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Effects of divalent cations on oscillations of membrane potentials (i.e., spontaneous repetitive hyperpolarizing responses) and on hyperpolarizing responses induced by electrical stimuli as well as on resting potentials were studied in large nondividing L cells. Deprivation of Ca2+ from the external medium inhibited these hyperpolarizing responses accompanying slight depolarization of the resting potential Sr2+ or Mn2+ applied to the external medium in place of Ca2+ was able to substitute for Ca2+ in the generation of hyperpolarizing responses, while Mg2+, Ba2+ or La3+ suppressed hyperpolarizing responses. The addition of A23187 to the bathing medium or intracellular injection of Ca2+, Sr2+, Mn2+ or La3+ induced membrane hyperpolarization. When the external Ca2+, Sr2+ or Mn2+ concentration was increased, the resting potential also hyperpolarized, in a saturating manner. The amplitude of maximum hyperpolarization produced by high external Ca2+ was of the same order of magnitude as those of hyperpolarizing responses and was dependent on the external K+ concentration. In the light of these experimental observations, it was deduced that the K+ conductance increase associated with the hyperpolarizing excitation is the result of an increase in the intracellular concentration of free Ca2+ mainly derived from the external solution.

Oscillation and activated hyperpolarizing responses induced by electrical stimuli (H.A. responses) were studied in large nondividing L ceils (giant L cells) under a variety of ionic conditions. When C1-in the bathing fluid was partially replaced with SO 2-at fixed external Na + and K + concentrations, the membrane potential depolarized transiently, but recovered to the original potential level after about 10 rain. Under such a steady state in a low-C1-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 C1conductances 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 -94mV 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.Nelson, Peacock and Minna (1972) found that L cells responded to an electrical, mechanical or chemical stimulus by producing a hyper-* Present Address: D6partment de physique, Universit6 de Montr6al,

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Dielectric dispersion measurements over a frequency range 0.01-100 MHz were made with the suspensions of a cultured cell line, mouse lymphoma L5178Y, and an attempt to explain the observed dielectric behavior by taking explicitly into consideration the possible involvement of cell nucleus has been presented. The use of a conventional "single-shell" model in which the cell is represented by a homogeneous sphere coated with a thin limiting shell phase did not duplicate the observed dispersion curves, whereas a "double-shell" model in which one additional concentric shell is incorporated into the "single-shell" model gave a much better fit between the observed and the predicted dispersion curves. Based on the latter model, we analyzed the raw data of dielectric measurements to yield a set of plausible electrical parameters for the lymphoma cell: CM approximately or equal to 1.0 muF/cm2, CN approximately or equal to 0.4 muF/cm2, epsilonk approximately or equal to 300, kc/ka approximately to or equal to 0.9, and kk/kc approximately or equal to 0.7. Here, CM and CN are the specific capacities of plasma and nuclear membranes; epsilon and k are the dielectric constant and conductivity with subscript a, c and k referring respectively to the extracellular, the cytoplasmic and the karyoplasmic phases.