Effects of Zero Gravity on Biochemical and Metabolic Properties of Skeletal Muscle Fiber-Types

Muscle physiology

Rattus norvegicus (Sprague-Dawley rat)
Flight: 6 Male

Delayed synchronous: 6, Vivarium: 6

Ambient Temperature Recorder, Animal Enclosure Module

Microgravity has a dramatic effect on skeletal muscle during space flight. The types and quantities of contractile proteins of skeletal muscle fibers play an important role in determining the muscle function. The objective of this study was to determine the effect of space flight on myosin composition of the soleus muscle through the investigation of mechanical properties of skeletal muscle determined by the types and quantity of myosin, the myosin heavy chain (MHC) protein isoform composition, and the MHC mRNA isoform content.

The soleus muscle was taken from rats after 6 days of space flight. Maximal isometric tension was measured and normalized to the physiological cross section area of the muscle. Maximal shortening velocity was determined and expressed as mm/s and muscle length/s. The percentage of slow type I and fast type IIA myosin heavy chain (MHC) protein isoforms were determined for the soleus muscle as well as the corresponding mRNA content for each MHC fiber type protein isoform.

A 5% reduction in maximal isometric tension was found in the soleus muscle, expressed in N/cm². In contrast to the decreased tension force, maximum shortening velocity increased 15% in the soleus muscle. It is speculated that to compensate for decreased isometric tension, maximum shortening velocity increases so the muscle can be stimulated at higher frequencies. Little change was seen in the percentage of slow Type I and fast Type IIA MHC protein isoforms. Fast Type IIX MHC protein isoform content increased by 10% of the total MHC protein isoform content. This increase may account for the increase in maximum shortening velocity. The mRNA content for fast Type IIX MHC isoform was also significantly increased. This is consistent with the increase in the fast Type IIX MHC protein isoforms. Based on this and other information, it appears muscle atrophy in microgravity is isoform-specific.

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