Effect of a 17 day spaceflight on contractile properties of human soleus muscle fibres

1. Soleus biopsies were obtained from four male astronauts 45 days before a nd within 2 h after a 17 day spaceflight. 2. For all astronauts, single chemically skinned post-flight fibres express ing only type I myosin heavy chain (MHC) developed less average peak Ca2+ a ctivated force (P-o) during fixed-end contractions (0.78 +/- 0.02 vs. 0.99 +/- 0.03 mN) and shortened at a greater mean velocity during unloaded contr actions (V-o) (0.83 +/- 0.02 vs. 0.64 +/- 0.02 fibre lengths s(-1))than pre flight type I fibres. 3. The flight-induced decline in absolute P-o was attributed to reductions in fibre diameter and/or P-o per fibre cross-sectional area. Fibres from th e astronaut who experienced the greatest relative loss of peak force also d isplayed a reduction in Ca2+ sensitivity. 4. The elevated V-o of the post-flight slow type I fibres could not be expl ained by alterations in myosin heavy or light chain composition. One altern ative possibility is that the elevated V-o, resulted from an increased myof ilament lattice spacing. This hypothesis was supported by electron microgra phic analysis demonstrating a reduction in thin filament density post-fligh t. 5. Post-flight fibres shortened at 30 % higher velocities than pre-flight f ibres at external loads associated with peak power output. This increase in shortening velocity either reduced (2 astronauts)or prevented (2 astronaut s) a post-flight loss in fibre absolute peak power (mu N (fibrelength) s(-1 )). 6. The changes in soleus fibre diameter and function following spaceflight were similar to those observed after 17 days of bed rest. Although in-fligh t exercise countermeasures probably reduced the effects of microgravity, th e results support the idea that ground-based bed rest can serve as a model of human spaceflight. 7. In conclusion, 17 days of spaceflight decreased force and increased shor tening velocity of single Ca2+-activated muscle cells expressing type I MRC . The increase in shortening velocity greatly reduced the impact that impai red force production had on absolute peak power.