Competing effects of activation history on force and cytosolic Ca2+ in intact single mice myofibers.

The purpose was to investigate the changes in cytosolic Ca²⁺ and force output during post-tetanic potentiation (PTP) during pre-fatigue and during prolonged low-frequency force depression (PLFFD) following fatigue. Intact single myofibers from the flexor digitorum brevis of mice were electrically stimulated to record force (n = 8) and free cytosolic Ca²⁺ concentration ([Ca²⁺]_c) with FURA-2 (n = 6) at 32 °C. Initially, force and [Ca²⁺]_c were measured during brief (350 ms) trains of stimuli at 30, 50, 70, and 200 Hz at ~ 2 s intervals (Force-frequency protocol, FFP). Then, a conditioning stimulus (CS) of six 120 Hz stimuli, separated by ~ 3 s, was used to induce PTP, immediately followed by an FFP. Myofiber fatigue was produced by 150 Hz trains every 3 s until peak force decayed 70% of the initial. Thirty minutes after the fatigue, the CS was repeated to assess the effect of PTP on force and [Ca²⁺]_c during PLFFD. The CS in unfatigued myofibers induced PTP as the submaximal force was enhanced and accompanied by increased peak [Ca²⁺]_c with no change in myofilament Ca²⁺ sensitivity. After fatigue, PLFFD was due to lowered peak [Ca²⁺]_c. Inducing PTP during PLFFD enhanced submaximal force primarily through greater peak [Ca²⁺]_c, mitigating the submaximal force deficits. Despite the impaired force during PLFFD, myofibers remained sensitive to PTP, and this mitigated the submaximal force deficits through increased peak [Ca²⁺]_c without a change in myofilament Ca²⁺ sensitivity. Therefore, force adjustments of intact single myofibers due to activation history are principally accomplished by opposing adjustments in [Ca²⁺]_c.