Figure 6. Superfast muscles have convergent elevated myoplasmic calcium transient dynamics.
(a) Representative force development and myoplasmic calcium concentration ([Ca2+]i) transients in songbird and toadfish SFMs. (b) Force and [Ca2+]i transient full-width-at-half-maximum (FWHM) values as a function of temperature. [Ca2+]i FWHM values converge to 1.5–2.0 ms for all SFMs (color left of ordinate) at their operating temperatures (shaded areas). Filled circle and crosses refer to toadfish and rattlesnake data from ref 6. (c) Calcium loading and unloading kinetics in isolated SR vesicles (see Materials and methods) yield similar values for bat SFMs compared to songbird and toadfish SFMs, and were significantly elevated (p<0.01, ANOVA) 5–10 times compared to intraspecific skeletal muscle in zebra finch and bat.
Figure 6—figure supplement 1. Calcium transient speed is stable over multiple stimulations.
(a) Representative traces of changes in fluorescence (dF) scaled to resting fluorescence (FR) during multiple stimulations in toadfish SFMs. The dF/FR signal inversely relates to the myoplasmic calcium concentration ([Ca2+]i). The first transient clearly shows an increased peak magnitude compared to the consecutive transients. Color coding from green (twitch 1) to red (twitch 10). (b) dF/FR transients overlaid for the 10 twitches and (c) corresponding dF/FR peak magnitude and FWHM as a function of twitch number. (d) In toadfish SFMs, dF/FR peak magnitude is higher for the first twitch compared to the consecutive ones confirming earlier observations (Harwood et al., 2011), but dF/FR FWHM does not change as a function of twitch number. (e–g) Results for zebra finch SFM. (h) Both dF/FR peak magnitude and FWHM do not significantly change as a function of twitch number (KW, p>0.05). The trend to longer duration FWHM with twitch number may be explained by measurement inaccuracy due to low signal to noise ratio.