Related Experiment Video
Updated: Jul 19, 2026
08:29
Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling
Published on: August 1, 2016
Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics
Pieter P de Tombe1, Alexandra Belus, Nicoletta Piroddi
1Center for Cardiovascular Research, Univ. of Illinois at Chicago, 835 S. Wolcott Avenue, MC901, Chicago IL 60612, USA. pdetombe@uic.edu
Summary
Incorporating slow skeletal troponin-I (ssTnI) into fast muscle myofibrils increases calcium sensitivity. However, this change in sensitivity does not affect the speed of force development, suggesting cross-bridge cycling is independent of thin-filament activation dynamics.
Area of Science:
- Muscle Physiology
- Biochemistry
- Molecular Biology
Background:
- Troponin-I (TnI) is a key regulatory protein in muscle contraction, with different isoforms found in cardiac and skeletal muscle.
- The interaction between TnI and other troponin subunits (TnC, TnT) influences myofilament calcium sensitivity and force generation dynamics.
- Understanding the functional consequences of TnI isoform substitution is crucial for elucidating muscle function and disease mechanisms.
Purpose of the Study:
- To determine if slow skeletal troponin-I (ssTnI) is sufficient to increase myofilament calcium sensitivity (EC50) in fast skeletal muscle.
- To investigate whether alterations in EC50 affect the dynamics of force development in single myofibrils.
- To explore the relationship between thin-filament activation dynamics and cross-bridge cycling rates.
Main Methods:
- Single rabbit psoas myofibrils were utilized, employing rapid solution switching techniques for activation.
- Troponin complexes were partially exchanged with recombinant cardiac Tn (cTn) or a chimera containing ssTnI.
- Myofilament calcium sensitivity (EC50) and force development dynamics were measured, with troponin exchange confirmed by SDS-PAGE.
Main Results:
- Exchange with cardiac TnI (cTnI) decreased EC50, while exchange with the ssTnI-chimera significantly further decreased EC50.
- Maximum tension generation remained unchanged across all experimental conditions.
- Despite significant alterations in EC50, dynamic parameters of activation and relaxation were unaffected.
Conclusions:
- The incorporation of ssTnI into fast skeletal muscle myofibrils is sufficient to enhance myofilament calcium sensitivity.
- Myofilament calcium sensitivity (EC50) is not correlated with the dynamics of actin-myosin interactions or force development.
- These findings suggest that the intrinsic cross-bridge cycling rate is independent of the thin-filament activation dynamics.
Related Concept Videos
01:29Relaxation of Skeletal Muscles
The period of muscle contraction primarily influences the duration of stimulation at the neuromuscular junction (NMJ), the presence of free calcium ions in the sarcoplasm, and the availability of energy or ATP to support contractions.
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.
When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated sodium channels. Sodium ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated calcium channels to open.
01:26Cross-bridge Cycle
As muscle contracts, the overlap between the thin and thick filaments increases, decreasing the length of the sarcomere—the contractile unit of the muscle—using energy in the form of ATP. At the molecular level, this is a cyclic, multistep process that involves binding and hydrolysis of ATP, and movement of actin by myosin.
01:25Smooth Muscle Contraction
Smooth muscle contraction is a complex process vital for various bodily functions, from maintaining blood vessel tension to facilitating the movement of food through the digestive tract. Unlike striated muscles, smooth muscle contraction begins more slowly and lasts longer.
The onset of contraction is triggered by an increase in calcium ions within the sarcoplasm, similar to the process in striated muscle. However, smooth muscles have a relatively smaller reservoir of the sarcoplasmic...
The onset of contraction is triggered by an increase in calcium ions within the sarcoplasm, similar to the process in striated muscle. However, smooth muscles have a relatively smaller reservoir of the sarcoplasmic...
01:16Actin and Myosin in Muscle Contraction
Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes...
01:20Excitation-Contraction Coupling in Skeletal Muscles
Excitation-contraction coupling is a series of events that occur between generating an action potential and initiating a muscle contraction. It occurs at the triad, a structure found in skeletal muscle fibers that comprise a T-tubule and terminal cisternae of the sarcoplasmic reticulum on each side. These triads are visible in longitudinally sectioned muscle fibers. They are typically located at the A-I junction — the junction between the A and I bands of the sarcomere.
When an action potential...
When an action potential...
01:16Calmodulin-dependent Signaling
Calmodulin (CaM) is a calcium-binding protein in eukaryotes that controls various calcium-regulated cellular processes. It has four calcium-binding sites that bind calcium to form the calcium-calmodulin ( Ca2+-CaM) complex. GPCR stimulation increases the calcium levels in the cells that bind to CaM and induces a conformational change.
The Ca2+-CaM complex does not have enzymatic activity by itself. Instead, the complex binds downstream target proteins, including membrane proteins or enzymes,...
The Ca2+-CaM complex does not have enzymatic activity by itself. Instead, the complex binds downstream target proteins, including membrane proteins or enzymes,...