Journal of Applied Physiology Millar Instruments
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

J Appl Physiol 100: 1037-1042, 2006. First published December 8, 2005; doi:10.1152/japplphysiol.00615.2005
8750-7587/06 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
100/3/1037    most recent
00615.2005v1
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bigbee, A. J.
Right arrow Articles by Edgerton, V. R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Bigbee, A. J.
Right arrow Articles by Edgerton, V. R.

Basal and evoked levels of bioassayable growth hormone are altered by hindlimb unloading

A. J. Bigbee,1 R. E. Grindeland,4 R. R. Roy,3 H. Zhong,2 K. L. Gosselink,2 S. Arnaud,4 and V. R. Edgerton1,2,3

Departments of 1Neurobiology and 2Physiological Science, and 3Brain Research Institute, University of California Los Angeles, Los Angeles; and 4Life Science Division, National Aeronautics and Space Administration Ames Research Center, Moffett Field, California

Submitted 24 May 2005 ; accepted in final form 1 December 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Bioassayable growth hormone (BGH) in rats is released in large quantities from the pituitary in response to the activation of large, proprioceptive afferent fibers from fast and mixed fiber-type hindlimb musculature. We hypothesized that hindlimb unloading (HU) of adult male rats would 1) reduce the basal levels of plasma BGH, and 2) abolish stimulus-induced BGH release. Rats were exposed to HU for 1, 4, or 8 wk. Plasma and pituitaries were collected under isoflurane anesthesia for hormone analyses. Additionally, at 4 and 8 wk, a subset of rats underwent an in situ electrical stimulation (Stim) of tibial nerve proprioceptive afferents. Basal plasma BGH levels were significantly reduced (–51 and –23%) after 1 and 8 wk of HU compared with ambulatory controls (Amb). Although Amb-Stim rats exhibited increased plasma BGH levels (88 and 143%) and decreased pituitary BGH levels (–27 and –22%) at 4 and 8 wk, respectively, stimulation in HU rats had the opposite effect, reducing plasma BGH (–25 and –33%) and increasing pituitary BGH levels (47 and 10%) relative to HU alone at 4 and 8 wk. The 22-kDa form of GH measured by immunoassay and the plasma corticosterone, T3, T4, and testosterone levels were unchanged by HU or Stim at all time points. These data suggest that BGH synthesis and release from the pituitary are sensitive both to chronically reduced neuromuscular loading and to acute changes in neuromuscular activation, independent of changes in other circulating hormones. Thus BGH may play a role in muscle, bone, and metabolic adaptations that occur in response to chronically unloaded states.

spaceflight; central nervous system; activity-dependence; proprioception; plasma hormones

CHRONIC UNLOADING OF THE musculoskeletal system during spaceflight elicits numerous physiological adaptations including altered muscle (7, 23) and bone (3) properties, as well as changes within the central nervous system (CNS) (4, 6, 23), each of which has an impact on the health, well-being, and performance of astronauts. The effects of actual and simulated microgravity on circulating hormones and metabolic factors also have been characterized (17, 22). In particular, growth hormone (GH) is of interest because of its role in modulating muscle, bone, and metabolic properties, both normally and in response to gravitational unloading.

Recent experiments from our laboratories (2, 9–11, 18–20) suggest that a growth-promoting, pituitary-derived peptide measured using a tibial growth assay (12), referred to as bioassayable growth hormone (BGH), differs from the 22- and 20-kDa forms of growth hormone measured immunologically (IGH). Previous experiments in rats showed that BGH can be released in large quantities in response to acute, low-threshold electrical stimulation of hindlimb fast or fast-slow mixed muscle compartment nerves, such as the tibial nerve (9, 10), acute tendon vibration of predominantly fast muscles (11), or an acute bout of treadmill exercise (2). Similarly, in humans, BGH is released in response to vibration of the tibialis anterior (TA) (20) or an acute plantarflexion exercise (18, 19). Furthermore, BGH is unique in its acute sensitivity to the activation of proprioceptive afferents. Although plasma BGH levels are increased by the stimuli listed above, they are unchanged in response to cutaneous nerve stimulation (9) and are significantly reduced in response to stimulation of a nerve innervating a predominantly slow muscle, such as the soleus (9), in rats. In all of those studies, plasma IGH levels were unchanged.

