Effects of the thymic peptide thymulin on in vitro and in vivo testicular steroid concentrations in White composite and Meishan boars

Immuno-peptides may have positive or negative effects on gonadal steroidoge nesis, but few have been tested outside of rodent species or in vivo. In Ex p. 1, thymulin, a secreted nonapeptide of the thymus, was incubated (1, 10, 100, or 1,000 ng/mL) with testicular minces (sampled at 3, 6, or 12 h) fro m Chinese Meishan boars of high gonadotropin/testicular steroidogenic funct ion (n = 8) and White composite boars of European origin(n = 8) to test the hypothesis that thymulin could augment hCG stimulation of testicular andro gen concentrations. Thymulin alone had few effects on androgen concentratio ns (testosterone, dehydroepiandrosterone+dehydroepiandrosterone sulfate [DH EA+DHEASO(4)]) in Meishan boar testicular incubates. In minces from White c omposites incubated with thymulin, testosterone concentrations were general ly below control values (P < .05), but DHEA+DHEASO(4) concentrations were u naffected. Thymulin had no effect on estrone concentrations in testicular i ncubates of White composite boars but stimulated estrone concentrations in Meishan testicular incubates. Thymulin plus hCG increased testosterone (3 a nd 6 h of incubation; P <.05) but not DHEA+DKEASO(4) concentrations in Whit e composite testicular incubates. Thymulin plus hCG did not alter androgen or estrogen concentrations from control values in Meishan testicular incuba tes. In Exp.2 with a protocol similar to that of Exp. 1 for testicular minc es from White composite boars (n = 30), thymulin increased testosterone con centrations during the early incubation period (1 to 3 h; P <.05) and depre ssed testosterone concentrations at later times (6 h; P < .05). Thymulin sy nergized with hCG in stimulating increases in testosterone and DHEA+DHEASO( 4) concentrations (P < .05) but had no effect on estrone concentrations in vitro. Thymulin was tested in vivo in boars from three genetic lines select ed for high, medium, or normal circulating LH concentrations (Meishan, sele ct White composites, and control White composites, respectively). Injection of thymulin i.v. (4.4, 44.4, or 444.4 ng/kg BW) generally increased circul ating testosterone concentrations (2 to 3 h later; P < .01), but the respon se was dependent on the boar's general circulating LH concentrations and do se of thymulin, Overall results from these studies support the hypothesis o f a thymulin augmentation of LH stimulation of androgen increases in vitro and in vivo in the testis of boars.