Graded lower body negative pressure induces intraventricular negative pressures and incremental diastolic suction: a pressure volume study in a porcine model

Lower body negative pressure (LBNP) has been a tool to study compensatory mechanisms to central hypovolemia for decades. However, underlying hemodynamic mechanisms were mostly assessed non-invasively and remain unclear. We hypothesized that incremental LBNP reduces diastolic filling and thereby affects left ventricular (LV) diastolic suction (DS). Here, we investigated the impact of graded LBNP at 3 different levels of seal as well as during beta-adrenergic stimulation by invasive pressure-volume (PV) analysis. Eight Landrace pigs were instrumented closed-chest for PV assessment. LBNP was applied at three consecutive locations: I) cranial, 10cm below xiphoid process; II) medial, half-way between cranial and caudal; III) caudal, at the iliac spine. Level III) was repeated under dobutamine infusion. At each level, baseline measurements were followed by application of incremental LBNP of -15, -30 and -45 mmHg. LBNP induced varying degrees of preload-dependent hemodynamic changes, with cranial LBNP inducing more pronounced effects than caudal. According to the Frank-Starling mechanism, graded LBNP progressively reduced LV stroke volume (LV SV) following a decrease in LV end-diastolic volume. Negative intraventricular minimal pressures were observed during dobutamine-infusion as well as higher levels of LBNP. Of note, incremental LV negative pressures were accompanied by increasing DS volumes, derived by extrapolating the volume at zero transmural pressure, the so-called equilibrium volume (V0), related to LV SV. In conclusion, graded preload reduction shifts the PV loop to smaller volumes and end-systolic volume below V0, which induces negative LV pressures and increases LV suction. Accordingly, LBNP induced central hypovolemia is associated with increased DS.

Muscle protection during hibernation of Daurian ground squirrels (Spermophilus dauricus): role of atrogin-1, MuRF1, and fiber-type transition

We investigated the mechanism of protection from skeletal muscle atrophy in the hind limb extensor digitorum longus (EDL) muscle of hibernating Daurian ground squirrels (Spermophilus dauricus Brandt, 1843). The effects of unrestrained hibernation and 14 day hind limb unloading (HLU) on EDL were studied in three seasons (summer, autumn, and winter). Atrogin-1 and MuRF1 mRNA skeletal muscle expression, wet muscle mass, and muscle to body mass ratios were unchanged during hibernation in all three seasons. EDL mass measurements decreased following HLU and atrogin-1 and MuRF1 mRNA expression increased. In summer, atrogin-1 and MuRF1 mRNA expression increased by 85% and 75%, respectively; in autumn, by 95% and 69%, respectively; and in winter, by 91% and 65%, respectively (P < 0.05). In the HLU group, microscopic skeletal muscle changes were present, including a reduction in the percentage of type-I skeletal muscle fibers. Fat storage in Daurian ground squirrels and a shorter photoperiod during hibernation did not affect the protective mechanisms that prevented skeletal muscle atrophy. The results of this study suggest that the stable expression of atrogin-1 and MuRF1 and the transition from fast glycolytic fibers to slow oxidative fibers are associated with a lack of skeletal muscle atrophy in the hibernating Daurian ground squirrel.

Effect of orthostasis on endothelial function: a gender comparative study

As the vascular endothelium has multiple functions, including regulation of vascular tone, it may play a role in the pathophysiology of orthostatic intolerance. We investigated the effect of orthostasis on endothelial function using EndoPAT®, a non-invasive and user-independent method, and across gender. As sex steroid hormones are known to affect endothelial function, this study examined the potential effect of these hormones on the endothelial response to orthostasis by including females at different phases of the menstrual cycle (follicular and luteal—where the hormone balance differs), and females taking an oral contraceptive. A total of 31 subjects took part in this study (11 males, 11 females having normal menstrual cycles and 9 females taking oral contraceptive). Each subject made two visits for testing; in the case of females having normal menstrual cycles the first session was conducted either 1–7 (follicular) or 14–21 days (luteal) after the start of menstruation, and the second session two weeks later, i.e., during the other phase, respectively. Endothelial function was assessed at baseline and following a 20-min orthostatic challenge (active standing). The EndoPAT® index increased from 1.71 ± 0.09 (mean ± SEM) at baseline to 2.07 ± 0.09 following orthostasis in females (p<0.001). In males, the index increased from 1.60 ± 0.08 to 1.94 ± 0.13 following orthostasis (p<0.001). There were no significant differences, however, in the endothelial response to orthostasis between females and males, menstrual cycle phases and the usage of oral contraceptive. Our results suggest an increased vasodilatatory endothelial response following orthostasis in both females and males. The effect of gender and sex hormones on the endothelial response to orthostasis appears limited. Further studies are needed to determine the potential role of this post orthostasis endothelial response in the pathophysiology of orthostatic intolerance.

