Diurnal pattern of plasma growth hormone-binding protein in man

The identification of specific GH-binding proteins (GH-BP) in human plasma, one of which is a fragment of the GH receptor, has added new complexity to the state of GH in the circulation. A major proportion of GH circulates in complexed form, which differs in kinetics and possibly bioactivity from free GH. Little is known about the regulation of the GH-BP, their constancy or variation in plasma, or plasma factors affecting GH binding. Consequently, the temporal pattern of bound and free GH in plasma is also unknown. Knowledge about possible spontaneous variability in GH-BP levels/activity is required for physiological investigations and comparative studies among different populations. To address these issues, we measured GH-binding activity in plasma every hour over a 24-h period in six normal adults (three men and three women). A standardized GH binding assay, employing incubation of plasma with [125I]GH and separation of bound from free GH by gel filtration, was used. GH-BP activity showed no significant diurnal variation in any subject. The average GH-BP activity was similar in all subjects, although statistically significant differences were found between some subjects. No age- or sex-related differences were identified. We conclude that in normal man plasma GH-BP activity is constant throughout the day, thereby implying 1) constancy of binding protein (and possibly GH receptor) concentration, and 2) absence of significant fluctuations in potential binding inhibitors/enhancers in plasma.