Decreased Serum Somatomedin C Concentrations during Sleep: Temporal Relationship to the Nocturnal Surges of Growth Hormone and Prolactin*

Abstract

Serum concentrations of immunoreactive somatomedin C, GH, and PRL were determined on samples obtained from 16 adult subjects throughout the day. Samples were collected by a portable continuous flow, blood withdrawal pump. Somatomedin C concentrations were stable during waking hours [1.13 +/- 0.09 U/ml (mean +/- SE)] but fell after the onset of sleep to a minimum value of 0.85 +/- 0.08 U/ml. After awakening, the concentration rose progressively to a mean of 1.26 +/- 0.10 U/ml between 1100-1200 h. These changes were statistically significant. There were no sex differences in the sleep-related decrement. The secretion of GH showed the expected surge in the first hours of sleep, and PRL concentrations were higher in the late phases of sleep or immediately after awakening. In sleep reversal experiments, three subjects who were deprived of sleep had a modest, slightly delayed fall in somatomedin C but no secretory peaks of GH or PRL. After the delayed onset of sleep, somatomedin C declined sharply as human GH concentrations rose.