Because load-related sensory input is modulated in response to spaceflight exposure, we hypothesized that the basal levels of plasma BGH would be reduced in response to chronic hindlimb unloading (HU) and that the previously observed BGH release induced by the nerve stimulation protocol (9, 10) would be blunted after 4 and 8 wk of HU. Therefore, the objectives of the present study were 1) to examine the physiological modulation of pituitary and plasma BGH and other plasma hormones to chronic simulated microgravity using an HU model at normogravity (1G) (32) and 2) to characterize the acute BGH response to low-threshold electrical stimulation of the tibial nerve after 4 and 8 wk of HU. These data are novel in that, whereas mammalian endocrine responses to microgravity have been examined (14, 21, 30, 31), and both pituitary IGH and BGH content and release modulation have been reported in vivo and in vitro in response to short-term spaceflight (13, 15, 16, 28), GH, thyroid hormones [triiodothyronine (T3) and thyroxine (T4)], corticosterone, and testosterone levels have not been reported in rats after exposure to extended periods (i.e., up to 8 wk) of unloading.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Experimental design and HU procedures.   Adult Sprague-Dawley male rats (420 ± 20 g; 4 mo of age) were housed individually in cages used specifically for HU (36 x 36 x 42 cm with Plexiglas walls and a grid floor) and given 1 wk to acclimate to those surroundings before beginning the study. The rats were kept on a 12:12-h light-dark cycle, in a room maintained at 24 ± 1°C. The hindlimbs of the HU rats were unloaded by using the technique described by Wronski and Morey-Holton (32). Briefly, the hindlimbs were suspended via a swivel track, such that the rats had mobility around the entire cage and free access to food and water. Care was taken to prevent the hindpaws from contacting the bottom or the sides of the cage. The rats were fed PMI Rat Diet (PMI Feeds, St. Louis, MO). The rats in the ambulatory control group (Amb; n = 8/time point) were pair-fed for the duration of the study, i.e., their food volume intake was restricted to that of each previous day for the HU rats (HU, n = 8 at 1 and 4 wk; n = 9 at 8 wk), so that nutrient intake remained consistent between groups. All animal experiments were carried out under National Institutes of Health guidelines and were approved by the National Aeronautics and Space Administration (NASA) Ames Research Center Animal Care and Use Committee.

Peripheral nerve stimulation protocol.   After 4 and 8 wk of HU, a subset of both Amb (Amb-Stim; n = 4) and HU (HU-Stim; n = 4 at 4 wk, n = 5 at 8 wk) rats underwent low-threshold electrical stimulation of the tibial nerve, using an activation pattern that simulated walking at 1.5 mph, as described in detail previously (9, 10). All of the rats at the 4 and 8 wk time points, regardless of whether they received the Stim procedure, were deeply anesthetized with isoflurane (2.5%), and the tibial nerve in one hindlimb was exposed and severed and the proximal end of the nerve placed on a bipolar electrode for 15 min. In the Stim rats, the proximal nerve trunk was stimulated in situ via the bipolar silver electrode by using a stimulator (Grass-Telefactor, West Warwick, RI). The stimulation paradigm was a 20-µs square wave pulse at a train frequency of 100 Hz (150 ms on-150 ms off, for 15 min) at a current strength that was two times the threshold required to elicit a visible reflex response. These procedures have been shown by Gosselink et al. (9, 10) to preferentially activate proprioceptive, i.e., muscle spindle and Golgi-tendon organ, afferents.

Sample collection.   At the 1-wk time point, HU and Amb rats were deeply anesthetized with isoflurane, and blood was collected via cardiac puncture into heparinized tubes. In the 4- and 8-wk experiments, blood was collected in the same manner immediately (i.e., within 1–2 min) after the termination of the 15-min Stim or sham-Stim protocol in the Amb, Amb-Stim, HU, and HU-Stim rats. Because the Stim procedure was not performed at 1 wk, the total time for anesthetic exposure was lower (<5 min) than in the 4- and 8-wk groups (~30 min). Comparisons were therefore made only between groups within a time point, but not across time points. Individual blood samples were collected into heparinized tubes and centrifuged at 4°C at 1,000 g for 25 min, and 1-ml plasma aliquots were taken for hormone measurement by radioimmunoassay (RIA). The remaining plasma was pooled by group at each time point for BGH analysis to provide a large enough volume for the in vivo tibial bioassay. The pituitaries were collected and the adenohypophyses were pooled by group and frozen at –80°C until further analyses of IGH and BGH, at which time the pituitaries were thawed and weighed, homogenized in a small amount of 0.01 M Na2CO3, and brought to stock dilution of 2 mg tissue/ml with 0.85% NaCl. Pooled samples represent the overall mean BGH value for each experimental group, i.e., each pooled sample from each experimental group was assayed in five bioassay rats, thus providing a probabilistic assessment of the difference between one pooled sample and another.

