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Evidence for t-tubular conduction failure in frog skeletal muscle induced by elevated extracellular calcium
Abstract:
In order to investigate the mechanism by which elevated extracellular calcium ions decrease tetanus tension in frog skeletal muscle, we made mechanical, electrophysiological and photographic measurements on single fibres or small bundles of fibres. Three lines of evidence point to t-tubular conduction failure as the primary mechanism of action of high calcium. They are a decrease in amplitude of the late afterpotential, attenuation or elimination of the notch and hump configuration of the early afterpotential, and the appearance of wavy myofibrils in the axial core of fibres during tetanus. These effects are fully reversible and are shared by other bivalent cations. High calcium concentration causes a change in the time course of the early afterpotential but does not alter the passive cell membrane characteristics, as reflected by the time course of decay of applied hyperpolarizing pulses.
Insights
Elevated extracellular calcium ions reduce frog skeletal muscle tension by causing t-tubular conduction failure. This mechanism, observed through electrophysiological and photographic analysis, is reversible and affects muscle fiber function.
Area of Science:
- Muscle physiology
- Skeletal muscle electrophysiology
- Calcium ion signaling
Background:
- Extracellular calcium ions play a crucial role in regulating skeletal muscle function.
- Previous studies suggest calcium influences muscle tension, but the precise mechanism remains unclear.
Purpose of the Study:
- To elucidate the mechanism by which elevated extracellular calcium ions decrease tetanus tension in frog skeletal muscle.
- To investigate the role of t-tubular conduction in calcium-induced changes in muscle tension.
Main Methods:
- Mechanical measurements on single muscle fibers and small fiber bundles.
- Electrophysiological recordings, including afterpotential analysis.
- Photographic documentation of myofibril structure during tetanus.
Main Results:
- Elevated extracellular calcium led to a decrease in tetanus tension.
- Evidence suggests t-tubular conduction failure, indicated by altered afterpotentials (late afterpotential decrease, early afterpotential notch/hump attenuation) and myofibril abnormalities.
- These effects were reversible and observed with other divalent cations, without altering passive membrane properties.
Conclusions:
- T-tubular conduction failure is the primary mechanism for high extracellular calcium-induced reduction in frog skeletal muscle tetanus tension.
- The findings highlight the critical role of t-tubular function in excitation-contraction coupling and muscle force generation.