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 (> 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).
Hollingworth
S
S
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia 19104-6085, USA.
Zhao
M
M
Baylor
S M
SM
D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004594:Electrophysiology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D007700:Kinetics; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D009119:Muscle Contraction / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology* / Q000502:physiology; D055095:Optics and Photonics; D013696:Temperature; D013997:Time Factors
D005456:Fluorescent Dyes; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D002118:Calcium; D016257:Fura-2