Changes in lower limb volume in humans during parabolic flight

Variations in gravity [head-to-footacceleration (G_z)] inducehemodynamic alterations as a consequence of changes in hydrostatic pressure gradients. To estimate the contribution of the lower limbs toblood pooling or shifting during the different gravity phases of aparabolic flight, we measured instantaneous thigh and calf girths byusing strain-gauge plethysmography in five healthy volunteers. Fromthese circumferential measurements, segmental leg volumes werecalculated at 1, 1.7, and 0 G_z.During hypergravity, leg segment volumes increased by 0.9% for thethigh (P < 0.001) and 0.5% for thecalf (P < 0.001) relative to1-G_z conditions. After suddenexposure to microgravity following hypergravity, leg segment volumeswere reduced by 3.5% for the thigh (P < 0.001) and 2.5% for the calf (P < 0.001) relative to 1.7-G_zconditions. Changes were more pronounced at the upper part of the leg.Extrapolation to the whole lower limb yielded an estimated 60-mlincrease in leg volume at the end of the hypergravity phase and asubsequent 225-ml decrease during microgravity. Although quantitativelyless than previous estimations, these blood shifts may participate inthe hemodynamic alterations observed during hypergravity and weightlessness.

     

Renal interstitial hydrostatic pressure and sodium excretion during acute volume expansion in diabetic rats

Experiments were performed to test the hypothesis that the renal interstitial hydrostatic pressure (RIHP) response to acute volume expansion is suppressed in diabetes mellitus. Sprague-Dawley rats received streptozotocin (STZ rats; 65 mg/kg ip) or vehicle (Sham rats). Two weeks later, RIHP and Na(+) excretion responses to acute graded volume expansion with isotonic saline were quantified under Inactin anesthesia (0.1 mg/kg ip). In Sham rats, acute graded volume expansion to 10% body wt produced increases in RIHP (Delta = 12.2 +/- 2.4 mmHg), urine flow (Delta = 54 +/- 8 microliter. min(-1). g(-1)), and Na(+) excretion (Delta = 11.5 +/- 1.9 mueq. min(-1). g(-1)). In STZ rats, these volume expansion-induced responses were significantly blunted (RIHP by 50%, urine flow by 81%, and Na(+) excretion by 76%). Renal decapsulation eliminated the differences between STZ and Sham rats with regard to volume expansion-induced increases in RIHP, urine flow, and Na(+) excretion. Renal denervation normalized the RIHP response to volume expansion and improved the diuretic and natriuretic responses in STZ rats. Moreover, diuretic and natriuretic responses to direct changes in RIHP (induced by renal interstitial volume expansion) were blunted in STZ rats. We conclude that diminished alterations in RIHP, as well as a reduced impact of RIHP on Na(+) excretion, contribute to the impaired diuretic and natriuretic responses to acute volume expansion during the early stage of diabetes.     

Dynamics of blood pressure, pulse wave transit time and systolic timei intervals during acute gravity changes induced by parabolic flight

To investigate cardiovascular adaptation to transient microgravity (Microgravity), we measured RR intervals (RRI), arterial blood pressure (BP), pulse wave transit time (PTT) and systolic time intervals (STI) during parabolic flight. Our results demonstrate that during microgram RRI, BP and PTT are subject to a rapid adaptation likely mediated by the baroreflex whereas STI changes with microgravity but does not present further adaptation.     

Shift in arm-pointing movements during gravity changes produced by aircraft parabolic flight.

It has been shown that target-pointing arm movements without visual feedback shift downward in space microgravity and upward in centrifuge hypergravity. Under gravity changes in aircraft parabolic flight, however, arm movements have been reported shifting upward in hypergravity as well, but a downward shift under microgravity is contradicted. In order to explain this discrepancy, we reexamined the pointing movements using an experimental design which was different from prior ones. Arm-pointing movements were measured by goniometry around the shoulder joint of subjects with and without eyes closed or with a weight in the hand, during hyper- and microgravity in parabolic flight. Subjects were fastened securely to the seat with the neck fixed and the elbow maintained in an extended position, and the eyes were kept closed for a period of time before each episode of parabolic flight. Under these new conditions, the arm consistently shifted downward during microgravity and mostly upward during hypergravity, as expected. We concluded that arm-pointing deviation induced by parabolic flight could be also be valid for studying the mechanism underlying disorientation under varying gravity conditions.     

