Bioactive growth hormone in older men and women: It's relationship to immune markers and healthspan

Objective The consequences of age-related decline in the somatotropic axis of humans are complex and remain largely unresolved. We tested the hypothesis that hGH measurements of plasma by bioassay vs immunoassay from samples obtained from free-living, elderly individuals would reveal a dichotomy in GH activities that are correlated with the functional status of the donors, i.e. their healthspan. Design Forty-one men and women of advanced age (men: N = 16, age, 80.5 ± 6.5 years; height, 173.1 ± 6.9 cm; body mass, 81.8 ± 13.0 kg) and (women: N = 25, age, 80.7 ± 7.2 years; height, 157.7 ± 6.0 cm; body mass, 68.8 ± 17 kg), were recruited for a cross-sectional study. Participants filled out PROMIS (Patient-Reported Outcomes Measurement Information System, U. S. Department of Health and Human Services) scales, undertook physical performance tests and had fasted blood samples obtained at rest for measurement of hormonal and immunology biomarkers. Results When measured by the well-established rat tibial line GH bioassay, one half of the plasma samples (n = 20) contained bioassayable GH (bGH), but the other half (n = 21) failed to mount increases in tibial plate width above saline injected controls. This difference did not correlate with the age, sex or physical functionality of the plasma donor. It also did not correlate with hGH concentrations measured by immunoassay. In those cases in which bGH was detected, various hierarchical regression models predicted that GHRH, c-peptide, VEGF, NPY, IL-4 and T-regulatory lymphocytes were associated with the difference and predicted bGH. Conclusion Results from this study suggest that the actions of bGH at the cellular level may be modified by other factors and that this may explain the lack of correlations observed in this study.