A non‐linear voltage dependent charge movement in frog skeletal muscle.

Abstract

Voltage-clamp experiments were carried out using the 3 microelectrode technique. Using this method membrane current density at V1 is proportional to .DELTA.V(= V2-V1) where V1 and V2 are voltages at distances l and 2l from the end of a fiber. Voltage dependent Na currents were blocked by tetrodotoxin, K currents by tetraethylammonium ions and Rb. Contraction was blocked by adding sucrose, 467 mM. The current .DELTA.V (control) associated with a positive voltage step from a hyperpolarized conditioning voltage to the holding potential, -80 mV, showed 2 components, a capacitative transient which decayed rapidly and a maintained steady level. The current .DELTA.V (test) associated with the same size positive step from the holding potential showed the same 2 components plus a 3rd one, a transient current which usually decayed more slowly than the capacitative transient. The 3rd component is best seen by subtracting the first 2 components, obtained from .DELTA.V (control), from .DELTA.V (test) to give .DELTA.V (test-control). The difference trace .DELTA.V (test-control) showed a transient outward current during pulse ''on'', and a transient inward current during pulse ''off''. The rates of decay of the ''on'' and ''off'' transients were, in general, different. The ''on'' rate depended on the voltage during the pulse, the ''off'' rate did not. The total charge which moved during the ''on'' transient, evaluated as the time integral of current, was equal in magnitude but opposite in direction to that which moved during the ''off''. The amount of charge movement Q bears a sigmoid relationship to voltage. Data from 6 fibers gave average values Qmax = 25 nC/.mu.F (farad) (normalized by fiber capacitance), V = -44 mV, k = 8 mV. There are probably a fixed number of charged groups confined to the membrane but free to move reversibly between at least 2 different sites which see different proportions of the total membrane potential. The average values for Qmax and k give a density of charged groups of 5.1 .times. 1010 groups/.mu.F. Using 0.9 .mu.F/cm2 for capacitance/area there are 459 groups/.mu.m2 averaged over surface and tubular membranes. If the groups are located only in the tubules the density would be 500-600/.mu.m2. An analysis of the passive electrical properties in terms of the Falk and Fatt model gave values for the tubular time constant which ranged from 1.50-3.04 ms. Theoretical reconstructions of charge movement currents were carried out assuming that the groups are distributed uniformly according to surface and tubular capacitance. Tubular delays probably distort the current transients if much of the charge is located in the tubule(T)-system. A theoretical analysis was carried out using 2 simple circuits consisting of a constant capacitance, due to either T-system or sarcoplasmic reticulum, in series with a voltage dependent conductance. Changes in the voltage dependent conductance, either instantaneous or time dependent, cannot account for the observed charge movement currents.