MUSCLE GROWTH IN RESPONSE TO MECHANICAL STIMULI

The relative merits of the separate and combined uses of stretch and e lectrical stimulation at 10 Hz in influencing the rates of protein syn thesis in vivo, proteolysis, and the growth of the extensor digitorum longus muscle have been investigated after 3 days in the rabbit. Conti nuous electrical stimulation failed to change muscle protein turnover or growth. Static stretch caused significant adaptive growth, with inc reases in c-fos, c-jun, and insulin-like growth factor I (IGF-I; 12-fo ld) mRNA levels, and protein (19%), RNA (128%), and DNA (45%) contents . Both the fractional (138%) and total (191%) rates of protein synthes is increased with stretch, correlating with increased ribosomal capaci ties. Combining stretch and electrical stimulation increased the mRNA concentration of IGF-I (40-fold). The adaptive growth was greater (35% ), with massive increases in the nucleic acids (185 and 300%), ribosom al capacities (230%), and the rates of protein synthesis (345 and 450% ). Large increases (i.e., 200-400%) in cathepsins B and L and dipeptid yl aminopeptidase I activities during stretch, with or without stimula tion, suggest a role for these enzymes in tissue remodeling during mus cle hypertrophy.