Evidence of a circadian rhythm for muscle sympathetic nerve activity in humans

Abstract

We previously observed that exogenous melatonin (3 mg) increases muscle sympathetic nerve activity (MSNA) at rest in humans (Cook and Ray, Clin Auton Res 19:303, 2009). Because melatonin exhibits a circadian rhythm in humans by increasing at night, it was hypothesized that MSNA at rest would be greater at night compared to day. Melatonin typically increases 8–10 times over daytime concentrations during the late evening hours. To test this hypothesis, arterial blood pressure, heart rate, calf blood flow and MSNA was measured in 9 healthy subjects (28 ± 1 yr; 5 male, 4 female) while in the prone position. Each subject was tested around noon and 10–12 hr later that evening (day: 11:34 ± 13 min, night: 22:10 ± 5 min). Subjects were instructed not to sleep between the two testing sessions. Contrary to our hypothesis, MSNA at rest was significantly decreased at night compared to day (8 ± 1 vs. 11 ± 2 bursts/min, respectively) (P < 0.05). Calf blood flow, heart rate, and arterial blood pressure were not significantly different at night compared to day (calf blood flow: 3.2 ± 0.3 vs. 2.8 ± 0.5 ml*100 ml−1*min−1, heart rate: 70 ± 4 vs. 66 ± 4 beats/min, arterial blood pressure: 91 ± 2 vs. 87 ± 1 mmHg, respectively). The data indicate that MSNA at rest decreases during the late evening hours suggesting there is diurnal variation in muscle sympathetic nerve activity in humans.Supported by: NIH HL077670 and a NASA Space Grant Fellowship