The first step in the macrophage activation process involves endocytosis of aloe carbohydrate and a rise in intracellular calcium. Thus AI can be observed within the cytoplasm of cultured macrophages as apple green fluorescence within an hour after exposure to fluorescein-labeled carbohydrate solution. When the location of this fluorescence is compared to labels for mitochondria and lysosomes, it shows greater than 98% correlation with lysosomal distribution (Monga etal, 1996; Burghardt etal, 1996). A significant increase in intracellular Ca2 + can be detected in macrophages following exposure to 50 ^g/ml AI. The Ca2 + flux occurs within seconds of addition of AI solution and appears as a single spike followed by a return to basal levels in less than one minute. No Ca2 + stimulatory activity can be detected in response to the pellet derived from centrifuged AI. Since this pellet consists of plant cell wall fragments rich in pectin (Ni, 2000) it is likely that the calcium flux is not simply due to ingestion of cell fragments by phagocytosis. When macrophage cultures are pretreated with the calcium chelating agent EGTA, the AI-induced Ca2 + response is completely abolished suggesting that the response to AI requires extracellular Ca2+. An AI solution can increase intracellular Ca2 + levels, not only in macrophages but also in uterine smooth muscle but not in liver epithelial cells. Free intracellular Ca2 + plays a pivotal role as a second messenger involved in signal transduction, and as the initiating step in macrophage activation.
Was this article helpful?