Effects of Acute Melatonin Supplementation on Cardiovascular Variables During Sympathetic Stimuli in Healthy Young Individuals

Melatonin supplementation has displayed antihypertensive and antioxidant functions in animals and clinical populations. While the exact mechanisms are unknown, animal studies have indicated melatonin may inhibit sympathetic responses by inhibiting vascular α-receptors. However, these effects are untested in humans. PURPOSE: To determine if acute melatonin supplementation alters vascular responses to sympathetic activation caused by mild hypovolemia at rest and during rhythmic exercise in healthy humans. METHODS: 14 healthy young adults (25±1 years, 6 females, tested in days 1-7 of menstrual cycle) participated in a randomized, single blind, crossover protocol. 5 mg of melatonin (MEL) or placebo (PLA) (mint water, mimics flavor of melatonin spray) were given as a sublingual spray 30 minutes before data collection to allow for peak absorption. Forearm blood flow (FBF, doppler ultrasound) and mean arterial pressure (MAP, finger photoplethysmography) were continuously measured during 5 min of supine rest, 2 min of −20mmHg lower body negative pressure (LBNP, causes simulated hypovolemia), 7 minutes of rhythmic handgrip exercise at 20% maximal effort, with minutes 5-7 having LBNP superimposed (Ex+LBNP). Each event was separated by 10 min of quiet rest. Forearm vascular conductance (FVC, FBF/MAPx100mmHg) was calculated. Paired t-tests were performed to compare data between conditions, significance was set as p ≤0.05. RESULTS: Data are presented as Δ% mean ± SD. No significant differences were found with small effect sizes for FBF (PLA:-23.64±22.46 vs. MEL: -34.14±27.17, p=0.33, r=0.18) and FVC (PLA:-22.97±24.16 vs. MEL:-32.98±28.01, p=0.36, r=0.17) from rest to LBNP. There were also no significant differences found, with small effect sizes for FBF (PLA:996.61±1385.63 vs. MEL: 643.24±588.87, p=0.30, r=0.19) and FVC (PLA:770.60±1071.85 vs. MEL:513.45±478.07, p=0.33, r=0.18) from rest to exercise. Lastly, no significant differences were found with small effect sizes for FBF (PLA:4.41±22.48 vs. MEL:4.57±20.08, p=0.98, r=0.01) and FVC (PLA:2.75±21.16 vs MEL:2.93±19.79, p=0.88, r=0.03) from exercise to exercise with LBNP. CONCLUSION: Our data indicates that melatonin does not alter vascular responses with sympathetic stimuli caused by mild hypovolemia, exercise, and combined hypovolemia and exercise. Thus, any vascular effects of melatonin supplementation are not likely caused by inhibition of alpha receptors in healthy humans and the observed improvements in clinical populations are likely attributable to other mechanisms. Non-applicable. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.