INTRODUCTION Due to the molecular heterogeneity that growth hor- mone exhibits, much attention has been given to its measurement and evaluation 1–3 . Previous studies have reported that different concentrations from the same sample can be obtained by using immunoassays that employ antibodies of varying epitope specificities 3–7 . In an attempt to provide a consensus for growth hormone measurement, Strasburger et al. 8 developed a novel enzyme-linked immunoabsorbent assay based on the molecular interaction between the hormone and its receptor necessary for receptor dimerization and subse- quent signal transduction. This “immunofunctional” (IF) assay uses an anti-human growth hormone (hGH) mono- clonal antibody and a biotinylated recombinant hGH binding protein (hGHBP) that bind, respectively, to hGH receptor binding sites 2 and 1, which is the first step in initiating the cascade of intracellular signalling events for hGH. Strasburger et al. 8 reported that this assay had a higher correlation than a conventional polyclonal assay when compared to the in vitro NB2 bioassay, lend- ing credence to the notion that this assay may be more Immunofunctional vs immunoreactive growth hormone responses after resistance exercise in men and women Bradley C. Nindl 1,3,4 , William J. Kraemer 1,3,5 and Wesley C. Hymer 1,2 1 Intercollege Graduate Program in Physiology, 2 Department of Biochemistry and Molecular Biology, 3 Laboratory for Sports Medicine, The Pennsylvania State University, University Park, PA 16801, USA, 4 Military Performance Division, United States Army Research Institute of Environmental Medicine, Natick, MA 01760, USA and 5 The Human Performance Laboratory, Ball State University, Muncie, IA, USA Summary Immunoassays for growth hormone (GH) may yield variable concentrations for the same sample due to the molecular heterogeneity of growth hormone and epitope specificity of their antibodies. Strasburger et al. developed an “immunofunctional” assay that only detects those GH molecules possessing intact sites 1 and 2, which are necessary for inducing receptor dimerization and subsequent signal transduction. This study compared the immunoreactive (IR) vs immunofunctional (IF) GH concentrations before and after acute resistance exercise (i.e. six sets of 10 repetition maximum squats separated by 2 min rest periods) in 8 men and 6 women. IF concentrations were determined by an ELISA DSL and IR GH by a monoclonal IRMA Nichols . Both men (M) and women (W) demonstrated similar increases for IR (M: 1.47 vs 25.0 ng/ml; W: 4.0 vs 25.4 ng/ml) and IF (M: 0.55 vs 11.66 ng/ml; W: 1.94 vs 10.41 ng/ml) GH following exercise. Post-exercise IF GH was significantly less than IR GH for both M and W. The ratio of IR/IF after exercise was ~2 and similar for both M and W. In summary, dynamic exercise elicited a similar rise in M and W for immunofunctionally active GH molecules, but the magnitude is lower than when detected with another conventional assay. Key words: somatotropin, hormone responses, immunoassays, gender differences. Received 14 February 2000 Revised 19 April 2000 Accepted 26 April 2000 Correspondence to: Bradley C. Nindl PhD, Military Performance Division, United States Army Research Institute of Environmental Medicine, Natick, MA 01760, USA. Tel: 508-233-5382: Fax: 508-233-4195; E-mail: Bradley.Nindl@NA.AMEDD.Army.Mil 1096–6374/00/020099+05 $35.00/0 © 2000 Harcourt Publishers Ltd © 2000 Harcourt Publishers Ltd Growth Hormone & IGF Research 2000, 10, 99–103 doi:10.1054/ghir.2000.0146, available online at http://www.idealibrary.com on