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

We measured the internal potassium activity, aⁱ_K, and membrane potential, E_m, simultaneously in 111 R2 giant neurons of Aplysia californica. aⁱ_K was 165.3 ± 3.4 mM, E_m was -47.8 ± 0.9 mv, and E_K calculated using the Nernst equation was -76.9 ± 0.05 mv. Such values were maintained for as long as 6 hr of continuous recording in untreated cells, aⁱ_K fell exponentially after the following treatments: cooling to 0.5°–4°C, ouabain, zero external potassium, 2,4-dinitrophenol, and cyanide. The effects of cooling and zero potassium were reversible. Potassium permeability was calculated from net potassium flux using the constant field equation and ranged from 2.6 to 18.5 x 10^-8 cm/sec. We conclude that potassium is actively transported into this neuron against a 30–40 mv electrochemical gradient.