Simulation of Ca²⁺ transients in the continuum of mammalian skeletal muscle fiber types

Abstract

ABSTRACT: Mathematical models have allowed a better understanding of the excitation-contraction coupling (ECC) during the activation of skeletal muscle fibers. Experiments with isolated mammalian muscle fibers loaded with Mag-Fluo-4 have revealed differences in the kinetics of the Ca²⁺ transients, as key experimental manifestation of the ECC, in the continuum of fiber types. However, a comprehensive mathematical description of the ECC, that considers most of the Ca²⁺-binding sites (e.g. parvalbumin, troponin, the dye, transporters, and pumps) in that continuum of fiber types has not been developed. Also, besides the classical mechanisms, namely parvalbumin (PV) and sarcoplasmic reticulum Ca²⁺ ATPase (SERCA), it has been determined that the Ca²⁺ removal from cytoplasmic compartment also depends on the mitochondrial Ca²⁺ movements and the Na⁺/Ca²⁺ exchanger (NCX) when the Ca²⁺ concentration is high in the cytoplasm and the PV and SERCA are saturated. In this work, changes in the concentration of Ca²⁺ through the sarcomere due to the ion’s movement were simulated for different mammalian fiber types (I, IIA, IIX/D and IIB) using single Ca²⁺ transients experimentally measured in mouse fibers loaded with the fast fluorescent indicator Mag-Fluo-4 AM at room temperature. Considering an estimated dye concentration of 246.5 µM, we found that the amplitude of the peak of the release rate of Ca²⁺ from the sarcoplasmic reticulum of the fibers type IIA, IIX and IIB, was 22.7%, 437.6% and 528.5%, respectively, higher than the type I. The cytoplasmic peak Ca²⁺ concentration was 14.32 µM, 14.32 µM, 10.96 µM and 8.63 µM. On the other hand, the mitochondrial peak Ca2+ concentration was 10.3%, 77.8% and 79.6% lower in the fibers type IIA, IIX and IIB than in the type I. The Ca²⁺ extruded from the cytoplasm by the NCX was 27.1%, 466.1% and 405.1% higher in fibers type I, IIX and IIB than in the type IIA. The decay phase of the Ca²⁺ transient can be fitted with a biexponential function (with τ1 and τ2). An increase of 50% in the parvalbumin content differentially changed the τ1 of the IIB-IIX group by -6.0% and of the IIA-I group by 16.2%, whilst the τ2 always decreased, by 26.1% in the IIB-IIX group and 6.1% in the IIA-I group. In conclusion, we presented for the first time a comprehensive model of Ca²⁺ movements based on kinetic parameters of the four mammalian fiber types and the molecular machinery involved in the ECC, including the mitochondria and the NCX. The proposed model allowed us to gain insight into the kinetics of the Ca²+ transients obtained with a fast Ca²+ dye, for the continuum of muscle fiber types.