Activity dependent potentiation is thought to result from phosphorylation of the regulatory light chains of myosin, increasing Ca²⁺ sensitivity. Yet, Ca²⁺ sensitivity decreases early in a period of intermittent contractions. The purpose of this study was to investigate the early change in Ca²⁺ sensitivity during intermittent submaximal tetanic contractions. Flexor digitorum brevis muscle fibres were dissected from mice after cervical disarticulation. Fibres were superfused with Tyrode solution at 32 °C. Length was set to yield maximal tetanic force. Indo-1 was microinjected into fibres and allowed to dissipate for 30 min. Fluorescence was measured at 405 and 495 nm wavelength and the ratio was used to estimate [Ca²⁺]. A control force-Ca²⁺ relationship was determined with stimulation over a range of frequencies, yielding constants for slope, max force, and half-maximal [Ca²⁺] (pCa^2 +₅₀). Data were collected for sequential contractions at 40 Hz at 2 s intervals. Active force decreased over the first 1–4 contractions then increased. A force-pCa²⁺ curve was fit to each contraction, using the control values for the Hill slope and max force by adjusting pCa²⁺₅₀ until the curve passed through the target contraction. Data are presented for three contractions for each fibre: first, maximum shift to the right, and last contraction. There was a significant shift to the right for pCa²⁺₅₀ (decreased Ca²⁺ sensitivity), usually early in the series of intermittent contractions, then pCa^2 +₅₀ shifted to the left, but remained significantly different from the control value. Although potentiation is associated with increased Ca²⁺ sensitivity, this increase begins only after Ca²⁺ sensitivity has decreased and, in most cases, Ca²⁺ sensitivity does not increase above the control level.

活性依赖性增强被认为是由肌球蛋白调节轻链的磷酸化引起的，增加了 Ca ²⁺敏感性。然而，Ca ²⁺敏感性在间歇性收缩期早期降低。本研究的目的是调查间歇性次最大强直性收缩期间Ca ²⁺敏感性的早期变化。在颈椎离断后从小鼠中解剖出趾短屈肌纤维。在 32°C 下用 Tyrode 溶液灌注纤维。长度设置为产生最大强直力。Indo-1 被显微注射到纤维中并使其消散 30 分钟。在 405 和 495 nm 波长处测量荧光，并且该比率用于估计 [Ca ²⁺]。控制力-Ca ²⁺关系通过在一定频率范围内的刺激来确定，产生斜率、最大力和半最大值[Ca ²⁺ ] (pCa ^{2 +} ₅₀ ) 的常数。以 40 Hz 的频率以 2 秒的间隔收集连续收缩的数据。主动力在前 1-4 次收缩时减少，然后增加。力-pCa ²⁺曲线适合每次收缩，使用希尔斜率和最大力的控制值，通过调整 pCa ²⁺ ₅₀直到曲线通过目标收缩。提供了每种纤维的三种收缩的数据：第一次，最大向右移动，最后一次收缩。pCa ²⁺右移显着₅₀（降低 Ca ²⁺敏感性），通常在一系列间歇性收缩的早期，然后 pCa ^{2 +} ₅₀向左移动，但仍与对照值显着不同。尽管增强与 Ca ²⁺敏感性增加有关，但这种增加仅在 Ca ²⁺敏感性降低后才开始，并且在大多数情况下，Ca ²⁺敏感性不会增加到高于对照水平。