Intermittent Thoughts on Building Muscle: Understanding the "Big T" - Testosterone Programs Stem Cells to Become Muscle not Fat + Keeps Satellite Cells & Motoneurons Alive

Image 1: Graphical summary of the probably best known function of testosterone - including who are not so "profane" as building muscle and getting ripped ;-)
In the last two installments of the Intermittent Thoughts, I have tried to convey a realistic perception of what exactly the effects of both supra- (that is below) and super- (that is above) physiological (that is "normal" in the sense that they represent the "average" male human being) levels of testosterone on body composition are. In this installment of the series I am now going to provide more information on the "exact" molecular underpinnings by which testosterone works its muscle building and fat burning magic. There is however one thing related to data I presented in the previous installments, I want to emphasize again: The use of a testosterone enanthate in the Bhasin study makes it very difficult to use the data to make prognoses with regard to the results you would see, when you use natural (or unnatural) supplements to raise the endogenous (produced by your testes) production of testosterone. And although there are certainly dozens of factors that would preclude respective inferences, I am going to address only those three, of which I believe that they are the most significant ones.

Three things to keep in mind, when you interpret the data from the last installment(s):
  1. With testosterone enanthate having a ~4-5 day half-life, the testosterone levels, which, in the Bhasin study, were measured on day 7 after the injection, represent only a <50% remainder of the testosterone levels we would see within 24 hours post injection.
    Figure 1: Hypothetical serum testosterone levels in the course of the first seven days after the injection of endogenous testosterone (blue) compared to the regular diurnal rhythm (green) and the levels in response to a pretty potent (+70%) natural testosterone booster (red; all data has illustrative value, only)
    In spite of the fact that the data in figure 1 is obviously not based on "real" experimental data, I hope that by taking a brief look at the ratios of the areas under the curve of the

    • "normal" testosterone level with its ~40% daily variation (green), the...
    • +70% (maximally) naturally boosted testosterone level (red) and a ...
    • testosterone enanthate injection (blue),
    all of you will understand why the "muscle building / fat burning" effects of a +70% boost in testosterone from whatever OTC product you may be taking can hardly compare to injectable testosterone.
  2. Another aspect that should be taken into account is the non-existent sex hormone binding globulin (SHBG) response in the Bhasin study, due to which the relative increases in bound and free (=unbound and purportedly "active") testosterone were identical. This can, but does not necessarily have to be the case, when you raise your testosterone levels "naturally". In that, the aromatization of testosterone to estrogen, appears to be one of the major correlates (I am deliberately not speaking of "causation" in this context) of increases in SHBG. In the worst case, you could thusly "boost" your total testosterone and end up with less free test due to a (possibly estrogen induced / related) increase in SHBG. That being said, I know a hand full of cases, where the exact opposite is the case. Especially very lean (yet still muscular) men tend to have low SHBG levels, so that despite "low-normal" total testosterone many of them have normal-high or even very high free testosterone levels.
  3. The last factor that makes a direct quantitative comparison of the effects "naturally" and "artificially" elevated testosterone levels questionable, to say the least, is the absence of the natural diurnal rhythm with exogenous testosterone administration. In the course of 24h the testosterone levels fluctuate by +/-40% with a spike in the morning (around 6-7am) and a trough in the early evening. Contrary to the "artificially enhanced" testosterone levels, the ones on the printout from your lab thusly represent either the daily max (if the blood was drawn early in the morning), an average (blood drawn around noon) or the nadir (blood drawn in the evening) of your 24h testosterone level.
    Just as an aside: Imagine you wanted to sell a "natural test booster". What would be the best way to get a "clinically proven" rise in testosterone? Right! You just get your "study" participants tested in the evening for baseline and in the morning for post-intervention levels and *bang* you got your "clinically proven" +40% increase in testosterone ;-)
    And even if you managed (by whatever means) to "naturally" raise your testosterone to a level that you would "on average" have +200% the natural negative feedback mechanism (inhibition of luteinizing hormone (LH) release) will soon put an end to your thusly short-dated testosterone boost.
All that does yet not change the observation we have made in the first installment of this (hitherto) three-part series about the effects of testosterone on skeletal muscle hypertrophy: Testosterone builds muscle! The underlying physiological processes, however, are not fully elucidated. The brief summary I have put together in the following paragraphs is thusly a "work in progress" not only because I am still trying to figure out "how testosterone works", but also because the complex interplay of hormones, protein signalling cascades and key players of the immune system simply has not been fully elucidated, yet.

Direct effects of testosterone on muscle cells

I don't know if you have ever heard the name "Vida", if not, then you have probably not delved into the depth of bro-scientific steriodology. Julius A. Vida's book Androgens and Anabolic Agents was published in 1969 is what some people would call the "steroid bible". It contains information about the structure and biological activity of 666 different steroids.
Figure 2: Scan from Vida's book showing data on the androgenic and anabolic activity of 19-Nortestosterone (Nandronole, aka DECA) from a rodent model.
With the latter being of particular interest for roid / pro-steroid producers and consumers, scans of the tables, that make up a good part of the original book can be found on bulletin boards all over the Internet (cf. figure 2). Vida obtained the data from rodent studies and estimated the "anabolic" effect of the tested compounds based on the hypertrophy response of the levator ani muscle of his lab animals. Now, you may rightly ask yourself, how that relates to the topic at hand... well, the reason Vida (and most other researchers) chose the levator ani muscle as a benchmark is its high responsiveness to androgens, because it has a much greater androgen receptor (AR) density than the most of the skeletal muscle you are probably trying to build, when you are at the gym (well, I assume you don't train the levator ani, do you? ;-).

