Acute response of aortic nerve activity to free drop-induced microgravity in anesthetized rats.

To test the hypothesis that arterial baroreflex was stimulated during microgravity (μG), arterial pressure (AP), intrathoracic pressure (ITP), and aortic nerve activity (ANA) were measured in anesthetized rats during 4.5 s of μG produced by free drop. A smooth and immediate reduction in G occurred during free drop, μG being achieved 100 ms after the start of the drop. Acute μG elicited an immediate and striking, but transient, increase in ANA, with no significant change in the AP, but a significant decrease in the end-expiratory ITP. The calculated transmural pressure of the aorta increased by 6.9 mmHg 2 s after the start of the drop. The increase in ANA lasted 2 s, then ANA returned to the control level, despite the calculated end-expiratory transmural pressure still being high. These results suggest that μG conditions stimulate the aortic baroreceptor by increasing transmural pressure by reducing the ITP. However, this effect is only transient, probably due to the high-pass property of the baroreceptors.