Active Transport of Chloride by the Giant Neuron of the Aplysia Abdominal Ganglion

Internal chloride activity, aⁱ_Cl, and membrane potential, E_m, were measured simultaneously in 120 R2 giant neurons of Aplysia californica. aⁱ_Cl was 37.0 ± 0.8 mM, E_m was -49.3 ± 0.4 mv, and E_Cl calculated using the Nernst equation was -56.2 ± 0.5 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated neurons. Cooling to 1°–4°C caused aⁱ_Cl to increase at such a rate that 30–80 min after cooling began, E_Cl equalled E_m. The two then remained equal for as long as 6 hr. Rewarming to 20°C caused aⁱ_Cl to decline, and E_Cl became more negative than E_m once again. Exposure to 100 mM K⁺-artificial seawater caused a rapid increase of aⁱ_Cl. Upon return to control seawater, aⁱ_Cl declined despite an unfavorable electrochemical gradient and returned to its control values. Therefore, we conclude that chloride is actively transported out of this neuron. The effects of ouabain and 2,4-dinitrophenol were consistent with a partial inhibitory effect. Chloride permeability calculated from net chloride flux using the constant field equation ranged from 4.0 to 36 x 10^-8 cm/sec.