Asymmetric charge movement in contracting muscle fibres in the rabbit

The Vaseline-gap technique was used to record asymmetric charge movement in small segments of muscle fibres from the white sternomastoid or the soleus muscle of the rabbit. At 22 degrees C, non-linear ionic currents (Na+, K+, Cl-, Ca2+) were virtually eliminated for potential steps to 0 mV or below by specific blocking agents or ion substitution. A Boltzmann fit of charge movement (Q) vs. potential (V) produced the mean values Qmax = 15.2 nC/microF, V = -26.8 mV and k = 15.3 mV for twenty-three sternomastoid fibres, and 4.8 nC/microF, -32 mV and 13.7 mV for seven soleus fibres. Qmax for the sternomastoid fibres was similar to that for other fast-twitch fibres when normalized by surface area rather than capacitance. Using a 55 ms step, the mean threshold potential (Vth) for contraction in twenty-eight fibres was -25.9 (+/- 2.9) mV (+/- S.E. of mean), and the mean amount of charge moved (qth) at the threshold potential was 8.5 (+/- 0.4) nC/microF. In some contracting fibres, a component of charge movement was observed which was analogous to q gamma in amphibian muscle in its time course and potential dependence. Addition of 80 mM-sucrose to the external solution increased the speed of both the asymmetric charge movement and the charging of the linear capacitance of each fibre. The effect was reversible. A clear relation between the time course of these two parameters was established, and this strongly indicated that the majority of the asymmetric charge was located in the transverse tubular system or beyond. Moreover, it was shown that at 22 degrees C nearly all asymmetric charge moved in less than 0.5 ms after depolarization of the T-system. Sucrose in the external solution affected the Q vs. V relation, steepening the curve and shifting it to more negative potentials, as well as slightly increasing Qmax. The actions of sucrose strongly suggest that it effectively dilates and/or shortens the transverse tubular system, probably by osmotic effects.