When Rodents Squat, Scientists Gain Insights into How Muscles Grow. IGF-1 Response to Exercise Does Matter - Locally, not Systemically, of Course!

You want to build big wheels? Look no further get yourself the "Squat T-Bar" with integrated 15mA electrical 'motivator' (Aguiar. 2012)

"A rodent study investigating strength workouts?" Yeah, I know it does not sound like that would be in any ways news-worthy, but if you take a look at the image on the right, you will immediately realize: This study is different! Instead of using a treadmill or simply stitching down (or rather up) one of the hindlimbs of the rodents to induce a chronic overload on the other one (don't laugh, many rodent studies have done just that), the study at hand (Aguiar. 2012), which is going to be published in the next issue of the International Journal of Sports Medicine, used a not innovative, but unfortunately largely forgotten (or overlooked?) torturing device that has been developed by Japanese researchers roughly 20 years a ago (Tamaki. 2012).

The rodent torture... ah pardon squat rack ;-)

After being fitted with a canvas jacket in a way that would enable the researchers to limit the twisting and flexion of their torsos (no, that was not a weight lifting belt ;-), the 32 male Wistar rats (80 days old, 250–300 g) were suspended in a standard position on their hind limbs and "encouraged" to exercise by "electrical stimulation [...] that was applied to the rat’s tail through a surface electrode"  (Aguiar. 2012).

Using their neat little toy, the eight researchers from the University Estadual Paulista, in Botucatu, Brazil, were able to submit the rats to a relatively realistic progressive resistance training regimen for either 8 or 12 weeks. Three times per week each rodent had to do 4 sets  of squats for 10-12 repetitions at 65-75% of its individual 1-RM (maximal weight the rodent could handle). During the study period, Aguiar et al. adjusted the weights twice a week to ensure the same training intensity throughout the experiment (something I would highly recommend to anyone of you, as well; try to pack on 1.25lbs - 2.5lbs at least every other week).

Figure 1: Body weight, muscle weight (plantaris, only) and food intake relative to body weight of the control (C8, C12) and trained (T8, T12) rats before and after the 8-week (C8, T8) and 12-week (C12, T12) intervention (data adapted from Aguiar. 2012)

As you can see in figure 1, this minimalist approach to leg training lead to an increase in both body weight and muscle weight that may initially look as if it was strongly linear. You do yet have to be careful about statements like that, because (a) the rodents did gain weight irrespective of whether they were training or not (80 day old rats are still growing!), so comparing the four bars next to each other and saying "yep, linear!" is not feasible, because this would mean linear as in not training for eight weeks < training for 8 weeks < not training for 12 weeks < training for 12 weeks, which is obviously nonsensical. That being said, there is simply (b) insufficient data to say anything about the linearity -- after all, we do have only three data points per group.

"All groups started the experiment with similar body weight. There was a significant increase (p < 0.05) in the body weight of the 4 groups in the resistance training program (C8: 35.5 %; T8: 27.7 %; C12: 46.9 %; and T12: 40.1 %) and final body weights were not significantly (p > 0.05) different between groups. Furthermore, no significant (p > 0.05) differences in the weekly food intakes were observed between the groups."(Aguiar. 2012)

What does yet stick out, is that the obviously age-dependent weight gain in the control groups C8 and C12 did not increase the weight of the plantaris muscle to a weight anywhere near to the muscle weight, the rats in the trained groups achieved.

Muscle gains and strength gains went hand in hand

In the rats who were subject to the three-times-per-week exercise regimen, on the other hand, those increases in muscle size went hand in hand with highly significant improvements in 1-RM squat power; While all groups had begun the training protocol with similar absolute 1-RMs of ~450g (that's about 130% body weight, pre) ...

