Differences in sodium voltage-gated channel properties according to myosin heavy chain isoform expression in single muscle fibres

The myosin heavy chain (MHC) isoform determines the characteristics and shortening velocity of muscle fibres. The functional properties of the muscle fibre are also conditioned by its membrane excitability through the electrophysiological properties of sodium voltage-gated channels. Macropatch-clamp is used to study sodium channels in fibres from peroneus longus (PL) and soleus (Sol) muscles (Wistar rats, n = 8). After patch-clamp recordings, single fibres are identified by SDS-PAGE electrophoresis according to their myosin heavy chain isoform (slow type I and the three fast types IIa, IIx, IIb). Characteristics of sodium currents are compared (Student's t test) between fibres exhibiting only one MHC isoform. Four MHC isoforms are identified in PL and only type I in Sol single fibres. In PL, maximal sodium current ( I _max), maximal sodium conductance ( g _Na,max) and time constants of activation and inactivation (τ _m and τ _h ) increase according to the scheme I→IIa→IIx→IIb ( P < 0.05). τ _m values related to sodium channel type and/or function, are similar in Sol I and PL IIb fibres ( P = 0.97) despite different contractile properties. The voltage dependence of activation ( V _a,1/2) shows a shift towards positive potentials from Sol type I to IIa, IIx and finally IIb fibres from PL ( P < 0.05). These data are consistent with the earlier recruitment of slow fibres in a fast-mixed muscle like PL, while slow fibres of postural muscle such as soleus could be recruited in the same ways as IIb fibres in a fast muscle.