In many cases, a functional role has been demonstrated on the basis of pharmacological blockade, knockout, or knockdown. For example, Craner et al. (2005)

demonstrated that sodium channel blockade with TTX attenuates phagocytosis by 40% in cultured microglia. They also showed that the phagocytic capacity of microglia derived from med mice, which lack Nav1.6 channels ( Kohrman et al., 1996), is 65% lower than that of microglia from wild-type (WT) mice. Black et al. (2009) observed that the clinically used sodium channel blocker phenytoin significantly reduces the phagocytic activity of microglia by 50%–60% ( Figure 2). These studies also showed that TTX and phenytoin Vorinostat in vivo attenuate the release of the proinflammatory cytokines interleukin 1-α (IL-1α), IL-1β, and tumor necrosis factor α (TNF-α) from stimulated 5-FU molecular weight microglia while having minimal effects on the release of IL-2, IL-4, IL-6, IL-10, monocyte chemotactic protein 1 (MCP-1), and transforming growth factor α (TGF-α). Phenytoin and TTX also significantly decrease ATP-induced migration of microglia. Supporting a role for Nav1.6 in the pathway leading to microglial migration, the level of ATP-induced migration of microglia cultured from med mice is significantly lower than that of cells from littermate WT mice ( Black et al., 2009). Another line of evidence of a novel contribution

of sodium channels—in this case the “cardiac” Nav1.5 channel—to the function of nonexcitable cells was provided by Carrithers et al. (2007), who demonstrated the expression of Nav1.5 within phagosomes of activated human macrophages by immunocytochemistry and immunogold electron microscopy (Figure 3). The expression of Nav1.5 was restricted to late endosomes of these cells and was not detected in early endosomes or on the macrophage plasma membrane. Evidence of a role for Nav1.5 in macrophage function was provided by the demonstration that TTX at 10 μM (a concentration that blocks Nav1.5) and gene knockdown of Nav1.5 with small hairpin RNA (shRNA) inhibit phagocytosis by these cells. Using time-resolved

fluorometry, Carrithers et al. (2007) also demonstrated from that the sodium channel activator veratridine reduces intraendosomal pH and intraendosomal [Na+] and that 10 μM TTX blocks lipopolysaccharide-induced acidification of the intraendosomal compartment in both purified endosomes and intact macrophages. These results suggest a model in which Nav1.5 channels, located intracellularly within the membrane of late endosomes, provide a route for Na efflux, which counterbalances proton influx, and thereby maintain electroneutrality during acidification, which is one of the final stages of phagocytosis. Noting that pH regulates the current of Nav1.5 (Khan et al., 2006), Carrithers et al. (2007) suggested that the expression and activity of Nav1.