Whole-cell currents in macrophages: II. Alveolar macrophages.

Although an outwardly rectifying K+ conductance has been described in murine peritoneal macrophages and a murine macrophage cell line, the expression of this conductance in human monocyte-derived macrophages (HMDMs) is rare. Whole-cell current recordings in this study were obtained from HMDMs differentiated in adherent culture for varying periods of time following isolation and compared to currents obtained in human alveolar macrophages (HAMs) obtained from bronchoalveolar lavage. These studies were undertaken to compare ionic current expression in the in vitro differentiated macrophage to that of a human tissue macrophage. HAMs are the major population of immune and inflammatory cells in the normal lung and are the most readily available source of human tissue macrophages. Of the 974 HMDMs in the study obtained from a total of 36 donors, we were able to observe the presence of the inactivating outward current (IA) which exhibited voltage-dependent availability in only 49 (or 5%) of the cells. In contrast, whole-cell current recordings from HAMs, revealed a significantly higher frequency of IA expression (50% in a total of 160 cells from 26 donors). In the alveolar cell, there was no correlation observed between cell size and peak IA amplitude, nor was there a relationship between peak IA amplitude and time in culture. The current in both cell types was K+ selective and 4-aminopyridine (4-AP) sensitive. IA in both cell types inactivated with a time course which was weakly voltage-dependent and which exhibited a time constant of recovery from inactivation of approximately 30 sec. The time course of current inactivation was dependent upon the external K+ concentration. An increase in the time constant describing current decay was observed in elevated K+. Current activation was half-maximal at approximately -18 mV in normal bathing solution. Steady-state inactivation was half-maximal at approximately -44 mV. The presence of the outwardly rectifying K+ conductance may alter the potential of the mononuclear phagocyte to respond to extracellular signals mediating chemotaxis, phagocytosis, and tumoricidal functions.