Hormone analyses.   The tibial bioassay method of Greenspan et al. (12) was used to determine plasma and pituitary BGH levels. Female rats were hypophysectomized (Hilltop Labs, Scotdale, PA) at 26 days of age and shipped to NASA Ames Research Center 3 days later. The rats were housed at 28 ± 1°C, on a 12:12-h light-dark cycle, and fed standard rat diet. The rats were given 1 wk to acclimate before beginning the assay. Bioassay control groups (n = 5/group) received intraperitoneal injections of a GH reference standard (bovine GH XIV-44-C5; 1.5 U/mg; 0-, 5-, 15-, or 45-µg total dose) purified from bovine pituitary extracts to obtain a standard curve (2). Experimental treatment groups (n = 5/group) received intraperitoneal injections of either the pooled plasma or pituitary homogenates from the Amb, Amb-Stim, HU, or HU-Stim rats. Each bioassay rat received an injection of 0.5 ml of GH standard, plasma, or pituitary homogenate/day for 4 days. The pituitary samples were diluted at two dose levels per experimental sample (0.67 and 0.22 mg tissue total dose). On the 5th day, the bioassay rats were killed via an overdose of carbon dioxide. The left tibia was removed from each rat, split longitudinally, and rinsed in 0.9% NaCl, followed by defatting in acetone and a final rinse in water. The bones were stained in 2.5% AgNO3 for 2.5 min to define the borders of the epiphyseal plate. An ocular micrometer mounted on a light microscope was used to make 10 width measurements along the length of each epiphysis, and the measurements were averaged for each rat. A standard curve was obtained by plotting the tibial widths from the control groups on the y-axis vs. their respective GH reference doses on the x-axis (2). Once the average tibial widths were determined for each experimental sample, the BGH values were obtained from the standard curve and are expressed in terms of rat GH (3.0 U/mg).

Plasma and pituitary IGH were measured using a competitive binding RIA (27) with a rat GH standard (VII-38-C, 3 U/mg) that was purified and iodinated in house. The primary antibody was monkey anti-rat GH (1:65,000 working dilution), with goat anti-monkey gamma globulin (1:20) as the secondary antibody. In addition to BGH and IGH, plasma corticosterone, T3, T4, and testosterone levels were determined. Corticosterone was measured with a double-antibody RIA kit (ICN Biomedicals, Costa Mesa, CA), and T3, T4, and testosterone were analyzed with solid-phase RIA kits (Diagnostic Products, Los Angeles, CA). IGH, corticosterone, T3, T4, and testosterone were assayed in duplicate. The inter- and intra-assay variations were ~6 and ~3% for IGH and <10% for the plasma hormones analyzed with commercially available kits.

Statistical methods.   HU- and/or Stim-induced changes in hormone levels were compared with those of their respective age-matched Amb controls. Statistical analyses of plasma hormone measurements other than BGH were determined by using a one-way ANOVA to determine overall differences, followed by a Tukey-Kramer post hoc test to identify group differences (Analyse-it Software, Leeds, UK), with P < 0.05 considered significant. Differences in pituitary BGH (2 dose levels) were determined by a four-point assay procedure, whereas differences in plasma BGH (1 dose level) were determined by a bracketed three-point assay method (29). Data are presented as means ± SE.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Basal plasma BGH levels were 51 and 23% lower in HU than Amb rats at 1 and 8 wk, respectively, (P < 0.05; Fig. 1A), whereas pituitary BGH content was twofold higher in HU than Amb rats at 4 wk (P < 0.05; Fig. 1B). Basal levels of plasma and pituitary IGH, and plasma corticosterone, T3, T4, and testosterone levels (Table 1) were unchanged in response to 1, 4, or 8 wk of HU relative to those of age-matched Amb rats.


