CHANGES IN THE HYPOPHYSEAL-GONADAL AXIS DURING THE ONSET OF PUBERTY IN YOUNG BULLS

The objectives of this study were to determine the extent of changes i n concentrations of testosterone, growth hormone (GH) and insulin-like growth factor I (IGF-I) concentrations in blood plasma and to charact erize the respective plasma-binding proteins of these two peptides dur ing the onset of puberty in male calves. The jugular veins of six male Holstein calves (II-weeks-old) were catheterized and blood was collec ted every 3 days (one sample every 30 min for gh). Hormone concentrati ons in plasma were determined by specific radioimmunoassay. After incu bation with [I-125]IGF-I, IGF-I-binding proteins (IGFBPs) were separat ed by gel filtration; radioactivity was determined in each fraction. W estern ligand blotting using radiolabelled hormones as ligand was also used to characterize the IGF-I- and GH-binding proteins in plasma. Pu berty was characterized by a rapid (in I or 2 days) increase in mean c oncentrations of testosterone in plasma (from 0.5 to >2 ng ml(-1)) and a pulsatile release of the hormone. During puberty, IGF-I concentrati ons also increased rapidly in 8-10 days from +/-50 ng ml(-1) to >150 n g ml(-1), whereas concentrations of GH in plasma remained relatively s table during the experimental period. A significant correlation was ob served between IGF-I and testosterone concentrations (r = 0.77; P < 0. 001) throughout the experimental period. Three different IGF-I-binding protein fractions with apparent moleclular masses of > 200, 150-170 a nd 45-65 kDa were found in plasma using gel filtration. The 150-170 kD a IGF-I-binding protein complex was more abundant after puberty, where as the lower molecular mass fraction was slightly decreased. This rele ase was confirmed by ligand blotting analysis. The binding subunit of the 150-170 kDa complex (molecular mass = 45-54 kDa; IGFBP-3) was more prominent after puberty, whereas the intensity of the 38 kDa band (IG FBP-2) was decreased. During this physiological step, migrant GH-bindi ng proteins at 190, 58 and 31 kDa presented similar band intensities i n all animals. In conclusion puberty in male calves was associated wit h subsequent increases in amounts of testosterone, IGF-I and IGFBP-3 c omplex, whereas IGFBP-2 was reduced. IGF-I and perhaps IGFBPs could be used as indicators of the establishment of puberty. In practice, this factor could be better than testosterone for evaluating hormonal stat us because of its nycthemeral stability.