Ligustrazine and the contractile properties of soleus muscle in hindlimb-unloaded rats

BACKGROUND

In this study we investigated the effects of different dosages of ligustrazine (tetramethylpyrazine, Tmp) on soleus function and sarco(endo)plasmatic reticulum Ca(2+)-ATPase (SERCA) activity in 14-d hindlimb-unloaded (HU) rats.

METHODS

Female Sprague-Dawley rats were randomly divided into 4 groups (8 rats in each group): synchronous control (CON); HU plus intragastric water instillation (HU-W); HU plus different dosages of instilled Tmp (high: 42.53 mg x kg(-1), HU-TmpH; low: 21.15 mg x kg(-1), HU-TmpL). Muscle contraction force was examined in the soleus muscle. SERCA activity was assayed according to the released inorganic phosphate content.

RESULTS

As expected, in HU-W, soleus peak twitch tension (Pt), peak tetanic tension (P0), time to 50% peak tension (TP50), time to peak tension (TPT), time from peak tension to 50% relaxation (RT50), and SERCA activity decreased, all compared with CON. HU-TmpH Pt and P0 values were 179% and 90% above HU-W, and 187% and 124% above HU-W in HU-TmpL, respectively. TP50 and TPT values were 148% and 80% slower than HU-W with HU-TmpH and 95% and 32% in the HU-TmpL group, respectively. RT50 was slower than HU-W by 21% in HU-TmpH; SERCA activity elevated by 56% with HU-TmpH and by 72% with HU-TmpL.

CONCLUSIONS

Ligustrazine may alleviate the decrease of muscle contractile force and increase of shortening velocity in atrophied soleus, possibly by means of elevated sarcoplasmic reticulum Ca(2+)-ATPase activity.

Maximizing information from space data resources: a case for expanding integration across research disciplines

Regulatory systems are affected in space by exposure to weightlessness, high-energy radiation or other spaceflight-induced changes. The impact of spaceflight occurs across multiple scales and systems. Exploring such interactions and interdependencies via an integrative approach provides new opportunities for elucidating these complex responses. This paper argues the case for increased emphasis on integration, systematically archiving, and the coordination of past, present and future space and ground-based analogue experiments. We also discuss possible mechanisms for such integration across disciplines and missions. This article then introduces several discipline-specific reviews that show how such integration can be implemented. Areas explored include: adaptation of the central nervous system to space; cerebral autoregulation and weightlessness; modelling of the cardiovascular system in space exploration; human metabolic response to spaceflight; and exercise, artificial gravity, and physiologic countermeasures for spaceflight. In summary, spaceflight physiology research needs a conceptual framework that extends problem solving beyond disciplinary barriers. Administrative commitment and a high degree of cooperation among investigators are needed to further such a process. Well-designed interdisciplinary research can expand opportunities for broad interpretation of results across multiple physiological systems, which may have applications on Earth.

Reactive hyperemia in the human liver

We tested whether hepatic blood flow is altered following central hypovolemia caused by simulated orthostatic stress. After 30 min of supine rest, hemodynamic, plasma density, and indocyanine green (ICG) clearance responses were determined during and after release of a 15-min 40 mmHg lower body negative pressure (LBNP) stimulus. Plasma density shifts and the time course of plasma ICG concentration were used to assess intravascular volume and hepatic perfusion changes. Plasma volume decreased during LBNP (-10%) as did cardiac output (-15%), whereas heart rate (+14%) and peripheral resistance (+17%) increased, as expected. On the basis of ICG elimination, hepatic perfusion decreased from 1.67 +/- 0.32 (pre-LBNP control) to 1.29 +/- 0.26 l/min (-22%) during LBNP. Immediately after LBNP release, we found hepatic perfusion 25% above control levels (to 2.08 +/- 0.48 l/min, P = 0.0001). Hepatic vascular conductance after LBNP was also significantly higher than during pre-LBNP control (21.4 +/- 5.4 vs. 17.1 +/- 3.1 ml.min(-1).mmHg(-1), P < 0.0001). This indicates autoregulatory vasodilatation in response to relative ischemia during a stimulus that has cardiovascular effects similar to normal orthostasis. We present evidence for physiological post-LBNP reactive hyperemia in the human liver. Further studies are needed to quantify the intensity of this response in relation to stimulus duration and magnitude, and clarify its mechanism.

Radix astragali and orthostatic response: a double-masked crossover study

AIM

Recent results from animal experiments have shown that radix astragali (RA), a traditional Chinese herbal tonic, alleviates muscle atrophy under simulated weightlessness conditions, rendering RA a candidate for human use as a countermeasure against muscular atrophy. Possible cardiovascular side effects have not yet been investigated. We analyzed the effects of RA on the orthostatic stability of healthy men.

