Abstract Skeletal muscle is a target tissue of GH. Based on its anabolic properties, it is widely ac
Skeletal muscle is a target tissue of GH. Based on its anabolic properties, it is widely accepted that GH enhances muscle performance in sports and muscle function in the elderly. This paper critically reviews information on the effects of GH on muscle function covering structure, protein metabolism, the role of IGF1 mediation, bioenergetics and performance drawn from molecular, cellular and physiological studies on animals and humans. GH increases muscle strength by enhancing muscle mass without affecting contractile force or fibre composition type. GH stimulates whole-body protein accretion with protein synthesis occurring in muscular and extra-muscular sites. The energy required to power muscle function is derived from a continuum of anaerobic and aerobic sources. Molecular and functional studies provide evidence that GH stimulates the anaerobic and suppresses the aerobic energy system, in turn affecting power-based functional measures in a time-dependent manner. GH exerts complex multi-system effects on skeletal muscle function in part mediated by the IGF system.
Skeletal muscles are specialised contractile tissues that control posture and physical activity while having an important role in energy metabolism. Their function is dependent on the composition and strength of fibre types that require energy to drive and sustain contractile work.
Muscle function is regulated by many factors including genes, nutrition, lifestyle and hormones. Many hormones including growth hormone (GH), thyroid hormones, testosterone and glucocorticoids exert major effects on skeletal muscle growth and function. The stimulation of muscle protein anabolism and growth by GH has led to widespread expectation that it increases muscle strength and power. GH is considered to be one of the most widely abused performance-enhancing agents in sports (Barroso et al. 2008, Holt & Sonksen 2008). Outside the sporting arena, GH is marketed as an antiageing therapy for frailty and disability secondary to loss of muscle mass. Despite its unequivocal protein accreting properties, evidence supporting a beneficial effect on muscle function is limited (Birzniece et al. 2011).
Muscle function is assessed in many ways, most commonly as strength and power (Abernethy et al. 1995). These endpoints reflect overlapping but distinct aspects of muscle function. Strength is dependent on muscle size, types and quality of contractile proteins. Muscle power, a measure of work performed per unit time, is assessed in different ways that vary in duration. The energy required to support muscle work can be drawn from pre-formed stores or generated from the metabolism of substrates (Wells et al. 2009). Energy metabolism can be anaerobic or aerobic. Muscle power is influenced by the availability of energy or energy type at the time of assessment. The recognition of mitochondrial myopathies as a class of functional muscle disorders arising from defects in mitochondrial respiratory chain enzymes highlights bioenergetics as an important mechanism influencing skeletal muscle function dependent on oxidative phosphorylation (Schaefer et al. 2001). The intersection between muscle structure, function and the muscle energy system has been a neglected area of active research. However, recent advances in GH research have highlighted that the bioenergetics of muscle is an important player determining aspects of muscle function.
This paper reviews the effects of GH on muscle structure and composition, and on protein and energy metabolism. By drawing together animal and human studies and relating the information on function to structure and bioenergetics, this review will give a new perspective on the regulation of skeletal muscle function by GH.
GH stimulates whole-body anabolism with protein accretion occurring in muscle and extra-muscular tissues. GHD results in a reversible loss of aerobic capacity arising secondarily from impaired cardiopulmonary and haematological status. GH regulates the bioenergetics of muscle that enhance anaerobic performance. GH increases muscle strength by increasing muscle mass without affecting contractile force or fibre composition.
In conclusion, GH is an anabolic hormone, which positively regulates muscle function. The contractile function of skeletal muscle is dependent on muscle size, fibre types and the availability of energy. Muscles utilise different forms of energy to carry out specific function. The effects on skeletal muscle bioenergetics highlight a novel aspect of GH metabolic action that provides a new direction for future research in this field.
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the review.
K K Y H is supported in part by the National Health and Medical Council of Australia. V C is supported by the Princess Alexandra Hospital Research Support Scheme.