Image 2: The levator ani muscle is especially prone to androgen induced hypertrophy, because it has a particularly high amount of androgen receptors. Whether this is something you are particularly happy about or not, does not matter, in 99% of the cases that you read about the "anabolic activity" of a given "designer steroid", the latter is usually provided relative to the testosterone-induced hypertophy response of this muscle.
Interestingly, the areas of the muscle with the highest androgen receptor expression are the myonuclei and the satellite cells. You know both of them from previous installments of this series and will certainly remember that the recruitement of new myonuclei from satellite cells was a necessary prerequisite for continuous muscle growth, because with ever-increasing myonuclear domain sizes, the muscle will eventually become disfuctional (cf. "Growing Beyond Limits"). It is thusly likely to assume that, next to IGF-1, testosterone provides a second, secondary or complementary growth stimulus to the otherwise quiescent satellite cells. From the fact that the subjects in the Bhasin study exhibited a marked hypertrophy response in the absence of adequate training stimuli, we may also further conclude that the action of testosterone, contrary to the previously discussed locally expressed IGF-1 splice variants (cf. MGF & Co), does (at least up to a certain degree) not depend on muscle damage / strength training. The results of a 2005 study from the Human Performance Laboratory at the University of Connecticut (Kraemer. 2005), which found a -46% reduction in androgen receptor expression in response to volume (not single set, though) training, would even suggest, that testosterone takes a backseat, whenever the MGF-pathway is doing its muscle building job.

Whether the latter, i.e. testosterone's job in building muscle, is identical to the one of IGF-1 and its splice variants is debatable, anyways. After all experiments with isolated bovine satellite cells have shown that incubation with the synthetic androgen trenbolone lead to dose-dependent increases in protein synthesis and decreases in protein degradation (Kamango-Sollo. 2011). The function of testosterone could thusly be to maintain myoblasts (=progenitor cells) in the proliferate state - or, put more simply, testosterone keeps the satellite cells alive and ready to be incorporated into the muscle, whenever this becomes necessary.

Testosterone turns potential fat into muscle

Despite the fact that the muscle building effects of testosterone are at the heart of this series, I guess that you were similarly impressed by the effect the administration of graded doses of testosterone enanthate had on the body fat levels of the subjects in the Bhasin study. One possible explanation for this effect would certainly be the increased energy demands of the additional skeletal muscle mass. This alone can however hardly explain the profundity of the negative effects Bhasin et al. observed in the low and very low dose testosterone enanthate groups.
Figure 3: Relative change in lean and fat mass in response to changes in serum testosterone levels; the green area indicates "normal" = physiological testosterone levels; the asterisks (*) denote statistically significant (p < 0.05) changes vs. baseline (calculated based on Bhasin. 2001)
I mean, if you take a close look at the data, even the low-dose groups effectively gained some muscle mass (<2% and statistically non-significant). A loss of skeletal muscle mass thusly cannot explain the 18-37% increase in fat mass (cf. figure 3). In a subsequent publication Bhasin et al. thusly propose a - I may say quite exciting - alternative explanation for this and similar observations in hypogonadal men (Bhasin. 2004):
[...the] reciprocal change in lean and fat mass induced by androgens is best explained by the hypothesis that androgens promote the commitment of mesenchymal pluripotent cells into myogenic lineage and inhibit adipogenesis through an androgen receptor mediated pathway.
This priming effect testosterone has on the "universal" stem cells from connective tissue would not only result in a greater amount of stem cells that are to become muscle cells (in other words: satellite cells), testosterone would also reduce the amount of "future adipocytes" and thusly inhibit the formation of new and the replenishment of apoptotic, i.e. dead, fat cells. This hypothesis is corroborated by  recent findings of Semirale et al. who report that reduced visceral and subcutaneous fat accumulation with a reciprocal increase in lean mass in male mice with targeted androgen receptor over-expression in mesenchymal stem cells (Semirale. 2011).

The role of testosterone in the mind-muscle connection

Its effect on the actual muscle cells and their progenitors aside, testosterone also binds to the androgen receptors on the motoneurons that innervate the muscle. Interestingly, the death of these motoneurons, is considered the primary cause for sarcopenia and the associated decrease in muscle mass in the course of the aging process (Narici. 2008). Direct treatment of motoneurons with different doses of testosterone leads to increases in motoneuron size and number (Fraley. 2002; Mansouri. 2003). The physiological equivalent of the latter may thusly well be responsible for the improvements in the "mind-muscle connection" users of performance enhancing drugs frequently report. It may also facilitate a greater / optimized activation of existing muscle fibers and could thusly contribute to strength gains which would not depend on previous muscle growth. The increase in strength, in turn, would allow athletes to lift heavier weight and provide a novel growth stimulus, and so on...

Whenever there is talk of androgens and the "mind-muscle connection", someone usually mentions the three letters D, H and T and thusly invokes the role of the most potent androgen, dihydrotestosterone, to which the "Big T" is nothing but a prohormone. Whether it really is DHT, a combination of both, or if one is just more potent in inducing these androgen-related neuronal effects, will however be a topic for the next installment of this series,  in which DHT and estrogen will round out a still very sketchy portray of the complex role the "sex hormones" play in an orchestrate that is so complex that the notion that one hormone, protein, amino acid, or inflammatory cytokine alone could make your muscle grow is simply ridicolous - even if this hormone is "The Big T" ;-)
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