Influence of Intravenous Norepinephrine Administration on Melatonin Plasma Concentration in Awake and Anaesthetised Healthy Adult Volunteers.

Melatonin (MEL) is an endogenous hormone that regulates circadian rhythm, immune function and oxidative stress responses. Norepinephrine (NE), an endogenous catecholamine, increases blood pressure through the sympathetic nervous system and is commonly administered intraoperatively or in the clinically ill patient for hemodynamic support. Although endogenous NE is known to stimulate MEL synthesis, the effects of exogenous intravenous NE on plasma MEL concentration, particularly under general anaesthesia (GA), remain poorly characterised. This study examined the influence of intravenous NE on plasma MEL concentrations in healthy volunteers, both awake and under general anaesthesia. A secondary analysis was conducted using data from a prospective, single-centre, cross-over study involving volunteers who received stepwise NE infusions while awake and under total intravenous general anaesthesia. Arterial blood samples were collected to determine plasma MEL and NE concentrations. Linear mixed-effects modelling was used to assess the relationship between NE and MEL concentrations. A total of 36 healthy volunteers were enroled, yielding 417 MEL-NE samples. Intravenous NE administration was significantly associated with higher plasma MEL concentration, corresponding to an estimated 26% increase in MEL per 1 standard-deviation increase in NE concentration (Estimate = 0.230, p < 0.001). MEL concentrations were approximately 43% higher in the awake phase compared to GA (Estimate = 0.356, p < 0.001). GA did not significantly modify the NE-MEL association (Estimate = -0.02877, p = 0.528). Substantial positive correlation was observed between baseline MEL concentrations and the magnitude of the response to NE administration (r = 0.74). In conclusion, intravenous NE increases plasma MEL concentrations in both awake and anaesthetised individuals. Substantial interindividual variation in MEL response suggests underlying physiological differences. Future studies should explore the long-term effects of NE administration on MEL concentrations. Trial Registration: The study is registered in the Dutch Trial Register (NL9312).