MYOGENIN MESSENGER-RNA IS ELEVATED DURING RAPID, SLOW, AND MAINTENANCE PHASES OF STRETCH-INDUCED HYPERTROPHY IN CHICKEN SLOW-TONIC MUSCLE

Biochemical changes that are associated with the growth phase of stret ch-induced skeletal muscle hypertrophy are better understood than even ts that maintain the increased muscle mass. One purpose of this study was to determine whether changes that occur during the period of rapid muscle hypertrophy persist during periods when muscle growth plateaus or the rate of enlargement slows. Serum response factor (SRF), myogen in, MyoD, and actin mRNA expression patterns were examined. SRF protei n interactions with serum response element-1 (SRE1) of the chicken ske letal alpha-actin gene were also characterized. Anterior latissimus do rsi (ALD) wet weight (132% and 122%) and total RNA concentration (29% and 19%) increased after 2 and 3 weeks of stretch overload, respective ly. Myogenin mRNA per microgram RNA increased after 3 (775%), 6 (1073% ), 14 (227%), and 21 days (133%) of stretch overload. At 6 days, myoge nin mRNA levels were increased in the distal, middle and proximal regi ons of the ALD. Serum response factor (SRF) mRNA per microgram total R NA was not increased after 2 or 3 weeks of stretch overload. MyoD and skeletal alpha-actin mRNAs per microgram total RNA were also unchanged after 2 and 3 weeks of stretch. Gel mobility shift assays demonstrate d that SRF bound to SRE1 from 14-day-stretched ALD nuclear extracts ha d an increased mobility compared to control, and this difference in mo bility was maintained in nuclear extracts from ALD muscle whose mass w as declining. These results indicate that the expression of myogenin m RNA and total RNA remains elevated during either slow or maintenance p eriods of stretch-induced increases in ALD mass, when SRF mRNA has ret urned to control levels. Additionally, stretch-induced alterations in SRF binding to SRE1, from the skeletal alpha-actin promoter, occur reg ardless of the rate of stretch-induced growth.