In this paper we examine the different voltage or calcium-dependent currents present in murine peritoneal macrophages, and in a macrophage-like cell line, J774. Three of these are K currents while the fourth is carried by Cl. One K current, activated by hyperpolarization, has all the characteristics of the inward rectifier found in egg or muscle cells. It appears in peritoneal macrophages only after several days in culture. A second K current, activated by depolarization, is a typical delayed rectifier. The amplitude of these currents and, as a consequence, the membrane potential of the cells, can be markedly changed by the movement of fluid around the cells. A third K current is activated by internal calcium levels in the micromolar range. It presents a low-voltage sensitivity and is blocked by 0.1-1 mM quinine. The Cl current flows through large-size channels (180-390 pS) that are active mainly in excised patches. These channels are unlikely to be half gap junctional channels, as suggested in former studies. The second goal of this paper is to examine if the activation of receptors for the Fc fragment of IgGs (Fc receptors) is associated with a change in the electrical properties of the membrane of macrophages. We have observed that the binding of multivalent ligands (the monoclonal antibody 2.4G2, aggregated IgGs, or sheep red blood cells coated with IgGs) to their Fc receptors on adherent macrophages did not trigger any change in resting potential. This is a surprising difference with former results obtained on non-adherent J774 cells (Young, J. D.-E., J. C. Unkeless, H. R. Kaback, and Z. A. Cohn, 1983, Proc. Natl. Acad. Sci. USA., 80:1357-1361) and on human alveolar macrophages (Nelson, D. J., E. R. Jacobs, J. M. Tang, J. M. Zeller and R. C. Bone, 1985, J. Clin. Invest., 76:500-507).

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