No abstract

We conclude that during the 20-min supine post-syncopal period, plasma volume, PRA and AVP return closer to baseline but aldosterone and ACTH continue increasing. The magnitude of observed concentration changes cannot be explained by haemoconcentration/haemodilution, rather it appears that the observed changes are indicative of hormone-specific endocrine activation patterns in the recovery phase.

AimsExposure to artificial gravity (AG) at different G loads and durations on human centrifuges has been shown to improve orthostatic tolerance in men. However, the effects on women and of an individual-specific AG training protocol on tolerance are not known.MethodsWe examined the effects of 90 minutes of AG vs. 90 minutes of supine rest on the orthostatic tolerance limit (OTL), using head up tilt and lower body negative pressure until presyncope of 7 men and 5 women. Subjects were placed in the centrifuge nacelle while instrumented and after one-hour they underwent either: 1) AG exposure (90 minutes) in supine position [protocol 1, artificial gravity exposure], or 2) lay supine on the centrifuge for 90 minutes in supine position without AG exposure [protocol 2, control]. The AG training protocol was individualized, by first determining each subject’s maximum tolerable G load, and then exposing them to 45 minutes of ramp training at sub-presyncopal levels.ResultsBoth sexes had improved OTL (14 minutes vs 11 minutes, p < 0.0019) following AG exposure. When cardiovascular (CV) variables at presyncope in the control test were compared with the CV variables at the same tilt-test time (isotime) during post-centrifuge, higher blood pressure, stroke volume and cardiac output and similar heart rates and peripheral resistance were found post-centrifuge.ConclusionsThese data suggest a better-maintained central circulating blood volume post-centrifugation across gender and provide an integrated insight into mechanisms of blood pressure regulation and the possible implementation of in-flight AG countermeasure profiles during spaceflights.

The purpose of this study was to test the hypothesis that plasma galanin concentration (pGal) is regularly increased in healthy humans with extensive orthostatic stress. Twenty-six test persons (14 men, 12 women) were brought to an orthostatic end point via a progressive cardiovascular stress (PCS) protocol consisting of 70 degrees head-up tilt plus increasing levels of lower body negative pressure until either hemodynamically defined presyncope or other signs of orthostatic intolerance occurred (nausea, clammy skin, excessive sweating, pallor of the skin). We further tested for possible gender, gravitational, and muscular training influences on plasma pGal responses: PCS was applied before and after 3 wk of daily vertical acceleration exposure training on a Human Powered Centrifuge. Test persons were randomly assigned to active (with bicycle work) or passive (without work) groups (seven men, six women in each group). Resting pGal was 26+/-3 pg/ml in men and 39+/-15 pg/ml in women (not significant); women had higher galanin responses (4.9-fold increase) than men (3.5-fold, P=0.017) to PCS exposure. Overall, PCS increased pGal to 186+/-5 pg/ml (P=0.0003), without significant differences between presyncope vs. orthostatic intolerance, pre- vs. postcentrifuge, or active vs. passive gravitational training. Increases in pGal were poorly related to synchronous elevations in plasma vasopressin. We conclude that galanin is regularly increased in healthy humans under conditions of presyncopal orthostatic stress, the response being independent of gravity training but larger in women than in men.

Abstract-The aim of this study was to evaluate the effect of orthostasis on the time course of plasma adrenomedullin concentration. On 5 different days, normotensive subjects were randomized to undergo for 30 minutes either 12°, 30°, 53°, or 70°passive head-up tilt or to remain supine. Venous blood was collected from each subject in the supine position before tilting, at 3 and 27 minutes during tilting, and at 2 and 50 minutes after orthostasis. Plasma adrenomedullin increased significantly with tilt of Ն30°in a stimulus-dependent manner. Approximately half of the increase seen at 27 minutes occurred during the first 2 minutes of upright positioning; the maximum effect with 70°tilt was ϩ70%. Elevations in norepinephrine, epinephrine, aldosterone, plasma renin activity, vasopressin, heart rate, and mean arterial pressure were also significant. Hematocrit, blood density, plasma density, and plasma volume loss rose (PϽ0.05) at 53°a nd 70°tilt. Our results indicate that adrenomedullin may play an important role in stabilization of hemodynamics during passive orthostasis. In conclusion, plasma adrenomedullin rapidly increases with orthostatic challenge in a stimulus-dependent manner and also swiftly returns to baseline levels after the subject resumes the supine position. Key Words: tilt, head-up Ⅲ adrenomedullin Ⅲ barorereflex Ⅲ volume, plasma Ⅲ catecholamines T he 52-amino acid peptide adrenomedullin (ADM) is a potent vasorelaxant 1 and natriuretic 2 peptide. The strong relationship between plasma ADM and vascular tone functions in water homeostasis by increasing the glomerular filtration rate and natriuresis while lessening renal vascular resistance. 2,3 Additionally, ADM might function as an endothelium-derived relaxation factor and an autocrine or paracrine factor for vascular smooth muscle. 4 Until now, no data were available on changes in plasma ADM levels induced by orthostasis, which redistributes blood from central-to lower-body vascular beds and unloads cardiopulmonary receptors 5 ; this causes constriction of resistance and capacitance vessels, 6 in part through the action of vasopressin, angiotensin, and endothelin-1. 7,8 The present investigation was designed to test the hypothesis that plasma ADM is influenced by head-up tilting (HUT) in normotensive, euhydrated humans; to study the time course of ADM during and after HUT; and to quantify effects in relation to the intensity of orthostatic challenge. As ancillary information, hemodynamic and other endocrine as well as blood volume indicators were determined. The present study shows that ADM responds quickly and in a dose-dependent manner to HUT, and its concentration rapidly declines afterward.