Effects of particulate and soluble (1-3)-beta-glucans on Ca2+ influx in NR8383 alveolar macrophages

Particulate and soluble (1-3)-beta-glucans are effective in preventing infections by enhancing macrophage and neutrophil functions. However, the mechanisms triggering these enhanced cellular responses are essentially unknown. We recently demonstrated that zymosan, a particulate (1-3)-beta-glucan receptor agonist, caused an influx of Ca2+ in NR8383 rat alveolar macrophages (AMs) and a resulting increase in intracellular Ca2+ (Zhang et al., J. Leukoc. Biol. 62 (1997) 341-348). Since Ca2+ is important in mediating leukocyte responses, we investigated whether other (1-3)-beta-glucans also alter Ca2+ mobilization in AMs. Particulate and soluble (1-3)-beta-glucans derived from Saccharomyces cerevisiae were used in these studies. Like zymosan, particulate (1-3)-beta-glucan (WGPs) caused a concentration-dependent increase in [Ca2+]i, which was inhibited by removal of extracellular Ca2+ and by SKF96365, an inhibitor of receptor-operated Ca2+ channels. When three different soluble (1-3)-beta-glucans, with molecular weights of approximately 11,000, 150,000, and 1,000,000 Da, were tested alone for effects on Ca2+ responses, the low molecular weight (1-3)-beta-glucan produced no effect and the intermediate and high molecular weight (1-3)-beta-glucans caused only a small increase in [Ca2+]i. Interestingly, however, all three soluble (1-3)-beta-glucans could significantly reduce the Ca2+ responses induced by a subsequent exposure to either WGPs or zymosan. These results demonstrate that: 1) particulate (1-3)-beta-glucan activates Ca2+ influx in NR8383 macrophages through receptor-operated Ca2+ channels; 2) soluble (1-3)-beta-glucans do not strongly activate Ca2+ influx in these cells; and 3) soluble (1-3)-beta-glucans significantly inhibit Ca2+ influx induced by WGPs or zymosan. Soluble (1-3)-beta-glucans are likely to prevent Ca2+ influx by competitively binding to the (1-3)-beta-glucan receptors recognizing zymosan and WGPs. The smaller Ca2+ influx induced by soluble (1-3)-beta-glucans may represent only a partial activation of post-receptor signal transduction pathways necessary for inducing Ca2+ influx.