METHODS

There were 10 test subjects who were assigned to a double-blinded, randomized crossover design using RA or placebo (PL) for 14 d each, respectively. Test runs were separated by a 14-d 'washout' interval. At the beginning and the end of every 14-d test run, graded orthostatic stress (GOS) consisting of head-up tilt (HUT) combined with lower body negative pressure (LBNP) was used to achieve a presyncopal endpoint. Orthostatic effects on cardiac and vascular function were continuously monitored.

RESULTS

There were no significant differences between the RA vs. PL groups: mean arterial blood pressure dropped by 13 vs. 17%, pulse pressure 46 vs. 35%, heart rate increased 108 vs. 117%, and stroke volume index decreased 54 vs. 49% from supine control to presyncope. Neither did RA influence standing time compared to PL (18 +/- 7 vs. 17 +/- 6 min), nor did progression from the first to the fourth trial (15 +/- 6 to 18 +/- 7 min).

CONCLUSION

RA does not influence resting cardiovascular variables and orthostatic capacity in humans. It can be expected that human studies of RA's musculo-skeletal countermeasure potential will not be compromised by any cardiovascular side effects at the dosage employed in this study.

Muscle composition after 14-day hindlimb unloading in rats: effects of two herbal compounds

AIM

We studied the potential of two herbal compounds (HC-1 and HC-2) in different dosages (HC-1, containing ligustrazine; HC-2, containing radix astragali) as a countermeasure against muscle atrophy in a hindlimb unloading rat model. Soleus muscle weight, fiber type distribution, cross-sectional area (CSA), and myosin ATPase activity were measured.

METHODS

Six rats each were assigned to a nine-group design: Control (CON); hindlimb unloading only (HLU); hindlimb unloading plus intragastric water instillation (HLU-W); and hindlimb unloading plus different dosages of instilled HC-1 (HLU-HC-1l, HLU-HC-1m, HLU-HC-1h) or HC-2 (HLU-HC-2l, HLU-HC-2m, HLU-HC-2h).

RESULTS

As expected, in HLU and HLU-W, soleus muscle-to-body weight ratio and CSA of type I and II fiber went down, and myosin ATPase activity and the percentage of type II fibers went up, all compared with CON. Compared to untreated rats, high-dose HC-1 enhanced type I and II fiber CSA by 77% and 55%, respectively; myosin ATPase activity (type II fiber percentage) were lower, but soleus muscle-to-body weight ratio did not change. High-dose HC-2 enhanced soleus muscle-to-body weight ratio by 33%; and type I / type II fiber CSA by 143% and 83% above HLU-W values, respectively, and resulted in a slower myosin ATPase activity (corresponding to a lower type II fiber percentage). Low- to mid-dose HC-1 and HC-2 showed some preventive effects as well.

CONCLUSION

HC-1 and HC-2 in proper dosages diminished muscle atrophy that otherwise occurs in simulated weightlessness.

Postural changes associated with public speech tests lead to mild and selective activation of stress hormone release

We tested whether simulation of postural changes, which occur during public speech test procedures, activates cardiovascular system and stress hormone release that could interfere with the effect of psychosocial stress load. Young healthy male volunteers (n=8) underwent procedure imitating exactly all postural changes present in the psychosocial stress model based on public speech used in this laboratory (namely changes from sitting to standing and repeated sitting). Postural changes were associated with increases in heart rate, blood pressure, plasma concentrations of noradrenaline and aldosterone and elevation in plasma renin activity. In contrast to cardiovascular parameters, adrenocorticotropic hormone, cortisol and adrenaline, the main characteristics of hormonal response during mental stress, were not significantly influenced. The overall magnitude of all observed alterations was much smaller than that seen following mental stress procedures in our previous studies. This study provides evidence that changes in body posture during public speech test procedure influence hemodynamics and endocrine responses in a mild manner. Though this influence may represent a source of unspecific variance, substantial confounding effects on responses to the psychosocial component of the procedure are unlikely. In any case, models combining mental stressors and changes in body posture must be interpreted as complex stress stimuli.

Renal adrenomedullin and high altitude diuresis

Previous investigations revealed that most of the fluid regulating hormones showed no consistent relationship to the hypoxic diuretic response (HDR). In this study we examined if adrenomedullin (AM), a hypoxia-mediated diuretic/natriuretic peptide is connected to HDR. Thirty-three persons were examined at low altitude (LA), on the third exposure day at 3440 m (medium altitude, MA) and on the fourteenth day at 5050 m (high altitude, HA). Nocturnal diuresis rose from 460 ml [interquartile range 302 ml] at LA to 560 [660] ml at MA to 1015 [750] ml at HA (p<0.005). Sodium excretion was similar at LA and MA (41.8 [27.0] vs. 41.4 [28.4] mM) and increased to 80.2 [29.1] mM at HA (p<0.005). Urinary AM excretion was 7.9 [3.9] at LA, 7.5 [5.7] pM at MA, and increased to 10.5 [5.1] pM (p<0.05) at HA. Urinary AM excretion was correlated to diuresis (r=0.72, p<0.005) and sodium excretion (r=0.57, p<0.005). Plasma AM concentration rose from 16.4 [3.1] to 18.8 [4.9] pM/l at MA (p<0.005) and to 18.3 [4.3] pM/l at HA (p<0.005). Plasma AM concentration and urinary AM excretion were not correlated, neither were plasma AM concentration and diuresis or natriuresis. Our data suggest the involvement of increased renal AM production in the pathophysiology of high altitude fluid and sodium loss.

