Responses of Regularly-Firing Aplysia R3-R13 Neurones to Normoxia and Hypoxia

Abstract

1. Exposure of 10 R3-R13 neurones to a 115-min period of hypoxia resulted in depolarization of their membrane potentials (EM) from a mean of −46.9±3.1 to −20.8±4.4mV (S.E.). 2. Intracellular potassium ion activities (aiK) decreased significantly from 118.9±5.1 to 67.7±8.5mM-K+. This is equivalent to a change in EK from −70.9 mV to −54.5 mV, which is insufficient to account for depolarization of approximately 26 mV. 3. During reoxygenation of the saline surrounding the ganglion, there was a continued depolarization of EM to −11.5 ± 3.2 mV and progressive fall in aiK to 49.2 ± 4.9 mM. 4. Decreases in the membrane slope resistance were also observed in these depolarizing neurones. The depression in resistance remained irreversible for as long as experiments were conducted. 5. Computations of PNa/PK ratios were made using a steady-state calculation. Increases in the PNa/PK ratio from 0.030 to 0.045 were observed during hypoxic depolarization using a modification of the Goldman equation which neglects the contribution of chloride ions. Subsequent depolarization and loss of aiK during reoxygenation elevated this value to 0.183. Whether or not the observed depression of the membrane resistance is linked to a change in either the sodium or potassium ion permeability is unknown. Release of neuro transmitter and related permeability changes cannot be ruled out as an effect of hypoxia.