Evidence for the presence of mobile charges in the cell membrane of Valonia utricularis

Abstract

Charge-pulse relaxation studies were performed on cells of the giant marine alga Valonia utricularis with microelectrodes inserted into the vacuole. If the cell was charged by short pulses of 200 ns duration, the decay of the initial membrane voltage could be described by two relaxation processes at normal pH (8.2). The fast exponential relaxation had a time constant of approximately 100 microseconds whereas the the time constant of the slow relaxation ranged between 2 and 15 ms. The ratio of the two amplitudes varied between 10 and 20 and was found to be independent of the initial voltage, up to 400 mV. In contrast to the time constants, the amplitude ratio was a function of the duration of the charge pulse. As the pulse length was increased to 10 ms, the fast relaxation disappeared. A change in pH of the natural sea water from 8.2 to 4 resulted in the disappearance of both exponential processes and the appearance of one single exponential with a 1-ms time constant over the whole pulse-length range. The analysis of the data in terms of a two-membrane model leads to unusual values and a pH-dependence of the specific capacitances (0.6 and 6 microF cm-2) of the two membranes, which can be treated as two serial circuits of a capacitor and a resistor in parallel. The charge-pulse and the current-clamp data are consistent with the assumption that the cell membrane of V. utricularis contains mobile charges with a total surface concentration of approximately 4 pmol cm-2. These charges cross the membrane barrier with a translocation rate constant around 500 s-1 and become neutralized at low pH. From our experimental results it cannot be completely excluded that the tonoplast has also a high specific resistance. But in this case it has to be assumed that the tonoplast and plasmalemma have very similar electrical properties and contain both mobile charges, so that the two membranes appear as a single membrane. Experiments on artificial lipid bilayer membranes in the presence of the lipophilic ion dipicrylamine, support our mobile charge concept for the cell membrane of V. utricularis.