Ca²⁺ and force during dynamic contractions in mouse intact skeletal muscle fibers

Muscle fiber force production is determined by the excitation frequency of motor nerves, which induce transient increases in cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_i) and the force-generating capacity of the actomyosin cross-bridges. Previous studies suggest that, in addition to altered cross-bridge properties, force changes during dynamic (concentric or eccentric) contraction might be affected by Ca²⁺-dependent components. Here we investigated this by measuring [Ca²⁺]_i and force in mouse muscle fibers undergoing isometric, concentric, and eccentric contractions. Intact single muscle fibers were dissected from the flexor digitorum brevis muscle of mice. Fibers were electrically activated isometrically at 30-100 Hz and after reaching the isometric force plateau, they were actively shortened or stretched. We calculated the ratio (relative changes) in force and [Ca²⁺]_i attained in submaximal (30 Hz) and near-maximal (100 Hz) contractions under isometric or dynamic conditions. Tetanic [Ca²⁺]_i was similar during isometric, concentric and eccentric phases of contraction at given stimulation frequencies while the forces were clearly different depending on the contraction types. The 30/100 Hz force ratio was significantly lower in the concentric (44.1 ± 20.3%) than in the isometric (50.3 ± 20.4%) condition (p = 0.005), whereas this ratio did not differ between eccentric and isometric conditions (p = 0.186). We conclude that the larger force decrease by decreasing the stimulation frequency during concentric than during isometric contraction is caused by decreased myofibrillar Ca²⁺ sensitivity, not by the decreased [Ca²⁺]_i.