Kinetic properties of intramembrane charge movement under depolarized conditions in frog skeletal muscle fibers.

Abstract

Intramembrane charge movement was measured on skeletal muscle fibers of the frog in a single Vaseline-gap voltage clamp. Charge movements determined both under polarized conditions (holding potential, V. = -100 mV; Qm,~ = 30.4 _ 4.7 nC/I~F, F = -44.4 mV, k = 14.1 mV; charge 1) and in depolarized states (V H = 0 mV; Qmax = 50.0 _____ 6.7 nC/IxF, V= -109.1 mV, k = 26.6 mV; charge 2) had properties as reported earlier. Linear capacitance (LC) of the polarized fibers was increased by 8.8 -+ 4.0% compared with that of the depolarized fibers. Using control pulses measured under depolarized conditions to calculate charge 1, a minor change in the voltage dependence (to V =-44.6 mV and k = 14.5 mV) and a small increase in the maximal charge (to Qm~x = 31.4 _ 5.5 nC/txF) were observed. While in most cases charge 1 transients seemed to decay with a single exponential time course, charge 2 currents showed a characteristic biexponential behavior at membrane potentials between -90 and -180 inV. The voltage dependence of the rate constant of the slower component was fitted with a simple constant field diffusion model (c% = 28.7 s -j, F = -124.0 mV, and k = 15.6 mV). The midpoint voltage (V) was similar to that obtained from the Q-V fit of charge 2, while the steepness factor (k) resembled that of charge 1. This slow component could also be isolated using a stepped OFF protocol; that is, by hyperpolarizing the membrane to -190 mV for 200 ms and then coming back to 0 mV in two steps. The faster component was identified as an ionic current insensitive to 20 mM Co 2+ but blocked by large hyperpolarizing pulses. These findings are consistent with the model implying that charge 1 and the slower component of charge 2 interconvert when the holding potential is changed. They also explain the difference previously found when comparing the steepness factors of the voltage dependence of charge 1 and charge 2.

Charge inactivation in the membrane of intact frog striated muscle fibers.

“…Feldmeyer et al (1990) described small, but statistically insignificant, reductions in charge II while charge I was reprimed by restoring the membrane potential. However, other studies did report charge interconversion and further implicated charge II as the electrical signal for the inactivated form of this voltage sensor stabilized by D600 binding (Caputo & Bolanos, 1989;Szucs et al 1991).…”

“…However, subsequent cut-fibre studies in both amphibian and mammalian systems provided conflicting evidence for (Caputo & Bolanos, 1989; Szucs, Papp, Csernoch & Kovacs, 1991) or against (Feldmeyer, Melzer & Pohl, 1990; Szucs et al 1991; see Discussion) the existence of charge I-charge II interconversion. Furthermore, recent articles raised the possibility that these discrepancies might result from cable nonuniformities in fibres in Vaseline-gap systems particularly in membrane areas under the seals .…”