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

Abstract

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.

Influence of oral contraceptive use on growth hormone in vivo bioactivity following resistance exercise: Responses of molecular mass variants

“…The lack of an acute increase in GH follows our previous work on the GH response to acute resistance exercise using the rat tibial line bioassay [7][8][9]. However, these results differ from the work of McCall and co-workers [10,11] who twice reported that unilateral plantar flexion exercise increased GH as measured by the rat tibial line assay. These very different exercise protocols might explain this discrepancy in findings as they resulted in divergent cortisol responses.…”

“…AHRET caused a significant increase in circulating cortisol [4,6] which is an inhibitor of bone growth [23]. In contrast, the protocol used by McCall et al [10] induced either no change or a decrease in cortisol pre-to post-exercise. Alternately, gender differences might explain the discrepancy in the bioactive GH response to exercise as measured by the rat tibial line assay.…”

“…Alternately, gender differences might explain the discrepancy in the bioactive GH response to exercise as measured by the rat tibial line assay. Women exhibit higher resting GH and bioactive GH compared to men [5,7,10]. Thus, women's ability to elevate bioactive GH in the circulation with exercise might be less than that of men.…”

“…In a continuing line of research on this topic, recent studies from our laboratory [7][8][9] extended this knowledge, utilizing size exclusion chromatography to separate plasma samples by molecular weight followed by assaying with different techniques. These studies along with the results of McCall and colleagues [10,11] demonstrated the complexities of human GH metabolism following exercise, especially when comparing immunological vs. biological activity of this hormone. Contrary to previous research [10,11], we found that an acute bout of heavy resistance exercise failed to increase circulating bioactive GH as measured by the rat tibial line bioassay.…”

“…These studies along with the results of McCall and colleagues [10,11] demonstrated the complexities of human GH metabolism following exercise, especially when comparing immunological vs. biological activity of this hormone. Contrary to previous research [10,11], we found that an acute bout of heavy resistance exercise failed to increase circulating bioactive GH as measured by the rat tibial line bioassay. In contrast, low (<30 kD) and mid (30-60 kD) molecular weight fractions responded to immunoassays following acute exercise stimulus.…”