Growth hormone (GH) exhibits a great deal of molecular heterogeneity (there are over 100 isoforms in circulation) and we are only beginning to understand how exercise influences concentrations of GH isoforms.

As the field of exercise endocrinology moves forward in the 21st century, greater recognition will be given to molecular heterogeneity that exists for many protein hormones within the paradigm of exercise and physical activity. Specifically, it is well established, but not well recognised, that GH exhibits a great deal of molecular heterogeneity, as over 100 molecular isoforms are thought to exist in the circulation. The importance of gaining a greater understanding of the exercise-mediated influences on GH molecular heterogeneity resides in the fact that the GH isoforms have diverse downstream metabolic and anabolic actions in target tissues.

GH MOLECULAR HETEROGENEITY

The human GH-N gene expresses the main pituitary molecular mass variant in the GH family, which is the 22 kDa form. Monomeric 22 kDa GH is a 191 amino acid sequence and represents ∼21% of all circulating plasma GH. The next most prevalent form is the monomeric 20 kDa molecule, representing ∼6% of all circulating plasma GH, formed through alternative mRNA splicing during which amino acid residues 32–46 are cleaved out. GH can also undergo post-translational modification and peripheral tissue proteolytic cleavage at its site of action to form variants and aggregates (ie, dimers, trimers, pentamers, oligomers) and fragments that exist in the circulation. The existence of high-affinity and low-affinity GH-binding proteins, which are released from the pituitary and/or cleaved from the extracellular domain of the GH receptor, adds further complexity to the nature and spatial arrangement of circulating GH moieties. GH should no longer be regarded as a single hormone, but rather as a …