[3][4] Receptors for myokines are found on muscle, fat, liver, pancreas, bone, heart, immune, and brain cells.
[1] They also participate in tissue regeneration and repair, maintenance of healthy bodily functioning, immunomodulation; and cell signaling, expression and differentiation.
Many proteins produced by skeletal muscle are dependent upon contraction; therefore, physical inactivity probably leads to an altered myokine response, which could provide a potential mechanism for the association between sedentary behaviour and many chronic diseases.
"[3]Physical exercise rapidly triggers substantial changes at the organismal level, including the secretion of myokines and metabolites by muscle cells.
[2] For instance, aerobic exercise in humans leads to significant structural alterations in the brain, while wheel-running in rodents promotes neurogenesis and improves synaptic transmission in particular in the hippocampus.
Moreover, physical exercise triggers histone modifications and protein synthesis which ultimately positively influence mood and cognitive abilities.
[14] Immunomodulation and immunoregulation were a particular focus of early myokine research, as, according to Dr. Bente Klarlund Pedersen and her colleagues, "the interactions between exercise and the immune system provided a unique opportunity to evaluate the role of underlying endocrine and cytokine mechanisms.
[7] As studies have demonstrated IL-6 has pro-inflammatory functions when evaluated in regard to sepsis and obesity, it was initially hypothesized that the exercise-induced IL-6 response was related to muscle damage.
It appears in muscle tissue and in the circulation during exercise at levels up to one hundred times basal rates, as noted, and may have a beneficial impact on health and bodily functioning with transient increases as P. Munoz-Canoves et al. write: "It appears consistently in the literature that IL-6, produced locally by different cell types, has a positive impact on the proliferative capacity of muscle stem cells.
To identify whether the brain is a source of BDNF during exercise, eight volunteers rowed for 4 h while simultaneous blood samples were obtained from the radial artery and the internal jugular vein.
Moreover, the importance of the cortex and hippocampus as sources of plasma BDNF becomes even more prominent in the response to exercise.”[20]With respect to studies of exercise and brain function, a 2010 report is of particular interest.
Erickson et al. have shown that the volume of the anterior hippocampus increased by 2% in response to aerobic training in a randomized controlled trial with 120 older adults.
The authors also summarize several previously-established research findings relating to exercise and brain function: (1) Aerobic exercise training increases grey and white matter volume in the prefrontal cortex of older adults and increases the functioning of key nodes in the executive control network.
(2) Greater amounts of physical activity have been associated with sparing of prefrontal and temporal brain regions over a 9-y period, which reduces the risk for cognitive impairment.
[22] Regarding the 2010 study, the authors conclude: "We also demonstrate that increased hippocampal volume is associated with greater serum levels of BDNF, a mediator of neurogenesis in the dentate gyrus.
These theoretically important findings indicate that aerobic exercise training is effective at reversing hippocampal volume loss in late adulthood, which is accompanied by improved memory function.
Kanzleiter et al have established that this myokine is secreted during muscular contraction against resistance, and plays a role in muscle growth.
In vivo overexpression of decorin in murine skeletal muscle promoted expression of the pro-myogenic factor Mighty, which is negatively regulated by myostatin.
[25][26][27] Irisin (fibronectin type III domain-containing protein 5 or FNDC5), a recently described myokine hormone produced and secreted by acutely exercising skeletal muscles, is thought to bind white adipose tissue cells via undetermined receptors.
The authors enquired whether the favorable impact of irisin on white adipose tissue might be associated with maintenance of telomere length, a well-established genetic marker in the aging process.
Boström et al. reported on December 14, 2012: "Since the conservation of calories would likely provide an overall survival advantage for mammals, it appears paradoxical that exercise would stimulate the secretion of a polypeptide hormone that increases thermogenesis and energy expenditure.
(Our data show) that even relatively short treatments of obese mice with irisin improves glucose homeostasis and causes a small weight loss.
The worldwide, explosive increase in obesity and diabetes strongly suggests exploring the clinical utility of irisin in these and related disorders.
"[24] While the murine findings reported by Boström et al. appear encouraging, other researchers have questioned whether irisin operates in a similar manner in humans.
A novel myokine osteonectin, or SPARC (secreted protein acidic and rich in cysteine), plays a vital role in bone mineralization, cell-matrix interactions, and collagen binding.
Osteonectin can be classed as a myokine, as it was found that even a single bout of exercise increased its expression and secretion in skeletal muscle in both mice and humans.