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Augmentation of the push-pull effect by terminal aortic occlusion during head-down tilt

Department of Exercise Science, The University of Iowa, Iowa City, Iowa, 52242

Submitted 25 November 2002 ; accepted in final form 26 February 2003

Tolerance to positive vertical acceleration (G_z) gravitational stress is reduced when positive G_z stress is preceded by exposure to hypogravity, which is called the "push-pull effect." The purpose of this study was to test the hypothesis that baroreceptor reflexes contribute to the push-pull effect by augmenting the magnitude of simulated hypogravity and thereby augmenting the stimulus to the baroreceptors. We used eye-level blood pressure as a measure of the effectiveness of the blood pressure regulatory systems. The approach was to augment the magnitude of the carotid hypertension (and the hindbody hypotension) when hypogravity was simulated by head-down tilt by mechanically occluding the terminal aorta and the inferior vena cava. Sixteen anesthetized Sprague-Dawley rats were instrumented with a carotid artery catheter and a pneumatic vascular occluder cuff surrounding the terminal aorta and inferior vena cava. Animals were restrained and subjected to a control gravitational (G) profile that consisted of rotation from 0 G_z to 90° head-up tilt (+1 G_z) for 10 s and a push-pull G profile consisting of rotation from 0 G_z to 90° head-down tilt (-1 G_z) for 2 s immediately preceding 10 s of +1 G_z stress. An augmented push-pull G profile consisted of terminal aortic vascular occlusion during 2 s of head-down tilt followed by 10 s of +1 G_z stress. After the onset of head-up tilt, the magnitude of the fall in eye-level blood pressure from baseline was -20 ± 1.3, -23 ± 0.7, and -28 ± 1.6 mmHg for the control, push-pull, and augmented push-pull conditions, respectively, with all three pairwise comparisons achieving statistically significant differences (P < 0.01). Thus augmentation of negative G_z stress with vascular occlusion increased the magnitude of the push-pull effect in anesthetized rats subjected to tilting.

arterial blood pressure; tilt; gravitational stress; orthostatic stress

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