The effects of 7-days of simulated spaceflight, achieved with the technique of "dry" water immersion, on human triceps surae muscle function have been investigated in six subjects. After immersion, the maximal voluntary contraction (MVC) was reduced by 19% (p<0.01), and the electrically evoked (150 Hz) maximal tetanic contraction (Po) was reduced by 8% (p>0.05). The difference between Po and MVC expressed as a percentage of Po and referred to as force deficiency has also been calculated. The force deficiency increased by 44% (p<0.01) after immersion. The decrease in Po was associated with increased maximal rates of tension development (7.2%) and of tension relaxation. The twitch time-to-peak was not significantly changed, and half relaxation and total contraction time were decreased by 5% and 3%, respectively, but the twitch tension (Pt) was not significantly changed and the Pt/Po ratio was decreased by 9%. The 60-s intermittent contractions (50 Hz) decreased tetanic force to 57% (p<0.05) of initial values, but force reduction was not significantly different in the two fatigue tests: fatigue index was 36.2 +/- 5.4% vs. 38.6 +/- 2.8%, respectively (p>0.05). While identical force reduction was present in the two fatigue test it would appear that concomitant electrical failure was considerably different. Comparison of the electrical and mechanical responses alterations recorded during voluntary contractions, and in contractions evoked by electrical stimulation of the motor nerve, suggests that immersion not only modifies the peripheral processes associated with contraction, but also changes central and/or neural command of the contraction. At peripheral sites, it is proposed that the intracellular processes of contraction play a role in the contractile impairment recorded during immersion.
An artificial gravity and ergometric exercise loading device for human use was manufactured. It has the capacity of a max 2 G-load at the heart level, and a max 150 W of work-load. Eight subjects (six completed) were subjected to four repeated trials with or without 20 W ergometric exercise. Anti-G score, defined as the G-load x running time to the endpoint, was significantly higher in the exercise trials than standing trials. Heart rate (HR), mean arterial pressure (MAP), thoracic fluid index (TFI) were significantly superior during the exercise trials. Artificial gravity by centrifuge at 1.2 or 1.4 G with 40 or 60 W of ergometric workload may be an excellent countermeasure against cardiovascular deconditioning after long exposure to microgravity.
Treatment with cyclosporin A (CsA) is accompanied by serious side effects such as hypertension and nephrotoxicity. Although these side effects are likely to be due to increased constriction of vascular smooth muscle cells, it has not been elucidated how CsA elicits vasoconstriction. We thus studied whether CsA affects the expression of cyclooxygenase (COX)-2, a rate-limiting enzyme for the synthesis of a vasoconstrictive hormone, thromboxane A2, in the primary culture of human aortic smooth muscle cells (HASMC). We also studied the effect of epidermal growth factor (EGF) on COX-2 expression because of its potent vasoconstrictive action in an experimental model of hypertension. Treatment of HASMC with CsA alone resulted in a significant increase in the COX-2 mRNA level. EGF also enhanced the mRNA level. CsA together with EGF further increased the COX-2 mRNA level. These results suggested that the CsA-dependent increase in COX-2 in vascular smooth muscle cells could be one of the molecular mechanisms for CsA-induced vasoconstriction.
It is known that estrogen deficiency results in osteoporosis in human and experimental animals. However, how this deficiency affects the development of disuse bone atrophy is not well understood. Recently, it has been reported that estrogen affects the production of cytokines such as interleukin 6 (IL-6) which acts as local bone-resorbing factor. We thus studied how estrogen deficiency caused by ovariectomy and estrogen supplements affects the expression of IL-6 mRNA in the femur of tail-suspended rats. Five-week old female Wistar rats were ovariectomized and divided into two groups. One group received an intramuscular injection of estradiol dipropionate once a week (OVX-E2 group), and the other received the vehicle alone (OVX group). After the third injection, the rats were subjected to tail suspension in metabolic cages for 1, 3, 5 and 7 days. The wet weight of femurs significantly decreased after day 3 of tail-suspension in the OVX group. However, no significant decrease was observed in the OVX-E2 group. The expression of IL-6 mRNA estimated by RT-PCR (reverse transcription coupled polymerase chain reaction) in the femur significantly increased on day 5 after tail suspension in the OVX group. In the OVX-E2 group, that level significantly decreased on day 1 after the commencement of tail suspension. During suspension the level tended to be lower than that in the OVX group, a significant difference being observed on days 5 and 7 of suspension. The present results suggest that estrogen administration to OVX rats prevents both IL-6 production in the femur and the development of disuse bone atrophy induced by tail suspension.
