Endocrine relationships during human spaceflight.

Human spaceflight is associated with a chronic loss of protein from muscle. The objective of this study was to determine whether changes in urinary hormone excretion could identify a hormonal role for this loss. Urine samples were collected from the crews of two Life Sciences Space Shuttle missions before and during spaceflight. Data are means ± SE with the number of subjects in parentheses. The first value is the mean preflight measurement, and the second value is the mean inflight measurement. Adrenocorticotropic hormone (ACTH) [27.7 ± 4.4 (9) vs. 25.1 ± 3.4 (9) ng/day], growth hormone [724 ± 251 (9) vs. 710 ± 206 (9) ng/day], insulin-like growth factor I [6.81 ± 0.62 vs. 6.04 ± 0.51 (8) nM/day], and C-peptide [44.9 ± 8.3 (9) vs. 50.7 ± 10.3 (9) μg/day] were unchanged with spaceflight. In contrast, free 3,5,3'-triiodothyronine [791 ± 159 (9) vs. 371 ± 41 (9) pg/day, P < 0.05], prostaglandin E2(PGE2) [1,064 ± 391 (8) vs. 465 ± 146 (8) ng/day, P < 0.05], and its metabolite PGE-M [1,015 ± 98 (9) vs. 678 ± 105 (9) ng/day, P < 0.05] were decreased inflight. The urinary excretion of most hormones returned to their preflight levels during the postflight period, with the exception of ACTH [47.5 ± 10.3 (9) ng/day], PGE2 [1,433 ± 327 (8) ng/day], PGF2α, [2,786 ± 313 (8) ng/day], and its metabolite PGF-M [4,814 ± 402 (9) ng/day], which were all increased compared with the preflight measurement ( P < 0.05). There was a trend for urinary cortisol to be elevated inflight [55.3 ± 5.9 (9) vs. 72.5 ± 11.1 μg/day, P = 0.27] and postflight [82.7 ± 8.6 (8) μg/day, P = 0.13]. The inflight human data support ground-based in vitro work showing that prostaglandins have a major role in modulating the changes in muscle protein content in response to tension or the lack thereof.

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