Weightlessness simulations for cardiovascular and muscle systems: validity of rat models.

Animal models are widely used to evoke responses comparable to those obtained during weightlessness. Two models are reviewed; one examines cardiovascular responses and cephalad fluid shifts in head down tilting (HDT), and the other examines atrophy in load bearing muscles by unloading the hind limbs. Cephalad fluid shifts result in diuresis, natriuresis, and kaliuresis. Reversals are rapid, within one week. Reports of cardiovascular responses are not similar among various laboratories, probably due to variations in protocols. Blood pressures (MAP, SP and DP) and heart rates measured with direct aorta cannulations become elevated as early as one and three days of HDT; recovery occurs within several hours; the response is a transient hypertension. The role of central and peripheral sympathetic nervous activity in flight and suspended rats is examined. Rats show little or no evidence of cardiac deconditioning. Direct blood pressures have not been made in flight rats, precluding direct comparisons with earth side experiments. Muscle atrophy and load bearing (slow twitch fibers) and non-load bearing (fast twitch fibers) muscle responses with hind limb unloading and recovery are compared with flight animal responses. Soleus muscle in response to whole body suspension (WBS), tail suspension (TS) or flight exposure consistently shows significant weight loss. In contrast, the extensor digitorum longus and vastus medialis show less marked responses. More specifically, slow twitch fibers in all these muscles show the greatest loss in mass (e.g. cross sectional areas). The conclusion is that both WBS or TS systems are useful in predicting and comparing changes due to weightless flight.