"[...] training for 8 and 12 weeks promoted a significant (p < 0.05) increase in the RM/BW ratio in the T8 (pre- vs. post-training: 35.7 % increase, p < 0.05) and T12 (pre- vs. post-training: 57.1 % increase, p < 0.05) groups, while no statistical (p > 0.05) difference was observed in their respective control groups." (Aguiar. 2012)

Consequently, the ratio of 1-RM to body weight was 36.1 % and 57.7 % higher in the groups who had been training fot the last 8 or 12 weeks than in the lazy controls and the time-effect yielded another +22% increase in strength in those rodents who trained for 12 and not just 8 weeks.

Figure 2: Strength gains (left) and increases in cross sectional area, as well as intramuscular IGF1, myogenin and myoD expression (data adapted from Aguiar. 2012)

Now you may have heard all that before, what really makes this study stand out, however, is the observation of statistically highly significant correlations of intra-muscular IGF1, myogenin and myoD  mRNA expression, which speaks in favor of my previous hypothesis (read up on that in the Intermittent Thoughts on Building Muscle) that muscle growth is triggered, driven and maintained almost exclusively at a local level.

What are myogenin and myoD? Both are myogenic regulation factors with myogenin actually being part of the myoD family of transcription factors that will make stem cells develop into myocytes (myo D is highest in recently activated satellite cells).

So, when you are looking for "hormonal" (or other pro-anabolic) ghosts (Phillips. 2012), it is imperative to look for them right where the spook, or, in this case, the muscle building magic happens. If you do just that (see figure 2) and correlate the intra-muscular mRNA expression of IGF-1, myogenin and myoD, you will find the "ghostly" explanation for strength and size gains, as well as the confounding structural changes in the architecture of the muscle, with corresponding correlations between the increases in muscle cross-sectional area (CSA) of r = 0.85 (p = 0.0001), r = 0.87 (p = 0.0001) and r = 0.88 (p = 0.0001) for myoD, myogenin and IGF-1, respectively.

Fiber type changes take their time and occur only within the type II spectrum

A neat side-finding, which is actually no news, though, pertains to the fiber-type conversions that took place in response to the exercise regimen. Firstly, the scientists confirmed the notion that these changes occur exclusively within a certain fiber type. In other words, while Aguiar et al. observed conversions from the metabollically more flexible type IIX/D to the highly glycolytic (power) IIA type, no conversions of the highly oxidative type I to type II fibers were observed. And though the results would generally suggest that fiber IIX/D-to-IIA type conversion, as they 

"[...] also appear to occur during endurance training in humans, so that it would [be] reasonable to think that any exercise stimulus (e. g., endurance or strength) that is sufficient in duration and/or intensity can potentially induce conversions within the fast fiber population from type IIX/D to type IIA" (Aguiar. 2012),

the time-frame in the course of which these changes took place -- namely 12 weeks -- would confirm that the common fear of strength and endurance athletes could provoke negative structral adaptations from doing a "cardio" or "strength" workout from time actually is actually unwarranted. Neither will the former turn a powerlifter into a weakling, nor will the latter make a marathon runner "bulky". Both powerlifter and marathoner are on the contrary going to benefit from the conditioning effect and increase in strength, respectively -- not to mention the important effects on overall health both and not as mainstream stupidity will tell you only the powerlifter can derive from, figuratively speaking, "killing some game in the other's territory"

Bottom line: More food for intermittent thoughts on building muscle ;-)

Figure 3: Correlations between acute GH (A), free testosterone (B), IGF-1 (C)  and cortisol (D) responses (area under the curve—AUC) and gains in type II fibre CSA (Burd. 2012).

Eventually, this study is an excellent example of a way to design a rodent study in a way that will render its results actually meaningful. And what's more, in this particularly case these results are not just meaningful, but can also help us to make some sense of a couple of things we have not fully understood / appreciated, as of yet.

