Dependence of Activity of Myofibrillar ATPase on Sarcomere Length and Calcium Ion Concentration***

In the heart, contractility may be varied over a wide range. It is determined mainly by the myoplasmic free calcium ion concentration. However, alteration of the responsiveness of the myofilaments to calcium may also be important in regulating cardiac contractility, in particular following changes in sarcomere length, or in hypoxia, or after interventions with certain novel cardiotonic drugs.

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Fluorescence microscope study of the binding of added C protein to skeletal muscle myofibrils.

The Journal of Cell Biology ◽

10.1083/jcb.90.1.25 ◽

1981 ◽

Vol 90 (1) ◽

pp. 25-31 ◽

Cited By ~ 47

Author(s):

C Moos

Keyword(s):

Skeletal Muscle ◽

Fluorescence Microscopy ◽

Calcium Ion ◽

Ion Concentration ◽

Free Calcium ◽

Myofibrillar Atpase ◽

C Protein ◽

Thick Filaments ◽

Calcium Ion Concentration

The binding of extra C protein to rabbit skeletal muscle myofibrils has been investigated by fluorescence microscopy with fluorescein-labeled C protein or unmodified C protein plus fluorescein-labeled anti-C protein. Added C protein binds strongly to the I bands, which is consistent with its binding to F actin in solution (Moos, C., C. M. Mason, J. M. Besterman, I. M. Feng, and J. H. Dubin. 1978. J. Mol. Biol. 124:571-586). Of particular interest, the binding to the I band is calcium regulated: it requires a free calcium ion concentration comparable to that which activates the myofibrillar ATPase. This increases the likelihood that C protein-actin interaction might be physiologically significant. When I band binding is suppressed, binding in the A band becomes evident. It appears to occur particularly near the M line, and possibly at the edges of the A band as well, suggesting that those parts of the thick filaments that lack C protein in vivo may nevertheless be capable of binding added C protein.

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Spontaneous and propagated contractions in rat cardiac trabeculae.

The Journal of General Physiology ◽

10.1085/jgp.93.5.943 ◽

1989 ◽

Vol 93 (5) ◽

pp. 943-961 ◽

Cited By ~ 51

Author(s):

B J Mulder ◽

P P de Tombe ◽

H E ter Keurs

Keyword(s):

Sarcoplasmic Reticulum ◽

Calcium Ion ◽

Sarcomere Length ◽

Calcium Release ◽

Ion Concentration ◽

Intracellular Calcium Ion ◽

Velocity Of Propagation ◽

Spontaneous Contractions ◽

Calcium Ion Concentration ◽

External Calcium

Sarcomere length measurement by microscopic and laser diffraction techniques in trabeculae of rat heart, superfused with Krebs-Henseleit solution at 21 degrees C, showed spontaneous local sarcomere shortening after electrically stimulated twitches. The contractions originated in a region of several hundred micrometers throughout the width of the muscle close to the end of the preparation that was damaged by dissection. The contractions propagated at a constant velocity along the trabeculae. The velocity of propagation increased from 0 to 10 mm/s in proportion to the number of stimuli (3-30) in a train of electrically evoked twitches at 2 Hz and at an external calcium ion concentration ([Ca++]o) of 1.5 mM. At a constant number of stimuli (n), the velocity of propagation increased from 0 to 15 mm/s with [Ca++]o increasing from 1 to 7 mM. In addition, increase of n and [Ca++]o led to an increase of the extent of local sarcomere shortening during the spontaneous contractions, and the occurrence of multiple contractions. Spontaneous contractions with much internal shortening and a high velocity of propagation frequently induced spontaneous synchronized contractions and eventually arrhythmias. Propagation of spontaneous contractions at low and variable velocity is consistent with the hypothesis that calcium leakage into damaged cells causes spontaneous calcium release from the overloaded sarcoplasmic reticulum in the damaged cells. This process propagates as a result of diffusion of calcium into adjacent cells, which triggers calcium release from their sarcoplasmic reticulum. We postulate that the propagation velocity depends on the intracellular calcium ion concentration, with increases with n and [Ca++]o.

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