The ATP to ADP ratio is, under normal conditions, very high. The ATP storage itself is quite small and therefore the muscles heavily rely on the quick regeneration of ATP from ADP (for which the catalysation by CK is perfectly adequate in doing so). The result of creatine suppletion is that it increases the total muscle creatine content, including phosphorylated creatine, resulting in a larger buffer to quickly regenerate ATP from ADP when its hydrolyzed during muscle contraction. This is the core ‘function’ on how creatine enhances performance.
In addition, it also exerts its effects by enhancing mitochondrial respiration and facilitating intracellular energy transport in which the PCr functions as an energy transporter from the mitochondria to the cytosol (Cr getting rephosphorylated in the mitochondria utilizing ATP derived from oxidative phosphorylation). Furthermore, it affects the expression of several genes, of which some are involved in skeletal muscle hypertrophy.
Measurement of the protein content of a wide variety of protein kinases in skeletal muscle after short-term Creatine Monohydrate (CrM) supplementation show, among others, upregulation of: p38 MAPK, ERK and Akt. Activation of the MAPK/ERK pathway can lead to phosphorylation of p70S6K (also one of mTORC1′s downstream effectors) which can increase gene transcription. Akt on the other hand works through the canonical PI3K/Akt/mTOR pathway, however, Akt negatively influences Raf (upstream regulator of MAPK) and drawing conclusions on available data remains difficult. Nevertheless, this could be another mode of action through which creatine positively influences protein synthesis.
Moreover, creatine induces proliferation and differentiation of satellite cells in vitro. Indeed, it has been shown in a clinical trial that creatine supplementation enhances the training-induced increase in satellite cell and myonuclei number in skeletal muscle. However, the exact biochemical mechanism through which creatine does so remains to be elucidated.
Another proposed mechanism is that of creatine being an ‘androgenic aid’, solely based on a single study showing a (testosterone indepedent) increase of dihydrotestosterone (DHT). DHT is the product of testosterone by 5α-reduction and has a higher potency in terms of androgen receptor activation. Now, this proposed mechanism of action is problematic in a multitude of ways. DHT is a 3-keto 5α-reduced steroid, and as most of these steroids, the enzyme 3α-HSD is happy to reduce the steroid on C3 resulting in a useless metabolite. One of the main sites of action of this enzyme is (don’t be shocked): skeletal muscle. So DHT gets metabolized rather quickly in the tissue where it should work according to this proposed mechanism. Now we aren’t done yet: skeletal muscle lacks significant amounts of any of the 5α-reductase isozymes. So there isn’t much conversion going on in the respective tissue to begin with. Surely the DHT diffuses from the serum to the skeletal muscle, yet these concentrations are quite low (around the 1 nmol/l mark). All in all, it is highly unlikely that this is one of the modes of action.
Finally, I would like to discuss its effects on serum myostatin, since creatine appears to decrease serum myostatin and it is currently another hot theory. For those of you who are unaware, myostatin is a protein involved in the regulation of muscle hypertrophy and hyperplasia. Now, I’m willing to go as far as to believe that this entire decrease measured in serum is solely the result of its decrease in skeletal muscle (which isn’t even a rare assumption I must admit, since its main site of synthesis is skeletal muscle). When looking at the big and bulky myostatin null mice in the original study wherein the protein was discovered (then still dubbed ‘GDF-8′) by McPherron et al. in 1997, it seems tempting to believe that any reduction in myostatin translates to gains. However, when closely examining their results and mechanism of action of the protein, I would hold back with drawing that conclusion. For starters, the myostatin null mice had NO myostatin, which makes it difficult to extrapolate the results to various concentrations. Nevertheless, this is a useful way to examine its biological function. As it seems in the myostatin null mice, its main function is the inhibition of myoblast proliferation and differentiation. Indeed the mouse also underwent significant hyperplasia. Myostatin therefore seems to put a brake on hyperplasia, as it does not occur under physiological conditions. Of course it also influences hypertrophy, but be it to a lesser extent. It mildly decreases Akt phosphorylation and some of its downstream substrates. Furthermore it upregulates a ubiquitine ligase (atrogine-1) via FoxO1, thus increasing protein breakdown. Nevertheless, when comparing the cross sectional area (CSA) increases of the mice both due to hyperplasia and hypertrophy, hyperplasia seems to take the crown. Although it would be stupid to say this mechanism of action isn’t involved in creatine’s mode of action, it is far from certain to conclude it actually is a significant mode of action.