Our previous studies demonstrated that estrogen (E2) prevents the development of disuse atrophy of the femur in tail-suspended rats. To elucidate the mechanisms of this E2 action, we investigated the effects of E2 on the expression of alkaline phosphatase (ALP, a marker for bone formation) and tartrate-resistant acid phosphatase (TRAP, a marker for bone resorption) in the femur of ovariectomized and tail-suspended rats. One group of ovariectomized rats received estradiol dipropionate (OVX-E2), and the other the vehicle alone (OVX). Each group was subjected to tail-suspension. After 1, 3, 5 or 7 days of suspension, ALP and TRAP mRNA levels were determined by Northern blot analysis. The ALP mRNA level was not altered by suspension in the OVX group, but it gradually increased in the OVX-E2 group, the highest level being observed at day 5 of suspension. In contrast, TRAP mRNA significantly increased at days 5 and 7 in the OVX group, while it is decreased significantly from day 3 to 7 in the OVX-E2 group. These results indicate that E2 prevents disuse atrophy of the femur in an ovariectomized and tail-suspended rat model by stimulating bone formation and by inhibiting bone resorption.
The purpose of the present study was to investigate the alterations in thermoregulatory control following 14 days of head-down bed rest (HDBR). The threshold temperature for sweating onset and sweating sensitivity were determined from sweating rates on the chest and forearm, and tympanic temperature as an index of core temperature (Tc) in nine healthy males exposed to a 60-min heat stress with a water-perfused blanket before and after HDBR. The threshold temperature for sweating onset, that is, the Tc at which sweating began on the chest and forearm was 36.75 +/- 0.14 and 36.72 +/- 0.13 degrees C before HDBR, respectively. The value significantly increased to 37.05 +/- 0.09 (p<0.05) for the chest and 37.04 +/- 0.08 degrees C (p<0.05) for the forearm after HDBR. On the other hand, the sweating sensitivity which was indicated as a slope of the Tc-sweating rate relationship significantly decreased from 4.20 +/- 1.15 to 2.32 +/- 1.18 for the chest (p<0.05) and from 4.20 +/- 1.06 to 2.92 +/- 0.98 mg/min/cm2/degrees C for the forearm (p<0.05) after HDBR. These findings suggest that the heat-dissipatory function was attenuated after 14 days of HDBR.
Velocity changes in the otolith-ocular reflex (OOR) during a step mode of lateral linear acceleration were examined by electrooculography (EOG) in 4 normal subjects. They were oscillated in darkness at a constant stimulus frequency of 0.175 Hz with 4 different G-levels between 0.2-0.4 G and at a constant G-level of 0.3 G with 4 different stimulus frequencies between 0.115-0.215 Hz. The OOR velocity was increased linearly by elevation of the G-level, though the increase varied considerably among subjects. On the other hand, changes in the stimulus frequency had minimal effect on the OOR velocity. The results indicate that the OOR response property is frequency-independent at a low stimulus-frequency range, even when the head was oscillated rectangularly instead of sinusoidally.
To clarify the response of muscle sympathetic nerve activity (MSNA) to static stimulation of otolith organs in a craniocaudal direction (+Gz) in humans, we examined the effect of otolith stimulation on MSNA without changing the effect of cardiopulmonary baroreceptors using a 6-8.5 degrees head-down tilt (HDT) and lower body negative pressure (LBNP) device. Before the study, we established that 6-8.5 degrees HDT with 10 mmHg LBNP caused a fluid shift to the degree that the thoracic impedance was the same as the supine position without LBNP. Subjects were young male volunteers aged 22.1 +/- 3.8 years who gave informed consent. MSNA was recorded from the tibial nerve by microneurography simultaneously with heart rate (ECG), thoracic fluid volume (impedance method), and blood pressure (tonometric method). During 6-8.5 degrees HDT with 10 mmHg LBNP, MSNA was suppressed slightly without significantly changing heart rate, thoracic impedance, or mean arterial blood pressure. The results suggest that the sympathosuppression was related not to the result of cardiopulmonary [correction of cardioplumonary] loading but to the -Gz change (caudocranial direction [correction of dirction]) of 0.1 G. It is estimated that the vestibulo-sympathetic reflex may suppress sympathetic outflow to muscles in humans.
