Spaceflight alters the fiber composition of the aortic nerve in the developing rat

Changes in blood pressure, blood flow towards the head and heart rate during 90 deg head-up tilting for 30 min in anaesthetized male rats

NEW FINDINGS

What is the central question of this study? How does the baroreceptor reflex contribute to systemic blood pressure (BP) control during prolonged 90 deg head-up tilting (HUT) in unconscious rats? What is the main finding and its importance? In intact rats, heart rate (HR) increased after the transition to upright by HUT, and BP was maintained in this posture throughout the 30 min experimental period. After sinoaortic denervation, which results in lack of discharge of the baroreflex afferents, HR and BP decreased gradually during 30 min of 90 deg HUT, suggesting that the baroreceptor reflex is an important factor for maintenance of BP during long-term upright posture. Changes in cardiovascular parameters during prolonged 90 deg head-up tilting (HUT) in animals have not been elucidated in detail. We clarified changes in systemic blood pressure (BP), blood flow towards the head (BF) and heart rate (HR) and the role of the baroreceptor reflex after a transition from the supine posture to 90 deg HUT for 30 min in anaesthetized rats (n = 13). In control rats with the baroreceptor reflex afferents intact, mean BP and BF after the onset of 90 deg HUT decreased significantly by -15.4 ± 5.9 and -26.2 ± 11.5% at 2.9 ± 1.1 s (mean ± SD, n = 12), respectively, compared with control values and then immediately increased and steadied at 30.7 ± 13.1 s (plateau; -2.8 ± 8.5% in BP and -17.5 ± 17.4% in BF compared with control values; BP was maintained during 90 deg HUT). After acute sinoaortic denervation in seven rats, initial reductions in BP and BF after 90 deg HUT were observed at 3.9 ± 1.0 s, similar to the reductions in the nerve-intact rats; the percentage changes from control were -19.2 ± 3.7 and -32.3 ± 8.4%, respectively. These parameters reached a plateau at 22.4 ± 5.8 s at -8.6 ± 7.7 and -29.5 ± 15.0%, respectively, and then BP decreased gradually throughout 90 deg HUT. Heart rate increased slightly after 90 deg HUT in nerve-intact rats, but after sinoaortic denervation this increase disappeared and HR decreased gradually during 90 deg HUT. These results suggest that the baroreceptor reflex contributes to the maintenance of adequate BP during long-term 90 deg HUT.

Hindlimb unloading: rodent analog for microgravity

The rodent hindlimb unloading (HU) model was developed in the 1980s to make it possible to study mechanisms, responses, and treatments for the adverse consequences of spaceflight. Decades before development of the HU model, weightlessness was predicted to yield deficits in the principal tissues responsible for structure and movement on Earth, primarily muscle and bone. Indeed, results from early spaceflight and HU experiments confirmed the expected sensitivity of the musculoskeletal system to gravity loading. Results from human and animal spaceflight and HU experiments show that nearly all organ systems and tissues studied display some measurable changes, albeit sometimes minor and of uncertain relevance to astronaut health. The focus of this review is to examine key HU results for various organ systems including those related to stress; the immune, cardiovascular, and nervous systems; vision changes; and wound healing. Analysis of the validity of the HU model is important given its potential value for both hypothesis testing and countermeasure development.

Seasonal oxidative capacity of skeletal muscles in hibernating Daurian ground squirrels (Spermophilus dauricus)

We investigated the mechanism of high oxidative capacity of skeletal muscles in hibernating Daurian ground squirrels (Spermophilus dauricus Brandt, 1843). Myoglobin (Mb) levels, as well as citrate synthase and lactate dehydrogenase (LDH) activities, were measured by spectrophotometry. Mb content in the soleus (SOL) muscle lasted from the beginning of hibernation to spring. Mb content in SOL was 87% higher in the hibernating group than in the summer group. Mb content in the extensor digitorum longus (EDL) muscle stayed at similar levels during the different periods of the year. Citrate synthase activity in SOL was 30% higher in the hibernating group than in the summer group. Meanwhile, citrate synthase activity in EDL did not change during hibernation. LDH activity in SOL was not different between the hibernating group and the summer active group, whereas LDH activity in EDL increased significantly (up to 11%) in the 2 days arousal after hibernation group compared with the hibernating group. We conclude that high oxidative capacity is provided by increased oxygen storage capacity of slow-twitch muscle fibers rather than from fast-twitch muscle fibers in hibernating animals.

