Functional and cellular adaptation to weightlessness in primates.

Considerable data has been collected on the response of hindlimb muscles to unloading due to both spaceflight and hindlimb suspension. One generalized response to a reduction in load is muscle fiber atrophy, although not all muscles respond the same. For example, predominantly slow extensor muscles like the Sol exhibit a large reduction in fiber size to unloading, while fast extensors like the plantaris and fast flexors like the tibialis anterior show little, if any, atrophy. Our understanding of how muscles respond to microgravity, however, has come primarily from the examination of hindlimb muscles in the unrestrained rat in space. The non-human primate spaceflight paradigm differs considerably from the rodent paradigm in that the monkeys are restrained, usually in a sitting position, while in space. Recently, we examined the effects of microgravity on muscles of the Rhesus monkey by taking biopsies of selected hindlimb muscles prior to and following spaceflights of 14 and 12 day durations (Cosmos 2044 and 2229). Our results revealed that the monkey's response to microgravity differs from that of the rat. The apparent differences in the atrophic response of the hindlimb muscles of the monkey and rat to spaceflight may be attributed to 1) a species difference, 2) a difference in the manner in which the animals were maintained during the flight (i.e., chair restraint or "free-floating"), and/or 3) an ability of the monkeys to counteract the effects of spaceflight with resistive exercise.