Figure 1
View larger version (12K):
[in this window]
[in a new window]
  		Fig. 1. Plasma (A) and pituitary bioassayable growth hormone (BGH; B) concentrations after 1, 4, and 8 wk of hindlimb unloading (HU). Values are means ± SE. *Significantly different from ambulatory control (Amb) at the same time point, P < 0.05.

 

View this table:
[in this window]
[in a new window]
  		Table 1. Plasma and pituitary IGH, and plasma corticosterone, T3, T4, and testosterone levels in ambulatory and hindlimb-unloaded rats

 
The nerve Stim protocol was performed after 4 and 8 wk of HU. This paradigm has been shown to induce BGH release in Amb rats (9, 10), a phenomenon that was confirmed in the present study [Fig. 2; compare Amb (–) Stim vs. Amb (+) Stim] at both 4 and 8 wk. Plasma BGH levels were 1.9- and 2.4-fold greater in Amb-Stim than Amb rats at 4 and 8 wk, respectively (P < 0.05; Fig. 2A), with concomitant reductions in pituitary BGH levels (25.5 vs. 35, and 44.5 vs. 56.5 µg/mg tissue, at 4 and 8 wk, respectively; P < 0.05; Fig. 2B). These changes are consistent with previous observations (9, 10). In contrast, nerve stimulation had the opposite effect on the BGH response in HU-Stim rats, where plasma BGH levels were reduced by 25 and 33% relative to HU rats at 4 and 8 wk, respectively (P < 0.05; Fig. 2A), and pituitary BGH levels were increased by 32% at 4 wk (P < 0.05) and slightly increased (9%; P > 0.05) at 8 wk (Fig. 2B). Importantly, the acute stimulation paradigm had no effect on plasma or pituitary IGH, or on plasma corticosterone, T3, T4, or testosterone levels in Amb or HU rats at either 4 or 8 wk (Table 1).


Figure 2
View larger version (13K):
[in this window]
[in a new window]
  		Fig. 2. Chronic unloading affects the plasma (A) and pituitary (B) BGH responses to stimulation of the proprioceptive afferents in the tibial nerve. The lines are used to illustrate the directional trends between the nonstimulated [(–) Stim] and stimulated [(+) Stim] groups. Values are means ± SE. *Significantly different from the same group (–) Stim at the same time point at P < 0.05.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Rodent basal endocrine response to HU.   The first objective of this study was to determine the effects of 1, 4, or 8 wk of chronic HU on the rodent endocrine system, namely plasma and pituitary BGH and IGH, and plasma corticosterone, T3, T4, and testosterone levels. These hormonal responses have not been reported previously for such prolonged periods of hindlimb unloading in rats. The basal levels of plasma and/or pituitary BGH, but not IGH, corticosterone, T3, T4, or testosterone, were differentially altered after 1, 4, or 8 wk of HU.

It should be noted that the Amb data at 1 wk were markedly increased relative to Amb plasma BGH levels at 4 and 8 wk (Fig. 1A). We attribute this to differences in anesthetic exposure time and therefore made comparisons only between groups within a given time point. With respect to HU-induced changes, a consistent reduction in plasma BGH was observed at all three time points, although not significantly at 4 wk. Surprisingly, BGH pituitary levels in HU rats were unchanged relative to Amb rats at 1 and 8 wk, whereas they were significantly higher at 4 wk, a result for which we have no explanation. It is not inconceivable that there could be an ongoing multiphasic response indicative of dynamic endocrine adaptations over the 8-wk period of HU. For example, at the 1-wk time point, gravitational loading-related stimuli normally associated with signaling BGH synthesis and release are only recently absent, thus plasma BGH levels are reduced, as are pituitary BGH levels, although to a lesser degree. At 4 wk, the pituitary may have increased BGH levels in response to the chronically altered loading state, possibly owing in part to increased BGH synthesis, and/or reduced BGH release. It is feasible that the increased pituitary BGH levels prevented a significant reduction in HU BGH plasma levels. Finally, at 8 wk, the HU pituitary BGH levels are similar to those in Amb rats, suggesting that synthesis continues. Plasma levels, however, are reduced, indicating a possible deficit in the BGH release mechanisms. The overall trend for reduced plasma and increased pituitary BGH at 4 and 8 wk is consistent with data from ~2-wk spaceflight and HU data from pituitary cells in vitro (15).

