Passive electrical properties and voltage dependent membrane capacitance of single skeletal muscle fibers

The passive membrane capacitance and conductance of isolated single muscle fibers were investigated using a vaseline gap method. The results obtained with this method are consistent with those obtained using the microelectrode technique. It was confirmed that the membrane capacitance of skeletal muscle consisted of a large capacitance of tubular membrane (7-10 microF/cm2) and a much smaller capacitance of surface membrane (1-2 microF/cm2). The relative time constants of these two components vary from one sample to another, resulting in one time and two time constant behaviors. Secondly, the capacitance of isolated skeletal muscle fibers was investigated during hyper- and depolarizing pulses, using the transient bridge technique with the vaseline gap method. Measurements were performed at two frequencies, i.e. 500 Hz and 20 kHz. It was found that the membrane capacitance increased by 15-20% with depolarizations. The voltage dependent membrane capacitance was not affected by the addition of tetrodotoxin in bathing solution blocking sodium current and muscle contraction. Also, blocking both Na and K current did not have an appreciable effect on the non-linear behavior of membrane capacitance. The origin of voltage dependent capacitance in muscle membrane appears to be distributed among several non-linear ionic processes such as Na and K currents and the flux of Ca and Cl ions and their accumulation.