It is well known that estrogen deficiency results in osteoporosis in human and experimental animals. However, how its deficiency affects the development of disuse atrophy is not well understood. We thus investigated how estrogen deficiency caused by ovariectomy and estrogen supplements affect serum levels of calcium, parathyroid hormone (PTH), and 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) in tail-suspended rats. Five-week-old female Wistar rats were ovariectomized and divided into two groups. One group received an intramuscular injection of estradiol dipropionate once a week (OVX-E2 group), and the other received the vehicle alone (OVX group). After the third injection, the rats were subjected to tail-suspension in metabolic cages for 1, 3, 5 or 7 days. In the OVX group, urinary excretion of deoxypyridinoline (D-Pyr) tended to increase on day 1 after tail-suspension. In the OVX-E2 group, basal excretion was lower than that in the OVX group, and no increase was observed after the suspension. Serum concentrations of ionized calcium significantly increased on day 1 after the suspension in both groups. However, in the OVX-E2 group, the level tended to be higher than those in the OVX group from day 0 to day 3. Serum PTH tended to decrease on day 1 after suspension in the OVX group. In the OVX-E2 group, it did not change during the suspension, but the levels were higher than those in the OVX group during the experiment. Serum 1,25(OH)2D3 transiently and significantly increased on day 1 after suspension in both groups. However, in the OVX group, the level was significantly higher than that in the OVX-E2 group. These data indicate that estrogen treatment of ovariectomized rats modifies the changes in calcium metabolism induced by tail-suspension.
We investigated the effect of estrogen (E2) and tail-suspension on expression of osteocalcin (OC) mRNA in the femur of ovariectomized rats. Five-week-old female Wistar rats were ovariectomized and divided into two groups: one group received estradiol dipropionate (OVX-E2), and the other received the vehicle (OVX). Each group was further divided into two subgroups, tail-suspended (S) and non-suspended (N), giving a total of four groups: OVX-E2-S, OVX-E2-N, OVX-S and OVX-N. After a 7-day suspension, femurs were excised, and OC mRNA levels were determined by Northern blot analysis. A significant decrease of OC mRNA in OVX-E2-N was observed when compared with that of OVX-N, indicating that E2 decreases the OC expression. Interestingly, tail-suspension further decreased the mRNA levels in both OVX-S and OVX-E2-S when compared with the levels of OVX-N and OVX-E2-N, respectively. Since glucocorticoids have been shown to decrease OC expression, we also measured the urinary excretion of corticosterone during the suspension period that reflects the serum levels of corticosterone, and found that it was increased by E2 and further increased by tail-suspension. These results indicate that estrogen and glucocorticoids exert additive effects in inhibiting OC expression in the rat femur.
To test our hypothesis that somatosensory inputs would influence postural modulation of soleus H-reflex, eleven subjects were investigated under the head-out water immersion (HOWI) conditions. Subjects were supine or standing on a tilting bed in each condition. They were instructed to maintain an upright posture with both legs. The water was filled to the subject's neck level in a test tank to reduce 95% of the gravitational effect by buoyancy. Surface electromyography of the soleus and tibialis anterior was measured. The soleus H-reflex was elicited at a stimulation intensity of 1.05 times the motor threshold. The recruitment profile of the motor response was unchanged between the conditions. The background activities of the soleus and tibialis anterior were not detected in any condition. The peak-to-peak amplitude of the H-reflex was significantly different between the conditions while the stimulation intensity (small M size) was not different. The soleus H-reflex during standing was significantly decreased compared with being supine in the control condition, whereas it did not in the HOWI condition. It was concluded that somatosensory inputs due to gravity exert an influence on postural modulation of the soleus H-reflex to maintain static posture in humans.
The purpose of the present study was to evaluate the effect of hypoperfusion to the static-exercised-muscle induced by arm elevation on muscle sympathetic nerve activity (MSNA) recorded from the right tibial nerve (n= 10) by microneurography. Subjects performed static handgrip exercises (SHG) at 30% of maximal voluntary contraction (MVC) for 2 min, followed by 2 min of posthandgrip muscle ischemia (PHGMI) at the heart level (control) and with the arm elevated (50 cm above heart level). Basal heart rate, mean blood pressure and MSNA responses expressed as burst rate (bursts/min) and total MSNA (%) were unaffected by the arm-position. Heart rate response during SHG was not influenced by the arm elevation, while mean blood pressure and MSNA responses were increased by it (P<0.05). MSNA responses were increased during the second minute of the SHG period (P<0.05) and the PHGMI period (P<0.05) in the elevated arm position. In conclusion, arm elevation increased MSNA responses to SHG and PHGMI, suggesting an increased magnitude of muscle metaboreflex.
