Functional properties of slow and fast gastrocnemius muscle fibers after a17-day spaceflight

The purpose of this investigation was to study the effects of a 17-day spac eflight on the contractile properties of individual fast- and slow-twitch f ibers isolated from biopsies of the fast- twitch gastrocnemius muscle of fo ur male astronauts. Single chemically skinned fibers were studied during ma ximal Ca2+-activated contractions with fiber myosin heavy chain (MHC) isofo rm expression subsequently determined by SDS gel electrophoresis. Spaceflig ht had no significant effect on the mean diameter or specific force of sing le fibers expressing type I, IIa, or IIa/IIx MHC, although a small reductio n in average absolute force (P-o) was observed for the type I fibers (0.68 +/- 0.02 vs. 0.64 +/- 0.02 mN, P < 0.05). Subject-by-flight interactions in dicated significant intersubject variation in response to the flight, as po stflight fiber diameter and P-o where significantly reduced for the type I and IIa fibers obtained from one astronaut and for the type IIa fibers from another astronaut. Average unloaded shortening velocity [V-o, in fiber len gths (FL)/s] was greater after the flight for both type I (0.60 <plus/minus > 0.03 vs. 0.76 +/- 0.02 FL/s) and IIa fibers (2.33 +/- 0.25 vs. 3.10 +/- 0 .16 FL/s). Postflight peak power of the type I and IIa fibers was significa ntly reduced only for the astronaut experiencing the greatest fiber atrophy and loss of P-o. These results demonstrate that 1) slow and fast gastrocne mius fibers show little atrophy and loss of P-o but increased V-o after a t ypical 17-day spaceflight, 2) there is, however, considerable intersubject variation in these responses, possibly due to intersubject differences in i n-flight physical activity, and 3) in these four astronauts, fiber atrophy and reductions in P-o were less for slow and fast fibers obtained from the phasic fast- twitch gastrocnemius muscle compared with slow and fast fibers obtained from the slow antigravity soleus.