Electrophysiological properties of Dictyostelium derived from membrane potential measurements with microelectrodes

Electrical membrane properties of the cellular slime mold Dictyostelium discoideum were investigated with the use of intracellular microelectrodes. The rapid potential transients (1 msec) upon microelectrode penetration of normal cells had a negative-going peak-shaped time course. This indicates that penetration of a cell with a microelectrode causes a rapid depolarization, which can just be recorded by the microelectrode itself. Therefore, the initial (negative) peak potential transient value Ep (-19mV) should be used as an indicator of the resting membrane potential Em of D. discoideum before impalement, rather than the subsequent semistationary depolarized value En (-5 mV). Using enlarged cells such as giant mutant cells (Ep = -39 mV) and electrofused normal cells (Ep = -30 mV) improved the reliability of Ep as an indicator of Em. From the data we concluded that Em of D. discoideum cells bathed in (mM) 40 NaCl, 5 KCl and 1 CaCl2 is at least -50 mV. This potential was shown to be dependent on extracellular potassium. The average input resistance Ri of the impaled cells was 56 M omega for normal D. discoideum. However, our analysis indicates that the membrane resistance of these cells before impalement is greater than 1 G omega. Specific membrane capacitance was 1-3 pF/cm2. Long-term recording of the membrane potential showed the existence of a transient hyperpolarization following the rapid impalement transient. This hyperpolarization was associated with an increase in Ri of the impaled cell. It was followed by a depolarization, which was associated with a decrease in Ri. The depolarization time was dependent on the filling of the microelectrode. The present characterization of the electrical membrane properties of Dictyostelium cells is a first step in a membrane electrophysiological analysis of signal transduction in cellular slime molds.