1. Fibroblastic L cells not only respond with a slow hyperpolarizing potential change to a mechanical or electrical stimulus but also show spontaneous, repetitive hyperpolarizations (i.e. membrane potential oscillation). 2. Almost all the cells can actively take up latex beads whose surfaces were treated by U.V. irradiation. 3. Non‐phagocytic L cells hardly showed hyperpolarizing responses, while hyperpolarizing responses were obtained in all the phagocytic L cells. The exposure of the cell surface to beads, however, did not trigger the generation of hyperpolarizing responses. 4. Metabolic inhibitors, low temperature and cytochalasin B inhibited both the uptake of beads and the hyperpolarizing responses. 5. Increasing the external concentration of Ca2+ induced a remarkable stimulation of the phagocytosis of beads. Mg2+ and Ba2+, which inhibited hyperpolarizing responses due to competition for Ca2+ sites on the outer surface of the membrane, significantly suppressed the uptake of beads. 6. Verapamil, a Ca2+ channel blocker, inhibited not only hyperpolarizing membrane responses but also ingestion of beads. 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.
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