Development of the Aortic Baroreflex under Conditions of Microgravity

Neurophysiology
Cardiovascular physiology

Rattus norvegicus (Sprague-Dawley rat)
Flight: 12 families (dam & neonates) Male/Female

Asynchronous: 12 families
Vivarium: 12 families

Research Animal Holding Facility

This study examined the effects of microgravity on the structural and functional development of the aortic baroreflex. The experiment objectives were to observe and analyze the effect of microgravity mainly on the baroreflex responses and on the fine structure of the aortic nerves.

Six 25-day-old rat neonates raised from 9 days old for 16 days in space were selected for examination on the day of landing (Recovery + 0 days = R+0), and another six neonates were reserved for experiments on 30 days postflight (R+30). Following anesthesia, blood pressure, heart rate, and aortic nerve activity were recorded, and baroreflex tests were performed. The animal body was perfused with a fixative and tissues were prepared for electron and light microscopy analysis. Reological properties of the aorta extirpated from other shared neonates on R+0 were also observed.

Flight pups (FLT) had significantly lower body weights than the ground controls, but there were no significant correlations between body weight and other parameters examined. Differences in blood pressure and heart rate seen at R+0 between the FLT and control groups dissipated by R+30. Each parameter of blood pressure, including mean blood pressure (MBP), systolic blood pressure, and diastolic blood pressure (DBP), was lower in the FLT group than controls at R+0, though there was no significant difference between FLT and control groups at R+30. Blood pressure in the R+30 FLT group was significantly higher than that measured on R+0. Similarly, basal heart rate (HR) at R+0 was higher in the FLT group than in the control groups. On R+30, all groups showed lower HR than at R+0. On R+0, the index of baroreceptor reflex sensitivity (delta HR%/ delta MBP%) showed significant differences between the FLT and control groups, with two FLT pups having the lowest value. At R+30, there was no significant index differences between the groups. Afferent sensitivity in the aortic baroreceptor reflex was also decreased in the FLT group on R+0, and again differences were not seen between any groups at R+30. On R+0, the numbers of unmyelinated fibers and the ratio of unmyelinated to all fibers were significantly less in the FLT group than in the controls, and these differences were still observed at R+30. However, myelination of the aortic nerve on both R+0 and R+30 was quite nominal. Aortic wall tension produced by strain was significantly smaller in FLT than in both control groups on R+0. The thickness of the aortic walls in FLT pups was about 70% of those in the other groups; the amount of smooth muscle cells and fine elastin fibers were markedly reduced. Examination and analysis of other varioius organs are still continued and interesting findings have been obtained.

Katsuda, S.I. et al.: Mechanical Tensile Properties of the Aortic Wall in the Premature Rat Exposed to the Microgravity Environment during Space Flight for 16 Days. Journal of Gravitational Physiology, vol. 7(2), Jul 2000, pp. P157-P158.

Miyake, M. et al.: Morphological Characteristics of the Kidney and Lung in the Neonatal Rats Observed after 16 Days Spaceflight. Biological Sciences in Space, vol. 17(3), 2003, pp. 173-174.

Shimizu, T, et al.: Development of the aortic baroreflex in microgravity. In: The Neurolab Spacelab Mission: Neuroscience Research in Space, eds. J.C. Buckey Jr. , J.L. Homick, Houston, Texas, NASA Johnson Space Center Press, pp.151-159.

Shimizu, T.: Development of the Aortic Baroreflex System under Conditions of Microgravity. Journal of Gravitational Physiology, vol. 6(1), Jul 1999, pp. P55-P58.

Waki, H. et al.: Effects of Spaceflight on Postnatal Development of Arterial Baroreceptor Reflex in Rats. Acta Physiologica Scandinavica [in Press]

Yamasaki, M. et al.: Effects of Space Flight on the Histological Characteristics of the Aortic Depressor Nerve in the Adult Rat: Electron Microscopic Analysis. Biol Sci Space, vol. 16(2), 2004, pp. 45-51.

Yamasaki, M. et al.: Spaceflight Alters the Fiber Composition of the Aortic Nerve in the Developing Rat. Neuroscience, vol. 128(4), 2004, pp. 819-829.