Combined, the hormone data suggest that the observed reduction in basal plasma BGH levels is not a part of a generalized endocrine response to HU. Furthermore, the absence of changes in the other hormones measured suggests that they are not directly involved in the modulation of pituitary BGH synthesis or release during chronic HU, at least under the conditions of the present experiment. The absence of an effect of HU on the IGH levels (Table 1) is in agreement with previous HU and spaceflight data. Plasma IGH levels were similar in vivarium control, spaceflight, and HU rats in the 14-day COSMOS 2044 mission (21) and in control and spaceflight rats in the 12.5-day COSMOS 1887 mission (14). However, considerable variability in rodent and human IGH data between studies has been noted previously and may be based, in part, on a variety of factors including reentry phenomena, e.g., hypergravity and/or stress, or the period of reambulation before sample collection (30).

BGH release via hindlimb nerve stimulation is abolished after chronic HU.   The second objective of this study was to determine whether the low-threshold stimulation-induced release of BGH known to occur in normally ambulating rats (9, 10) would be altered by chronic HU. The present results clearly show that, in the Amb-Stim situation, pituitary levels were depleted by ~30%, and plasma BGH was doubled relative to Amb non-Stim. Our interpretation of these control data is that the tibial nerve afferent stimulus in some way signals the pituitary to release large quantities of BGH into the plasma. The opposite occurred, however, in the HU-Stim group, where pituitary BGH levels were acutely increased in response to Stim and plasma levels decreased. Thus it appears that the rat hindlimb proprioceptive afferent-pituitary axis is altered after 4 and 8 wk of HU in a manner that inhibits activity-dependent BGH release from the tibial nerve. The plasma BGH response in HU-Stim rats is consistent with recent studies in human subjects, where a short bout of plantarflexion exercise elicited BGH release in normally ambulating human subjects at 1G, but not in subjects who exercised during ~2 wk of bed rest (18) or spaceflight (19). Combined, these data suggest that activity-dependent BGH release is sensitive to chronic musculoskeletal unloading.

One plausible interpretation of the HU-Stim BGH data is that the acute Stim paradigm indeed remained effective in signaling the pituitary to manufacture BGH, resulting in acutely increased pituitary BGH levels. However, the concomitant decrease in plasma BGH levels suggests that the pituitary in the physiologically unloaded state may be compromised in its capacity to release BGH. This is feasible, as growth hormone releasing factor (GRF) levels in the arcuate nucleus of the hypothalamus are reduced after 2 wk of spaceflight (26), although it is unknown whether the same occurs at more prolonged periods of HU at 1G. Even if Stim-induced release was abolished relative to Amb levels, one might expect that basal release would still continue, resulting in constant plasma BGH levels between the Stim and non-Stim conditions. However, this did not occur. It also may be possible that the Stim protocol on the unloaded system influences BGH bioactivity, so that it may be released from the pituitary in a different form from that detected by the tibial bioassay. Speculation as to this type of modification to BGH bioactivity has been previously proposed in in vitro pituitary studies in spaceflight (15). If this were the case, the increased pituitary levels of BGH would suggest that changes to the molecule occurred after BGH release.

Potential CNS sites involved in HU-related BGH modulation.   The exact mechanisms for the neuromuscular-mediated release of BGH are unknown. We previously proposed a spinally mediated pathway, which could possibly involve hypothalamic GRF and somatostatin (SS) that facilitate and inhibit IGH secretion, respectively, because these regulatory factors possess the capability to modulate BGH (24, 25). Although GRF and SS immunoreactivity were previously shown to be reduced in the median eminence and preproGRF mRNA and SS in the arcuate nucleus of male rats of similar age to those in the present study after 14 days of spaceflight (26), no changes were observed in GRF or SS after 2 wk of HU (26), and no study to date has measured these hypothalamic factors at longer time points of unloading. Thus, although we cannot conclusively determine whether they played a role in the altered BGH levels observed after the extended time points of HU in the present study, their potential role in BGH synthesis and release warrants future consideration.

