Changes of plasma concentrations of insulin-like peptide 3 and testosterone, and their association with scrotal circumference during pubertal development in male goats.

Insulin-like peptide 3 (INSL3) has been used as a testis-specific biomarker for puberty in several species, but the secretory profile of INSL3 during pubertal development in small ruminants is unknown. Here we sought to determine the age-related changes in the plasma concentrations of INSL3 and testosterone and their association with scrotal circumference during pubertal development in five male Shiba goats. Blood samples and scrotal circumference measurement were taken every 2 weeks from week 10 to week 52 of each goat's lifespan. Based on the changes in scrotal circumference, data were grouped into early pubertal (10-22 weeks), late pubertal (22-34 weeks) and post-pubertal (34-52 weeks) categories. The plasma concentrations of testosterone and luteinizing hormone (LH) were measured by enzyme-immunoassays (EIAs), and we used a time-resolved fluorescence immunoassay (TRFIA) to measure plasma INSL3. The biweekly sampling showed that the plasma INSL3 secretions maintained a moderate increase during and after puberty, whereas the plasma testosterone secretions fluctuated over the same period. The comparison of the three age categories revealed a significant increase (p < 0.01) in the mean plasma INSL3 concentrations during the late and post-pubertal periods compared to the early pubertal period. There was no difference in the mean plasma testosterone concentrations between the early and late pubertal periods, but a significant increase (p < 0.01) was observed during the post-pubertal period compared to early and late pubertal periods. The mean plasma LH concentrations increased significantly (p < 0.05) from the early pubertal to late pubertal and from the late pubertal to post-pubertal periods. A significant increase (p < 0.05) in the mean scrotal circumference from the early pubertal to late pubertal and from the late pubertal to post-pubertal periods was observed. The R2 value of the best regression curves between scrotal circumference and INSL3 (0.513; p < 0.001) was higher than that between scrotal circumference and testosterone (0.162; p < 0.01) from 10 to 52 weeks of age. In conclusion, in male goats, plasma concentrations of INSL3 increased continuously during and after puberty, whereas testosterone secretions were fluctuated. The scrotal circumference was more highly correlated with the INSL3 concentrations than with testosterone, implying that INSL3 is superior as a biomarker of testicular total Leydig cell volume.