| Abstract: | Enhanced function of the respiratory burst, measured as stimulated release of superoxide anion (O2-) or hydrogen peroxide, characterizes activated macrophages. Activated macrophages undergo a decline in their capacity to release O2- (a deactivation) when placed in culture for 3 days. To better understand the molecular basis for the enhanced respiratory burst of activated macrophages, we explored the mechanisms underlying deactivation of activated mouse peritoneal macrophages. Deactivation was observed when the assay was performed in a physiologic Na+ buffer, and by day 3 of culture, release of O2- from activated macrophages stimulated with phorbol myristate acetate (PMA) was almost identical to that in resident (nonactivated) macrophages. In contrast, when the assay was performed in a buffer in which Na+ was replaced by K+, release of O2- from activated macrophages on day 3 was equal to or greater than that on day 0, suggesting that the enzyme responsible for the respiratory burst was not altered during culture. The number and affinity of PMA receptors were not changed during culture and were not affected by high external K+. Continuous assay of O2- release by coverslip-adherent macrophages in a cuvette indicated that the lag time between addition of stimulus and release of O2- was reduced, and the initial rate of O2- release was enhanced in K+ buffer. The potency of monovalent cations to support O2- release was K+ greater than Rb+ greater than choline+ greater than Cs+ = Na+ greater than Li+, suggesting that characteristics such as ionic radius or molecular size influence this effect, and the effect is not due simply to absence of Na+. Extracellular Ca2+ or Mg2+ was required for the maximal effect of high external K+, and enhancement by high K+ and divalent cations increased progressively during culture. These findings suggest that deactivation is caused primarily by changes in signal transduction from PMA receptors to the respiratory burst enzyme, rather than by changes in these receptors or the enzyme itself, and that signal transduction can differ in different macrophage populations. |
