Tumor Necrosis Factor-α Affects Growth Hormone Secretion by a Direct Pituitary Interaction

Abstract

Abstract

Administration of 50, 250, and 1,250 ng/kg iv of recombinant bovine tumor necrosis factor-α (RBTNF) did not affect basal plasma concentrations of growth hormone (GH) or thyroid-stimulating hormone in male calves. However, when administered 30 min before challenge with 1 μg/kg iv of thyrotropin-releasing hormone (TRH), 250 ng/kg of RBTNF increased the subsequent incremental GH response. At 1,250 ng/kg of RBTNF, GH response to TRH was significantly blunted. For each dose of RBTNF administered, the incremental change in plasma thyroid-stimulating hormone following TRH was not significantly different from control. To examine direct effects of RBTNF on pituitary function, fresh bovine pituitaries were sliced into 1-mm cubes and incubated with 0 or 10^–8, 10^–9, or 10^–10 M RBTNF. Additional cultures were treated with 10^–8 or 10^–9 M GH-releasing factor or 10^–8 M TRH and 0 or 10^–8 M RBTNF. Media GH increased in cultures with 10^–10 M RBTNF and declined linearly as RBTNF concentration increased. RBTNF blocked GH release from GH-releasing factor- and TRH-challenged pituitary slices. Membranes prepared from homogenized bovine pituitaries had specific saturable binding characteristics for monomeric ¹²⁵I-RBTNF. Membranes treated with 4 M MgCI₂ for 10 min and washed free of Mg²⁺ produced Scatchard plots fit to a two-site model (high affinity site K_d = 6.6 nM), while Scatchards of non-Mg²⁺-treated membranes fit a single site (K_d = 8.9 nM). Polyacrylamide gel electrophoresis separation of ¹²⁵I-RBTNF cross-linked pituitary membranes showed specific binding of monomeric ¹²⁵I-RBTNF to protein components ranging in molecular weight from 19,000 to 77,000. The data suggest that RBTNF has modulatory effects on the regulation of GH secretion acting directly at the pituitary through specific receptors.