A variety of studies in animal models have suggested that acute (and chronic) administration of b2-adrenoceptor (AR) agonists increases skeletal muscle mass. In vitro studies have supported the hypothesis that clenbuterol activates the PI3-kinase/Akt signalling pathway promoting protein synthesis and cell survival. However, many of the agonists employed in those experiments had actions on both b2 and b3-ARs, we advised to examine the in vitro effects of b2 and b3-AR stimulation on activation of PI3K-mTOR-p70S6 kinase. Rat skeletal myocytes (L6 cells) were incubated with b2 and b3-AR agonists and antagonists; for all treatments, evaluation of p70S6k phosphorylation (P-p70S6k) by Western blot analysis was performed.

Incubation with clenbuterol (1 mM) induced a significant increase (about 50%) of P-p70S6k; the addition of ICI 118551, b2-AR antagonist, didn't exerted any inhibitory effect. When SR 59230A (1mM), a specific b3-AR antagonist, was added to clenbuterol no increase in the phosphorylation was observed. Addition of CL316.243 increased the P-p70S6k to 150%; this effect was totally blocked by SR 59230A, rapamycin (mTor inhibitor) and wortmannin (PI3-kinase inhibitor).

Our results strongly suggest that the selective stimulation of the b3 adrenoceptors plays a major role in triggering the activation of the PI3K-mTOR-p70S6k kinase pathway.