Characteristics of circulating growth hormone in women after acute heavy resistance exercise

The effects of exercise on th molecular nature of secreted human growth hor mone (GH) or its biological activity are not well understood. Plasma fron w omen (average age 23.6 yr, n = 35), drawn before and after an acute heavy r esistance exercise test, was fractionated by size exclusion chromatography into three size classes namely, >60 kDa (fraction A), 30-60 kDa (fraction B ), and <30 kDa (fraction C), before GH assay. Concentrations of GI in these fractions, as well as in unfractioned plasma, were measured by the Nichols immunoradiometric assay, Nation Institute of Diabetes and Digestive and Ki dney Disease (NIDDK) polyclonal competitive RIA, Diagnostic System Laborato ry's immunofunctional assay (measures dimerization-capable species), and th e rat tibial bioassay. Significantly increased circulating GH concentration s of two-to fourfold were observed when immunoassays in unfractionated plas ma samples were used, but they showed no significant change with use of the rat tibial bioassay. Significant exercise-induced increases in GH were fou nd in fractions and C but not in fraction A. Because chemical reduction the samples before GH immunoassay significantly increase GH concentrations in fractions B and C (Nichols and NIDDK kits) after exercise, it is concluded that exercise may specifically increase release of disulfide-linked hormone molecule and/or fragments. Finally, because most of the GH release after e xercise was able to dimerize the GH receptor in vitro, it is also concluded that these forms have the two intact binding sites required to initiate si gnal transduction in target cells.