Quantitative analysis of synaptic contacts made between functionally identified oralis neurons and trigeminal motoneurons in cats

A previous study revealed that rostrodorsomedial oralis (Vo.r) neurons synapsing on trigeminal motoneurons use GABA and/or glycine as neurotransmitters. To determine the number and spatial distribution of contacts, injections of biotinamide and horseradish peroxidase were made into a Vo.r neuron and an alpha-motoneuron in the jaw-closing (JC) and jaw-opening (JO) motor nucleus, respectively, in 39 cats. All Vo.r neurons responded to low-threshold mechanical stimulation of the oral tissues. Single Vo.r neurons terminating in the JC nucleus (Vo.r-dl neurons; n = 5) issued, on average, 10 times more boutons than Vo.r neurons terminating in the JO nucleus (Vo.r-vm neurons; n = 5; 4437 vs 445). The Vo.r-dl neuron-JC alpha-motoneuron pairs (n = 4) made contacts on either the soma-dendritic compartment or dendrites, and the Vo.r-vm neuron-JO motoneuron pairs (n = 2) made contacts on dendrites, with a range of two to seven contacts. In five of the six pairs, individual or groups of two to three terminals contacted different dendritic branches of a postsynaptic cell. The Vo.r-dl neurons innervated a greater number of counter-stained motoneuronal somata than did the Vo.r-vm neurons (216 vs 26). Total number of contacts per Vo.r neuron was higher for the Vo.r-dl than Vo.r-vm neurons (786 vs 72). The present study demonstrates that axonal branches of Vo.r neurons are divided into two types with different innervation domains on the postsynaptic neuron and that they are highly divergent. The overall effect exerted by these neurons is predicted to be much greater within the JC than JO motoneuron pool.

[Counteraction effect of intermittent -Gx on changes of responsiveness of basilar artery in tail-suspended rats]

OBJECTIVE

To investigate counteracting effects of intermittent -Gx exposure on changes of responsiveness of basilar artery in tall-suspended rats.

METHOD

Rats tail-suspended for 3 wk were daily subjected to 1 h 1.5 G or 2.6 G centrifugation, simulating -Gx exposure of various G values. Responses of perfused isolated basilar arterial rings to KCl was examined.

RESULT

Maximal isometric contractile response to KCl were significantly enhanced in arterial rings isolated from 3 wk tail-suspended rats as compared with that from simultaneous control rats (P < 0. 05). However, maximal isometric contractile responses to KCl of arterial rings isolated from 3 wk tail-suspended plus daily -Gx exposure rats were no difference as compared with that from simultaneous control rats, and were significantly decreased (P < 0.05) as compared with that from 3 wk tail-suspended rats.

CONCLUSION

Daily intermittent -Gx exposure as a countermeasure to prevent the effects of simulated weightless on changes in responsiveness of basilar artery from tail-suspended rats was valid.

[Changes of human recombination bone morphogenetic protein-2 in bone and marrow in tail suspended rats]

OBJECTIVE

To study human recombination bone morphogenetic protein-2 (rhBMP-2) change in bone and marrow under simulated weightlessness.

METHOD

5 SD rats were tail suspended for 14, 28 d with another 5 freely active rats as control. Histological samples were in situ hybridized.

RESULT

rhBMP-2 expression of bone and marrow were lower in tail suspended rats than control levels (P < 0.05). rhBMP-2 expression tail suspended rats in 14 day were higher than that in 28 day suspended group (P < 0.05).

CONCLUSION

rhBMP-2 levels in rats bone and marrow were lower after tail suspension.

Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exists in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural features of trigeminal primary afferent neurons in the cat dorsal principal nucleus (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal labeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the boutons in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprimary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profiles per labeled bouton did not differ in the Vpd and Vo.r. Most labeled boutons in the two nuclei were postsynaptic to unlabeled axon terminals with pleomorphic vesicles (p-ending). The number of p-endings per labeled bouton was higher in the Vpd than Vo.r A morphometric analysis indicated that labeled bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these parameters were larger than those of p-endings in each nucleus. In both labeled boutons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficacy of transmission at primary afferent synapses does not differ.

[A simulation study of effects of depressed myocardial contractility on cardiovascular response to lower body negative pressure]

OBJECTIVE

To study the effects of depressed myocardial contractility induced by microgravity on cardiovascular response to orthostatic stress, and to investigate the role played by the changes of myocardial contractility in the mechanism of cardiovascular deconditioning and orthostatic intolerance induced by space weightlessness.