Circulatory galanin levels increase severalfold with intense orthostatic challenge in healthy humans

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° 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.

The possible role of adrenomedullin in adaption to high altitude

Diuresis and natriuresis are well-known responses to acute hypoxic exposure. Even though there have been many studies on the hormonal regulation of the hypoxic diuretic response (HDR), most of the fluid-regulating hormones showed no consistent correlation to HDR. The pathophysiology of high altitude related syndromes was often seen to be linked to an impairment of volume regulating processes. Adrenomedullin (AM) is a recently discovered polypeptide with diuretic, natriuretic and vasodilatory properties whose production is basically mediated by hypoxia. As yet, little is known about the action of AM under hypoxic conditions in vivo. We present evidence that AM may play a role in the physiology and pathophysiology of human adaption to high altitude.

Sodium intake does not influence bioimpedance-derived extracellular volume loss in head-down rest

INTRODUCTION

There is disagreement regarding the impact of dietary sodium on alterations in extracellular volume during head-down bed rest (HDBR). The primary purpose of this study was to assess the effects of salt intake on extracellular volume (ECV) during HDBR.

METHODS

We performed whole-body bioimpedance spectroscopy with controlled sodium intake during 4 d of ambulation and 8 d of -6 degrees HDBR in 10 normotensive men. Each subject performed an initial 12-d familiarization run with moderate sodium (246 +/- 12 mmol x L(-1) x d(-1) excreted) during which no measurements were made. They then participated in treatment runs involving low sodium (LS: 143 +/- 10 mmol x L(-1) x d(-1) Na+ excreted) and high sodium (HS: 434 +/- 17 mmol x L(-1) x d(-1) Na+ excreted). The different treatments were separated by > or =1 mo and the order of LS and HS was balanced among the subjects. These treatments were based on controlled food and drink supplies as prepared by a dietitian. We monitored sodium output and measured aldosterone, plasma renin activity (PRA), and vasopressin. Bioimpedance was measured every second day in supine position using tetrapolar electrodes.

RESULTS

Based on exponential data fitting, we calculated an ECV decrease of 0.79 +/- 0.32 L (-5.8%; p = 0.018) in LS, and 1.21 +/- 0.31 L (-4.0%; p = 0.002) in HS during HDBR. LS and HS were not different (p > 0.1); 4 d pre-HDBR sodium adjustment produced a fall in ECV in the LS group only (-3.7%, p = 0.023). Hormone levels were not changed by HDBR. Plasma aldosterone was lower in HS (69 +/- 7 pg x ml(-1)) than in LS (180 +/- 24 pg x ml(-1)).

DISCUSSION

Our bioimpedance data confirm that low sodium intake decreases ECV in ambulatory conditions and indicate that 8 d of HDBR produce a loss of ECV of about 5% (p < 0.05). The loss did not seem to be influenced by sodium intake between approximately 3 and approximately 10 g x d(-1).

Post-exercise decrease of plasma hyaluronan: increased clearance or diminished production?

The exercise-induced increase and post-exercise decrease of plasma hyaluronan concentration were studied in human subjects. Six well trained men performed incremental exercise until exhaustion (MAX), intensive (submaximal, SUB) and extensive exercise (moderate, MOD) on a bicycle ergometer, defined as work at 100, 77 and 50% of maximal oxygen consumption. Hyaluronan was analyzed using a high-sensitivity, proteoglycan-dependent time-resolved immunoassay and hemoglobin, hematocrit and plasma protein levels were assessed using standard laboratory procedures. Compared to resting control levels, the plasma hyaluronan concentration (pHA) increased (p < 0.05) by 76% (65.0 +/- 6.1 vs. 37.0 +/- 1.0 microg/l) during 15 min MAX, by 44% (56.4 +/- 2.6 vs. 39.2 +/- 3.8 microg/l) during 30 min SUB and by 27% (46.3 +/- 7.8 vs. 36.4 +/- 4.3 microg/l) during 90 min MOD. The increase with time averaged 4.03%.min(-1) during MAX, 1.35%.min(-1) during SUB and 0.35%.min during MOD. After exercise (15 and 30 min), pHA decreased by 43% below resting levels after MAX (p < 0.05) and by 36% after SUB, respectively. In conclusion, pHA steadily rose with time during physical exertion, with a non-linear increase of concentration/time slope with exercise intensity; second, the magnitude of the post-exercise pHA decrease was proportional to the exercise-induced pHA increase, suggesting elevated hyaluronan clearance with rising plasma levels after physical exertion.

