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This Article Full Text Free Full Text (PDF) Free All Versions of this Article: 100/3/1037    most recent 00615.2005v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Bigbee, A. J. Articles by Edgerton, V. R. Search for Related Content PubMed PubMed Citation Articles by Bigbee, A. J. Articles by Edgerton, V. R.

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

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

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