METHOD

On the basis of our previous model used to simulate the cardiovascular response to lower body negative pressure (LBNP), the factor of changes of myocardial contractility was incorporated into the model by multiplying a coefficient to the maximum elastance of the heart working sub-model. By decreasing the coefficient progressively, then the changes of heart rate (HR), blood pressure (BP), and cardiac output (CO) during LBNP after 0 - 30% of myocardial contractility depression combined with 12% decrease of total blood volume were simulated.

RESULT

Simulation results indicated that depressed myocardial contractility induces more augment of HR, and more decrease of BP and CO during LBNP.

CONCLUSION

The depression of myocardial contractility degenerated cardiovascular response to orthostatic stress.

Simulated microgravity enhances vasoconstrictor responsiveness of rat basilar artery

Recently, hypertrophy and increased myogenic tone of brain vessels have been observed in rats after simulated microgravity. It is expected that simulated microgravity may also induce hyperreactivity of brain vessels. To test this hypothesis, Sprague-Dawley rats were subjected to a 4-wk tail-suspended hindlimb unloading (TS) to simulate the cardiovascular deconditioning effect of microgravity. After 4 wk, the vasoreactivity of isolated basilar arterial rings from TS rats to both receptor- and non-receptor-mediated vasoconstrictors, such as KCl, arginine vasopressin, or 5-hydroxytryptamine (5-HT), and vasodilators such as ACh, thrombin, adenosine, or sodium nitroprusside were examined and compared with those from simultaneous control (Cn) rats. In the first part of this study, it was found that the maximal isometric contractile responsiveness evoked by vasoconstrictors such as KCl, arginine vasopressin, or 5-HT was enhanced in basilar arterial rings from TS rats, whereas vasodilatory responsiveness to vasodilators showed no significant difference between TS and Cn rats. In the second part of this study, it was found that removal of the endothelium had no effects on the contractile responsiveness to 5-HT in basilar arterial rings from TS rats but enhanced markedly the responsiveness in basilar arterial rings from Cn rats to an extent comparable with that of TS rats. Application of tetraethylammonium also had no effects on the contractile response to 5-HT in basilar arterial rings from TS but significantly increased the responsiveness of basilar arterial rings from Cn rats with endothelium intact. These results showed that 4-wk simulated microgravity enhanced the vascular contractile responsiveness of basilar arterial rings to both receptor- and non-receptor-mediated vasoconstrictors, and the enhancement of 5-HT-induced contraction in TS rat basilar arteries was due to an impairment of endothelium-dependent mechanism. These results suggest that endothelium-derived hyperpolarizing factors are responsible for this endothelium-dependent attenuating modulatory mechanism in contractile responsiveness of rat basilar arteries to 5-HT.

[Transition of soleus troponin I isoforms and atrophy of testis in tail-suspended rats]

Objective. To observe the transition time of soleus I (TnI) isoforms and to elucidate the relationship between soleus TnI transition and atrophy; and to analyze the time course between testis atrophy and soleus atrophy. Method. Eight groups of male rats were suspended for 3, 4, 5, 7, 14, 21, 28 and 42 d, respectively. Besides, three groups of female rats were suspended for 3, 4 and 5 d respectively. Wet and relative weights (wet weight/body weight) of testis and soleus were measured. The expression of TnI was observed by Western blot. Result. The relative weight of soleus of tail-suspended male rats decreased significantly after 4 d of suspension as compared with control. The degree of rats soleus atrophy in the first 14 d of suspension was greater than that after 14 d. The relative weight of testis showed the same change as that of soleus. There was no significant change in the relative weight of soleus in 4 d of tail-suspended female rats. The significant decrease in the relative weight of tail-suspended female rats began on the 5th day. The Western blot showed that the transition from slow skeletal TnI (ssTnI) to fast skeletal TnI (fsTnI) in the soleus occurred in 14 d of tail-suspension. Conclusion. The overt atrophy of tail-suspended male rats occurs on the 4th day. The soleus TnI transition from ssTnI to fsTnI is on the 14th day. It is suggested that the TnI is not the sensitive protein to gravity. The overt atrophy of female tail-suspended rats occurs at the 5th day. This indicated that the decrease in testosterone level may accelerate the atrophy of the soleus.

