Oscillations of membrane potential in L cells

Summary

Oscillation and activated hyperpolarizing responses induced by electrical stimuli (H.A. responses) were studied in large nondividing L cells (giant L cells) under a variety of ionic conditions. When Cl⁻ in the bathing fluid was partially replaced with SO ²⁻₄ at fixed external Na⁺ and K⁺ concentrations, the membrane potential depolarized transiently, but recovered to the original potential level after about 10 min. Under such a steady state in a low-Cl⁻ medium, the amplitudes of oscillations and H.A. responses remained almost identical with those in the control medium. On exposure to a low-Na⁺ medium, both membrane potentials in the resting and hyperpolarized states were slightly hyperpolarized, but the pattern and the amplitude of oscillations and H.A. responses remained much the same. Changes in external K⁺ concentrations remarkably affected the amplitudes of oscillations and H.A. responses: the amplitudes decreased with increases in external K⁺ concentration. Calculation of the changes in K⁺, Na⁺ and Cl⁻ conductances during oscillations and H.A. responses under these various ionic conditions showed that the change in K⁺ conductance is the only factor responsible for the oscillation and the H.A. response. The reversal potential for the potential oscillation is about −94 mV under normal conditions, this value being quite close to that of the equilibrium potential of K⁺. The reversal potentials in various external K⁺ concentrations satisfied the Nernst equation for a K⁺ electrode. Valinomycin induced remarkable hyperpolarization of the resting potential, resulting in an inhibition of oscillations. The level of valinomycin-induced hyperpolarization of the resting potential required to inhibit H.A. responses was the same as that of the peak potentials of the oscillation and H.A. response. In the light of these observations, it is concluded that the spontaneous potential oscillation and the H.A. response are caused solely by increase in the K⁺ conductance of the cell membrane.