DISTINCT MODES OF MELATONIN SECRETION IN NORMAL MEN

As for many hormones, melatonin levels in the blood suggest that it is discharged from the pineal gland in a pulsatile manner. Recently, the existence of short-term episodes, superimposed on the circadian patte rn of circulating melatonin, has been questioned. Because plasma melat onin levels reflect not only the secretory process, but also the effec ts of distribution and degradation, secretory rates were estimated fro m peripheral levels, using a deconvolution procedure. Fourteen healthy volunteers were studied during the night, while sleeping in the dark (2300-0700), and seven of them subsequently were used in a replicate s tudy. Plasma melatonin levels were measured at 10-min intervals by a d irect, specific radioimmunoassay. Pulse analysis was performed using t he computer program UL-TRA. Approximately 30% more pulses were detecte d on the overall secretory profiles than on plasma profiles. The pulse s occurred at random intervals and were often superimposed on tonic ba sal secretion. Their number, amplitude, and distribution over time wer e variable depending on subjects. Also the mean melatonin secretory ra te varied more than threefold across individuals. Despite the large in terindividual variability, the subjects, who were used in replicate ex periment, displayed a rather similar secretory profile. We conclude th at in normal adult men, melatonin secretion undergoes two distinct sec retory modes, in which episodic secretion is superimposed on tonic sec retion in subject-dependent variable proportions.