Changes in pituitary growth hormone cells prepared from rats flown on Spacelab 3

McCall, G. E., C. Goulet, R. E. Grindeland, J. A. Hodgson, A. J. Bigbee, and V. R. Edgerton. Bed rest suppresses bioassayable growth hormone release in response to muscle activity. J. Appl. Physiol. 83(6): 2086–2090, 1997.—Hormonal responses to muscle activity were studied in eight men before (−13 or −12 and −8 or −7 days), during (2 or 3, 8 or 9, and 13 or 14 days) and after (+2 or +3 and +10 or +11 days) 17 days of bed rest. Muscle activity consisted of a series of unilateral isometric plantar flexions, including 4 maximal voluntary contractions (MVCs), 48 contractions at 30% MVC, and 12 contractions at 80% MVC, all performed at a 4:1-s work-to-rest ratio. Blood was collected before and immediately after muscle activity to measure plasma growth hormone by radioimmunoassay (IGH) and by bioassay (BGH) of tibia epiphyseal cartilage growth in hypophysectomized rats. Plasma IGH was unchanged by muscle activity before, during, or after bed rest. Before bed rest, muscle activity increased ( P < 0.05) BGH by 66% at −13 or −12 days (2,146 ± 192 to 3,565 ± 197 μg/l) and by 92% at −8 or −7 days (2,162 ± 159 to 4,161 ± 204 μg/l). After 2 or 3 days of bed rest, there was no response of BGH to the muscle activity, a pattern that persisted through 8 or 9 days of bed rest. However, after 13 or 14 days of bed rest, plasma concentration of BGH was significantly lower after than before muscle activity (2,594 ± 211 to 2,085 ± 109 μg/l). After completion of bed rest, muscle activity increased BGH by 31% at 2 or 3 days (1,807 ± 117 to 2,379 ± 473 μg/l; P < 0.05), and by 10 or 11 days the BGH response was similar to that before bed rest (1,881 ± 75 to 4,160 ± 315 μg/l; P < 0.05). These data demonstrate that the ambulatory state of an individual can have a major impact on the release of BGH, but not IGH, in response to a single bout of muscle activity.

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confidence: 58%

Bed rest suppresses bioassayable growth hormone release in response to muscle activity

McCall¹,

Goulet²,

Grindeland

et al. 1997

“…Perspective on BGH regulation and microgravity. Several rat studies have reported that spaceflight or hindlimb unloading decreases BGH secretion by pituitary somatotrophs in vitro and in vivo (15)(16)(17)(18)(19). In addition, human studies have shown that the normal exercise-induced plasma BGH increases that occur at 1 G are inhibited by spaceflight (23) or bed rest (22).…”

Section: Discussionmentioning

confidence: 99%

“…This response of BGH to exercise was inhibited by chronic muscle unloading during 17 days of bed rest (22) or spaceflight (23) and returned within 2-4 days of reloading. A variety of in vitro and in vivo data from rats have also shown that the release of BGH from the pituitary was inhibited by 14 days of hindlimb unloading (18) or by 7-14 days of spaceflight (15,18,19). Moreover, our laboratory recently demonstrated in rats that muscle afferent activity stimulated pituitary BGH release into the circulation and proposed that a muscle afferent-pituitary axis can modulate BGH release (11)(12)(13).…”

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confidence: 99%

Muscle afferent activity modulates bioassayable growth hormone in human plasma

McCall¹,

Grindeland

Roy

et al. 2000

Immunoassayable and bioassayable growth hormone responses to vibration-induced activation of muscle spindle afferents of the soleus (Sol) or tibialis anterior (TA) muscles were studied in 10 men. Subjects were supine while a 10-min vibration stimulus (100 Hz; 1.5-mm amplitude) was applied to the muscle, with each of the muscles tested on separate days. Blood samples were collected before, during, immediately after, and after 5 and 10 min of vibration. Plasma growth hormone concentrations were determined by radioimmunoassay (IGH) for all sampling periods and by bioassay (BGH; measurement of tibial epiphysial cartilage growth in hypophysectomized rats) for samples obtained before and immediately after vibration. Plasma IGH concentrations were similar at all time points during the Sol or TA experiments. After 10 min of muscle vibration, mean plasma BGH was elevated 94% [1,216 +/- 148 (SD) to 2, 362 +/- 487 microg/l; P = 0.0001] for TA and decreased 22% (1,358 +/- 155 to 1,058 +/- 311 microg/l; P = 0.09) for Sol. These data demonstrate that activation of TA muscle spindle afferents increases circulating BGH but not IGH. The absence of a similar vibration-induced BGH response for the Sol indicates a differential regulation of BGH release by these two predominantly slow muscles, perhaps related to their respective flexor and extensor functions. These data indicate that a muscle afferent-pituitary axis modulates the release of BGH, but not IGH, from the pituitary in humans and that this axis is muscle specific, similar to that observed in rats.

“…Chronic unloading of the musculature achieved by 17 days of spaceflight (29) or bed rest (28) eliminated the normal BGH response to exercise in humans, with recovery of the response occurring after 2-4 days of reambulation. In rats, hindlimb unloading (22) or spaceflight (21)(22)(23) depressed pituitary BGH release in both in vivo and in vitro experiments. Furthermore, our laboratory demonstrated that a single bout (15 min) of intermittent electrical stimulation of large-diameter, low-threshold fibers from predominantly fast-twitch ankle extensor muscles induced BGH release in anesthetized rats (19), whereas stimulation of these afferents from a predominantly slow-twitch ankle extensor inhibited BGH secretion (18).…”

mentioning

confidence: 93%

Vibration-induced activation of muscle afferents modulates bioassayable growth hormone release

Gosselink¹,

Roy

Zhong

et al. 2004

The effects of tendon vibration on bioassayable growth hormone (BGH) secretion from the pituitary gland were investigated in anesthetized adult male rats. The tendons from predominantly fast-twitch ankle extensor muscles (gastrocnemius and plantaris) or a predominantly slow-twitch ankle extensor (soleus) were vibrated by using a paradigm that selectively activates group Ia afferent fibers from muscle spindles. The lower hindlimb was secured with the muscles near physiological length, and the tendons were vibrated for 15 min at 150 Hz and a displacement of 1 mm. Control rats were prepared similarly, but the tendons were not vibrated. Compared with control, vibration of the tendons of the fast ankle extensors markedly increased (160%), whereas vibration of the slow soleus decreased (68%), BGH secretion. Complete denervation of the hindlimb had no independent effects on the normal resting levels of BGH, but it prevented the effects of tendon vibration on BGH secretion. The results are consistent with previous findings showing modulation of BGH release in response to in vivo activation or in situ electrical stimulation of muscle afferents (Bigbee AJ, Gosselink KL, Grindeland RE, Roy RR, Zhong H, and Edgerton VR. J Appl Physiol 89: 2174-2178, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, and Edgerton VR. J Appl Physiol 88: 142-148, 2000; Gosselink KL, Grindeland RE, Roy RR, Zhong H, Bigbee AJ, Grossman EJ, and Edgerton VR. J Appl Physiol 84: 1425-1430, 1998). These data provide evidence that this previously described muscle afferent-pituitary axis is neurally mediated via group Ia afferents from peripheral skeletal muscle. Furthermore, these data show that activation of this group Ia afferent pathway from fast muscles enhances, whereas the same sensory afferent input from a slow muscle depresses, BGH release.