The Effects Of Loading And Stretch On Transcriptional And Translational Markers

Abstract

INTRODUCTION To date, literally thousands of studies have examined issues related to muscle atrophy and hypertrophy as reflected by changes in radial size. In contrast, less than 100 studies have examined issues related to the underlying biology of longitudinal growth. Given this consideration, the objective of this study was to contrast changes in transcriptional and translational capacity following: 1) recovery of cross-sectional area via reloading of atrophied muscles; and 2) longitudinal growth as induced by chronic stretch via distraction. METHODS Female Sprague-Dawley rats were randomly assigned to the following groups: 1) control (CON); 2) reloading (RELOAD); and 3) distracted (DIS). Animals assigned to the RELOAD group underwent 4 wks of hindlimb suspension prior to the reloading phase. They were then allowed to undergo normal cage activity for either 4 or 8 days, at which time they were sacrificed. Animals in the DIS group were sacrificed 4 and 8 days following the onset of stretch. The soleus muscle was stretched at a rate of 0.5 mm/day. RESULTS As compared to the CON group, there was a large increase in the concentration of total RNA of the soleus muscle at both the 4 and 8-day time-points for the RELOAD group. Similarly, there was a large increase in the concentration of oligo dT. In contrast, stretch actually produced a small but significant reduction in total RNA, whereas the concentration of oligo dT was unaffected. Reloading also produced large increases in the concentrations of both actin and MHC mRNA levels, whereas just the opposite was observed in the stretched muscles. DISCUSSION Reloading of skeletal muscle produced significant increases in four key markers of growth: i) muscle weight; ii) total RNA pool; iii) the total amount and concentration of mRNA as reflected by oligo dT; and iv) a large increase in the amounts and concentrations of actin and myosin mRNA. Stretch, in contrast, failed to elicit an anabolic profile as defined by our markers. This may be due, in large part, to the joint immobilization that accompanies the stretch model used in this study.