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Effect of adenosine triphosphate analogues on skeletal muscle fibers in rigor
1Laboratory of Physical Biology, National Institutes of Arthritis and Musculoskeletal and Skin Diseases, Bethesda, Maryland 20892.
Abstract:
It is commonly believed, for both vertebrate striated and insect flight muscle, that when the ATP analogue adenyl-5'-yl imidodiphosphate (AMPPNP) is added to the muscle fiber in rigor, it causes the fiber to lengthen by 0.15%. This has been interpretated (Marston S.B., C.D. Roger, and R.T. Tregear. 1976. J. Mol. Biol. 104:263-267) as suggesting (a) that in rigor the crossbridge is fixed to, i.e., almost never detaches from the actin filament; (b), that the crossbridge remains fixed to the actin filament after AMPPNP addition; and (c) that the ability of AMPPNP to cause apparent lengthening of a muscle fiber is due to its ability to cause a conformational change in the myosin crossbridge that has an axial component of approximately 1.6 nm/half-sarcomere. The present study, done only on chemically-skinned rabbit psoas fibers, confirms that AMPPNP can cause muscle fibers to lengthen by 0.15% but only for a narrow set of experimental conditions. When experimental conditions are varied over a wider range, it becomes apparent that the extent of lengthening of a rigor muscle fiber upon AMPPNP addition depends almost entirely on the strain present in the rigor fiber before AMPPNP addition. Addition of AMPPNP to an unstrained rigor fiber (one supporting zero tension), induces zero length change while addition of AMPPNP to very highly strained rigor fibers induces length changes greater than 0.15%. The data thus do not support the hypotheses that the crossbridges remain fixed to the actin filament after AMPPNP addition and that the size of the apparent length change induced by AMPPNP is related to the size of the axial component of a conformational change. Instead, the data support the idea that the ability of AMPPNP to cause lengthening of a rigor muscle fiber is related to its ability to accelerate the rate at which strained crossbridges detach from actin and reattach in positions in lesser strain. The data do not rule out a conformational change upon AMPPNP binding, they simply make clear that any attempt to measure a force response conceivably due to a conformational change, would be more than obscured by the force changes due to crossbridges detaching and reattaching in positions of lesser strain.
Insights
The ATP analogue AMPPNP can lengthen muscle fibers, but this effect depends on the fiber's strain. Lengthening occurs as strained crossbridges detach and reattach, not due to a fixed conformational change.
Area of Science:
- Muscle physiology
- Biophysics
- Molecular motors
Background:
- The ATP analogue adenyl-5'-yl imidodiphosphate (AMPPNP) is widely believed to cause muscle fiber lengthening upon addition to rigor muscle.
- This phenomenon has been interpreted as evidence for fixed crossbridges and conformational changes in myosin.
Purpose of the Study:
- To investigate the mechanism behind muscle fiber lengthening induced by AMPPNP in chemically-skinned rabbit psoas fibers.
- To re-evaluate the interpretation of AMPPNP-induced length changes in relation to crossbridge behavior and conformational changes.
Main Methods:
- Experiments were conducted on chemically-skinned rabbit psoas fibers.
- Fibers were subjected to varying degrees of strain in rigor before the addition of AMPPNP.
- Length changes were measured under different experimental conditions.
Main Results:
- AMPPNP-induced lengthening of rigor muscle fibers is highly dependent on the pre-existing strain in the fiber.
- Unstrained rigor fibers show no length change upon AMPPNP addition, while highly strained fibers show greater than 0.15% lengthening.
- The data do not support the hypothesis of fixed crossbridges or a direct link between length change and axial conformational change size.
Conclusions:
- AMPPNP-induced muscle fiber lengthening is primarily due to accelerated detachment and reattachment of strained crossbridges.
- The observed length changes are not solely caused by a fixed conformational change in the myosin crossbridge.
- While AMPPNP binding may induce conformational changes, their effects are obscured by the dominant mechanism of crossbridge detachment and reattachment dynamics.