Cardiovascular and Valsalva responses during parabolic flight

We investigated the integrated cardiovascularresponses of 15 human subjects to the acute gravitational changes(micro- and hypergravity portions) of parabolic flight. Measurementswere made with subjects quietly seated and while subjects performed controlled Valsalva maneuvers. During quiet, seated, parabolic flight,mean arterial pressure increased during the transition into microgravity but decreased as microgravity was sustained. Thedecrease in mean arterial pressure was accompanied by immediate reflexive increases in heart rate but by absent (orlater-than-expected) reflexive increases in total vascular resistance.Mean arterial pressure responses in Valsalva phasesII_l, III, and IV wereaccentuated in hypergravity relative to microgravity(P < 0.01, P < 0.01, andP < 0.05, respectively), butaccentuations differed qualitatively and quantitatively from thoseinduced by a supine-to-seated postural change in 1 G. This study is thefirst systematic evaluation of temporal and Valsalva-related changes incardiovascular parameters during parabolic flight. Results suggest thatarterial baroreflex control of vascular resistance may be modified byalterations of cardiopulmonary, vestibular, and/or otherreceptor activity.

     

Changes in Doppler mitral inflow patterns during parabolic flight

Aim of the study was to evaluate by transthoracic Doppler the alterations in mitral inflow velocity pattern caused by acute changes in loading conditions occurring during parabolic flights. Each parabola included normogravity (1 Gz, 1 min), mild hypergravity (1.8 Gz, 20 sec), microgravity (0 Gz, 24 sec) and mild hypergravity (1.8 Gz, 20 sec) phases. Pulsed-Doppler images were digitally acquired in 11 unmedicated subjects (46 +/- 5 years), in standing upright position and supine resting. Doppler profiles were semi-automatically traced and inflow parameters extracted and averaged onto three consecutive beats. Only in standing position, significant alterations during microgravity (p<0.05) were noted in several parameters.     

Effects of parabolic flight on abdominal arterial pressure in conscious rats.

Abdominal arterial pressure during parabolic flight was measured using a telemetry system to clarify the acute effect of microgravity on hemodynamics in conscious rats. The microgravity condition was elicited by three different levels of entry gravity, i.e. 2 G, 1.5 G and 1 G. On exposure to 2 G, mean aortic pressure (MBP) increased up to 118.7 mm Hg +/- 7.3 compared with the value at 1 G (107.0 +/- 6.3 mm Hg, n=6). The value at microgravity preceded by 2 G was 118.0 mmHg +/- 5.2 mm HG and it was still higher than at 1 G. When 1.5 G was elicited before microgravity exposure, MBP also increased (1.5 G: 114.9 +/- 5.3 vs 1 G: 105.8+/-5.0 mm Hg) and the value at microgravity was 117.3 + /- 5.3 mmHg. During pre-microgravity maneuver with 1 G, no changes were observed compared with the control level at 1 G (pre-microgravity: 105.0 +/- 5.0 vs 1G: 104.8 +/- 5.1 mm Hg ), whereas the MBP increased up to 117.0 +/- 6.5 mm Hg on exposure to microgravity. From these results, we found that in conscious rat MBP increase during acute microgravity exposure with either 1 G or hyper-G entry.     

Feasibility of real-time 3D echocardiography in weightlessness during parabolic flight

Aim of the study was to test the feasibility of transthoracic real-time 3D (Philips) echocardiography (RT3D) during parabolic flight, to allow direct measurement of heart chambers volumes modifications during the parabola. One RT3D dataset corresponding to one cardiac cycle was acquired at each gravity phase (1 Gz, 1.8 Gz, 0 Gz, 1.8 Gz) during breath-hold in 8 unmedicated normal subjects (41 +/- 8 years old) in standing upright position. Preliminary results, obtained by semi-automatically tracing left ventricular (LV) and left atrial (LA) endocardial contours in multiple views (Tomtec), showed a significant (p<0.05) reduction, compared to 1 Gz, of LV and LA volumes with 1.8 Gz, and a significant increase with 0 Gz. Further analysis will focus on the right heart.     

