Myosin heavy chain isoform composition and Ca2+ transients in fibres from enzymatically dissociated murine soleus and extensor digitorum longus muscles

摘要

Electrically elicited Ca²⁺ transients reported with the fast Ca²⁺ dye MagFluo-4 AM and myosin heavy chain (MHC) electrophoretic patterns were obtained in intact, enzymatically dissociated fibres from adult mice extensor digitorum longus (EDL) and soleus muscles. Thirty nine fibres (23 from soleus and 16 from EDL) were analysed by both fluorescence microscopy and electrophoresis. These fibres were grouped as follows: group 1 included 13 type I and 4 type IC fibres; group 2 included 2 type IIC, 3 IIA and 1 I/IIA/IIX fibres; group 3 included 4 type IIX and 1 type IIX/IIB fibres; group 4 included 2 type IIB/IIX and 9 type IIB fibres. Ca²⁺ transients obtained in groups 1, 2, 3 and 4 had the following kinetic parameters (mean ±s.e.m.): amplitude (ΔF/F): 0.61 ± 0.05, 0.53 ± 0.08, 0.61 ± 0.06 and 0.61 ± 0.03; rise time (ms): 1.64 ± 0.05, 1.35 ± 0.05, 1.18 ± 0.06 and 1.14 ± 0.04; half-amplitude width (ms): 19.12 ± 1.85, 11.86 ± 3.03, 4.62 ± 0.31 and 4.23 ± 0.37; and time constants of decay (τ1 and τ2, ms): 3.33 ± 0.13 and 52.48 ± 3.93, 2.69 ± 0.22 and 41.06 ± 9.13, 1.74 ± 0.06 and 12.88 ± 1.93, and 1.56 ± 0.11 and 9.45 ± 1.03, respectively. The statistical differences between the four groups and the analysis of the distribution of the parameters of Ca²⁺ release and clearance show that there is a continuum from slow to fast, that parallels the MHC continuum from pure type I to pure IIB. However, type IIA fibres behave more like IIX and IIB fibres regarding Ca²⁺ release but closer to type I fibres regarding Ca²⁺ clearance. In conclusion, we show for the first time the diversity of Ca²⁺ transients for the whole continuum of fibre types and correlate this functional diversity with the structural and biochemical diversity of the skeletal muscle fibres.