The role of Kupffer cell α 2-adrenoceptors in norepinephrine-induced TNF-α production

Abstract

Although previous studies have demonstrated that plasma levels of the proinflammatory cytokine tumor necrosis factor-α (TNF-α) increase during early sepsis, the precise mechanism responsible for its upregulation remains to be elucidated. Since recent studies have shown that the gut is an important source of norepinephrine (NE) release during early sepsis and enterectomy prior to the onset of sepsis attenuates TNF-α production, we hypothesized that gut-derived NE plays a major role in upregulating TNF-α via the activation of α 2-adrenoceptors on Kupffer cells. To confirm that NE increases TNF-α synthesis and release, Kupffer cells were isolated from normal rats and incubated with NE (20 or 50 nM) or another α 2-adrenergic agonist clonidine (50 nM) without addition of Escherichia coli endotoxin. Supernatant levels of TNF-α were then measured. In additional animals, intraportal infusion of NE (20 μM) with or without the specific α 2-adrenergic antagonist yohimbine (1 mM) at a rate of 13 μl/min was carried out for 2 h. Plasma and Kupffer cell levels of TNF-α were assayed thereafter. Moreover, the effects of NE and yohimbine on TNF-α production was further examined using an isolated perfused liver preparation. The results indicate that both NE and clonidine increased TNF-α release by approximately 4–7-fold in the isolated cultured Kupffer cells. Similarly, intraportal infusion of NE in vivo or in isolated livers increased TNF-α synthesis and release which was inhibited by co-infusion of yohimbine. Furthermore, the increased cellular levels of TNF-α in Kupffer cells after in vivo administration of NE was also blocked by yohimbine. These results, taken together, suggest that gut-derived NE upregulates TNF-α production in Kupffer cells through an α 2-adrenergic pathway, which appears to be responsible at least in part for the increased levels of circulating TNF-α observed during early sepsis as well as other pathophysiologic conditions such as trauma, hemorrhagic shock, or gut ischemia/reperfusion.