Influence of oral contraceptive use on growth hormone in vivo bioactivity following resistance exercise: Responses of molecular mass variants William J. Kraemer a, * , Bradley C. Nindl b , Jeff S. Volek a , James O. Marx c , Lincoln A. Gotshalk d , Jill A. Bush e , Jill R. Welsch f , Jakob L. Vingren a , Barry A. Spiering a , Maren S. Fragala a , Disa L. Hatfield a , Jen-Yu Ho a , Carl M. Maresh a , Andrea M. Mastro f , Wesley C. Hymer f a Human Performance Laboratory, Department of Kinesiology, Department of Physiology and Neurobiology, University of Connecticut Storrs, CT 06269, USA b Military Performance Division, United States Army Research, Institute of Environmental Medicine Natick, Massachusetts 01760, USA c Department of Biology, The University of Pennsylvania, Philadelphia, PA 19104, USA d Department of Health and Physical Education, University of Hawaii at Hilo, Hilo, HI 96720, USA e Laboratory of Integrated Physiology, Department of Health and Human Performance, University of Houston, Houston, TX 77204-6015, USA f Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA Received 21 May 2007; revised 26 September 2007; accepted 5 October 2007 Available online 26 November 2007 Abstract The purpose was to examine effects of oral contraceptive (OC) use on plasma growth hormone (GH) responses to heavy resistance exercise. Sixty untrained women were placed into one of two groups: currently using OC (Ortho Tri-Cyclen Ò )(n = 25; mean ± SD: 24.5 ± 4.2 y, 160.4 ± 7.1 cm, 64.1 ± 11.3 kg) or not currently using OC (NOC) (n = 35; 23.6 ± 4.6 y, 165.9 ± 6.0 cm, 65.7 ± 10.3 kg). Participants performed an acute heavy resistance exercise test (AHRET; six sets of 10 repetition squats; 2 min rest between sets) during days 2–4 of the follicular phase (NOC group) or of inactive oral contraceptive intake (OC group). Plasma was obtained before and immediately after AHRET and subsequently fractionated based on apparent molecular weight (>60 kD, 30– 60 kD, and <30 kD). GH was determined in unfractionated plasma and each plasma fraction using 4 methods: (1) Nichols Institute Diagnostics immunoradiometric assay (Nichols), (2) National Institute of Diabetes and Digestive Kidney Diseases (NIDDK) com- petitive radioimmunoassay, (3) DSL immunofunctional enzyme-linked immunoabsorbent assay (IFA) and (4) rat tibial line bioas- say. GH increased (P < 0.05) in all fractions post-AHRET for the Nichols, NIDDK, and IFA. The OC group displayed higher resting GH for the NIDDK, and higher exercise-induced GH for the IFA, Nichols, and NIDDK in unfractionated plasma and >60 kD subfraction compared to NOC group. No differences were observed for the tibial line bioassay. OC use augmented immu- nological GH response to AHRET in unfractionated plasma and >60 kD molecular weight subfraction. However, OC use only increased biological activity of GH in one of two bioassays. These data demonstrated that GH concentrations at rest and following exercise are assay-dependent. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Birth control; Estradiol; Reproductive hormones; Strength; Women 1096-6374/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ghir.2007.10.001 * Corresponding author. Tel.: +1 860 486 6892; fax: +1 860 486 6898. E-mail address: william.kraemer@uconn.edu (W.J. Kraemer). www.elsevier.com/locate/ghir Available online at www.sciencedirect.com Growth Hormone & IGF Research 18 (2008) 238–244