Plasticity of monkey triceps muscle fibers in microgravity conditions

We examined the changes in functional properties of triceps brachii skinned fibers from monkeys flown aboard the BION 11 satellite for 14 days and aft er ground-based arm immobilization. The composition of myosin heavy chain ( MHC) isoforms allowed the identification of pure fibers containing type I ( slow) or type IIa (fast) MHC isoforms or hybrid fibers coexpressing predomi nantly slow (hybrid slow; HS) or fast (hybrid fast) MHC isoforms. The ratio of HS fibers to the whole slow population was higher after flight (28%) th an in the control population (7%), and the number of fast fibers was increa sed (up to 86% in flight vs. 12% in control). Diameters and maximal tension s of slow fibers were decreased after flight. The tension-pCa curves of slo w and fast fibers were modified, with a decrease in pCa threshold and an in crease in steepness. The proper effect of microgravity was distinguishable from that of immobilization, which induced less marked slow-to-fast transit ions (only 59% of fast fibers) and changed the tension-pCa relationships.