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

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) microg/l, P = 0.019] and secretory rate [CON: 0.68 (0.29) > EX: 0.48 (0.23) microg/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 (approximately 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.