MEMBRANE CAPACITANCE IN FROG CUT TWITCH FIBERS MOUNTED IN A DOUBLE VASELINE-GAP CHAMBER

In experiments on cut muscle fibers mounted in a double Vaselinegap chamber, electrical measurements are usually made by measuring the voltage V1(t) in one end pool and by passing current I2(t) from the other end pool to the central pool, which is usually clamped to earth potential. The voltage in the current-passing end pool is denoted by V2(t). This article describes how the value of the holding current, Ih, and the values of ΔV2(∞)/ΔV1(Δ) and ΔI2(∞)/ΔV1(∞) that are associated with a small change in V1(t) can be used to estimate the linear cable parameters rm, ri, and re in a cut fiber that has been equilibrated with a Cscontaining internal solution, rm, ri, and re represent, respectively, the resistance of the plasma membranes, the internal longitudinal resistance, and the external longitudinal resistance under the Vaseline seals, all for a unit length of fiber. The apparent capacitance, Capp, of the preparation is defined to equal ∫0∞I2.tr(t) dt/ΔV1(∞), in which ΔI2.tr(t) represents the transient component of current that is associated with a change in V1(t) of amplitude ΔV1(∞). A method is described to estimate cm, the capacitance of the plasma membranes per unit length of fiber, from Capp and the values of rm, ri, and re. In experiments carried out with a tetraethylammonium chloride (TEA. C1) solution at 13-14°C in the central pool, cm remained stable for as long as 3-4 h. The values of Cm, 0.19 µF/cm on average, and their variation with fiber diameter are similar to published results from intact fibers. This article also describes the different pathways that are taken by the current that flows from the current-passing end pool to the central pool. Approximately two-thirds of ΔI2, tr(t) flOWS across the capacitance of the plasma membranes in the central-pool region. The rest flows either across plasma membranes that are under the two Vaseline seals or directly from the currentpassing end pool to the central pool, across the external longitudinal resistance under the Vaseline seal. [There is also a current that flows directly from the voltage-measuring end pool to the central pool but this does not contribute to ΔI22, tr(t)]. © 1990, Rockefeller University Press., All rights reserved.