Hemodynamic consequences of rapid changes in gravity: The push-pull effect

Brief exposure to -Gz ("push") reduces eye-level blood pressure (ELBP) during subsequent exposure to +Gz ("pull"), called the push-pull effect. Anesthetized Sprague-Dawley rats were subjected to 90° head-up tilt (+1Gz) and 10 s of +1Gz stress immediately preceded by 2 s of head-down tilt (-1Gz).; In Study 1, we investigated the response of the autonomic nervous system by studying responses before and after hexamethonium inhibited autonomic function. Hexamethonium lowered ELBP and abolished the push-pull effect, the lack of which persisted when ELBP was restored to baseline by phenylephrine infusion. Lowering ELBP by nitroprusside infusion or by hemorrhage when autonomic function was intact also abolished the push-pull effect.; In Study 2, carotid hypertension was augmented when head-down tilt was combined with terminal aortic occlusion. In addition to control and push-pull g-profiles, an augmented push-pull g-profile consisted of vascular occlusion during 2 s of head down tilt followed by 10 s of head-up tilt. Following the onset of head-up tilt, the fall in ELBP was progressively greater for the control, push-pull, and augmented push-pull conditions. We also found that push-pull stress can be simulated by vascular occlusion in the absence of head-down tilt.; In Study 3, the influence of gender and the axis of rotation on the push-pull effect were evaluated. Gender exerted a significant effect on baseline ELBP. In males and females, ELBP rose to a similar extent during "push", fell to a similar extent during control stress, and fell to a similar extent during push-pull stress. Altering the axis of rotation between the x-axis (roll) and the y-axis (pitch) did not influence the results.; In summary, intact autonomic function and a normal baseline arterial pressure are needed for expression of the push-pull effect, the magnitude of which can be increased by augmenting -Gz stress with vascular occlusion. Gender and axis of rotation do not influence the push-pull effect in anesthetized rats subjected to tilting.