Effects of spaceflight on postnatal development of arterial baroreceptor reflex in rats

Aim:  It has been reported that spaceflight attenuates the arterial baroreceptor reflex. As this reflex function changes dramatically during postnatal development, we hypothesized that space flight depresses the developmental changes of the reflex system. To test this hypothesis, we evaluated the baroreceptor reflex function in rats, which were exposed to a microgravity environment on a space shuttle 9–25 days after birth. Methods:  Baroreceptor reflex sensitivity and the afferent sensitivity were evaluated by measuring heart rate (HR) and aortic nerve activity (ANA) changes in response to an increase in mean arterial pressure (MBP) derived by phenylephrine injection (20–50 μg kg−1) under urethane-anaesthesia. Results:  Baroreceptor reflex sensitivity (% change of HR/% change of MBP) was lower in the flight group (FLT: −0.19 ± 0.04, n = 4) than either the asynchronous ground control group (AGC: −0.47 ± 0.06, n = 6, P < 0.01) or the vivarium group (VIV: −0.41 ± 0.07, n = 6, P < 0.05). This was similar to the differences of the afferent sensitivity (% change of ANA/% change of MBP) between FLT (2.07 ± 0.30) and the control groups (AGC: 2.71 ± 0.22, n.s.; VIV: 3.00 ± 0.32, P < 0.05). At the end of 30 days of recovery under normal gravity conditions, however, there were no significant group differences in these parameters. Conclusion:  These results suggest that the space environment attenuates the postnatal development of the arterial baroreceptor reflex function in rats, which may be partially because of a depression of the postnatal development of the baroreceptor afferents. These functional alterations, however, recover to their normal level on re-exposure to the Earth's gravity.