Bed rest immobilization with various oral sodium supply: plasma hormones and body fluids

OBJECTIVE

To answer the question if plasma hormone concentrations (plasma renin activity--PRA, vasopressin--pAVP, and aldosterone concentration) due to antiorthostatic immobilization (8 days -6 degree head-down tilt bed rest--HDBR) are altered by oral salt load, we provided constant sodium supply during 4 days ambulatory conditions followed by 8 days HDBR in 10 normotensive men.

METHODS

A low' (LS: 143 10 mM) and high' (HS: 434+/-17 mM Na+/d excreted) sodium treatment were provided in randomized order, separated > or = 1 mo. Before and at the end of HDBR, hemodynamic variables and thoracic impedance were determined, and blood was taken for aldosterone and PRA, venous hematocrit, and plasma mass density. Extracellular fluid volume and pAVP were determined every second day. Whole body electrical impedance spectroscopy was employed to assess changes in extracellular volume, hormone determinations were done with radioimmunoassay, mass density measurements with the mechanical oscillator technique.

RESULTS

Extracellular volume decreased with HDBR (LS: -4.0%, p=0.002; HS: -5.8%, p=0.018) without significant difference between salt treatments. Resting hormone levels were not altered by HDBR, but pAVP was lower (5.5+/-0.1 pg/ml) in HS than in LS (7.2+/-0.3 pg/ml) as was plasma aldosterone (HS: 69+/-7 pg/ml, LS: 180+/-24 pg/ml). On the other hand, HDBR reduced extracellular volume by ?5% irrespective of dietary sodium supply.

CONCLUSIONS

Our data support the hypothesis that hormonal activities are more affected by oral salt load than by simulated short-term space flight, and suggest that the reduction of extracellular fluid volume due to head down bed rest is not influenced by moderate changes of dietary sodium supply.

Is there "cardiovascular drift" during and after simulated orthostasis in humans?

This study investigates the time course of hemodynamic variables during and after simulated orthostasis (lower body suction at 55mmHg for 30 mins: LBNP-55). Individual strategies of blood pressure defense could be observed; LBNP-55 increased heart rate and reduced stroke volume, but was non-hypotensive. During 20 minutes post-LBNP, heart rate was decreased as well as stroke volume index below pre-LBNP values. The reduced cardiac index together with unchanged mean arterial pressure gave a highly significant increase of peripheral resistance after LBNP. The validity of stroke volume results under the experimental conditions given needs to be clarified.

Orthostasis and transcapillary fluid shifts

Postural blood volume changes aggravate the regulation of arterial blood pressure and perfusion vis-a-vis the hydrostatic effects of orthostasis, ie, blood pooling below the hydrostatic indifferent points and reduced cardiac preload. Corresponding problems surface with extended passive standing, particularly in highly trained, dehydrated, or otherwise compromised subjects, or after long-lasting immobilization, as with space flight.

Permanent depression of plasma cGMP during long-term space flight

The purpose of this study was to investigate plasma concentrations of cyclic guanosine monophosphate (cGMP) and atrial natriuretic peptide (ANP) during and after real and simulated space flight. Venous blood was obtained 3 min after the beginning and 2 min after the lower body negative pressure maneuver in two cosmonauts preflight (supine), inflight, and postflight (supine) and in five other subjects before, at the end, and 4 days after a 5-day head-down tilt (-6 degrees) bed rest. In cosmonaut 1 (10 days in space), plasma cGMP fell from preflight 4.3 to 1.4 nM on flight day 6, and was 3.0 nM on the fourth day after landing. In cosmonaut 2 (438 days in space), it fell from preflight 4.9 to 0.5 nM on on flight day 3, and stayed <0.1 nM with 5, 9, and 14 months in space, as well as on the fourth day after landing. Three months after the flight his plasma cGMP was back to normal (6.3 nM). Cosmonaut 2 also displayed relatively low inflight ANP values but returned to preflight level immediately after landing. In a ground-based simulation on five other persons, supine plasma cGMP was reduced by an average of 30% within 5 days of 6 degrees head-down tilt bed rest. The data consistently demonstrate lowered plasma cGMP with real and simulated weightlessness, and a complete disappearance of cGMP from plasma during, and shortly after long-duration space flight.

