SUMMARY1. In fibroblastic L cells, spontaneously repeated hyperpolarizing responses (oscillation of membrane potential) and hyperpolarizing responses evoked by electrical stimuli were suppressed by the external application of a K+ channel blocker, nonyltriethylammonium (C9). This hydrophobic TEA-analogue also inhibited the hyperpolarization induced by intracellular Ca2+ injection.2. Quinine or quinidine, known inhibitors of the Ca2+-activated K+ channel of red cells, instantaneously inhibited these hyperpolarizations. Thus, these hyperpolarizations are likely to be caused by the operation of Ca2+-sensitive K+ channels.3. Azide, which is known to inhibit the mitochondrial Ca2+ uptake in fibroblasts, and caffeine, dantrolene Na and oxalate, which affect the microsomal Ca2+ transport, did not exert any effects upon the electrical potential profiles.4. On the other hand, Ca2+ channel blockers (nifedipine, D 600 and Co2+) suppressed the hyperpolarizing responses, but not the hyperpolarizations produced by intracellular Ca2+ injection, suggesting that the calcium ions responsible for the hyperpolarizing responses are mainly derived from outside the cell through Ca2+ channels.5. Flavones of plant origin, which are known to inhibit Ca2+-ATPase, prolonged the duration of the hyperpolarizing phase of the oscillation or produced a sustained hyperpolarization.6. It is concluded that the Ca2+ channel and the Ca2+ pump play essential roles in the generation of the hyperpolarizing response and of the membrane potential oscillation in L cells, and that these hyperpolarizations are brought about by a transient elevation of cytosolic Ca2+ level which, in turn, activates Ca2+-dependent K+ channels.

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,

7. It is concluded that the Ca2+ inflow on the hyperpolarizing membrane responses is closely associated with the phagocytic activity in L cells, probably through activation of the microfilament assembly.

Summary. The transepithelial resistance, the cell membrane resistance and the ratio of resistances of the serosal (baso-lateral) to the mucosal (brush border) cell membrane were measured in rat duodenum, jejunum and ileum by means of microelectrode techniques. These measured values were not affected in the presence of actively transported solutes in the mucosal bathing fluid.Contribution of an electrical conductance through the extracellular shunt pathway to the total transepithelial conductance was quantitatively estimated using an electrically equivalent circuit analysis. These values estimated in respective tissues of small intestine were approx. 95% of the total transepithelial conductance, remaining unaffected by an active solute transport.From these data, the changes in emf's of the mucosal and serosal membrane induced by D-glucose or glycine were separately evaluated.