Ionic movements in a single neuron.

Intracellular ionic activities of K and C1 were measured in the giant cell of Aplysia californica before and after the inhibition of cellular metabolism. Ion-specific liquid ion-exchanger microelectrodes were used to make these measurements. The equilibrium potential for K, calculated from directly measured intracellular and extracellular ionic activities was -80 mV, while the resting membrane potential was -45 to -55 mV. This indicates the presence of an active transport system which pumps K into the cell. Cooling below 3°C, ouabain (2 x 10[-4]M), 2,4-dinitrophenol (0.2mM), and cyanide (5-10mM) all caused a net efflux of K. The efflux, which was generally mono-exponential, had a rate constant which varied between 4.2 x 10[-5]sec[-1] and 5 x 10[-5]sec[-1] from cell to cell, with no significant difference among the fluxes consequent to the various treatments. Therefore, it was concluded that each of these treatments completely inhibited all active K influx. The permeability coefficient for K, calculated on the basis of the net efflux of intracellular K, was 1.5 x 10[-8]cm/sec. The calculated ionic conductance (chord conductance) was 1.5 x 10[-5]mho/cm[2]. These values are in good agreement with values obtained using electrical measurements. The equilibrium potential for CI was from 2 to 18 mV more negative than Em, which indicated the presence of an outwardly directed C1 pump. Cooling the cell below 4°C caused a passive influx of C1 and EC1 equaled Em within 20-80 minutes. The C1 influx was mono-exponential and had a rate constant of -1.4 x 10[-4]sec[-1]. Although ouabain increased C1 influx, its effect was much smaller than that of cooling. It is possible that the fall in Em caused by ouabain was responsible for the somewhat parallel decrease in EC1. Studies on the effects of DNP and cyanide on intracellular chloride activity were inconclusive. The passive net influx of C1 yielded a calculated permeability coefficient and calculated ionic conductances which were almost two orders of magnitude greater than the values calculated from independent electrical measurements. It is suggested the C1 crosses the membrane primarily as a neutral complex with some carrier cation.