Analysis of 24-hour plasma profiles of growth hormone (GH)-binding protein, GH/GH-binding protein-complex, and GH in healthy children.

Human blood contains a high affinity GH binding protein (GHBP) which corresponds to the extracellular domain of he GH-receptor. It has been suggested that GHBP can modify the biological actions of GH, and alter the distribution of GH in the body. To study the hormonal regulation of growth, it is therefore necessary to measure GHBP as well as GH. We recently developed a ligand-mediated immunofunctional assay (LIFA) which allows separate quantitation of total GHBP (free and GH-bound) and the complex formed by GH and GHBP (GH/GHBP-complex) in human blood. We have now used the ligand-mediated immunofunctional assay to measure GHBP levels in plasma profiles from healthy children. GH was measured by immunoradiometric assay. Fifteen 24-h plasma profiles from 12 healthy children (3 girls and 9 boys) of different ages (6-15 yr), heights (-2.5 to +3.0 SD scores) and pubertal stages (1-4) were examined. Blood was withdrawn continuously for 24 h and collected in 20-min fractions. Time series for GH, GHBP, and GH/GHBP-complex were analyzed by cross-correlation and Fourier analysis. GH was secreted in a pulsatile fashion in all subjects. The concentration of the GH/GHBP-complex varied during the sampling period, and the changes correlated significantly with the GH pulses with correlation coefficients reaching maximum at zero time lag. In contrast, the changes in the total GHBP concentration were minor (coefficients of variation approximately 10%), and not correlated to GH pulses. Fourier analysis showed similar spectral power patterns for GH and GH/GHBP-complex, suggesting a diurnal rhythm (12- to 24-h periods) as well as components of higher frequencies (around 4-h periods). Although there were only subtle fluctuations in the total GHBP concentration, Fourier transformation revealed a diurnal rhythm with nadir during the night, while components of higher frequencies were much less abundant. We conclude that variations in total GHBP as measured by LIFA during a 24-h sampling period are small and that the concentration can be estimated from a single random blood sample.