Nocturnal growth hormone secretory dynamics are altered after resistance exercise: deconvolution analysis of 12-hour immunofunctional and immunoreactive isoforms

1 Military Performance Division, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts; 2 Human Performance Laboratory, Department of Kinesiology, University of Connecticut, Storrs, Connecticut; 3 Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania; and 4 Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania Submitted 6 December 2005 ; accepted in final form 5 July 2006 To characterize the effects of daytime exercise on subsequent overnight growth hormone (GH) secretion and elimination dynamics, serum was sampled, and GH was measured every 10 min for 12 h (1800 to 0600) in a control (CON) condition and after a 50-set resistance exercise protocol (EX) from 1500 to 1700. GH was measured with a conventional immunoreactive (IR) and an immunofunctional (IF) assay, and values were analyzed via a multi-parameter deconvolution analysis. EX resulted in a higher overnight secretory burst frequency [CON: 7.6 (SD 2.4) < EX: 9.4 (2.2) bursts per 12 h, P = 0.005] but lower mean burst mass [CON: 9.2 (4.7) > EX: 6.0 (2.9) µg/l, P = 0.019] and secretory rate [CON: 0.68 (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 (1.1) < IR 19.8 (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. pulsatile; somatotropin; kinetics; approximate entropy; weight lifting Address for reprint requests and other correspondence: B. C. Nindl, Military Performance Division, U.S. Army Research Institute of Environmental Medicine, Natick, MA (e-mail: bradley.nindl{at}us.army.mil )