EFFECT OF 17 DAYS OF BED REST ON PEAK ISOMETRIC FORCE AND UNLOADED SHORTENING VELOCITY OF HUMAN SOLEUS FIBERS

The purpose of this study was to examine the effect of prolonged bed r est (BR) on the peak isometric force (P-o) and unloaded shortening vel ocity (V-o) of single Ca2+-activated muscle fibers. Soleus muscle biop sies were obtained from eight adult males before and after 17 days of 6 degrees head-down BR. Chemically permeabilized single fiber segments were mounted between a force transducer and position motor, activated with saturating levels of Ca2+, and subjected to slack length steps. V-o was determined by plotting the time for force redevelopment vs. th e slack step distance. Gel electrophoresis revealed that 96% of the pr e- and 87% of the post-BR fibers studied expressed only the slow type I myosin heavy chain isoform. Fibers with diameter >100 mu m made up o nly 14% of this post-BR type I population compared with 33% of the pre -BR type I population. Consequently, the post-BR type I fibers (n = 14 7) were, on average, 5% smaller in diameter than the pre-BR type I fib ers (n = 218) and produced 13% less absolute P-o. BR had no overall ef fect on P-o per fiber cross-sectional area (P-o/CSA), even though half of the subjects displayed a decline of 9-12% in P-o/CSA after BR. Typ e I fiber V-o increased by an average of 34% with BR. Although the rat io of myosin light chain 3 to myosin light chain 2 also rose with BR, there was no correlation between this ratio and V-o for either the pre -or post-BR fibers. In separate fibers obtained from the original biop sies, quantitative electron microscopy revealed a 20-24% decrease in t hin filament density, with no change in thick filament density. These results raise the possibility that alterations in the geometric relati onships between thin and thick filaments may be at least partially res ponsible for the elevated V-o of the post-BR type I fibers.