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Updated: Jul 21, 2026

Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles
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Published on: November 1, 2012

Extra calcium on shortening in barnacle muscle. Is the decrease in calcium binding related to decreased cross-bridge

A M Gordon1, E B Ridgway

  • 1Department of Physiology and Biophysics, University of Washington, Seattle 98195.

The Journal of General Physiology
|September 1, 1987
PubMed
Summary

This study investigates how calcium is released during muscle shortening in barnacle muscle fibers. Using a calcium-sensitive protein called aequorin, the researchers observed that extra calcium is released when muscles shorten. They found that the amount of this extra calcium depends on the initial muscle length and the size of the shortening step. The results suggest that calcium binding is more closely related to muscle force or cross-bridge attachment than to muscle length alone. The study also found that the source of force—whether from stretch or increased stimulation—can affect calcium release. These findings help clarify the complex relationship between calcium binding and muscle contraction.

Keywords:
muscle contraction mechanismscalcium signaling in musclebarnacle muscle physiologyaequorin calcium monitoring

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Area of Science:

  • Muscle physiology and biophysics
  • Calcium signaling in contractile systems
  • Molecular mechanisms of muscle contraction

Background:

Prior research has shown that calcium plays a central role in muscle contraction by binding to regulatory proteins. However, the exact relationship between calcium binding, muscle length, and force remains unclear. Earlier studies demonstrated that extra calcium is released during muscle shortening under controlled conditions. Yet, the source of this calcium and the mechanisms governing its release are not fully understood. Some theories suggest that calcium release is linked to cross-bridge cycling or changes in filament geometry. However, the specific contribution of length, force, or cross-bridge attachment to this process remains uncertain. This uncertainty motivated further investigation into how these variables interact during muscle shortening. The current paper builds on previous work by examining the effect of muscle length and force on calcium release. The absence of a clear consensus on the causal relationships between calcium binding and contractile parameters highlights a gap in the field. Addressing this gap could provide insights into the biophysical mechanisms of muscle contraction.

Purpose Of The Study:

The aim of this study was to determine whether the observed decrease in calcium binding during muscle shortening is due to changes in muscle length, force, or cross-bridge attachment. The researchers used barnacle muscle fibers microinjected with aequorin to monitor calcium levels. They tested how varying initial muscle lengths and shortening steps affect calcium release. The motivation for this study stems from the need to clarify the interplay between calcium binding and contractile parameters. Previous findings indicated a correlation between calcium release and muscle shortening, but the underlying mechanism was unclear. By manipulating initial length and shortening steps, the authors sought to isolate the effects of length and force. The study also aimed to compare calcium release under different force conditions, such as those caused by stretch versus increased stimulation. This approach allows for a more precise understanding of the variables influencing calcium binding during contraction.

Main Methods:

The researchers used barnacle single muscle fibers microinjected with aequorin to measure intracellular calcium levels. They applied voltage clamp and length control techniques to stimulate muscle fibers during the declining phase of the calcium transient. Shortening steps were introduced at different times during the calcium transient to observe the effects on calcium release. The initial muscle length was varied to assess its impact on calcium binding. Shortening steps of different magnitudes were tested to determine the relationship between step size and calcium release. The study compared calcium release when force was increased through stretch versus increased stimulation. Data were collected using photometric measurements of aequorin luminescence. The time course and amplitude of calcium release were analyzed in relation to muscle length and force.

Main Results:

The amount of extra calcium observed during shortening steps depends on initial muscle length, decreasing at shorter lengths. The relationship between initial length and extra calcium parallels that between initial length and peak active force. The ratio of extra calcium to active force is nearly independent of initial length. The amplitude of extra calcium increases with larger shortening steps, approaching saturation at steps of 10% or more. Extra calcium is less pronounced when force is increased through stretch rather than stimulation. The time course of extra calcium lies between those of free calcium and muscle force. These findings suggest that calcium binding is influenced by muscle force or cross-bridge attachment. The data do not distinguish between direct and indirect effects of length on calcium binding.

Conclusions:

The data suggest that calcium binding during muscle shortening is related to muscle force or cross-bridge attachment, rather than length alone. The observed decrease in calcium binding may reflect changes in cross-bridge attachment or force generation. The relationship between calcium release and initial length parallels that of active force, indicating a possible link between the two. However, the study does not conclusively determine whether length affects calcium binding directly or indirectly. The findings support the idea that calcium binding is influenced by contractile parameters such as force and cross-bridge dynamics. The results do not rule out a role for length in calcium binding but suggest it is secondary to force or attachment. The comparison of force conditions indicates that the source of force affects calcium release. These conclusions are based on the observed patterns of calcium release under controlled experimental conditions.

The study suggests that calcium binding during shortening is more closely related to muscle force or cross-bridge attachment than to muscle length alone.

Calcium release was measured using microinjected aequorin, a calcium-sensitive photoprotein that emits light when calcium binds.

Muscle length was varied to determine if calcium binding is influenced by length, as well as to compare it with the effect of force.

The ratio being nearly independent of initial length suggests that calcium binding is more closely tied to force than to length.

The amplitude increases with larger shortening steps, reaching saturation at steps of 10% or more.

The study implies that calcium release differs when force is increased through stretch versus increased stimulation.