What I am particularly thinking about here, is the contrast between the in-vitro effects of IGF-1 and the (more or less absent) real-world effects of the IGF-1 response to exercise (=systemic increase), as it was observed by West and Phillips in a 2012 study. In their well-powered longitudinal study, neither the acute increase in systemic testosterone, nor the exercise induced increases in systemic IGF-1 showed significant correlations with the gains in type II CSA in a cohort (n = 56) of young men in response to 12 weeks of resistance training (West. 2012; see figure 3).

Another interesting finding of the West study was that, contrary to the circulating testosterone and IGF-1 levels, GH and cortisol did show direct correlations with increased muscle cross sectional areas in type II fibers.

And while the former correlation may be explained by the influence of growth hormone (GH) on the local expression of IGF-1 (Hameed . 2004), there is another open question left: How does cortisol actually figure in here? I mean, the chronic elevation / exogenous adminstration of cortisol, has been show to do the exact opposite, i.e. it decreases the local IGF-1 mRNA expression (Inder. 2010).

Figure 4: Graphical summary of what you should have learned Intermittent Thoughts on Building Muscle ... you didn't 'cause you are new to the SuppVersity or simply forgot about it? No problem read the preliminary summary and browse the individual chapters here!

What was missing in the Inder study, however, was the exercise component: Working out does not just exert protective effects against the negative side effects of the provision of exogenous "cortisol" (in this case Dexamethason), as they were observed in the afore referenced study by Inder et al., exercise will also lead to profound increases in local IGF-1 mRNA expression (e.g. +60% in Bamann. 2001), despite the fact that it will also increase the release of the falsely vilified anti-inflammatory glucocorticoid, cortisol... acute vs. chronic, local vs. system, peak values and amplitudes vs. plateaus and AUC values - you got to keep all these contrastive, yet complementary pairs in mind, when you are thinking about the endocrine and intracrine (within the cell) mediators of skeletal muscle hypertrophy.... what? Sounds familiar? Well, you must have been following the Intermittent Thoughts on Building Muscle, then ;-)

References:

• Aguiar AF, Vechetti-Júnior IJ, Alves de Souza RW, Castan EP, Milanezi-Aguiar RC, Padovani CR, Carvalho RF, Silva MD. Myogenin, MyoD and IGF-I Regulate Muscle Mass but not Fiber-type Conversion during Resistance Training in Rats. Int J Sports Med. 2012 Oct 11.
• Bamman MM, Shipp JR, Jiang J, Gower BA, Hunter GR, Goodman A, McLafferty CL Jr, Urban RJ. Mechanical load increases muscle IGF-I and androgen receptor mRNA concentrations in humans. Am J Physiol Endocrinol Metab. 2001.
• Ding H, Gao XL, Hirschberg R, Vadgama JV, Kopple JD. Impaired actions of insulin-like growth factor 1 on protein Synthesis and degradation in skeletal muscle of rats with chronic renal failure. Evidence for a postreceptor defect. J Clin Invest. 1996 Feb 15;97(4):1064-75. 
• Inder WJ, Jang C, Obeyesekere VR, Alford FP. Dexamethasone administration inhibits skeletal muscle expression of the androgen receptor and IGF-1--implications for steroid-induced myopathy. Clin Endocrinol (Oxf). 2010 Jul;73(1):126-32.
• Phillips SM. Strength and hypertrophy with resistance training: chasing a hormonal ghost. Eur J Appl Physiol. 2012 May;112(5):1981-3-
• Sculthorpe N, Solomon AM, Sinanan AC, Bouloux PM, Grace F, Lewis MP. Androgens affect myogenesis in vitro and increase local IGF-1 expression. Med Sci Sports Exerc. 2012 Apr;44(4):610-5.
• Tamaki T, Uchiyama S, Nakano S. A weight-lifting exercise model for inducing hypertrophy in the hindlimb muscles of rats. Med Sci Sports Exerc. 1992 Aug;24(8):881-6.
• West DW, Phillips SM. Associations of exercise-induced hormone profiles and gains in strength and hypertrophy in a large cohort after weight training. Eur J Appl Physiol. 2012 Jul;112(7):2693-702.