Inhibition by nociceptin on excitatory non-adrenergic non-cholinergic response in guinea pig airways

AIM

To study the effect of nociceptin (NC), a newly discovered heptadecapeptide, and U-50488H, a kappa-opioid receptor agonist, on excitatory non-adrenergic non-cholinergic (eNANC) constriction responses in guinea pig isolated bronchus.

METHODS

An eNANC response was induced by electric field stimulation (EFS) in the preparation via activation of the sensory nerve terminals. The effect of NC and U-50488H was analyzed on the response.

RESULTS

Nociceptin 0.001 - 0.1 micromol/L inhibited the eNANC constriction which was induced by EFS but not by capsaicin in guinea pig bronchus. The constriction inhibited by NC 0.01 micromol/L was (43 +/- 31) % compared with the control. After pretreatment with naloxone 0.1 micromol/L, the constriction was inhibited by (46 +/- 28) %, without marked change compared with the above figure. IC50 (95 % of confidence limits) was 6.12 (3.8 - 9.9) nmol/L. U-50488H also inhibited the EFS-evoked eNANC constriction and the effect was abolished after pretreatment with naloxone. IC50 (95 % of confidence limits) was 1.08 (0.5 - 2.2) micromol/L. Capsaicin 0.01 - 1 micromol/L caused a cumulative constriction response in the preparation. Moreover, the effect of capsaicin was not affected by pretreatment with NC 0.01 micromol/L or U-50488H 0.1 micromol/L. The constriction induced by exogenous neurokinin A, were also unaffected by treatment with NC 0.01 micromol/L or U-50488H 0.1 micromol/L in isolated bronchus.

CONCLUSION

Nociceptin inhibits EFS-induced eNANC constriction, which is not reversed by naloxone, while U-50488H inhibits EFS-induced eNANC response via activation of opioid receptor in guinea pig airways.

[Estimating cardiovascular age of civil flying personnel by means of heart rate and blood pressure variability analysis]

Objective. To estimate the cardiovascular age of civil flying personnel by means of heart rate and blood pressure variability analysis and to evaluate its significance in aviation medicine. Method. First, heart rate variability (HRV), blood pressure variability (BPV) and spontaneous baroreflex sensitivity (BRS) were analyzed among 89 healthy civil flying personnel by using conventional AR spectral analysis and sequence method respectively. Then, principal component analysis was conducted over original and derived variables of HRV and BPV spectral and BRS data. Finally, by the use of multiple regression in which the chronological age acted as the dependent variable and the components significantly related to age were used as the regressors, the equation for estimating the cardiovascular age was established. Result. Only seven principal components can exactly reflect the same information of autonomic regulatory function which was embodied in the 17 variables of HRV and BPV spectral and BRS parameters. Among the seven principal components, the PC2orig, PC4orig and PC2deri were negatively correlated with chronological age (P<0.05), whereas the PC3orig was positively correlated with the chronological age (P<0.01). The cardiovascular age derived from the equation was significantly correlated with the chronological age of the civil flying personnel (r= 0.73, P<0.01). Conclusion. The cardiovascular age estimated by means of a multi-variate analysis of HRV, BPV and BRS can be treated as a comprehensive indicator reflecting the age dependency of autonomic regulatory function of cardiovascular system in healthy civil flying personnel, and its interpretation and significance in application are surely worthy of further and fully dedicated efforts.

Peripheral effector mechanism hypothesis of postflight cardiovascular dysfunction

Studies on the mechanisms of cardiovascular dysfunction after space-flight are important to illustrate the cardiovascular effect of microgravity and develop appropriate multi-system countermeasures for future long-duration spaceflights. Over the past 10 yr, we have systematically studied the adaptational changes in structure and function of both the heart and vessels, using the tail-suspension rat model to simulate microgravity effects. Our results indicate that simulated microgravity induced atrophic changes and reduced contractility of the heart muscle, and upward- and downward-regulation in structure, function, and innervation state of vessels in the brain and hind body of the rat. In addition, more recent advances in relevant ground-based and space-flight studies from different laboratories have also been reviewed. Based on these studies, it has been speculated that, in addition to hypovolemia, the microgravity-induced adaptational changes in the structure and function of the two main effectors of the cardiovascular system, i.e., the arterial smooth muscle and the cardiac muscle, might be among the most important mechanisms responsible for postflight cardiovascular dysfunction and orthostatic intolerance. In this paper we will review the available evidence with comments.