The research on the formation mechanism of extraordinary oxidative capacity of skeletal muscle in hibernating ground squirrels (Spermophilus dauricus)

BACKGROUND

Previous studies indicate that hibernating animals, under conditions of torpor for long periods, show an increased oxidative muscle fibers (type I) ratio and a decreased glycolytic muscle fibers (type II) ratio in skeletal muscle and accompanied by extraordinary oxidative ability. This observation is completely contrasted with non-hibernators, which show a shift of oxidative muscle fibers (type I) to glycolytic muscle fibers (type II). Presently, the mechanisms by which these changes occur remain unclear. To investigate the mechanism of high oxidative capacity of the skeletal muscles in hibernating ground squirrels, capillary density (CD), and capillary/fiber (C/F) were measured by immunohistochemistry. mRNA expression levels of hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) were determined using real-time quantitative PCR assay. Spectrophotometry was applied to determine the activities of hexokinase (PK), pyruvate kinase (HK), and cytochrome c oxidase (CcO).

RESULTS

Inthe soleus muscle (SOL), mRNA expression levels of HIF-1αandVEGF in torpor became slightly lower but were not statistically significant; they were, however, significantly higher in the arousal group. In hibernating animals, no significant change occurred in CD but C/F increased by 15 %. CcO showed the highest activity in torpor. There were no significant differences in the activities of HK and PK between the torpid animals and summer active animals in SOL. However, PK activity increased by 34 % after hibernation.

CONCLUSIONS

Oxidative capacitymay be ensured by an increase of capillary supply of skeletal muscle in hibernating animals.

Spaceflight affects postnatal development of the aortic wall in rats

We investigated effect of microgravity environment during spaceflight on postnatal development of the rheological properties of the aorta in rats. The neonate rats were randomly divided at 7 days of age into the spaceflight, asynchronous ground control, and vivarium control groups (8 pups for one dam). The spaceflight group rats at 9 days of age were exposed to microgravity environment for 16 days. A longitudinal wall strip of the proximal descending thoracic aorta was subjected to stress-strain and stress-relaxation tests. Wall tensile force was significantly smaller in the spaceflight group than in the two control groups, whereas there were no significant differences in wall stress or incremental elastic modulus at each strain among the three groups. Wall thickness and number of smooth muscle fibers were significantly smaller in the spaceflight group than in the two control groups, but there were no significant differences in amounts of either the elastin or collagen fibers among the three groups. The decreased thickness was mainly caused by the decreased number of smooth muscle cells. Plastic deformation was observed only in the spaceflight group in the stress-strain test. A microgravity environment during spaceflight could affect postnatal development of the morphological and rheological properties of the aorta.

An afferent explanation for sexual dimorphism in the aortic baroreflex of rat

Sex differences in baroreflex (BRx) function are well documented. Hormones likely contribute to this dimorphism, but many functional aspects remain unresolved. Our lab has been investigating a subset of vagal sensory neurons that constitute nearly 50% of the total population of myelinated aortic baroreceptors (BR) in female rats but less than 2% in male rats. Termed "Ah," this unique phenotype has many of the nonoverlapping electrophysiological properties and chemical sensitivities of both myelinated A-type and unmyelinated C-type BR afferents. In this study, we utilize three distinct experimental protocols to determine if Ah-type barosensory afferents underlie, at least in part, the sex-related differences in BRx function. Electron microscopy of the aortic depressor nerve (ADN) revealed that female rats have less myelin (P < 0.03) and a smaller fiber cross-sectional area (P < 0.05) per BR fiber than male rats. Electrical stimulation of the ADN evoked compound action potentials and nerve conduction profiles that were markedly different (P < 0.01, n = 7 females and n = 9 males). Selective activation of ADN myelinated fibers evoked a BRx-mediated depressor response that was 3-7 times greater in female (n = 16) than in male (n = 17) rats. Interestingly, the most striking hemodynamic difference was functionally dependent upon the rate of myelinated barosensory fiber activation. Only 5-10 Hz of stimulation evoked a rapid, 20- to 30-mmHg reduction in arterial pressure of female rats, whereas rates of 50 Hz or higher were required to elicit a comparable depressor response from male rats. Collectively, our experimental results are suggestive of an alternative myelinated baroreceptor afferent pathway in females that may account for, at least in part, the noted sex-related differences in autonomic control of cardiovascular function.

