Responses of growth hormone aggregates to different intermittent exercise intensities

The purpose of this study was to determine the impact of high-intensity intermittent exercise on the presence of circulating growth hormone (GH) aggregates measured using two different assay techniques. Six male subjects with endurance training background participated in this study under both exercise and no-exercise control conditions. After resting blood sampling, subjects completed an intermittent treadmill exercise protocol at four speeds predicted to elicit a specific VO(2):60% VO(2max) for 10 min, 75% for 10 min, 90% for 5 min, and 100% for 2 min. After each exercise intensity was completed treadmill speed was reduced to a walk (3.5-4 min) for blood sampling. Sampling continued every 15 min for 1 h into recovery. All samples were then measured for GH concentrations using Nichols immunoradiometric assay (IRMA) and Diagnostic Systems Laboratory's immunofunctional assay (IFA). A second set of samples was chemically reduced using reduced glutathione (GSH; 10 mM for 18 h at room temperature) to break disulfide bonds between possible oligomeric GH complexes, and subsequently assayed using the same GH assays. With the IRMA, GH was significantly elevated ( P<0.05) after the 75% workload and remained elevated through 30 min post-exercise. After adding GSH to the sample, the IRMA indicated significant increases in GH as early as the 60% exercise intensity and remained elevated through 45 min into recovery. At 75%, the GSH assay run was significantly higher than the non-GSH assay run. With the IFA, GH was significantly elevated at 60% in the non-GSH condition, whereas the GSH assay run indicated significant elevations at 75%. Both GSH and non-GSH conditions remained elevated through 30 min into recovery. These data indicate that the addition of GSH to serum samples prior to assay via an IRMA may break existing disulfide bonds between aggregated GH molecules, thus altering the apparent assay signal to reveal greater total GH in the sample.