A proteolytic NH2-terminal truncation of cardiac troponin I that is up-regulated in simulated microgravity

In a tail suspension rat model, we investigated changes in myofilament protein during cardiac adaptation in simulated microgravity. Contractile force and velocity of cardiac muscle were decreased in the tail suspension rats as compared with the control. Ca(2+)-dependent actomyosin ATPase activity was also decreased; however, sensitivity of cardiac muscle to Ca(2+) activation was unchanged. There was no change in expression of myosin heavy chain, tropomyosin, troponin T, or troponin I isoforms in hearts of tail suspension rats. A novel finding is a fragment of cardiac troponin I (cTnI) that had increased amounts in the heart of tail suspension rats. Binding of this cTnI fragment by a monoclonal antibody that specifically recognizes the COOH terminus indicates an intact COOH terminus. NH(2)-terminal sequence analysis of the cTnI fragment revealed truncations primarily of amino acids 1-26 and 1-27 and smaller amounts of 1-30, including Ser(23) and Ser(24), which are substrates of protein kinase A phosphorylation. This cTnI fragment is present in normal cardiac muscle and incorporated into myofibrils, indicating a role in regulating contractility. This proteolytic modification of cTnI up-regulated during simulated microgravity suggests a potential role of the NH(2)-terminal segment of cTnI in functional adaptations of cardiac muscle.

Effect of repeated +Gz exposures on energy metabolism and some ion contents in brain tissues of rats

BACKGROUND

It has been demonstrated that during +Gz exposure cerebral blood flow is significantly reduced, resulting in brain ischemia. In pilots, such conditions could recur several times during centrifuge training and combat maneuvers and could possibly cause reversible change in brain energy metabolism.

HYPOTHESIS

In rats there is an association between +10 Gz exposure and the decreased brain metabolism, as indicated by decreased adenosine triphosphate (ATP) and ATPase activity, and increased adenosine diphosphate (ADP) and lactate, etc. The aim of the present study was to examine the time course and recovery of brain energy metabolism, lactate, ATPase activity, Na+, K+, Ca2+ and water contents after three +10 Gz exposures in rats.

METHODS

There were 64 male Sprague-Dawley rats that were restrained and placed on an animal centrifuge. They were divided into groups of 16. Control rats were exposed to +1 Gz and experimental rats were exposed to +10 Gz three times each for 3 min at 30-min intervals. After being euthanized, rat brains were removed 0 h, 1 h, or 6 h after the last centrifuge run. Brain samples were analyzed for energy metabolism, lactate, Na+-K+-ATPase activity, water and electrolytes contents.

RESULTS

The cortical ATP content, Na+-K+-ATPase and lactate dehydrogenase (LDH) activities decreased significantly, whereas the cortical ADP, adenosine monophosphate (AMP) and lactate contents increased significantly 0 h after three +10 Gz exposures, as compared with those of control. The ATP, ADP, and AMP contents returned to their control levels 1 h after the +10 Gz exposures, however, lactate content, Na+-K+-ATPase and LDH activities delayed recovery 6 h after +10 Gz exposures. The cortical K+ content increased significantly 0 h and 1 h after +10 Gz exposures, and returned to the control level 6 h after +Gz exposures. Na+ and water contents increased significantly 1 h and 6 h after the +10 Gz exposures. There was no significant change in Ca2+ content after +Gz exposures.

CONCLUSIONS

Three +10 Gz (3 min each) exposures were associated with transient depression of brain metabolism as indicated by a decrease in ATP, Na+-K+-ATPase activity, and an accumulation of lactate, and disturbance of ion homeostasis. It is suggested that a causal relationship might exist between repeated high +Gz exposures and brain metabolism.

[Heat stress-induced HSP70 expression in heart and vessels of simulated weightless rats]

To examine the effect of simulated weightlessness on inducible HSP70 expression in the heart and vessel tissues of rats, a tail-suspension rat model was used to simulate weightlessness. HSP72 mRNA and HSP70 expression in heart and vessel tissues of both simulated weightless and control rats exposed to heat stress (ambient temperature, Ta = 43 degrees C) and recovered at Ta of 25 degrees C for 1 h (CON-H1, SUS-H1) or 2 h (CON-H2, SUS-H2) were analyzed using Northern blot and Western blot. The expression of HSP72 mRNA in the myocardium significantly decreased in SUS-H2, as compared with that of CON-H2 rats. The amount of HSP72 in the myocardium tended to decrease in both SUS-H1 and SUS-H2 groups, as compared with the corresponding control groups, but the differences were not statistically significant. The levels of inducible HSP70 expression in the vessels were related with their anatomical locations, for that the expression of both HSP72 mRNA and HSP72 significantly increased in basilar arteries, whereas it showed a slight decline in femoral arteries. The blunted HSP70 expression in myocardium suggests that simulated weightlessness may induce myocardial changes similar to those in aging. However, the HSP70 expression changes in arteries are in accord with the trend of differential adaptation changes in vessels to simulated weightlessness.