Cardiovascular and hormonal changes with different angles of head-up tilt in men

The purpose of this study was to assess the endocrine status, thoracic impedance, blood concentration, and hemodynamic dose-responses using different angles of passive head-up tilt (HUT) ranging from 12 degrees to 70 degrees in the same subjects. Measurements were performed during 20 min supine position (pre-HUT), 30 min upright (HUT12, HUT30, HUT53, or HUT70), and 20 min supine (post-HUT); subjects 70 min in the supine position only (HUT0) served as resting controls. Norepinephrine increased above resting control values by 19, 44, 80, and 102%; epinephrine by 30, 41, 64, and 68%; aldosterone by 29, 62, 139, and 165%; plasma renin activity n. s., 41, 91, and 89%; vasopressin n.s., 27, 47, and 59%; thoracic bioimpedance n. s., 8, 13, and 16%; heart rate n. s., 5, 26, and 45%, and mean arterial pressure n. s., 5, 7, and 10%; at min 27 of HUT12, HUT30, HUT53, and HUT70, respectively. Pulse pressure decreased with HUT53 and HUT70 by 4 and 10%. Hematocrit increased by 0.2, 1.7, 6.3, and 7.2%, respectively. Blood density increased by 2.3 and 3.0 g/l, plasma density by 1.7 and 1.8 g/l with HUT53 and HUT70. After finishing HUT, heart rate fell to values which stayed below pre-HUT, and also below resting control levels for > or = 5 min ("post-orthostatic bradycardia") even after the lowest orthostatic load (HUT12). Thoracic impedance and arterial pressure remained increased after terminating HUT30, HUT53, and HUT70. In conclusion, passive orthostatic loading of different extent produces specific dose-responses of different magnitude in the endocrine system, blood composition, thoracic impedance, and hemodynamic variables. The heart rate is depressed even after HUT12, while arterial blood pressure and thoracic impedance exceed pre-stimulus levels after greater head-up tilt, indicating altered cardiovascular response after passive orthostasis.

Cardiovascular and humoral readjustment after different levels of head-up tilt in humans

PURPOSE

To get a more complete picture of cardiovascular regulation after postural changes, this investigation directly monitored volume-related, hemodynamic, and endocrine variables during and after 30 min of passive head-up tilt (HUT) of various degrees. It was hypothesized that the return of variables to pre-tilt control level is of system-specific duration and different from what is found after lower body negative pressure (LBNP).

DESIGN

We tested 7 persons on 5 different days using, in random order, no (HUT0) or different intensity (12 degrees , 30 degrees , 53 degrees , and 70 degrees ) of passive orthostasis (HUT12, HUT30, HUT53, HUT70). Data were collected before (supine), during, and after (supine) HUT and compared with synchronous data from HUT0.

RESULTS

There was graded alteration with the sine of tilt angle for all hormones and directly volume-related variables. The effects of HUT70 were of the same magnitude as previously documented by others. After HUT, hemodynamic variables and catecholamines returned to control levels most rapidly. Heart rate depression, as observed in a companion LBNP study in the same subjects, did not occur. Vasopressin, PRA, plasma volume and Z0 returned to nominal values more slowly. Plasma aldosterone was still elevated 50 min after reassuming supine posture.

CONCLUSION

Besides specific dose-responses within hemodynamic, volume-dependent, and hormonal variables after orthostatic loading of different degree, the return to control levels after HUT occurs with distinctly different time-courses, which are not identical with those seen after LBNP-simulated orthostasis.

Orthostatic stimuli rapidly change plasma adrenomedullin in humans

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 degrees, 30 degrees, 53 degrees, or 70 degrees 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 >/=30 degrees 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 degrees tilt was +70%. 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 (P<0.05) at 53 degrees and 70 degrees 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.

Endocrine status and LBNP-induced hormone changes during a 438-day spaceflight: a case study

We investigated basal levels and lower body negative pressure (LBNP)-induced changes of volume regulating (PRA, aldosterone, AVP, ANP99-126) and other stress-sensitive hormones (catecholamines, cortisol, ACTH) in venous plasma from one cosmonaut before (-45 d), during (3, 170, 287, 430 d) and after (+4, +90 d) a record-breaking long-term (438 d) spaceflight. Blood was taken at the beginning and immediately after ending LBNP (-15/-30/-35 mm Hg for 15/15/10 min, respectively) preflight supine, inflight, and postflight supine. PRA, aldosterone, and vasopressin levels stayed within normal boundaries during the entire flight and after landing. Catecholamines exceeded reference limits (epinephrine > 140 pg x ml(-1), norepinephrine >1000 pg x ml(-1) 5 and 9 mo inflight, and 4 d postflight. ANP and cGMP were lower inflight (p<0.04) than pre- or postflight. Cortisol and ACTH were not consistently altered. LBNP-induced hormonal changes were not different (p>0.05) in microgravity and 1-G. Based on data from one cosmonaut, we conclude that long-term spaceflight up to 430 d duration appeared to lower plasma ANP and cGMP during flight and occasionally elevate catecholamine levels, without significantly altering LBNP-induced relative hormone changes as compared with those observed on the ground.

