Hollingworth S, Zhao M, Baylor S M
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia 19104-6085, USA.
J Gen Physiol. 1996 Nov;108(5):455-69. doi: 10.1085/jgp.108.5.455.
Bundles of 10-100 fibers were dissected from the extensor digitorum longus muscle of mouse, mounted in an apparatus for optical recording, and stretched to long sarcomere length (> or = 3.6 microns). One fiber within the bundle was microinjected with furaptra, a fluorescent indicator that responds rapidly to changes in myoplasmic free [Ca2+] (delta [Ca2+]). Twitches and brief tetani were initiated by external stimulation. At myoplasmic furaptra concentrations of approximately 0.1 mM, the indicator's fluorescence signal during fiber activity (delta F/F) was well resolved. delta F/F was converted to delta [Ca2+] under the assumption that furaptra's myoplasmic dissociation constant for Ca2+ is 98 microM at 16 degrees C and 109 microM at 28 degrees C. At 16 degrees C, the peak amplitude of delta [Ca2+] during a twitch was 17.8 +/- 0.4 microM (+/-SEM; n = 8) and the half-width of delta [Ca2+] was 4.6 +/- 0.3 ms. At 28 degrees C, the peak and half-width values were 22.1 +/- 1.8 microM and 2.0 +/- 0.1 ms, respectively (n = 4). During a brief high-frequency tetanus, individual peaks of delta [Ca2+] were also well resolved and reached approximately the same amplitude that resulted from a single shock; the initial decays of delta [Ca2+] from peak slowed substantially during the tetanus. For a single twitch at 16 degrees C, the amplitude of delta [Ca2+] in fast-twitch fibers of mouse is not significantly different from that recently measured in fast-twitch fibers of frog (16.5 +/- 0.9 microM; Zhao, M., S. Hollingworth, and S.M. Baylor. 1996. Biophys. J. 70:896-916); in contrast, the half-width of delta [Ca2+] is surprisingly brief in mouse fibers, only about half that measured in frog (9.6 +/- 0.6 ms). The estimated peak rate at which Ca2+ is released from the sarcoplasmic reticulum in response to an action potential is also similar in mouse and frog, 140-150 microM/ms (16 degrees C).
从小鼠趾长伸肌中分离出由10 - 100根肌纤维组成的肌束，将其安装在用于光学记录的装置中，并拉伸至长肌节长度（≥3.6微米）。向肌束中的一根肌纤维微量注射氟罗帕特拉，这是一种对肌浆游离[Ca2+]（Δ[Ca2+]）变化反应迅速的荧光指示剂。通过外部刺激引发单收缩和短暂强直收缩。在肌浆氟罗帕特拉浓度约为0.1 mM时，纤维活动期间指示剂的荧光信号（ΔF/F）清晰可辨。假设氟罗帕特拉在16℃时对Ca2+的肌浆解离常数为98 μM，在28℃时为109 μM，将ΔF/F转换为Δ[Ca2+]。在16℃时，单收缩期间Δ[Ca2+]的峰值幅度为17.8±0.4 μM（±SEM；n = 8），Δ[Ca2+]的半高宽为4.6±0.3毫秒。在28℃时，峰值和半高宽值分别为22.1±1.8 μM和2.0±0.1毫秒（n = 4）。在短暂的高频强直收缩期间，Δ[Ca2+]的各个峰值也清晰可辨，且达到的幅度与单次电击产生的幅度大致相同；强直收缩期间，Δ[Ca2+]从峰值开始的初始衰减显著减慢。对于16℃时的单次单收缩，小鼠快肌纤维中Δ[Ca2+]的幅度与最近在青蛙快肌纤维中测得的幅度（16.5±0.9 μM；Zhao, M., S. Hollingworth, and S.M. Baylor. 1996. Biophys. J. 70:896 - 916）无显著差异；相比之下，小鼠纤维中Δ[Ca2+]的半高宽出奇地短，仅约为青蛙中测得值的一半（9.6±0.6毫秒）。在小鼠和青蛙中，响应动作电位时从肌浆网释放Ca2+的估计峰值速率也相似，为140 - 150 μM/毫秒（16℃）。