Quantitative ultrastructure of synapses on functionally identified primary afferent neurons in the cat trigeminal mesencephalic nucleus

Though a number of studies have reported the presence of synapses on neurons in the trigeminal mesencephalic nucleus (Vmes), there have been no quantitative studies of either the density of innervation, or the ultrastructure, of the synapses on single, physiologically identified neurons in this nucleus. In this study we recorded from single neurons in the Vmes, identified them as being either muscle spindle afferents (MS) or periodontal ligament mechanoreceptor afferents (PL), and then labeled the neurons by intra-axonal injection of horseradish peroxidase (HRP). The material was first processed to reveal the HRP activity, following which ultrathin sections through the labeled somata were cut and examined under the electron microscope. Complete serial reconstructions were made through the soma of one MS neuron and one PL neuron, and the contacts on the neurons reconstructed. Boutons were found on the soma, spines, appendages and the axon hillock and the initial segment of the axon. The numbers of boutons terminating on the two neurons were 198 (PL) and 424 (MS), giving a packing density of 4.4 and 10.7 boutons respectively (i.e., number of boutons/100 micron 2 of the postsynaptic membrane). Boutons could be separated into two types on the basis of their vesicles: those containing clear, round vesicles (i.e., S-type) and those containing a mixture of round, oval and flattened vesicles (P-type). Ninety-five (PL neuron) and 99% (MS neuron) of terminals on the two neurons were P-type. All the S-type boutons and 80% of the P-type boutons formed asymmetric synaptic contacts while 10% of the P-type boutons made symmetric contacts. Quantitative measurements of the P-type boutons on the labeled neurons, in which the data of MS and PL neurons were pooled, revealed that bouton volume was highly correlated with bouton surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume. However, comparing the quantitative measurements of the P-type boutons with those of previously reported vibrissa afferent terminals and their associated axon terminals revealed that all the parameters were smaller for the P-type boutons (on Vmes neurons) than those of the vibrissa afferent terminals but similar to those of axon terminals presynaptic to the vibrissa afferents. Taken together, our results emphasize the wide scope for synaptic interactions in the Vmes and suggest that it may be more fruitful to view the Vmes as an integrating center.

[Biomedical problems of artificial gravity: overview and challenge]

The limitations of the currently used exercise-based countermeasures and the scientific rationale for the gravity-based countermeasures are addressed. Then, an overview of the physiological effects and requirements for the two types of artificial gravity (AG), i.e., the continuous AG provided by full-time spinning of the entire space system/habitat and the intermittent AG by incorporating a short-arm centrifuge into the spacecraft, is presented. Finally, significance and challenge faced in future research on biochemical problems of artificial gravity are discussed.

[Peripheral effector mechanism hypothesis on cardiovascular dysfunction after spaceflight]

In the years of 1990's, we systematically studied the adaptational changes in structure and function of both the heart and the vessels during simulated weightlessness. In our serial work, the tail-suspension rat model was used to simulate the microgravity-induced cephalad shift and redistribution of blood. On the basis of the facts we observed and the more recent advances in space and ground-based studies in 1990's, we put forward a hypothesis to offer a possible explanation for the frequent occurrence of postflight cardiovascular dysfunction. It states that, in addition to the factor of hypovolemia, the microgravity-induced adaptational changes in the structure and function of the two main effectors of the cardiovascular system, i.e., the arterial smooth muscle and the cardiac muscle might be one of the most important mechanisms accounting for postflight cardiovascular dysfunction.

