K. L. Gosselink

Muscle Afferent–pituitary Axis: A Novel Pathway for Modulating the Secretion of a Pituitary Growth Factor

McCALL, G.E., K.L. GOSSELINK, A.J. BIGBEE, R.R. ROY, R.E. GRINDELAND, and V.R. EDGERTON. Muscle afferent–pituitary axis: A novel pathway for modulating the secretion of a pituitary growth factor. Exerc. Sport Sci. Rev., Vol. 29, No. 4, pp 164–169, 2001. Evidence is presented for a novel muscle-neuro-endocrine pathway for modulating the secretion of an unidentified pituitary growth factor by proprioceptive input from skeletal muscle afferents. Exercise stimulates the release of this growth factor, whereas chronic unloading associated with bed rest or spaceflight abrogates the normal exercise stimulus.

Synergistic ablation does not affect atrophy or altered myosin heavy chain expression in the non-weight bearing soleus muscle.

The purpose of this study was to investigate whether the soleus muscle undergoes atrophy and alterations in myosin heavy chain (MHC) composition during non-weight bearing in the absence of synergists. Thirty-two female rats were randomly assigned to four groups: control (C), synergistic ablation (ABL) of the gastrocnemius and plantaris muscles to overload the soleus muscle, hindlimb suspension (HLS), or a combination of synergistic ablation and hindlimb suspension (HLS-ABL). After 28 days of hindlimb suspension, soleus atrophy was more pronounced in HLS (58%) than in HLS-ABL (43%) rats. Compared to C rats, non-weight bearing decreased mixed and myofibrillar protein contents and Type I MHC 49%, 45%, and 7%, respectively, in HLS animals. In addition, de novo expression of fast Type IIx and Type IIb MHC (5% and 2%, respectively) was observed in HLS animals. Similarly, when compared to C rats, mixed and myofibrillar protein contents and Type I MHC decreased 43%, 46%, and 4%, respectively, in HLS-ABL animals. Also, de novo expression of Type IIx (4%) and IIb (1%) MHC was observed. Collectively, these data indicate that the loss of muscle protein and Type I MHC, and the de novo expression of Type IIx and Type IIb MHC in the rat soleus occur independently of the presence of synergists during non-weight bearing. Furthermore, these results confirm the contention that soleus mass and MHC expression are highly sensitive to alterations in mechanical load.