Figure 3.
Fast-twitch muscle fibers showed biphasic regulation of Gm during AP firing. The voltage deflections, ΔV, in AP-firing fibers associated with the small-amplitude current injections were used for calculation of Gin and Gm using Eq. 3. This approach resulted in a temporal resolution of Gin and Gm of 7 s. (A and B) The mean values of Gin (A) and Gm (B) from 18 experiments that were performed as described for the fiber in Fig. 2. To quantify the data, phase 1 was fitted to Eq. 5, and phase 2 was fitted to Eq. 6. (C) Gm from a representative experiment (black circles) in which Eq. 5 was fitted to phase 1 (red line) and extended beyond phase 1 to illustrate the starting value for the fit of phase 2 to Eq. 6 (blue line). (D) The mean parameters obtained by fitting Eqs. 5 and 6 to the individual observations in the 18 fibers whose mean, Gm, is shown in B. These mean parameters were subsequently used to calculate Gm during the firing of 5,000 APs using Eqs. 5 and 6. The outcome of this calculation has been included as a red line in B. All mean data have been presented as means ± SEM.