Effects of depressed myocardial contractility induced by microgravity on cardiovascular response to orthostatic stress: a computer simulation

The aim of present study is to investigate the role played by the depression of myocardial contractility in the mechanism of cardiovascular deconditioning and orthostatic intolerance (OI) induced by space weightlessness. Based on our previous model, which was used to simulate cardiovascular response to lower body negative pressure (LBNP), we incorporated the factor of changes of myocardial contractility into the model by multiplying a coefficient to the time-varying elastance that represents the changes of cardiac contractility. By decreasing the coefficient progressively, we simulated the changes of heart rate (HR), blood pressure (BP), and cardiac output (CO) during LBNP after 0-30% of myocardial contractility depression combined with 12% decrease of the total blood volume. Simulation results indicate that depressed myocardial contractility induces more augment of HR, and more decrement of BP and CO during LBNP and suggest that the depression of myocardial contractility degenerate cardiovascular response to orthostatic stress.

Fos protein expression in the medulla oblongata and changes in size of spinal lateral horn neurons after 4-wk simulated weightlessness in rats

Responses of the neurons in medulla oblongata and C8-T1 spinal cord lateral horn of rats induced by simulated weightlessness were investigated using anti-Fos protein and anti-tyrosine hydroxylase (TH) double staining immunohistochemical methods, and Nissl-staining technique respectively. After four weeks of tail-suspension, many Fos-like positive neurons were localized in the medullary visceral zone (MVZ), predominantly in the nucleus of tractus solitarii and ventrolateral medulla, and some of them showed TH-like immunoreactivity. Sizes of the cell bodies of the lateral horn neurons in C8-T1 segment were significantly increased in 4-wk tail-suspended rats (P<0.05) as compared with that in controls. The results suggest that the neurons in MVZ and the spinal lateral horn may be involved in the adaptation of central cardiovascular regulation during weightlessness.

[Counteracting effects of intermittent head-up tilt on simulated-weightlessness induced atrophy of anti-gravity muscles]

Objective. To study the efficacy of intermittent + Gz (45 degrees head-up tilt, HUT) exposures in preventing or alleviating atrophic changes in hind limb muscles induced by simulated weightlessness. Method. Male Sprague-Dawley (SD) rats were assigned randomly to one of three groups: simultaneous control (CON), simulated weightlessness (SUS), and SUS plus 6 h/d HUT (SUS + HUT). Muscles examined included soleus (SOL), medial gastrocnemius (correction from grastrocnemius) (MG), lateral gastrocnemius (LG) and extensor digitorum longus (EDL). Sections were treated with an adenosinetriphosphatase (ATPase) stain or alkaline phosphatase stain. The cross-sectional areas (CSA) of fibers, the relative proportion of type I fiber and the ratio of capillaries/fibers (C/F) were measured using Leica image analysis system. Result. Compared with CON, the wet weight of hind limb muscles in SUS were significantly reduced. The changes of wet weight in different groups were various. The C/F ratios of all muscles were significantly reduced. SUS + HUT rats showed significant increases in SOL and MG wet weight, and the relative counter-effects of intermittent HUT were 93.4% and 34.8%, respectively. In SUS + HUT group, the CSA of both type I and II fibers and relative proportion of type I fibers were completely recovered in SOL, and partially recovered in MG, while the counter-effects were much less obvious in the fibers of LG and EDL. However, HUT resulted in a significant recovery of the C/F ratios in all muscles. Conclusion. The present study demonstrated that intermittent HUT is effective in counteracting the atrophy induced by simulated weightlessness. The result that reactivity to HUT varied among different muscles suggests that the intermittent artificial gravity should be complemented with other countermeasures.

[Age dependency of heart rate variability, blood pressure variability and baroreflex sensitivity]

Objective. To compare the cardiovascular autonomic regulatory function between young and middle-aged male subjects and to assess the effects of aging. Method. Spectral indices of short term heart rate variability (HRV), systolic blood pressure variability (SBPV), and baroreflex sensitivity (BRS) in both supine and upright positions were obtained by AR spectral analysis and the sequential method, respectively, for both groups. Result. HRV spectral indices in both supine and upright, and the BRS in supine were lower in the middle-aged subjects than in the young subjects (P< 0.05, or P< 0.01). From supine to upright position total power (TP), low frequency power (LF) and high frequency power (HF) of HRV spectrum did not show any significant changes in the middle-aged subjects, whereas in the young subjects typical changes were observed. SBPV spectral indices did not show significant differences between the two groups in both supine and upright positions. However, SBPV HF was an exception, being lower in supine position in the middle-aged than in the young subjects (P< 0.01). For the young subjects, in supine position, BRS was correlated with LF and HF of HRV, respectively, and the spectral indices of HRV in supine position were also correlated with their corresponding indices in upright position. For the middle-aged subjects these correlations did not exist. Conclusion. In the middle-aged, both HRV and BRS are reduced and the correlations between HRV indices and BRS are not existent. However, the short-term blood pressure variability seems to be not age-dependent. It suggests that different regulatory mechanisms might account for it and that under resting condition the feed forward regulation from HRV to SBPV might be unimportant.

