The amplitude and time course of the myoplasmic free [Ca2+] transient in fast-twitch fibers of mouse muscle

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 (greater than or equal to 3.6 mu m). 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 similar to 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 mu M at 16 degrees C and 109 mu M at 28 degrees C. At 16 degrees C, the peak amplitude of Delta[Ca2+] during a twitch was 17.8+/-0.4 mu M (+/-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 mu M 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 mu M; 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 mu M/ms (16 degrees C).