Daily 4-h head-up tilt is effective in preventing muscle but not bone atrophy due to simulated microgravity

To assess the potential value of intermittent artificial gravity as an efficient countermeasure, our previous studies have showed that daily 4-h standing (STD) is sufficient in counteracting muscle atrophy but not bone atrophy induced by simulated microgravity. The aim of the present study was to determine whether intermittent gravitational loading by daily 2-h or 4-h, +45 degrees head-up tilt (HUT) is more effective than STD in counteracting muscle and, particularly, bone atrophy due to simulated microgravity. Sprague-Dawley male rats weighing 290-300 g were subjected to a 28-d tail-suspension to simulate microgravity deconditioning. Daily HUT for 2, or 4 h was used to provide intermittent gravitational loading in foot-ward and tail-ward directions. The results showed that 4 h/d HUT was sufficient, and 2 h/d was less effective, in preventing adverse changes in muscle weights, fiber types, and cross-sectional areas (CSA) of muscles due to a 28-d simulated microgravity. The % protections by 4 h/d HUT in maintaining the CSAs of type I fibers in soleus, medial and lateral gastrocnemius and extensor digitorum longus muscles were 103%, 82%, 102%, and 83%, respectively. However, according to changes in physical and mechanical properties of femur, daily 4-h HUT was ineffective in attenuating the adverse changes in bone due to a 28-d simulated microgravity. Reductions in wet, dry, and ash weights and decreases in mechanical strength of femur did not show significant improvement by daily 2-h or 4-h HUT. Taken together, the findings indicate that the countermeasure effectiveness of daily 2-h or 4-h HUT for muscles is comparable with that by daily STD with the same durations. Daily 4-h HUT, as 4-h STD, is also ineffective in attenuating adverse changes in bone mass, but seems partially effective in preventing declines in mechanical properties due to simulated microgravity.