Simulation of Myoplasmic Calcium Transients in Mouse Slow-Twitch Muscle Fibers

Abstract

ABSTRACTFiber bundles were dissected from soleus muscles of adult mice and individual slow-twitch fibers were micro-injected with furaptra, a low-affinity rapidly-responding fluorescent Ca indicator. Fiber activity was elicited by action potential stimulation at 16 oC and dfCaD, the fraction of the indicator in the Ca-bound form, was measured. The dfCaD waveform was simulated with a multi-compartment reaction-diffusion model that provides estimates of the amount and time course of Ca release from the sarcoplasmic reticulum (SR), the binding and diffusion of Ca in the myoplasm, the re-uptake of Ca by the SR Ca pump, and the myoplasmic free Ca transient itself (d[Ca]). In response to one action potential (AP), the following spatially-averaged estimates were obtained (concentration units are referred to the myoplasmic water volume): 107 micro-molar for the amount of Ca release; 57 micro-molar/ms and 1.7 ms for the peak and full-duration at half maximum (FDHM) of the release flux; 7.6 micro-molar and 4.9 ms for the peak and FDHM of d[Ca]. In response to five APs at 67 Hz, d[Ca] summated somewhat in response to later action potentials in the train, while the second and subsequent releases declined progressively, from 0.3 to 0.1 times that of the first release. Two important parameters of the model are the on- and off-rate constants of the reaction between Ca and the regulatory sites on troponin. Values of 0.4x108 M−1 s−1 and 26 s−1, respectively, were found to be consistent with the measurements of dfCaD. The peak troponin occupancy is estimated to be 79% and 93% in response to one and five APs, respectively.