Norepinephrine (NE) modulates the responsiveness of macrophages to proinflammatory stimuli through the activation of adrenergic receptors (ARs). Being part of the stress response, early increases of NE in sepsis sustain adverse systemic inflammatory responses. The intestine is an important source of NE release in the early stage of cecal ligation and puncture (CLP)-induced sepsis in rats, which then stimulates TNF-α production in Kupffer cells (KCs) through the activation of the α2-AR. It is important to know which of the three α2-AR subtypes (i.e., α2A, α2B or α2C) is responsible for the upregulation of TNF-α production. The aim of this study was to determine the contribution of α2A-AR in this process.
Adult male rats underwent CLP and KCs were isolated 2 h later. Gene expression of α2A-AR was determined. In additional experiments, cultured KCs were incubated with NE with or without BRL-44408 maleate, a specific α2A-AR antagonist, and intraportal infusion of NE for 2 h with or without BRL-44408 maleate was carried out in normal animals. Finally, the impact of α2A-AR activation by NE was investigated under inflammatory conditions (i.e., endotoxemia and CLP). Gene expression of the α2A-AR subtype was significantly upregulated after CLP. NE increased the release of TNF-α in cultured KCs, which was specifically inhibited by the α2A-AR antagonist BRL-44408. Equally, intraportal NE infusion increased TNF-α gene expression in KCs and plasma TNF-α which was also abrogated by co-administration of BRL-44408. NE also potentiated LPS-induced TNF-α release via the α2A-AR in vitro and in vivo. This potentiation of TNF-α release by NE was mediated through the α2A-AR coupled Gαi protein and the activation of the p38 MAP kinase. Treatment of septic animals with BRL-44408 suppressed TNF-α, prevented multiple organ injury and significantly improved survival from 45% to 75%.
Our novel finding is that hyperresponsiveness to α2-AR stimulation observed in sepsis is primarily due to an increase in α2A-AR expression in KCs. This appears to be in part responsible for the increased proinflammatory response and ensuing organ injury in sepsis. These findings provide important feasibility information for further developing the α2A-AR antagonist as a new therapy for sepsis.