Relationship between transmural pressure and aortic diameter during free drop-induced microgravity in anesthetized rats

Abstract

To test the hypothesis that the aortic wall is stretched without increasing aortic pressure (AP) during microgravity (μG), the AP, intrathoracic pressure (ITP), and aortic diameter (AD) were measured in anesthetized Sprague-Dawley rats during 4.5s of μG produced by freefall. A smooth and immediate reduction in gravity (G) occurred during freefall, μG being achieved 100 ms after the start of the drop. Acute μG elicited an immediate increase in AD, which was not accompanied by an increase in AP. However, the ITP decreased during μG resulted in an increase in the calculated transmural pressure (TP=AP-ITP) of the aortic wall. A simple linear regression analysis showed that the slopes of the plot of AP vs. AD differed at 1 G and μG, whereas those for the plot of TP vs. AD did not. Thus, the increase in AD during μG was accounted for by the increase in TP. These results suggest that a decrease in ITP, resulting in an increase in TP of the aorta, is a key issue in understanding cardiovascular responses to μG.