MODELING THE CURRENT-VOLTAGE CHARACTERISTICS OF CHARA MEMBRANES .1. THE EFFECT OF ATP REMOVAL AND ZERO TURGOR

摘要

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 mod eled by including an inward current, i(excit), with p.d.-dependence described by a combination of hyperbolic tangent functions. An inward current, i(no-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. [References: 37]