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

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 (delta F/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 (tau(1) and tau(2), 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.