Human vagal baroreflex mechanisms in space

Although astronauts' cardiovascular function is normal while they are in space, many have altered haemodynamic responses to standing after they return to Earth, including inordinate tachycardia, orthostatic hypotension, and uncommonly, syncope. Simulated microgravity impairs vagal baroreceptor-cardiac reflex function and causes orthostatic hypotension. Actual microgravity, however, has been shown to either increase, or not change vagal baroreflex gain. In this study, we tested the null hypothesis that spaceflight does not impair human baroreflex mechanisms. We studied 11 American and two German astronauts before, during (flight days 2-8), and after two, 9- and 10-day space shuttle missions, with graded neck pressure and suction, to elicit sigmoid, vagally mediated carotid baroreflex R-R interval responses. Baseline systolic pressures tended to be higher in space than on Earth (P = 0.015, repeated measures analysis of variance), and baseline R-R intervals tended to be lower (P = 0.049). Baroreceptor-cardiac reflex relations were displaced downward on the R-R interval axis in space. The average range of R-R interval responses to neck pressure changes declined from preflight levels by 37% on flight day 8 (P = 0.051), maximum R-R intervals declined by 14% (P = 0.003), and vagal baroreflex gain by 9% (P = 0.009). These measures returned to preflight levels by 7-10 days after astronauts returned to Earth. This study documents significant increases of arterial pressure and impairment of vagal baroreflex function in space. These results and results published earlier indicate that microgravity exposure augments sympathetic, and diminishes vagal cardiovascular influences.

Chronic inhibition of standing behaviour alters baroreceptor reflex function in rats

AIM

To investigate whether daily orthostatic stress during development is an important factor affecting arterial baroreceptor reflex function, we examined the effect of chronic inhibition of upright standing behaviour on the baroreceptor reflex function in rats.

METHODS

Upright standing behaviour was chronically inhibited during the developmental period between 3 and 8 weeks of age in Sprague-Dawley rats and heart rate (HR) and aortic nerve activity in response to increased and decreased mean arterial pressure (MAP) was measured after the treatment period.

RESULTS

The baroreceptor cardiac gain in the rats grown without standing behaviour was significantly lower than the control rats grown in a normal commercial cage (1.0 +/- 0.1 beats min(-1) mmHg(-1) vs. 1.6 +/- 0.2 beats min(-1) mmHg(-1), P < 0.05). The range of HR change in the MAP-HR functional curve was also lowered by chronic inhibition of orthostatic behaviour (56.2 +/- 5.9 beats min(-1)) compared with that of the control rats (76.8 +/- 6.9 beats min(-1), P < 0.05). However the aortic afferent function remained normal after the treatment period, indicating that the attenuated baroreceptor reflex function may be due to other mechanisms involving functional alterations in the cardiovascular centres, efferents and/or peripheral organs. Body weight and adrenal weight were not affected by the inhibition of orthostatic behaviour, suggesting that the animals were not exposed to specific stress by this treatment.

CONCLUSION

These results indicate that active haemodynamic changes induced by orthostatic behaviour are an important factor for setting the basal level of reflex function during development. Moreover, our experimental model may be useful for studying mechanisms of attenuated baroreceptor reflex observed after exposure to a chronic inactive condition.

