The Short Term Secretion Pattern of Human Serum Melatonin Indicates Apulsatile Hormone Release*

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Abstract
It is well known that many hormones are secreted in a pulsatile fashion. Changes in pulse frequency and/or amplitude can have severe physiological and pathological consequences. Attempts to assess the short term secretion pattern of the pineal hormone melatonin (MLT) provided inconsistent results. The development of objective statistic-and computerbased pulse detection programs and the advent of more precise and specific MLT assay systems during recent years prompted us to reexamine the short term secretion of the pineal hormone. We investigated 16 healthy volunteers (8 males and 8 females), aged 28.1+−6.7 yr (mean+−SD). Five-milliliter blood specimens were collected at 10-min intervals from 1900-0900 h (n = 8), 3-min intervals from 2300-0300 h (n = 4), 10-min intervals from 0800-2200 h (n = 2), and 3-min intervals from 1100-1500 (n = 2). The serum MLT concentration in each specimen was measured with a previously described RIA. The sensitivity of the assay varied between 3.0-7.5 pg (0.013-0.033 pmol) MLT/tube. The intraassay variance was assessed on 10 serum pools to which varying amounts of MLT had been added, covering the entire range of the standard curve; the coefficient of variance varied between 16.6-6.1%. The sequence of MLT levels in each subject was examined for the existence of pulses by visual inspection and by the computer programs Pulsar and Cluster. Apart from the circadian rhythm, the three methods did not reveal clear pulses but, rather, continuous release of the pineal hormone during the day and night. We conclude that the pineal hormone MLT is, in contrast to many other hormones, secreted in an apulsatile manner. Very frequent blood sampling may be unnecessary for full characterization of the secretion pattern without loss of information. In addition to the circadian pacemaker, the pineal does not appear to be under the control of the proposed hypothalamic or an intrinsic pulse generator.