To elucidate the effect of aging on blood pressure regulation, especially cardiopulmonary baroreflex in humans, we measured the resting value of muscle sympathetic nerve activity (MSNA), heart rate (HR), blood pressure (BP), peripheral venous pressure (PVP), and calf blood flow (CBF) in eight healthy young male subjects and eight healthy aged subjects, and compared the responses to the low level of lower body negative pressure (LBNP) between the two groups. The resting MSNA was higher, and the response of MSNA to LBNP was lower in the aged group than in the young group; the response of PVP to LBNP was also lower in the aged than in the young group. Delta MSNA/delta PVP showed no significant differences between the two groups. The present findings indicate that the effect of LBNP on cardiopulmonary baroreflex is greater in young people than in aged people, but that cardiopulmonary baroreflex may be preserved with advancing age.
To investigate whether handedness influences sympathetic nerve responses during exercise, muscle sympathetic nerve activity (MSNA), heart rate, and blood pressure during maximal voluntary handgrip exercise under ischemic conditions (MVHG) and during post-exercise ischemia (PEI) were compared between the dominant (DA) and non-dominant arm (NDA) in eight left-handed subjects. MSNA expressed as burst frequency or total activity (TSNA) and blood pressure increased significantly during MVHG and PEI. Although the heart rate rose during MVHG, it returned to resting level during PEI. The increased MSNA was less in DA exercise than the NDA, while no significant differences in heart rate and blood pressure response were observed. The results suggest that handedness seems to influence reflex sympathetic activation during muscular contraction, but has no significant effect on heart rate or blood pressure.
Muscle sympathetic nerve activity (MSNA) is activated during mild-intensity isometric handgrip exercise, but not during isometric leg exercise at a similarly relative intensity. It is unclear whether MSNA is increased by leg exercise at heavy intensity in humans. We aimed to examine how MSNA responds to high-intensity, isometric leg exercise in humans. Eight young healthy male volunteers performed isometric plantar flexions of high intensity (70% of MVC) for 2 min, followed by 2 min of post-exercise muscle ischemia (PEMI; isolated muscle metaboreflex stimulation), with monitoring of muscle sympathetic nerve activity (MSNA), heart rate and arterial blood pressure. MSNA was increased at the onset of exercise (p<0.05), and further increased to 512 +/- 111% of baseline at the second min of exercise (p<0.05). Furthermore, MSNA remained elevated above baseline during PEMI (259 +/- 41 % of baseline, p<0.05). Heart rate also rose at the onset of the test (P<0.05) and gradually increased all through the test (p<0.05), but decreased to baseline level during PEMI. Mean blood pressure was progressively increased from the onset to the end of test (p<0.05), and remained elevated above baseline during PEMI (p<0.05). In summary, MSNA was increased progressively throughout high-intensity, isometric plantar flexion in humans, primarily due to muscle metaboreflex.
To test the hypothesis that increased leg venous compliance (LVC) is one of the contributory factors to orthostatic intolerance (OI) after simulated microgravity, 28 healthy young males were exposed to a 14-day head-down bed rest, and LVC was measured by venous occlusion plethysmography. Orthostatic tolerance was evaluated by a 60 degree head-up tilt (HUT) for 15 min. Sixteen subjects suffered from OI after the bed rest. They were then divided into orthostatic tolerance (non-fainters, n=12) and intolerance (fainters, n=16) groups. We found that fainters had significantly larger LVC before bed rest (0.055 +/- 0.003 vs. 0.065 +/- 0.002 ml 100 ml-1 mm Hg-1, non-fainter vs. fainter; P < 0.05). After bed rest, LVC markedly increased in both groups. In all the subjects calf circumference was reduced on average by 4.7% and the percent change in LVC was negatively correlated with the percent change in calf circumference when all subjects' data were combined after bed rest (r = -0.42, P < 0.05). Our results did not support the hypothesis that increased LVC is the contributory factor to OI after a 14-day bed rest; however, the mechanisms behind the large LVC in the fainters before bed rest are unclear, and the initial LVC might be a predictive indicator for OI after microgravity exposure.
