THE AMPLITUDE AND TIME-COURSE OF THE MYOPLASMIC FREE [CA2-TWITCH FIBERS OF MOUSE MUSCLE(] TRANSIENT IN FAST)

Bundles of 10-100 fibers were dissected from the extensor digitorum lo ngus muscle of mouse, mounted in an apparatus for optical recording, a nd stretched to long sarcomere length (greater than or equal to 3.6 mu m). One fiber within the bundle was microinjected with furaptra, a fl uorescent indicator that responds rapidly to changes in myoplasmic fre e [Ca2+] (Delta[Ca2+]). Twitches and brief tetani were initiated by ex ternal stimulation. At myoplasmic furaptra concentrations of similar t o 0.1 mM, the indicator's fluorescence signal during fiber activity (D elta F/F) was well resolved. Delta F/F was converted to Delta[Ca2+] un der the assumption that furaptra's myoplasmic dissociation constant fo r 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-freque ncy tetanus, individual peaks of Delta[Ca2+] were also well resolved a nd reached approximately the same amplitude that resulted from a singl e shock; the initial decays of Delta[Ca2+] from peak slowed substantia lly during the tetanus. For a single twitch at 16 degrees C, the ampli tude of Delta[Ca2+] in fast-twitch fibers of mouse is not significantl y 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. Bi ophys. J. 70:896-916); in contrast, the half-width of Delta[Ca2+] is s urprisingly brief in mouse fibers, only about half that measured in fr og (9.6+/-0.6 ms). The estimated peak rate at which Ca2+ is released f rom the sarcoplasmic reticulum in response to an action potential is a lso similar in mouse and frog, 140-150 mu M/ms (16 degrees C).