Baroreflexes of the rat. V. Tetanus-induced potentiation of ADN A-fiber responses at the NTS

In a long-term neuromuscular blocked (NMB) rat preparation, tetanic stimulation of the aortic depressor nerve (ADN) enhanced the A-fiber evoked responses (ERs) in the cardiovascular region, the nucleus of the solitary tract (dmNTS). The potentiation persisted for at least several hours and may be a mechanism for adaptive adjustment of the gain of the baroreflex, with functional implications for blood pressure regulation. Using a capacitance electrode, we selectively stimulated A-fibers and acquired a stable 10-h "A-fiber only" ER baseline at the dmNTS. Following baseline, an A+C-fiber activating tetanus was applied to the ADN. The tetanus consisted of 1,000 "high current" pulses (10 trains; 300 mus, 100 Hz, 1 s), with intertrain interval of 9 s. A 10-h A-fiber only posttetanic test phase repeated the stimulus pattern of the baseline. Fourteen tetanus experiments were done in 12 rats. Compared with the baseline before tetanus, the A-fiber ER magnitudes of posttetanus hours were larger [F(13, 247) = 3.407, P < .001]; additionally, the 10-h posttetanus magnitude slopes were more positive than during 10 h before tetanus (df = 13; t = -3.47; P < 0.005); thus, an ADN A+C fiber-activating tetanus produced increases in the magnitude of the A-fiber ERs in the dmNTS that persisted for several hours. In an additional rat, application of an NMDA receptor antagonist, prior to the tetanus, blocked the potentiation effect. The stimulus protocols, magnitude and duration of the effect, and pharmacology resemble associative long-term potentiation (LTP).

Effects of spaceflight on postnatal development of arterial baroreceptor reflex in rats

AIM

It has been reported that spaceflight attenuates the arterial baroreceptor reflex. As this reflex function changes dramatically during postnatal development, we hypothesized that space flight depresses the developmental changes of the reflex system. To test this hypothesis, we evaluated the baroreceptor reflex function in rats, which were exposed to a microgravity environment on a space shuttle 9-25 days after birth.

METHODS

Baroreceptor reflex sensitivity and the afferent sensitivity were evaluated by measuring heart rate (HR) and aortic nerve activity (ANA) changes in response to an increase in mean arterial pressure (MBP) derived by phenylephrine injection (20-50 microg kg(-1)) under urethane-anaesthesia.

RESULTS

Baroreceptor reflex sensitivity (% change of HR/% change of MBP) was lower in the flight group (FLT: -0.19 +/- 0.04, n = 4) than either the asynchronous ground control group (AGC: -0.47 +/- 0.06, n = 6, P < 0.01) or the vivarium group (VIV: -0.41 +/- 0.07, n = 6, P < 0.05). This was similar to the differences of the afferent sensitivity (% change of ANA/% change of MBP) between FLT (2.07 +/- 0.30) and the control groups (AGC: 2.71 +/- 0.22, n.s.; VIV: 3.00 +/- 0.32, P < 0.05). At the end of 30 days of recovery under normal gravity conditions, however, there were no significant group differences in these parameters. conclusion: These results suggest that the space environment attenuates the postnatal development of the arterial baroreceptor reflex function in rats, which may be partially because of a depression of the postnatal development of the baroreceptor afferents. These functional alterations, however, recover to their normal level on re-exposure to the Earth's gravity.

Effects of space flight on the histological characteristics of the aortic depressor nerve in the adult rat: electron microscopic analysis

The effects of microgravity on the histological characteristics of the aortic depressor nerve, which is the afferent of the aortic baroreflex arc, were determined in 10 female adult rats. The rats were assigned for nursing neonates in the Space Shuttle Columbia or in the animal facility on the ground (NASA Neurolab, STS-90), and were housed for 16 days under microgravity in space (microg, n=5) or under one force of gravity on Earth (one-g, n=5). In the Schwann cell unit in which the axons of unmyelinated fibers are surrounded by one Schwann cell, the average number of axons per unit in the microg group was 2.1 +/- 1.6 (mean +/- SD, n=312) and significantly less than that in the one-g group (3.0 +/- 2.9, n=397, p<0.05). The proportion of unmyelinated fibers in the aortic depressor nerve in the microg group was 64.5 +/- 4.4% and significantly less than that in the one-g group (74.0 +/- 7.3%, p<0.05). These results show that there is a decrease in the number of high-threshold unmyelinated fibers in the aortic depressor nerve in adult rats flown on the Shuttle Orbiter, suggesting that the aortic baroreflex is depressed under microgravity during space flight.