VELOCITY, FORCE, POWER, AND CA2-MUSCLE FIBERS( SENSITIVITY OF FAST AND SLOW MONKEY SKELETAL)

In this study, we determined the contractile properties of single chem ically skinned fibers prepared from the medial gastrocnemius (MG) and soleus (Sol) muscles of adult male rhesus monkeys and assessed the eff ects of the spaceflight living facility known as the experiment suppor t primate facility (ESOP). Muscle biopsies were obtained 4 wk before a nd immediately after an 18-day ESOP sit, and fiber type was determined by immunohistochemical techniques. The MG slow type I fiber was signi ficantly smaller than the MG type II, Sol type I, and Sol type II fibe rs. The ESOP sit caused a significant reduction in the diameter of typ e I and type I/II (hybrid) fibers of Sol and MG type II and hybrid fib ers but no shift in fiber type distribution. Single-fiber peak force ( mN and kN/m(2)) was similar between fiber types and was not significan tly different from values previously reported for other species. The E SOP sit significantly reduced the force (mN) of Sol type I and MG type II fibers. This decline was entirely explained by the atrophy of thes e fiber types because the force per cross-sectional area (kN/m(2)) was not altered. Peak power of Sol and MG fast type II fiber was 5 and 8. 5 times that of slow type I fiber, respectively. The ESOP sit reduced peak power by 25 and 18% in Sol type I and MG type II fibers, respecti vely and, for the former fiber type, shifted the force-pCa relationshi p to the right, increasing the Ca2+ activation threshold and the free Ca2+ concentration, eliciting half-maximal activation. The ESOP sit ha d no effect on the maximal shortening velocity (V-o) of any fiber type . V-o of the hybrid fibers was only slightly higher than that of slow type I fibers. This result supports the hypothesis that in hybrid fibe rs the slow myosin heavy chain would be expected to have a disproporti onately greater influence on V-o.