Changes in the pituitary ultrastructure also may be involved in BGH regulatory mechanisms. Shellenberger et al. (28) proposed that when dense secretory granules, where BGH is thought to predominantly reside, are exposed to a gravitational perturbation, BGH packaging and release mechanisms may be altered, along with changes in intracellular signaling pathways. Our data are consistent with this proposed mechanism. The elevated pituitary BGH levels in the 4-wk HU and HU-Stim groups relative to the Amb group (Fig. 2B) suggest that BGH continues to be manufactured in the presence of altered afferent signals related to hindlimb loading. However, the concomitant decrease in plasma BGH levels (Fig. 2A, compare 4-wk HU and HU-Stim) suggests that it is not being released from the pituitary. Combined, these observations suggest that the mechanism(s) for BGH release may be subject to modulation by gravitational unloading. BGH synthesis also may be affected by HU. Although the increased pituitary BGH levels in 8-wk HU-Stim relative to HU is not as pronounced as that observed at 4 wk, this may be due to ongoing physiological adaptations over time, possibly to unloading and/or aging-related phenomena.

Finally, given that BGH release can be mediated by neuromuscular afferent activity, changes in the properties of the neuromuscular afferents themselves likely contributed to its reduced release in response to afferent stimulation after chronic HU. Consistent with this, soleus and TA electromyographic activities are known to be altered during HU (1), and De-Doncker et al. (5) recently showed altered firing properties of Ia afferents in response to soleus muscle stretch after 14 days of HU. Most likely, it is a combination of HU-related adaptations throughout the neuromuscular proprioceptive afferent-pituitary axis that results in altered processing of hindlimb afferent input to the CNS and reduced basal and stimulus-induced BGH release. Interestingly, the tibial nerve stimulation-induced response in HU rats in the present study is reminiscent of that observed in response to soleus nerve stimulation at 1G (9). It may be of interest in future studies, therefore, to determine the BGH response to stimulation of afferents from a predominantly slow muscle such as the soleus, because this muscle shows extensive atrophy and fiber-type changes in response to HU.

Physiological significance of BGH in chronic unloading.   The question remains as to the physiological significance of altered BGH modulation in response to gravitational unloading. We speculate that it may be possible that changes in BGH influence muscle and/or bone mass changes that occur in response to unloading. Clearly, BGH influences bone growth in young rats, as shown in principle by the tibial bioassay. It also may play a role in the dynamic relationship between bone maintenance and turnover that occurs during unloading. Perhaps BGH is involved in muscle growth, maintenance, or metabolism, or in determining the synaptic efficacy of the neuromuscular junction as well, as all are altered in response to gravitational unloading.

Another possibility, from the neuromuscular afferent activation standpoint, is that acute BGH release from the pituitary may be prompted during heightened activity levels (i.e., during which the tibial nerve afferents would be activated), but not during quiet postural conditions such as standing, where the soleus (i.e., soleus nerve afferents) is predominantly active, suggesting that the mechanism for afferent-induced release may be conserved to act during a situation when systemic metabolic challenge arises. The stimulation paradigm used in the present study can facilitate [via stimulation of primarily fast muscles (10)] or inhibit [via stimulation of the slow soleus muscle (9)] BGH release, suggesting that the predominant fiber type or function (i.e., flexor vs. extensor) of a muscle may have an effect on BGH release. We postulate that, because the soleus is tonically active during common postural tasks and activation of the soleus nerve inhibits BGH release in rats (9), it is not metabolically efficacious for BGH to be released continually. Rather, BGH release may be facilitated in times of acute metabolic stress and/or activation of more nonpostural muscles such as the TA. BGH, therefore, may play a role in controlling carbohydrate metabolism, mobilizing energy resources during threats to homeostasis, or preserving glucose as an energy source for the nervous system during metabolically challenged states. In support of this view, plasma BGH levels are increased in response to both fasting and cold exposure (8). Clearly, unloading-induced changes in BGH release may have significant physiological consequences for long-term spaceflight.

In summary, the present results show that chronic unloading of the hindlimbs markedly alters the neuromuscular afferent-pituitary axis response to proprioceptive signals that normally facilitate BGH release. Further examination is required to fully understand the biochemical nature and physiological significance of the load-sensitive mechanisms that modulate BGH. Such understanding may help to identify therapeutic targets toward ameliorating muscle atrophy and bone loss observed in spaceflight, as well as in chronically bed-rested persons and other individuals with chronically reduced levels of neuromuscular activity.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This study was supported by NASA grant NAG2-1188 and National Institute of Dental Research Grant DE-N0489. A. J. Bigbee was supported by NASA Graduate Student Research Program NGT2-52265.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
We offer sincere thanks to the laboratory of Dr. Charles Wade at NASA Ames Research Center for technical support and to the NASA-ARC Animal Care Support Staff. Thanks go also to the members of Dr. Allan Tobin's laboratory at UCLA for insightful comments.