Plasma hyaluronan concentration: no circadian rhythm but large effect of food intake in humans

This study was designed to determine if a circadian rhythm in plasma hyaluronan concentration [HA] exists in the absence of physical activity, and if plasma [HA] is associated with feeding in human subjects. Five persons were studied under standardized conditions, blood samples being taken between 0600 and 2200 hours at 30-min intervals. Any orthostatic challenge and muscle activity was abolished by immobilization by a 6 degrees head-down bed-rest, and the effect of a quasi-continuous ingestion of energy compared a normal, three-portion diet of equivalent energy content or to fasting. Reproducibility of HA profiles on two consecutive half-days was also studied. A highly sensitive immunoassay was used to determine plasma [HA]. The data indicated that without physical activity and without food ingestion, [HA] was unchanged and displayed no diurnal rhythm. In addition, we observed that [HA] increased after the first food intake, peaking after 60 min, and concluded from our results that without ingestion of a larger meal, and sessions of postural or muscle activity, no circadian plasma [HA] rhythm exists.

Cardiovascular changes during and after different LBNP levels in men

BACKGROUND

This study quantifies hemodynamic and thoracic impedance (TI) changes with four levels of lower body negative pressure (LBNP) from -15 to -65 mm Hg in seven healthy men in supine position 20 min before (pre-LBNP), 30 min during, and 20 min after suction (post-LBNP) as well as without suction (LBNP-0, rest control).

RESULTS

LBNP > 15 mm Hg increased basic TI by up to 2.2 omega (+9.5%). TI-computed stroke volume index (SVI) continuously decreased with time up to -12%, -28%, -36%, and -40% at the end of LBNP-15, -35, -55, and -65. TI-computed cardiac index decreased most (-14%) at LBNP-15 and -35, resulting in a 19% increase of calculated total peripheral resistance index at those intensities. Mean arterial pressure (MAP) did not change in any systematic way with lower LBNP levels, but increased +4.7% and +7.4% at the end of LBNP-55 and -65, respectively. Heart rate remained unchanged at LBNP-15, but continuously increased to reach +22%, +42%, and +55% at the end of LBNP-35, -55, and -65. After finishing LBNP, heart rate fell to values below both pre-LBNP and rest control for > or = 5 min (post-LBNP bradycardia). SVI transiently returned to, and MAP increased above, pre-LBNP levels after suction.

CONCLUSION

In conclusion, different levels of lower body subatmospheric pressure produced quantitatively different time course and dose-response patterns and remained non-hypotensive up to -65 mm Hg suction. Further, heart rate was depressed after LBNP, while arterial BP and TI-computed total peripheral resistance exceeded pre-stimulus levels, indicating an altered cardiovascular state after 30 min of simulated orthostasis.

Head-up tilt and lower body suction: comparison of hormone responses in healthy men

The purpose of this study was to compare, in the same subjects, hormonal responses to 30-min head-up tilt (HUT) and lower body suction (LBNP) of different intensity (24 degrees and 70 degrees, and 15 and 35 mm Hg, respectively). Basal pooled individual data from -10 min (n = 32) were within normal reference limits: norepinephrine (NE) averaged 318 +/- 23 pg/ml; epinephrine, 34.0 +/- 5.5 pg/ml; plasma renin activity (PRA), 0.72 +/- 0.08 ng ATII/ml/h; aldosterone, 164 +/- 20 pg/ml; atrial natriuretic peptide (ANP), 29.9 +/- 2.0 pg/ml; cGMP, 6.29 +/- 0.59 mmol/l; cortisol, 95.7 +/- 5.8 ng/ml; and ACTH, 50.3 +/- 2.6 pg/ml. The low-level stimuli failed to induce consistent changes in hormone levels. From the onset of the stimulus (minute 0) to its termination (minute 30), norepinephrine (NE) increased by 101% with LBNP-35, and by 70% with HUT70, respectively. The NE increase with LBNP-35 was higher (p < 0.05) than with HUT70. Epinephrine rose with HUT70 (by 162%) only. PRA increased by 157% with LBNP-35, and by 119% with HUT70, respectively; these responses were not significantly different. Aldosterone rose equally (by 85 and 89%) with LBNP-35 and HUT70 but not with the low-level stimuli. No consistent changes were observed in ANP, c-GMP or ACTH concentrations. Cortisol values fell during the LBNP and HUT24 situations but rose transiently after HUT70. We conclude that the hormones investigated respond differently to head-up posture and lower body suction and in a specific manner. Greater effects of high-level stimuli (HUT70, LBNP-35) were noted as compared to low-level stimuli (HUT24, LBNP-15). The application of combined sets of models stimulating the cardiovascular system may aid in the analysis of responses of hormonal systems in man.

