Fiber composition of the afferent of aortic baroreflex correlates with the reflex indexes: A space experiment of neonatal rats

Abstract

To study the development of the aortic baroreceptor reflex(ABR)under conditions of microgravity, we examined the fiber composition of the left aortic nerve(LAN) from rats aged 25 days raised in microgravity on the Space Shuttle Columbia(FLT group) for 16 days. Two ground control groups (same cage as that for FLT and commercial cage) were compared with FLT. We have confirmed the alteration of the LAN under microgravity, to study their relationship with the functional characteristics of the ABR in anesthetized rats(urethane, 1.2-1.8 g/kg). It was difficult to obtain both histologic and functional data from the individual animals because of the some limitations in the space experiment. Thus, we had original idea to investigate the relationship between percentile proportion of unmyelinated fibers(UMF) and indexes of the ABR at the peak increase in arterial pressure; we calculated averages of factors obtained by each experiment to normalize the characteristics of the three groups. These results showed that the FLT group indicated the lower peak increase in mean arterial pressure for a given dose of phenylephrine and the fewer aortic UMF, the fewer UMF leaded the lower aortic afferent sensitivity, and the fewer UMF related with the lower index of baroreflex sensitivity. [Jpn J Physiol 54 Suppl:S101 (2004)]