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Whole-cell currents in macrophages: II. Alveolar macrophages
D J Nelson1, B Jow, K J Popovich
1Department of Medicine, University of Chicago, Illinois 60637.
Abstract:
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.
Insights
Human alveolar macrophages (HAMs) express an inactivating outward potassium current (IA) significantly more often than human monocyte-derived macrophages (HMDMs). This difference in IA expression may impact macrophage immune functions.
Area of Science:
- Immunology
- Cell Physiology
- Ion Channel Research
Background:
- Outwardly rectifying K+ conductance is known in murine macrophages but is rare in human monocyte-derived macrophages (HMDMs).
- Human alveolar macrophages (HAMs) are a primary immune cell population in the lung and a readily available source of human tissue macrophages.
Purpose of the Study:
- To compare the expression of an inactivating outward potassium current (IA) between in vitro differentiated HMDMs and ex vivo isolated HAMs.
- To characterize the properties of IA in both macrophage types.
Main Methods:
- Whole-cell patch-clamp recordings were performed on HMDMs cultured for various durations and on HAMs obtained from bronchoalveolar lavage.
- Ionic currents were analyzed for voltage dependence, ion selectivity, and sensitivity to 4-aminopyridine (4-AP).
Main Results:
- IA was observed in only 5% of HMDMs (49/974 cells) but in 50% of HAMs (160/26 donors).
- The IA current in both cell types was K+ selective, 4-AP sensitive, and showed weakly voltage-dependent inactivation with a recovery time constant of ~30 sec.
- Current inactivation kinetics were dependent on external K+ concentration, and activation/inactivation occurred at specific voltage potentials.
Conclusions:
- HAMs exhibit a significantly higher frequency of IA expression compared to HMDMs.
- The presence of IA may influence mononuclear phagocyte functions such as chemotaxis, phagocytosis, and tumoricidal activity.