Modeling the current-voltage characteristics of Chara membranes: I. The effect of ATP removal and zero turgor.

Abstract

We have obtained and modeled the electrical characteristics of the plasma membrane of Chara internodal cells: intact, without turgor and perfused with and without ATP. The cells were voltage and space-clamped to obtain the I/V (current-voltage) and G/V (conductance-voltage) profiles of the cell membrane. The intact cells yielded similar I/V characteristics with resting p.d.s. of -221 +/- 12 mV (cytoplasmic clamp, 5 cells) and -217 +/- 12 mV (vacuolar clamp, 5 cells). The cut unperfused cells were depolarized at -169 +/- 12 mV (7 cells) compared to the vacuole-clamped intact cells. The cells perfused with ATP fell into three groups: hyperpolarized group with resting p.d. -175 +/- 12 mV (4 cells) and I/V profile similar to the intact and cut unperfused cells; depolarized group with resting p.d. of -107 +/- 12 mV (6 cells) and I/V profiles close to linear; and excited cells with profiles showing a negative conductance region and resting p.d. at -59 +/- 12 mV (5 cells). The cells perfused with medium containing no ATP showed upwardly concave I/V characteristics and resting p.d. at -81 +/- 12 mV (6 cells). The I/V curves were modeled employing the "Two-state" model for the H+ pump (Hansen et al., 1981). The inward and outward rectifiers were fitted to exponential functions and combined with a linear background current. The excitation state in perfused cells was modeled by including an inward current, iexcit, with p.d.-dependence described by a combination of hyperbolic tangent functions. An inward current, ino-ATP, with a smaller amplitude, but very similar p.d.-dependence was also included in the simulation of the I/V curves from cells without ATP. This approach avoided I/V curve subtraction. The modeling of the total I/V and G/V characteristics provided more information about the parameters of the "Two-state" pump model, as well as more quantitative understanding of the interaction of the major transport systems in the plasmalemma in generation of the resting potential under a range of circumstances. ATP had little effect on nonpump currents except the excitation current; depolarization profoundly affected the pump characteristics.