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Variations in the cytosolic free Ca2+ concentration ( Ca2+i) upon LPS exposure were studied in single rat peritoneal macrophages loaded with fura-2 under carefully controlled conditions. Of a total of 60 cells examined, 47% responded to LPS (1 microgram/ml) with an increase in Ca2+i. Macrophages were heterogeneous with regard to the LPS response, with individual cells exhibiting single rapid and transient increases in Ca2+i, multiple transients, or slower and more sustained variations. In 62% of the responding cells, a second exposure to LPS elicited a Ca2+i rise, although usually to a slightly lower peak value. Thus, rapid desensitization to LPS does not occur in the majority of these macrophages. EGTA did not abolish the response of those cells that exhibited a single rapid transient in Ca2+i, indicating that the source of the initial Ca2+i rise was the intracellular stores. There was no obvious correlation between the type of response to LPS and the initial morphologic features (rounded vs polarized) of the cells. Our present work shows unequivocally that LPS induces increases in macrophage Ca2+i and, thereby, lends substantial support to the hypothesis that Ca2+i is a second messenger in LPS-mediated activation of the macrophage.
Letari et al. (Thu,) studied this question.