Electrical Properties of Neurospora crassa : Effects of external cations on the intracellular potential

Glass micropipette electrodes have been employed to study the transsurface potential difference of Neurospora crassa. For mature hyphae grown in agar cultures, the internal potential is large and negative, often exceeding -200 mv. The potential is sensitive to the concentrations of extracellular potassium, sodium, hydrogen, and calcium ions, but does not vary in a manner which is readily explained by ionic diffusion potentials. With extracellular solutions containing only potassium chloride (or sulfate) and sucrose, the internal potential shifts toward zero (becomes less negative) at 45 mv per tenfold increase of potassium, over the range 0.1 to 10 mM. A similar result has been found with sodium, though the slope is only 33 mv/log unit. Calcium (1 mM) diminishes the influence of potassium and sodium by 60 to 70 per cent. As potassium or sodium is raised above 20 mM, the slope of the internal potential increases sharply to 85 to 90 mv/log unit, both in the presence and absence of calcium. With increasing hydrogen ion concentration, too, the internal potential shifts toward zero; in this case the slope is about 12 mv/pH unit at pH 9 and rises smoothly to 33 mv/pH unit at pH 3. All these phenomena are probably properties of the plasma membrane. The polysaccharide cell wall contains few fixed negative charges, has a low transverse resistance, and supports very little potential difference when separated from the plasma membrane.