POSTURE INFLUENCES MELATONIN CONCENTRATIONS IN PLASMA AND SALIVA IN HUMANS

Melatonin is extensively used as a circadian marker rhythm and thus an y factors influencing its concentrations other than endogenous rhythmi city must be assessed. We report here the effects of posture on melato nin concentrations in plasma and saliva. The study was performed durin g the rising phase of the melatonin rhythm between 21:00 and 01:30 h. From 23:00 h, 7 healthy subjects remained sitting until 24:00 h and in dim light (< 10 lux) until 01:30 h. From 24:00 h, they assumed a diff erent postural position in one of the following stages: Stage 1a: 24:0 0-00:30 h - standing, 00:30-01:00 h - supine, 01:00-01:30 h - standing ; Stage 1b - reverse of Stage 1a; Stage 2a: 24:00-01:30 h - supine; an d Stage 2b: 24:00-01:30 h - standing. Blood and saliva samples were ob tained every hour from 21:00-24:00 h and then every 10 min from 24:00- 01:30 h. Plasma and salivary melatonin concentrations, measured by rad ioimmunoassay, increased when moving from a supine to a standing posit ion and decreased when these positions were reversed. These changes, a s seen with other blood components, can be explained through the influ ence of gravity which causes a decrease in plasma volume on standing a nd an increase in plasma volume on lying down.