[Effects of intermittent +45 degrees head-up-tilt on mechanical parameters of femoral (correction of femural) bone in tail-suspended rats]

Objective. To study the effects of different intermittent +45 degrees head-up-tilt (IHUT) on tail-suspended rats. Method. 28 male SD rats were randomly and equally divided into: control group (C), tail-suspended group (S), IHUT 2 h group (H2), and IHUT 4 h group (H4). On the basis of tail-suspension to simulate weightlessness, H rats were given 2 h or 4 h IHUT each day. The effects of 3 wk IHUT on tail-suspended rats were evaluated by measuring physical and mechanical parameters of the femur. Result. Femoral mass (fresh, dry and ash), diameter, and density (fresh, except H2 group) in S, H2 and H4 rats were declined significantly compared to C rats (P<0.01 or P<0.05). Diameter (P<0.05) and density of femur (P<0.01) in H2 rats were significantly improved compared to S rats. Compared to C rats, the strength and stiffness of femora were obviously weakened (P<0.01) in S rats. Elastic load in H2 rats improved markedly (P<0.05) than those in S group; in H4 group, elastic load, maximum load, and bending toughness coefficient improved significantly (P< 0.01 or P<0.05). Conclusion. Using IHUT, mechanical parameters of weight bearing bones of tail-suspended rats can be improved apparently, by extending the exposure time, mechanical parameters can be improved.

Are learning approaches and thinking styles related? A study in two Chinese populations

This article presents the results of an investigation of the construct validity of J. B. Biggs's (1987) theory of learning approaches and of R. J. Sternberg's (1988) theory of thinking styles in two Chinese populations. The study is also an examination of the nature of the relations between the two theories. University students from Hong Kong (n = 854) and from Nanjing, mainland China (n = 215), completed the Study Process Questionnaire (J. B. Biggs, 1992) and the Thinking Styles Inventory (R. J. Sternberg & R. K. Wagner, 1992). Results indicated that both inventories were reliable and valid for assessing the constructs underlying their respective theories among both Hong Kong and Nanjing university students. Results also showed that the learning approaches and thinking styles are related in the hypothesized ways: The surface approach was hypothesized to be positively and significantly correlated with styles associated with less complexity, and negatively and significantly correlated with the legislative, judicial, liberal, and hierarchical styles. The deep approach was hypothesized to be positively and significantly correlated with styles associated with more complexity, and negatively and significantly correlated with the executive, conservative, local, and monarchic styles. Implications of these relations are discussed.

[A simulated study of effects of simulated hypovolemia on cardiovascular response to orthostatic stress]

Objective. To study the effects of hypovolemia on cardiovascular response to orthostatic stress, and to investigate the role of hypovolemia in the mechanism of cardiovascular deconditioning and orthostatic intolerance induced by space weightlessness. Method. The effects of loss of blood volume had been incorporated in the sub-model of blood redistribution in the model developed for simulating the cardiovascular response to lower body negative pressure (LBNP). With the help of the model, we simulated the changes of heart rate (HR) and blood pressure (BP) during LBNP after 0-25% loss of blood volume. Result. When the amount of decrease of blood volume was less than 5% of the total blood volume, HR and BP could be maintained in normal range during LBNP through baroreflex regulation. When the amount of the decrease of blood volume was more than 15% of the total blood volume, HR and BP could be kept in normal range when the subject was supine and at rest. But BP fell sharply and the cardiovascular system almost collapsed during orthostatic exposure. Conclusion. Decrease of blood volume causes significant degradations of cardiovascular response to orthostatic stress.

Validity of the Learning Process Questionnaire with students of lower academic attainment

The Learning Process Questionnaire was administered to 692 secondary school students (308 boys, 380 girls, and 4 unspecified) in the Philippines. The ages of the participants ranged from 14 to 22 years, with an average of 15 yr. An exploratory factor-analysis procedure was conducted for the entire sample, for the lower achievers (n = 333), and for higher achievers (n = 359). The factor structure of the Learning Process Questionnaire was uninterpretable with data obtained from students of lower academic achievement.