Bacterial endotoxin induces [Ca2+]i transients and changes the organization of actin in microglia

We have employed amoeboid microglia purified from primary cultures of neonatal rat brain to examine the effect of bacterial lipopolysaccharide (LPS), a potent activator of immune cells, on intracellular calcium concentration ([Ca2+]i) in brain macrophages. In single brain macrophages loaded with indo 1, pulse administration of LPS elicited a rapid and transient increase in [Ca2+]i. From a total of 70 cells examined, all responded to LPS with a similar [Ca2+]i transient, indicating a good homogeneity of the cell population with regard to the LPS response. It was concluded that the rise of cytosolic [Ca2+]i originated from intracellular stores because the response to LPS occurred similarly in the presence or in the absence of extracellular Ca2+. A second administration of LPS to the same cells resulted in a second but reduced [Ca2+]i transient. In contrast to the first response to LPS, this second response was totally dependent on the presence of Ca2+ in the extracellular medium. The first response to LPS was strongly inhibited by ruthenium red and could be suppressed in a reversible manner by preincubating the cells with caffeine in the absence of Ca2+ in the extracellular medium. These results indicate that caffeine-sensitive intracellular Ca2+ stores may be the major source of Ca2+ in the response of brain macrophages to LPS. The possible release of Ca2+ from phosphatidylinositol(3,4,5)-trisphosphate (IP3)-sensitive stores in brain macrophages was also evaluated by stimulating cells with the IP3-mobilizing agonist histamine. Brain macrophages were heterogeneous with regard to the histamine response since histamine induced a [Ca2+]i rise in only 30% of cells examined.(ABSTRACT TRUNCATED AT 250 WORDS)