   FOOTNOTES
 

Address for reprint requests and other correspondence: R. R. Roy, Univ. of California, Los Angeles, 1804 Life Science Bldg., 621 Charles E Young Dr., Los Angeles, CA 90095 (e-mail: rrr{at}ucla.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

  1. Alford EK, Roy RR, Hodgson JA, and Edgerton VR. Electromyography of rat soleus, medial gastrocnemius, and tibialis anterior during hind limb suspension. Exp Neurol 96: 635–649, 1987.[CrossRef][Web of Science][Medline]
  2. Bigbee AJ, Gosselink KL, Roy RR, Grindeland RE, and Edgerton VR. Bioassayable growth hormone release in rats in response to a single bout of treadmill exercise. J Appl Physiol 89: 2174–2178, 2000.[Abstract/Free Full Text]
  3. Bikle DD, Sakata T, and Halloran BP. The impact of skeletal unloading on bone formation. Gravit Space Biol Bull 16: 45–54, 2003.[Medline]
  4. D'Amelio F, Fox RA, Wu LC, and Daunton NG. Quantitative changes of GABA-immunoreactive cells in the hindlimb representation of the rat somatosensory cortex after 14-day hindlimb unloading by tail suspension. J Neurosci Res 44: 532–539, 1996.[CrossRef][Web of Science][Medline]
  5. De-Doncker L, Picquet F, Petit J, and Falempin M. Effects of hypodynamia-hypokinesia on the muscle spindle discharges of rat soleus muscle. J Neurophysiol 89: 3000–3007, 2003.[Abstract/Free Full Text]
  6. Edgerton VR, McCall GE, Hodgson JA, Gotto J, Goulet C, Fleischmann K, and Roy RR. Sensorimotor adaptations to microgravity in humans. J Exp Biol 204: 3217–3224, 2001.
  7. Edgerton VR and Roy RR. Neuromuscular adaptations for actual and simulated spaceflight. In: Handbook of Physiology. Environmental Physiology. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 4, vol. I, chapt. 32, p. 721–763.
  8. Ellis S and Grindeland RE. Dichotomy between bio- and immunoassayable growth hormone. In: Growth Hormone and Related Peptides. New York: Elsevier, 1976, p. 75–83.
  9. Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, and Edgerton VR. Afferent input from rat slow skeletal muscle inhibits bioassayable growth hormone release. J Appl Physiol 88: 142–148, 2000.[Abstract/Free Full Text]
  10. Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, Grossman EJ, and Edgerton VR. Skeletal muscle afferent regulation of bioassayable growth hormone in the rat pituitary. J Appl Physiol 84: 1425–1430, 1998.[Abstract/Free Full Text]
  11. Gosselink KL, Roy RR, Zhong H, Grindeland RE, Bigbee AJ, and Edgerton VR. Vibration-induced activation of muscle afferents modulates bioassayable growth hormone release. J Appl Physiol 96: 2097–2102, 2004.[Abstract/Free Full Text]
  12. Greenspan FS, Li CH, Simpson ME, and Evans HM. Bioassay of hypophyseal growth hormone; the tibia test. Endocrinology 45: 455–463, 1949.
  13. Grindeland R, Hymer WC, Farrington M, Fast T, Hayes C, Motter K, Patil L, and Vasques M. Changes in pituitary growth hormone cells prepared from rats flown on Spacelab 3. Am J Physiol Regul Integr Comp Physiol 252: R209–R215, 1987.[Abstract/Free Full Text]
  14. Grindeland RE, Popova IA, Vasques M, and Arnaud SB. Cosmos 1887 mission overview: effects of microgravity on rat body and adrenal weights and plasma constituents. FASEB J 4: 105–109, 1990.[Abstract]
  15. Hymer WC, Grindeland R, Krasnov I, Victorov I, Motter K, Mukherjee P, Shellenberger K, and Vasques M. Effects of spaceflight on rat pituitary cell function. J Appl Physiol 73: 151S–157S, 1992.[Abstract]
  16. Hymer WC, Grindeland RE, Salada T, Nye P, Grossman EJ, and Lane PK. Experimental modification of rat pituitary growth hormone cell function during and after spaceflight. J Appl Physiol 80: 955–970, 1996.[Abstract/Free Full Text]
