Although previous studies have demonstrated that plasma levels of the proinflammatory
cytokine tumor necrosis factor-alpha (
TNF-alpha) 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-alpha production, we hypothesized that gut-derived NE plays a major role in upregulating
TNF-alpha via the activation of alpha(2)-adrenoceptors on Kupffer cells. To confirm that NE increases
TNF-alpha synthesis and release, Kupffer cells were isolated from normal rats and incubated with NE (20 or 50 nM) or another alpha(2)-adrenergic agonist
clonidine (50 nM) without addition of
Escherichia coli endotoxin. Supernatant levels of
TNF-alpha were then measured. In additional animals, intraportal infusion of NE (20 microM) with or without the specific alpha(2)-adrenergic antagonist
yohimbine (1 mM) at a rate of 13 microl/min was carried out for 2 h. Plasma and Kupffer cell levels of
TNF-alpha were assayed thereafter. Moreover, the effects of NE and
yohimbine on
TNF-alpha production was further examined using an isolated perfused liver preparation. The results indicate that both NE and
clonidine increased
TNF-alpha 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-alpha synthesis and release which was inhibited by co-infusion of
yohimbine. Furthermore, the increased cellular levels of
TNF-alpha 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-alpha production in Kupffer cells through an alpha(2)-adrenergic pathway, which appears to be responsible at least in part for the increased levels of circulating
TNF-alpha observed during early
sepsis as well as other pathophysiologic conditions such as
trauma,
hemorrhagic shock, or gut
ischemia/reperfusion.