Determination of cable parameters in skeletal muscle fibres during repetitive firing of action potentials

Abstract

Key points: Current methods for determination of the resting membrane conductance in active, action potential-firing muscle fibres are based on the assumptions that the volume and the cytosolic composition of the fibres remain constant during activity. Here, we present a microelectrode-based method that without such assumptions measures a range of cable parameters including the resting membrane conductance together with action potential characteristics of individual fibres during intermittent action potential firing. The method can be used to study the role of activity-induced regulation of the resting membrane conductance for action potential excitation and propagation in active muscles. Recent studies in rat muscle fibres show that repetitive firing of action potentials causes changes in fibre resting membrane conductance (Gm) that reflect regulation of ClC-1 Cl- and KATP K+ ion channels. Methodologically, these findings were obtained by inserting two microelectrodes at close proximity in the same fibres enabling measurements of fibre input resistance (Rin) in between action potential trains. Since the fibre length constant (λ) could not be determined, however, the calculation of Gm relied on the assumptions that the specific cytosolic resistivity (Ri) and muscle fibre volume remained constant during the repeated action potential firing. Here we present a three-microelectrode technique that enables determinations of multiple cable parameters in action potential-firing fibres including Rin and λ as well as waveform and conduction velocities of fully propagating action potentials. It is shown that in both rat and mouse extensor digitorum longus (EDL) fibres, action potential firing leads to substantial changes in both muscle fibre volume and Ri. The analysis also showed, however, that regardless of these changes, rat and mouse EDL fibres both exhibited initial decreases in Gm that were eventually followed by a ∼3-fold, fully reversible increase in Gm after the firing of 1450-1800 action potentials. Using this three-electrode method we further show that the latter rise in Gm was closely associated with excitation failures and loss of action potential signal above -20 mV.