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Potassium contractures and mechanical activation in mammalian skeletal muscles
Abstract:
Potassium (K-) contractures were recorded from slow-twitch (mouse soleus and fast-twitch (mouse extensor digitorum longus (EDL) and rat sternomastoid) muscles. The mouse limb muscles responded to a maintained increase in external potassium concentration with a rapid increase in tension (fast contracture) which inactivated and was followed by a slow contracture. Rat sternomastoid muscles responded with fast contractures only. The threshold potassium concentration for contraction was higher in fast-twitch muscles than in soleus muscles, at 22 and at 37 degrees C. After corrections had been made for the more rapid depolarization of soleus fibers, the threshold potential for soleus fiber contraction was 15mV closer to the resting membrane potential than the threshold for fast-twitch fiber contraction. The K-contracture results were confirmed by two microelectrode voltage-clamp experiments. Activation of fast twitch fibers required depolarizing pulses that were 15 to 20mV greater than the pulses required to activate soleus fibers. When the time courses of K-contractures were compared it was evident that inactivation with prolonged depolarization was much faster in the fast-twitch muscles than in the soleus muscles. The results suggest that the voltage dependence and kinetics of the process coupling T-tubule depolarization with calcium release from the sarcoplasmic reticulum may depend on fiber atype in mammalian skeletal muscle.
Insights
Mammalian skeletal muscles show fiber-type differences in potassium-induced contractions. Fast-twitch fibers require greater depolarization for activation and inactivate faster than slow-twitch fibers.
Area of Science:
- Muscle Physiology
- Skeletal Muscle Contraction
- Cellular Electrophysiology
Background:
- Skeletal muscle contraction is initiated by depolarization of the sarcolemma and T-tubules.
- Different muscle fiber types (slow-twitch vs. fast-twitch) exhibit distinct electrophysiological and contractile properties.
- Understanding the coupling between membrane depolarization and calcium release is crucial for muscle function.
Purpose of the Study:
- To investigate the differences in potassium-induced contractures between slow-twitch and fast-twitch mammalian skeletal muscles.
- To determine the voltage dependence and kinetics of excitation-contraction coupling in different fiber types.
- To elucidate the role of fiber type in the process linking T-tubule depolarization to calcium release.
Main Methods:
- Potassium (K-) contractures were recorded from isolated slow-twitch (mouse soleus) and fast-twitch (mouse EDL, rat sternomastoid) muscles.
- Two microelectrode voltage-clamp experiments were performed to confirm K-contracture findings.
- Measurements included tension development, depolarization thresholds, and inactivation kinetics.
Main Results:
- Fast-twitch muscles (EDL, sternomastoid) exhibited faster inactivation of potassium-induced contractures compared to slow-twitch soleus muscles.
- The threshold potassium concentration and depolarization required for contraction were higher in fast-twitch fibers.
- Soleus fibers required less depolarization (15mV closer to resting potential) for activation than fast-twitch fibers.
Conclusions:
- Mammalian skeletal muscle fiber type influences the voltage dependence and kinetics of excitation-contraction coupling.
- Differences in T-tubule depolarization to calcium release coupling exist between slow-twitch and fast-twitch fibers.
- These findings suggest fiber-type specific mechanisms in the regulation of calcium release from the sarcoplasmic reticulum.