Cardiovascular stability and unchanged muscle sympathetic activity during xenon anaesthesia: role of norepinephrine uptake inhibition.

Background: Intraoperative hypotension is associated with increased risk of perioperative complications. The N-methyl-d-aspartate (NMDA) receptor (NMDA-R) antagonist xenon (Xe) induces general anaesthesia without impairment of cardiac output and vascular resistance. Mechanisms involved in cardiovascular stability have not been identified.
Methods: Muscle sympathetic activity (MSA) (microneurography), sympathetic baroreflex gain, norepinephrine (NE) plasma concentration (high-performance liquid chromatography), anaesthetic depth (Narcotrend(®) EEG monitoring), and vital parameters were analysed in vivo during Xe mono-anaesthesia in human volunteers (n=8). In vitro, NE transporter (NET) expressing HEK293 cells and SH-SY5Y neuroblastoma cells were pre-treated with ketamine, MK-801, NMDA/glycine, or vehicle. Subsequently, cells were incubated with or without Xe (65%). NE uptake was measured by using a fluorescent NET substrate (n=4) or [(3)H]NE (n=6).
Results: In vivo, Xe anaesthesia increased mean (standard deviation) arterial pressure from 93 (4) to 107 (6) mm Hg and NE plasma concentration from 156 (55) to 292 (106) pg ml(-1), P<0.01. MSA and baroreflex gain were unaltered. In vitro, ketamine decreased NET activity (P<0.01) in NET-expressing HEK293 cells, while Xe, MK-801, and NMDA/glycine did not. Xe reduced uptake in SH-SY5Y cells expressing NET and NMDA-Rs (P<0.01). MK-801 (P<0.01) and ketamine (P<0.01) also reduced NET activity, but NMDA/glycine blocked the effect of Xe on [(3)H]NE uptake.
Conclusions: In vivo, Xe anaesthesia does not alter sympathetic activity and baroreflex gain, despite increased mean arterial pressure. In vitro, Xe decreases the uptake of NE in neuronal cells by the inhibition of NET. This inhibition might be related to NMDA-R antagonism and explain increased NE concentrations at the synaptic cleft and in plasma, contributing to cardiovascular stability during Xe anaesthesia.