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1 Military Performance Division, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts, United States
2 Human Performance Laboratory, Department of Kinesiology, University of Connecticut, Storrs, Connecticut, United States
3 Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania, United States
4 Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania, United States
* To whom correspondence should be addressed. E-mail: bradley.nindl{at}us.army.mil.
To characterize the effects of daytime exercise on subsequent overnight growth hormone (GH) secretion and elimination dynamics, serum was sampled and GH measured every 10 min for 12 h (1800 to 0600 h) in a control (CON) condition and following a 50-set resistance exercise protocol (EX) from 1500 to 1700 h. GH was measured with a conventional immunoreactive (IR) assay and an immunofunctional (IF) assay and values were utilized in multi-parameter deconvolution analysis. EX resulted in a higher overnight secretory burst frequency (CON: 7.6 (SD 2.4) < EX: 9.4 (SD 2.2) bursts per 12 h, P = 0.005) but lower mean burst mass (CON: 9.2 (SD 4.7) > EX: 6.0 (SD 2.9) µg/L, P = 0.019) and secretory rate (CON: 0.68 (SD 0.29) > EX: 0.48 (0.23) µg/L/min; P = 0.015; ANOVA main effect means presented). Approximate entropy (ApEn) was greater after EX, indicating a less orderly GH release process than CON. The estimated half-life of IF GH was significantly lower than IR GH (IF: 15.3 (SD 1.1) < IR 19.8 (SD 1.6) min, P < 0.001) but similar between the CON and EX conditions (~17 min). Despite the changes in secretory dynamics, 12-h mean and integrated GH concentrations were similar between conditions. The results suggest that although quantitatively similar total amounts of GH are secreted overnight in CON and EX conditions, resistance exercise alters the dynamics of secretion by attenuating burst mass and amplitude yet increasing burst frequency.
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