Heat shock proteins (HSPs) have been identified in all cells, prokaryotic and eukaryotic, to protect the cells from harmful insults and stress. Increased HSP synthesis can also result during normal cellular functions and also respond to exposure from environmental stress and infection. Although the molecular mechanisms responsible for HSP cellular protection are still not fully understood, their expression is critical for cellular survival and can be modified by cell signal transducers such as intracellular pH, cyclic AMP, Ca2+ Na+, inositol [correction of inostitol] triphosphate, protein kinase C, and protein phosphates. Most HSPs interact, assemble, fold, unfold, bind, transport, translocate and 'chaperone' other proteins in the cell and alter their function.
To evaluate the fluid shift and leg venous compliance during orthostatic stress with advancing age, 12 aged and 5 young healthy males were subjected to graded lower body negative pressures (LBNP) of -5, -10, and -15 mmHg. Cardiovascular variables were monitored continuously, and leg venous compliance was determined by venous occlusion plethysmography. Neither heart rate nor mean arterial pressure changed significantly in any subject during LBNP. A progressive decrease in the thoracic fluid volume index and a gradual increase in the leg fluid volume index, indicating a fluid shift towards the lower body were observed significantly in the young group (p<0.05), while these changes were not significant in the aged group. A linear reduction in peripheral venous pressure could be seen during graded LBNP in all subjects, but the reduction rate was smaller in the aged group. Baseline leg venous compliance was reduced in the elderly (p<0.05). During LBNP, venous compliance decreased in all subjects, but the decrease was significantly smaller in the aged group (p<0.05). It is suggested that the smaller fluid shift and smaller decreased leg venous compliance in aged people during gravitational stress were mainly due to the vascular and ventricular stiffness induced by an age-related reduction in visco-elasticity of the peripheral venous and ventricular walls.
Studies carried out in space flights and in altered gravitational environments have shown that exposure to altered gravity conditions results in alterations in cellular structure and function. In the present study, we used a clinostat to generate a vector-averaged gravity environment, and evaluated the responses of osteoblast-like ROS 17/2.8 cells subsequent to rotation at 50 r.p.m from 24 to 72 hr. We found that the cells started to detach during the first 24 hr of culture in clinostat, but not in stationary and horizontal rotation (the latter serving as a control for turbulence, shear forces and vibrations). At 24 hr, there was a significant decrease in the number of adherent cells under clino-rotation (2.75 +/- 0.5 x 10(5) in stationary culture versus 2.02 +/- 0.27 x 10(5) under clino-rotation), and 19.8% of adherent cells were trypan-blue positive when cultured in 2% fetal bovine serum. All the detached cells were trypan-blue positive. At 72 hr, the cells became confluent in all three groups. These results suggest that vector-averaged gravity could cause the death of osteoblasts during the first 24 hr of clino-rotation. We hypothesize that this cell death might play a role in the pathogenesis of osteoporotic bone loss as observed in actual space flight.
Functional nystagmic eye movements are compensatory responses meant to stabilize the gaze during head movement. Niven et al. (1966) have reported that otolithic nystagmus can be elicited by linear acceleration in humans only along interaural (Y-axis) acceleration (Gy), but not along occipitonasal (X-axis) or head-to-foot (Z-axis) acceleration. This result was reexamined in the present study in normal subjects who elicited lateral (Gy) nystagmus frequently. The nystagmic elicitation was examined by EOG in 4 healthy subjects who were fastened tightly in the chair of a linear accelerator and instructed to look straight ahead at an imaginary target in the dark. The stimulation was five oscillations of a constant 10-m stroke at two different acceleration levels of 0.3 and 0.5 G in a step mode. The X- and Z-axis accelerations did not induce any horizontal or vertical nystagmus, while the Y-axis acceleration did, in agreement with the above report. However, it was found that nystagmic elicitation due to Gy loading tended to be lower in the supine than in the upright sitting position. The present finding that nystagmic slow-phase velocity (SPV) increased with an increase in the G-load also contradicted the above report. Upright subjects who showed G-directional preponderance (DP) in nystagmic elicitation did not always maintain DP when supine. No relation could be seen between DP and SPV, suggesting that DP might be generated by a central mechanism different from SPV control.
