cross Bridge ◽

Skinned Muscle ◽

Specific Effects ◽

Cross Bridges ◽

Bridge Models

Chemically skinned muscle fibers, prepared from the rat medial gastrocnemius and soleus, were subjected to four sequential slack tests in Ca2+-activating solutions containing 0, 15, 30, and 0 mM added Pi. Pi (15 and 30 mM) had no effect on the unloaded shortening velocity ( V o) of fibers expressing type IIb myosin heavy chain (MHC). For fibers expressing type I MHC, 15 mM Pi did not alter V o, whereas 30 mM Pireduced V o to 81 ± 1% of the original 0 mM Pi value. This effect was readily reversible when Pi was lowered back to 0 mM. These results are not compatible with current cross-bridge models, developed exclusively from data obtained from fast fibers, in which V o is independent of Pi. The response of the type I fibers at 30 mM Pi is most likely the result of increased internal drag opposing fiber shortening resulting from fiber type-specific effects of Pi on cross bridges, the thin filament, or the rate-limiting step of the cross-bridge cycle.

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Quantitative electrophoretic analysis of myosin heavy chains in single muscle fibers

Journal of Applied Physiology ◽

10.1152/jappl.2001.90.5.1927 ◽

2001 ◽

Vol 90 (5) ◽

pp. 1927-1935 ◽

Cited By ~ 51

Author(s):

Boris A. Tikunov ◽

H. Lee Sweeney ◽

Lawrence C. Rome

Keyword(s):

Muscle Fibers ◽

Electrophoretic Analysis ◽

Single Fiber ◽

Fiber Types ◽

Single Muscle ◽

Myosin Heavy Chains ◽

Single Fibers ◽

Heavy Chains ◽

Cross Bridges ◽

Whole Muscle

To better understand the molecular basis of the large variation in mechanical properties of different fiber types, there has been an intense effort to relate the mechanical and energetic properties measured in skinned single fibers to those of their constituent cross bridges. There is a significant technical obstacle, however, in estimating the number of cross bridges in a single fiber. In this study, we have developed a procedure for extraction and quantification of myosin heavy chains (MHCs) that permits the routine and direct measurement of the myosin content in single muscle fibers. To validate this method, we also compared MHC concentration measured in single fibers with the MHC concentration in whole fast-twitch (psoas and gracilis) and slow-twitch (soleus) muscles of rabbit. We found that the MHC concentration in intact psoas (184 μM) was larger than that in soleus (144 μM), as would be expected from their differing mitochondrial content and volume of myofibrils. We obtained excellent agreement between MHC concentration measured at the single fiber level with that measured at the whole muscle level. This not only verifies the efficacy of our procedure but also shows that the difference in concentration at the whole muscle level simply reflects the concentration differences in the constituent fiber types. This new procedure should be of considerable help in future attempts to determine kinetic differences in cross bridges from different fiber types.

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