EFFECT OF SPACEFLIGHT ON SKELETAL-MUSCLE - MECHANICAL-PROPERTIES AND MYOSIN ISOFORM CONTENT OF A SLOW MUSCLE

This study examined changes in contractile, biochemical, and histochem ical properties of slow antigravity skeletal muscle after a 6-day spac eflight mission. Twelve male Sprague-Dawley rats were randomly divided into two groups: flight and ground-based control. Approximately 3 h a fter the landing, in situ contractile measurements were made on the so leus muscles of the flight animals. The control animals were studied 2 4 h later. The contractile measurements included force-velocity relati onship, force-frequency relationship, and fatigability. Biochemical me asurements focused on the myosin heavy chain (MHC) and myosin light ch ain profiles. Adenosinetriphosphatase histochemistry was performed to identify cross-sectional area of slow and fast muscle fibers and to de termine the percent fiber type distribution. The force-velocity relati onships of the flight muscles were altered such that maximal isometric tension (P,) was decreased by 24% and maximal shortening velocity was increased by 14% (P < 0.05). The force-frequency relationship of the flight muscles was shifted to the right of the control muscles. At the end of the 2-min fatigue test, the flight muscles generated only 34% of P-o, whereas the control muscles generated 64% of P-o. The flight m uscles exhibited de novo expression of the type IIx MHC isoform as wel l as a slight decrease in the slow type I and fast type IIa MHC isofor ms. Histochemical analyses of flight muscles demonstrated a small incr ease in the percentage of fast type II fibers and a greater atrophy of the slow type I fibers. The results demonstrate that contractile prop erties of slow antigravity skeletal muscle are sensitive to the microg ravity environment and that changes begin to occur within the Ist wk. These changes were at least, in part, associated with changes in the a mount and type of contractile protein expressed.