Effects of the head-down tilt posture on postnatal development of the aortic baroreflex in the rabbit

To clarify the effects of the head-down tilt (HDT) posture on the postnatal development of the aortic baroreflex, we raised rabbits from 3-4 weeks of age for 34-36 d in this posture, which simulates the headward shift of body fluid in space, and examined the structural and functional characteristics of the aortic nerves and baroreflex responses. The rabbits were divided into 3 different groups: 20 degrees HDT, environmental control (EC), and vivarium control (VIV). Left aortic nerve activity (ANA) at basal arterial pressure was defined as 100%, and the maximum ANA at maximal increase in arterial pressure was 265+/-103% (mean+/-SD, n=5) in the HDT, 482+/-170% (n=4) in the EC, and 677+/-491% (n=4) in the VIV groups. The minimum ANA at maximal decrease in arterial pressure was 67.3+/-13.8% in the HDT, 40.1+/-10.2% in the EC, and 38.6+/-13.8% in the VIV groups. The maximal change of ANA in response to maximal change in arterial pressure in the HDT group was significantly less than that of either the EC or VIV group (p<0.05). The average baroreflex gain in the HDT group was significantly less than in the VIV group (p<0.05). The number of unmyelinated fibers in the left aortic nerve in the HDT group was significantly less than that of each control group (p<0.05). Thus the development of the aortic baroreflex system is apparently depressed by the HDT posture, suggesting that similar consequences will be observed in mammals that develop in space.