Functional properties of slow and fast gastrocnemius muscle fibers after a 17-day spaceflight

Abstract

The purpose of this investigation was to study the effects of a 17-day spaceflight on the contractile properties of individual fast- and slow-twitch fibers isolated from biopsies of the fast-twitch gastrocnemius muscle of four male astronauts. Single chemically skinned fibers were studied during maximal Ca2+-activated contractions with fiber myosin heavy chain (MHC) isoform expression subsequently determined by SDS gel electrophoresis. Spaceflight had no significant effect on the mean diameter or specific force of single fibers expressing type I, IIa, or IIa/IIx MHC, although a small reduction 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 indicated significant intersubject variation in response to the flight, as postflight 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 lengths (FL)/s] was greater after the flight for both type I (0.60 +/- 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 significantly reduced only for the astronaut experiencing the greatest fiber atrophy and loss of P(o). These results demonstrate that 1) slow and fast gastrocnemius fibers show little atrophy and loss of P(o) but increased V(o) after a typical 17-day spaceflight, 2) there is, however, considerable intersubject variation in these responses, possibly due to intersubject differences in in-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 [J. J. Widrick, S. K. Knuth, K. M. Norenberg, J. G. Romatowski, J. L. W. Bain, D. A. Riley, M. Karhanek, S. W. Trappe, T. A. Trappe, D. L. Costill, and R. H. Fitts. J Physiol (Lond) 516: 915-930, 1999].

Hepatocyte Growth Factor in Mouse Soleus Muscle Increases with Reloading after Unloading

“…Decreases in muscle activity due to spaceflight 1) , unloading 2) and fixation of joints 3) often cause skeletal muscle atrophy. This is associated with a loss of contractile protein content due to decreases in overall protein synthesis 4) and increases in proteolysis 5) .…”

Effect of long-term muscle paralysis on human single fiber mechanics

“…Human studies involving 17 days of bed rest (27,28) or spaceflight (26) revealed that maximal single fiber force (P 0 ) from the soleus muscle was decreased, mainly as a result of a decline in fiber cross-sectional area (CSA), and that maximal unloaded shortening velocity (V 0 ) was increased. Consequently, single fiber power was either maintained or depressed, depending on the study participant or fiber type (29), with fibers expressing type I MHC being generally more affected. Muscle unloading up to 4 mo in a long-term bed-rest study induced a similar pattern of adaptation in the functional properties of type I fibers from the soleus muscle (31), with changes seemingly proportional to the duration of unloading.…”

Distinctive VEGFR2 Endocytic Delivery during Muscle Mobilization

“…Lately, it was reported that there is a differential turnover of myosin heavy chain (MyHC) proteins during muscle wasting [25][26][27][28][29]. Then, mobilization of gatrocnemius and soleus was established in the present study, wherein distinctive VEGFR2 trafficking was evoked.…”

“…Also, motor endplates on fast and slow muscle fibers show distinct transmitter release characteristics [25]. Accumulating evidence indicated that the degree of muscle denervation depends on muscle type, with atrophy more prominent in type I than in type II muscle fibers [26][27][28][29]. There is also a differential turnover of MyHC proteins during muscle wasting after a bed-rest period, which is indicative of muscle fibers in a transitional state [30].…”