Stretch-induced growth in chicken wing muscles: Role of soluble growth-promoting factors

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Abstract
The involvement of soluble growth‐promoting factors in stretch‐induced hypertrophy of the Patagilis muscle (PAT) in the chicken wing was investigated. Soluble extracts were prepared from young chicken PAT muscle made hypertrophic by passive stretch and from unstretched contralateral controls. Extracts were tested for their ability to stimulate cell proliferation and creatine phosphokinase (CPK) activity in primary monolayer cultures of chick embryo muscle cells. Factors were present in muscle extracts which showed a dose‐dependent stimulation of cell proliferation and CPK activity in vitro. Passive stretch for 5 days produced a rapid hypertrophy of the PAT which was accompanied by a dramatic increase in the activity of the growth factor(s). Release of stretch resulted in an arrest of growth and an immediate fall in growth factor activity. The difference in growth‐stimulating activity between control and stretched PAT extracts could be demonstrated in chicken transferrin‐sensitive chick myoblast cultures. Stretch thus induces an increase in a class‐specific growth factor, possibly Transferrin, in the PAT. Stretched PAT extracts stimulated: (a) chick myoblast proliferation to a greater extent than an optimum concentration of chick embryo extract, and (b) CPK activity in vitro to a greater extent than excess Transferrin. Both control and stretched PAT extracts supported the growth of rat myoblasts. We conclude that PAT muscle extracts also contain unknown growth factor(s) which are different from Transferrin.