Early-age heat exposure affects skeletal muscle satellite cell proliferation and differentiation in chicks

Exposure of young chicks to thermal conditioning (TC; i.e., 37 degreesC for 24 h) resulted in significantly improved body and muscle growth at a later age. We hypothesized that TC causes an increase in satellite cell prolifer ation, necessary for further muscle hypertrophy. An immediate increase was observed in satellite cell DNA synthesis in culture and in vivo in response to TC of 3-day-old chicks to levels that were significantly higher than th ose of control chicks. This was accompanied by a marked induction of insuli n-like growth factor-I (IFG-I), but not hepatocyte growth factor in the bre ast muscle. No significant difference between treatments in plasma IGF-I le vels was observed. A marked elevation in muscle regulatory factors on day 5 , followed by a decline in cell proliferation on day 6 together with contin uous high levels of IGF-I in the TC chick muscle may indicate accelerated c ell differentiation. These data suggest a central role for IGF-I in the imm ediate stimulation of satellite cell myogenic processes in response to heat exposure.