Recording of blood pressure,heart rate and aortic nerve activity during parabolic flight in the rat via radio-telemetry

Exposure to microgravity induces cardiovascular deconditioning characterized by orthostatic hypotension when astronauts return to the earth. In order to understand the mechanism of cardiovascular deconditioning, it is necessary to clarify the changes in hemodynamics and the cardiovascular regulation system over the period of space flight. The telemetry system applied to freely moving animals will be a useful and appropriate technique for this kind of long term study of the cardiovascular system in the conscious animal during space flight. The purpose of the present study is twofold: firstly, to observe the detailed changes of arterial pressure and heart rate (HR) during microgravity elicited by the parabolic flight in order to study the acute effect of microgravity exposure on the cardiovascular system; and secondly, to test the feasibility of the telemetry system for recording blood pressure, HR and autonomic nervous activities continuously during space flight.     

Lung volume changes in response to altered breathing gas pressure during upright immersion

Since elastic and flow-resistive respiratory work are volume dependent, changes in lung volume during immersion affect respiratory effort. This investigation examined changes in lung volume with air delivery pressure modifications during upright immersion. Static pressure-volume relaxation relationships and lung volumes were obtained from ten immersed subjects breathing air at four delivery pressures: mouth pressure, lung centroid pressure (PLC), and 0.98 kPa above and below PLC. The PLC is the static lung pressure which returns the respiratory relaxation volume (VR) to normal and was previously determined to be +1.33 kPa relative to pressure at the sternal notch. Lung volume changes observed when breathing air at mouth pressure were reversed when air was supplied at PLC. The expiratory reserve volume (ERV) and VR were reduced by 58% and 87%, respectively, during uncompensated immersion. These differences indicated an active defence of ERV and implied that additional static respiratory work was required to overcome transrespiratory pressure gradients.     

Structural changes in lipid bilayers and biological membranes caused hydrostatic pressure

By use of neutron diffraction, the structural parameters of oriented multilayers of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine with deuteriocarbon chains/cholesterol (molar ratio 70:30), multilamellar lipid vesicles composed of pure lipids and lipid/cholesterol mixtures, and crystalline purple membrane patches from Halobacterium halobium have been measured at pressures up to 2 kbar. Pressurization of the oriented 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine/cholesterol multilayers results in an in-plane compression with the mean deuteriocarbon chain spacing of 4.44 A obtained under ambient conditions decreasing by 3-7% at 1.9 kbar. The thickness for this bilayer increases by approximately equal to 1.5 A, but the net bilayer volume decreases and the isothermal compressibility is estimated to be in the range (-0.1 to -0.6) X 10(-4)/bar at 19.0 degrees C. The d spacings for multilamellar vesicles composed of lipids in the liquid crystalline state and lipid/cholesterol mixtures increase linearly as a function of pressure, suggesting that these bilayers are also compressed in the membrane plane. For 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and 1,2-distearoyl-sn-glycero-3-phosphatidylcholine MLVs in the gel state, the d spacing decreases with pressure. For 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine, the hexagonally packed chains are anisotropically compressed in the bilayer plane, resulting in a pseudohexagonal chain packing at 1.9 kbar. The bilayer compressibility is (-0.4 or -0.5) X 10(-4)/bar depending on whether the chain tilt increases with pressure or terminal methyl groups of apposing lipid monolayers approach each other.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of gravity changes induced by parabolic flight on cytokine production in mouse spleen

Dysregulation of the immune system has been a well-documented effect of human exposure to a microgravity environment during space flight. These effects have included altered cytokine production, reduced proliferative responses, altered signal transduction pathways and altered distribution of peripheral immune cells. Recent reports have documented immunologic studies performed in-flight. When tested during space flight, delayed-type hypersensitivity was reduced, indicating a dysregulation of cell-mediated immune function. However, the mechanisms by which this occurs remain unclear. The production of cytokines plays a critical role in the ability of a host to mount an immune response against an invading microorganism. In this study, the alteration of cytokine responses in mice following parabolic flight (PF) was investigated.