Stretch-induced myogenin, MyoD, and MRF4 expression and acute hypertrophy in quail slow-tonic muscle are not dependent upon satellite cell proliferation

The objectives of these studies were to determine if (1) hypertrophy-stimul ated myogenic regulatory factor (MRF) mRNA increases occur in the absense o f proliferating satellite cells, and (2) acute hypertrophy occurs without s atellite cell proliferation. Adult and aged quails were exposed to 0 or 250 0 Rads gamma irradiation, and then wing muscles were stretch-overloaded for 3 or 7 days. MRF mRNA levels in stretch-overloaded and contralateral anter ior latissimus dorsi (ALD) muscles were determined after 3 days; hypertroph y was determined after 7 days. The elimination of proliferating cells in ir radiated muscles was verified histologically by bromodeoxyuridine incorpora tion. Relative levels of MRF4, MyoD, and myogenin mRNA were elevated 100%-4 00% in stretch-overloaded ALD muscles from irradiated adult quails indicati ng that satellite cell proliferation was not a prerequisite for MRF mRNA in creases. Myogenin was the only MRF that exhibited mRNA increases that were lowered by irradiation. This suggests that satellite cells contribute only to myogenin mRNA increases in non-irradiated adult muscles following 3 days of stretch-overload. Stretch-overloaded ALD muscles from aged quails had a relative increase in myogenin mRNA of similar to 150%. The myogenin increa se was the same in non-irradiated and irradiated aged animals and also the same as that in stretch-overloaded muscles from irradiated adult quails. To gether, these data indicate that attenuated increases in MRF expression in muscles from aged animals are attributable to lower satellite cell MRF expr ession. ALD muscle masses and protein contents in adult irradiated quails a pproximately doubled after 7 days of stretch-overload demonstrating hypertr ophy despite the elimination of satellite cell proliferation.