Effects of the Head-Down Tilt Posture on Postnatal Development of the Aortic Baroreflex in the Rabbit.

Yamasaki, Masao; Shimizu, Tsuyoshi

doi:10.2170/jjphysiol.52.149

Cited by 10 publications

(16 citation statements)

References 23 publications

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“…Similarly, an attenuated gain of body weight during HDT was found in previous experiments performed without hind-limb unloading on rabbits [35].…”

Section: Discussionsupporting

confidence: 82%

Does Chronic Experimental Head-Down Tilt Alter Intramural Innervation Density of Limb Blood Vessels?

Lorant¹,

Raffai²,

Nádasy³

et al. 2005

JJP

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Earlier, substantial increases in the intramural sympathetic innervation density of rat hind-limb blood vessels were found after 2 weeks of experimental orthostasis with tubular 45° headup tilt cages. In the present study, we presumed that chronic head-down tilting induces opposite changes in the innervation density. Tilted rats were kept 45° head-down in long tubular cages for either 2 or 4 weeks (HDT2, HDT4), and the control animals were maintained in horizontal tilt cages for the same period (HOR2, HOR4). Segments of the saphenous and brachial veins and arteries were used for quantitative electron microscopic examinations. Intramural innervation density was defined by nerve terminal density (NTD) and synaptic microvesicle count (SVC) within the vascular Our previous work, applying a novel chronic 45°head-up tilt model of orthostasis [1,2], revealed several long-term vascular mechanisms supporting orthostatic tolerance in the hind limbs [3]. These mechanisms include an enhancement of the acute-pressure-induced myogenic response of saphenous vein with increases in smooth muscle cell count and in passive vessel diameter without much change in the wall thickness [1,[4][5][6]. Chronic head-up tilt also influences the microcirculatory network properties. A significant rarefaction of microvessels was found in the hind-limb oxidative skeletal muscles of rat after two weeks of orthostasis [7]. Branching angles decreased, and vessel diameters increased in the superficial venous system of the saphenous region of rats in response to long-term orthostasis [8]. The amount of electron-dense vesicles of ap- adventitia. Neither HDT2 nor HDT4 resulted in a decrease of NTD or SVC of the saphenous and brachial veins or arteries; instead, a tendency to increase was observed in some cases. Thus, in contrast to the large increases we found earlier in hind-limb vascular innervation density after 2 weeks of head-up tilting, head-down tilting of the same duration-or even twice as long-did not decrease the adventitial innervation density in our model. We assume that the quasi-free locomotor exercise the tilted animals in the long tubular cages were allowed may counteract a possible suppressive effect of chronic head-down tilt on hind-limb vascular innervation density. [The Japanese Journal of Physiology 55: [127][128][129][130][131][132][133][134] 2005] Key words: brachial vein, head-down tilt, saphenous artery, saphenous vein, vascular innervation.proximately 0.1-0.2 µm diameter (which we identified in the endothelium cells of rat saphenous vein) greatly decreased during two weeks of orthostatic body position [9,10].After 2 weeks of 45° head-tilting, among the most interesting observations we made in this model of orthostasis was a surprisingly large increase (nearly double) in the adventitial (intramural) sympathetic innervation density of hind-limb blood vessels, especially the veins [2]. This increase included both the density of nerve terminals (NTD) and the number of synaptic microvesicles in the terminals (SVC). The...

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“…Similarly, an attenuated gain of body weight during HDT was found in previous experiments performed without hind-limb unloading on rabbits [35].…”

Section: Discussionsupporting

confidence: 82%

Does Chronic Experimental Head-Down Tilt Alter Intramural Innervation Density of Limb Blood Vessels?

Lorant¹,

Raffai²,

Nádasy³

et al. 2005

JJP

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“…Thoren and colleagues investigated the activation of the baroreceptor C-fibers (unmyelinated fibers) in rats and rabbits, and their results suggested that the unmyelinated fibers in the aortic depressor nerve are stimulated by changes in intramural stress rather than stretch (1999). We suggested that the reduction in intramural stress might be caused by the narrow range of arterial pressure alterations as a consequence of the absence of gravitational potential energy and hydrostatic pressure gradients in any vessel under the conditions of microgravity in space (Yamasaki and Shimizu, 2002). Therefore, the decrease in number of unmyelinated fibers, which mainly discharge at high-pressure levels, probably occurs in accordance with a reduction in intramural stress under microgravity during space flight.…”

Section: Discussionmentioning

confidence: 90%

“…We have suggested that there is a high possibility that the conditions of microgravity reduce the role of the aortic depressor nerve and the baroreceptor reflex in circulatory regulation. This hypothesis is based on the results obtained by our headdown tilt experiment that simulates the headward shift of body fluid under microgravity Yamasaki and Shimizu, 2002). To confirm this possibility, we carried out space experiments to study the aortic baroreceptor reflex in rat neonates using the Space Shuttle Columbia (STS-90, Neurolab mission), and demonstrated a reduced number of unmyelinated fibers in the aortic depressor nerve (Yamasaki et al, 1999a).…”

Section: Introductionmentioning

confidence: 88%

“…Specimens were fixed continuously by immersion in 1% solution of paraformaldehyde and glutaraldehyde (pH 7.4 with 0.12 M PBS) for 2 days and post-fixed in a solution of 1% OsO 4 dissolved in PBS for 2 hours after a 1-hour wash with 10% dextrose PBS. Subsequent histological processing for the electron microscopic examination was carried out in part using an auto tissue processor (RX-90; Rotex, Australia) according to our modified method (Yamasaki and Shimizu, 2002). A series of thin cross-sections (60−80 nm) in each aortic depressor nerve were cut with an ultra-microtome (MT-X; RMC, USA) and were corrected by bringing them into contact with the polyvinyl formal (Formvar) membrane attached to a single grid (1 x 2 mm).…”

Section: Electron Microscopic Analysismentioning

confidence: 99%

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Effects of Space Flight on the Histological Characteristics of the Aortic Depressor Nerve in the Adult Rat: Electron Microscopic Analysis

et al. 2004

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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 (µg, 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 µg 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 µg 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 highthreshold 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.

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“…In our research group, postnatal development of the baroreflex system has been studied under ordinary gravitational conditions [9] and simulated microgravity conditions such as head-down tilt (HDT) [21, 22] and parabolic flight [22, 23]. Yamasaki and Shimizu [24] showed previously in 3-4-week-old rabbits raised in HDT posture in a simulated μ G environment for 34–36 days that the number of unmyelinated fibers of the left aortic nerve was significantly reduced compared to the control rabbits, which suggested that development of the aortic baroreflex sensitivity was depressed by exposure to HDT posture. It is thus possible that a similar phenomenon is observed during spaceflight in neonate animals.…”

Section: Introductionmentioning

confidence: 99%

Spaceflight Affects Postnatal Development of the Aortic Wall in Rats

Katsuda

Yamasaki

Waki

et al. 2014

BioMed Research International

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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.

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Effects of the Head-Down Tilt Posture on Postnatal Development of the Aortic Baroreflex in the Rabbit.

Cited by 10 publications

References 23 publications

Does Chronic Experimental Head-Down Tilt Alter Intramural Innervation Density of Limb Blood Vessels?

Does Chronic Experimental Head-Down Tilt Alter Intramural Innervation Density of Limb Blood Vessels?

Effects of Space Flight on the Histological Characteristics of the Aortic Depressor Nerve in the Adult Rat: Electron Microscopic Analysis

Spaceflight Affects Postnatal Development of the Aortic Wall in Rats

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