Abstract
To study the development of regulatory function of the circulation during and after spaceflight, the 9-day-old rats were raised in microgravity on the Space Shuttle (FLT) for 16 days and age-matched rats remained on the ground (1G). On the landing day (Recovery day 0, R+0) and 30th day after landing (R+30), we performed the functional and histologic experiments in the anesthetized rats (urethane, 1.2-1.8 g/kg). We reported previously that the FLT rats on R+0 indicated the fewer unmyelinated fibers (UMF) in the afferent of aortic baroreflex (aortic nerve, AN), which related with the lower index of baroreflex sensitivity (IBRS) at the peak increase in arterial pressure (phenylephrine, iv), compared to 1G rats. On R+30, though the AN in the FLT remained fewer UMF, there was no significant difference in the IBRS between the FLT and 1G. The ratio of heart weight to body weight on R+0 or R+30 was larger in the FLT than in the 1G. The basal heart rates in the FLT were higher on R+0 and significantly lower on R+30 compared to 1G, and no statistical difference in the basal blood pressure between the two groups was observed on each day. These results suggest that the regulatory function of blood circulation can develop compensatively for the modification of AN in the young rat which returns to Earth form space while growing. [Jpn J Physiol 55 Suppl:S99 (2005)]