Plasma volume with alternative tilting: effect of fluid ingestion

The present study determines the effect of repeated 70 degrees head-up tilt (HUT) on plasma volume (PV) shifts by measuring blood density (BD), plasma density (PD), and hematocrit (Hct). Eight men (18-26 yr) underwent a predrink period with two supine (P1 and P3) and two HUT (P2 and P4) phases of 45 min each. At the end of P4 they drank 10 ml/kg body wt of isotonic (290 mosmol/kg) sodium chloride (Iso) or hypotonic (< 10 mosmol/kg) unsweetened tea (Hypo) or nothing [control (Con)]. The following periods continued the supine (P5, P7)/upright (P6) sequence. BD and PD were measured from ear lobe blood; they were different (P < 0.05) between Con, Hypo, and Iso P6 and P7. The density of fluid that moved between intra- and extravascular compartments was 1,008.2 +/- 0.4 g/l and did not differ with test situations. In Con (P3, P5, P7), supine PV steadily decreased compared with P1 (P < 0.05). PV in P1, P2, and P3 of all treatments averaged 120 +/- 1, 101 +/- 1, and 115 +/- 1%, respectively, of PV in P4. Tilt-induced PV shifts ranged from -9.7 to -16.7% compared with PV during the respective previous phases. After drinking, PV increased (P < 0.05) above Con values at the end of P7 by 12.9% with Iso and by 6.6% with Hypo. Progressive hemoconcentration occurred in the nondrink supine periods; isotonic saline ingestion increased supine PV to Con level but did not stop or reverse the decrease of upright hemoconcentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in blood and plasma composition with lower body negative pressure on the ground and in space in one subject

In one cosmonaut, we investigated lower body negative pressure (LBNP)-induced capillary fluid shifts which we hypothesized would be changed by microgravity (micrograms) adaptation. Sound pulse velocity (SV) was determined in whole blood and plasma samples, using a new method that could detect 0.1% protein concentration changes. Experiments were performed 3 months preflight (supine), during space flight (6th day in orbit), and postflight (supine; 4th day after landing). Antecubital blood was taken at the beginning (3 min: time a) and after shut-down (+2 min: time b) of 40 min LBNP (-15/-30/-35 mm Hg for 15/15/10 min, respectively), since in control experiments with multiple sampling on Earth, the largest difference (increase) between blood and plasma SV was observed between a and b. Our cosmonaut had a 1.6 m/s increase in blood sound pulse velocity (BSV) preflight and a 4.0 m/s increase postflight, whereas BSV stayed unchanged in flight. Plasma sound pulse velocity (PSV) increased 1.2 m/s preflight and 1.7 m/s postflight, whereas PSV did not rise (-0.4 m/s) in flight. This would indicate profoundly altered LBNP-induced fluid dynamics in flight, compared to control (1-g) conditions. On the 4th day postflight, blood and plasma sound velocity increased more with LBNP than preflight, indicating greater hemoconcentration than under control conditions. In summary, the data suggest: 1) altered fluid shifts between blood and interstitial compartments during LBNP with 6 d adaptation to microgravity; and 2) increased hemoconcentration during LBNP early after a 10-d spaceflight.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal changes with lower body negative pressure on the 6th day in microgravity in one cosmonaut

We measured volume regulating and stress hormones (AVP, aldosterone, ANP, c-GMP, angiotensin II, PRA, epinephrine, norepinephrine, ACTH, cortisol) in venous blood twice during a lower body negative pressure (LBNP) maneuver in one cosmonaut (31 years, 75 kg, 180 cm) preflight (supine), inflight (6th d in orbit), and on the 4th d (supine) after a 10-d flight. Antecubital blood was taken at the beginning (3 min: "a") and after ceasing (2 min: "b") 40 min LBNP (-15/-30/-35 mm Hg for 15/15/10 min). At the beginning of LBNP, no big changes of resting hormone levels are to be expected. Comparison of "a" values: Inflight, there was a 4-5-fold increase in vasopressin and epinephrine, a slight increase in aldosterone, ANP, norepinephrine, cortisol and ACTH, and a decrease in PRA levels. Postflight, vasopressin was almost as much increased as inflight, and aldosterone and ANP levels were higher than pre- or inflight. PRA, epinephrine, norepinephrine, and cortisol were moderately increased, whereas ACTH and angiotensin II were diminished. Comparison of "b" to "a" values (2 min after LBNP to 3 min intra-LBNP): Preflight, ANP, PRA, and epinephrine rose more than 100%. The inflight response was higher for aldosterone but lower for all other volume active hormones. Postflight, the increase in PRA was pronounced, whereas little change occurred in other hormones. Cortisol and ACTH fell similarly during LBNP under all conditions. In summary, the data provide evidence that not only the endocrine status but also the neuroendocrine responsiveness to stimulation; i.e., the hormone response during cardiovascular load, are altered by the stay in microgravity and readaptation to normal conditions.