Role of sympathetic responses on the hemodynamic consequences of rapid changes in posture in humans

Abstract

Tolerance to +G z gravitational stress is reduced when +G z stress is preceded by exposure to hypogravity (fractional, 0, or negative Gz). For example, there is an exaggerated fall in eye-level arterial pressure (ELAP) early on during +G z stress (head-up tilt; HUT) when this stress is immediately preceded by −G z stress (head-down tilt; HDT), termed the “push-pull effect.” The aim of the present study was to test the hypothesis that sympathetic responses contribute to the push-pull effect. Young, healthy subjects ( n = 7 males and 3 females) were subjected to 30 s of 30° HUT from a horizontal position and to 30 s of 30° HUT when HUT was immediately preceded by 20 s of −15° HDT. Four bouts of HDT-HUT were alternated between five bouts of HUT in a counterbalanced design, and 1 min was allowed for recovery between tilts. This protocol was repeated during clonidine administration (2.5 μg/kg bolus over 30 min and then continuously at 0.36 μg·kg−1·h−1). Clonidine blunted the vasomotor responses to tilting, and this led to exaggerated changes in arterial pressure. Clonidine exerted little specific influence on the push-pull effect. Thus sympathetic responses appear neither to contribute to, nor protect against, the push-pull effect for the rate and duration of tilting imposed in the present study.