Background: Undercarboxylated osteocalcin (ucOC) is implicated in cell growth and strength in skeletal muscle. However, whether muscle loss, during atrophic conditions, is related to a reduction in ucOC remains unclear. We tested the hypothesis that in rats, both hindlimb immobilisation and testosterone depletion (via castration), leads to serum ucOC reduction and this reduction is associated with the degree of muscle atrophy as well as changes in atrophy signaling pathways.
Methods: Rats were subject to a 10-day hindlimb immobilisation 7 days after castration or sham surgery. We measured ucOC, testosterone, and insulin levels as well as mass and strength of EDL and soleus muscles in rats with or without the intervention. We examined the expression and activity of the putative ucOC-sensitive receptor GPRC6A in muscle and its two possible downstream kinases, ERK and AMPK, as well as the expression and activity of proteins in the muscle atrophy signaling network.
Results: Hindlimb immobilization resulted in lower ucOC levels (by 30%, p<0.05) compared with non-immobilised rats. Lower ucOC correlated with lower muscle mass and muscle strength in both EDLand in all animals. Although testosterone levels were significantly reduced post castration (by 90%, p<0.001), no significant changes caused by testosterone depletion in serum ucOC and muscle mass were observed. In EDL muscle, ucOC levels were associated with p-ERK and p-AMPK, and lower phosphorylated ERK or AMPK correlated with lower phosphorylated mTOR, P70S6K, and ULK1. In the much more severely atrophic soleus, both ucOC level and GPRC6A expression were associated with phosphorylated AMPK, and lower phosphorylated AMPK was correlated with lower phosphorylated FOXO1 and ULK1 but higher Fbx32 expression and higher insulin levels.
Conclusion: Our data indicate that reduced ucOC level, decreased muscular GPRC6A expression, and attenuated muscular ERK and AMPK activities in atrophic condition is related to muscle loss in a muscle-type specific manner.