  17. Macho L, Fickova M, Nemeth S, Svabova E, Serova L, and Popova I. The effect of space flight on the board of the satellite Cosmos 2044 on plasma hormone levels and liver enzyme activities of rats. Acta Astronaut 24: 329–332, 1991.[Medline]
  18. McCall GE, Goulet C, Grindeland RE, Hodgson JA, Bigbee AJ, and Edgerton VR. Bed rest suppresses bioassayable growth hormone release in response to muscle activity. J Appl Physiol 83: 2086–2090, 1997.[Abstract/Free Full Text]
  19. McCall GE, Goulet C, Roy RR, Grindeland RE, Boorman GI, Bigbee AJ, Hodgson JA, Greenisen MC, and Edgerton VR. Spaceflight suppresses exercise-induced release of bioassayable growth hormone. J Appl Physiol 87: 1207–1212, 1999.[Abstract/Free Full Text]
  20. McCall GE, Grindeland RE, Roy RR, and Edgerton VR. Muscle afferent activity modulates bioassayable growth hormone in human plasma. J Appl Physiol 89: 1137–1141, 2000.[Abstract/Free Full Text]
  21. Merrill AH Jr, Wang E, Mullins RE, Grindeland RE, and Popova IA. Analyses of plasma for metabolic and hormonal changes in rats flown aboard COSMOS 2044. J Appl Physiol 73: 132S–135S, 1992.[Abstract]
  22. Morey-Holton ER, Whalen RT, Arnaud SB, and Van Der Meule MC. The skeleton and its adaptation to gravity. In: Handbook of Physiology. Environmental Physiology. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 4, vol. I, chapt. 31, p. 691–719.
  23. Roy RR, Baldwin KM, and Edgerton VR. Response of the neuromuscular unit to spaceflight: what has been learned from the rat model. Exerc Sport Sci Rev 24: 399–425, 1996.[Medline]
  24. Russell SM, Hughes JP, and Nicoll CS. Differential effects of hypothalamic extract on release of bioactive and immunoactive rat growth hormone in vitro. Neuroendocrinology 30: 355–361, 1980.[Medline]
  25. Russell SM, Vodian MA, Hughes JP, and Nicoll CS. Electrophoretic separation of forms of rat growth hormone with different bioassay and radioimmunoassay activities: comparison of intraglandular and secreted forms. Life Sci 23: 2373–2382, 1978.[Medline]
  26. Sawchenko PE, Arias C, Krasnov I, Grindeland RE, and Vale W. Effects of spaceflight on hypothalamic peptide systems controlling pituitary growth hormone dynamics. J Appl Physiol 73: 158S–165S, 1992.[Abstract]
  27. Schalch DS and Reichlin S. Plasma growth hormone concentration in the rat determined by radioimmunoassay: influence of sex, pregnancy, lactation, anesthesia, hypophysectomy and extrasellar pituitary transplants. Endocrinology 79: 275–280, 1966.[Abstract/Free Full Text]
  28. Shellenberger KE, Grindeland RE, and Hymer WC. Rat anterior pituitary hormone cells: responses to variable gravity. Aviat Space Environ Med 69: A37–A44, 1998.[Medline]
  29. Steele RGD and Torrie JH. Linear Regression.. New York: McGraw-Hill, 1960.
  30. Strollo F. Hormonal changes in humans during spaceflight. Adv Space Biol Med 7: 99–129, 1999.[Medline]
  31. Vasques M, Lang C, Grindeland RE, Roy RR, Daunton N, Bigbee AJ, and Wade CE. Comparison of hyper- and microgravity on rat muscle, organ weights and selected plasma constituents. Aviat Space Environ Med 69: A2–A8, 1998.[Medline]
  32. Wronski TJ and Morey-Holton ER. Skeletal response to simulated weightlessness: a comparison of suspension techniques. Aviat Space Environ Med 58: 63–68, 1987.[Medline]




This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow All Versions of this Article:
100/3/1037    most recent
00615.2005v1
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Bigbee, A. J.
Right arrow Articles by Edgerton, V. R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Bigbee, A. J.
Right arrow Articles by Edgerton, V. R.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2006 by the American Physiological Society.