Bioactive growth hormone in humans: Controversies, complexities and concepts.
OBJECTIVE
To revisit a finding, first described in 1978, which documented existence of a pituitary growth factor that escaped detection by immunoassay, but which was active in the established rat tibia GH bioassay.


METHODS
We present a narrative review of the evolution of growth hormone complexity, and its bio-detectability, from a historical perspective.


RESULTS
In humans under the age of 60, physical training (i.e. aerobic endurance and resistance training) are stressors which preferentially stimulate release of bioactive GH (bGH) into the blood. Neuroanatomical studies indicate a) that nerve fibers directly innervate the human anterior pituitary and b) that hind limb muscle afferents, in both humans and rats, also modulate plasma bGH. In the pituitary gland itself, molecular variants of GH, somatotroph heterogeneity and cell plasticity all appear to play a role in regulation of this growth factor.


CONCLUSION
This review considers more recent findings on this often forgotten/neglected subject. Comparison testing of a) human plasma samples, b) sub-populations of separated rat pituitary somatotrophs or c) purified human pituitary peptides by GH bioassay vs immunoassay consistently yield conflicting results.
['Hymer|Wesley C|WC|', 'Kennett|Mary J|MJ|', 'Maji|Samir K|SK|', 'Gosselink|Kristin L|KL|', 'McCall|Gary E|GE|', 'Grindeland|Richard E|RE|', 'Post|Emily M|EM|', 'Kraemer|William J|WJ|']
D000344:Afferent Pathways D000818:Animals D001681:Biological Assay D000066670:Cell Plasticity D000076663:Endurance Training D015444:Exercise D013006:Growth Hormone D019382:Human Growth Hormone D006801:Humans D007031:Hypothalamus D007118:Immunoassay D018482:Muscle, Skeletal D010805:Physical Conditioning, Animal D010903:Pituitary Gland, Anterior D051381:Rats D055070:Resistance Training D052683:Somatotrophs
Bioassayable growth hormone Somatotroph functional heterogeneity Somatotroph subpopulations