AGE-RELATED-CHANGES IN THE SECRETION OF LH IN-VIVO AND IN-VITRO IN INFANTILE AND PREPUBERTAL HOLSTEIN BULL CALVES

The objectives of this study were (i) to determine whether age-related changes in the secretion of LH are associated with alterations in sec retory activity or numbers of gonadotrophs, and (ii) to determine whet her gonadotrophs obtained from the pars distalis and pars tuberalis un dergo similar age-related changes in function. Blood samples were coll ected from Holstein bull calves every 15 min for 12 h at < 1, 12, and 42 weeks of age (n = 5 per age group) to characterize the secretion of LH. Calves were killed 3-5 days later. The pars distalis and pars tub eralis were enzymatically dispersed into suspensions of single cells. Cells from the pars distalis were (i) extracted with 0.01 mol NaHCO3, l(-1), (ii) fixed for immunocytochemical analysis, and (iii) cultured in six-well plastic plates at a density of 500 000 cells per well in m edia containing 2.5% homolo,oous calf serum for 18 and 72 h. Cells fro m the pars tuberalis were cultured as for pars distalis cells. As expe cted, LH pulse frequency increased (P < 0.01) between one and 12 weeks of age and then declined. The percentage of cells from the pars dista lis containing immunoreactive LH averaged 8.4%, and did not change wit h age. The mass of the pars distalis and the total number of cells rec overed increased with age (P < 0.05); consequently, the number of gona dotrophs recovered also increased. The initial content of LH of pars d istalis cells changed with age and was greatest at 12 weeks. At < 1. a nd 12 weeks of age, the difference in initial content was reflected in the amount of LH released in vitro after both 18 and 72 h in culture. In addition, at these ages the total amount of LH present (media plus cell extract) at the end of the culture period was similar to the ini tial cell content. In contrast, the total LH content of cells obtained from the pars distalis of calves at 42 weeks of age increased by appr oximately 3.5 times during the first 18 h of culture. The total conten t of LH from pars tuberalis cells after 18 h in culture increased slig htly with age but this increase was not significant. In summary, age-r elated differences in the numbers and secretory activity of gonadotrop hs located within the pars distalis contribute to age-related increase s in concentrations of LH in plasma. Changes in the synthesis and rele ase of LH from pars distalis and pars tuberalis cells appear to differ in culture.