Bioactivity and immunoactivity of bovine luteinizing hormone isoforms as influenced by in vivo progestin treatment

Abstract

Experiments were conducted to test two hypotheses: 1) the luteinizing hormone isoform profile in the bovine pituitary shifts toward the more basic forms by 24 hours after the removal of an exogenous source of a progestin, and 2) the sensitivity of a cytochemical assay will exceed that of the mouse interstitial cell testosterone assay for assessment of in vitro bioactivity of bovine luteinizing hormone. Beef heifers (n=12) that had been synchronized to allow natural luteolysis to occur during a 9-d duration of norgestomet treatment were allotted to one of two groups defined by interval to slaughter following removal of an implant containing a progestin: the 0 hour group and the 24 hour group. Post-perfusion hemipituitaries were homogenized and chromatofocused. Chromatofocusing fractions were analyzed for immunoactive LH concentrations using a double antibody LH RIA and the isoform peaks were determined. Isoform peak fractions were analyzed for bioactive LH concentrations in two in vitro bioassay systems: a mouse interstitial cell testosterone assay (MICT) and a cytochemical bioassay (CYTO). The distribution of immunoactive LH isoforms did not differ between treatment groups. The distribution of bioactive LH isoforms as determined by the CYTO assay also did not differ between treatment groups, but the inability to detect any effects of treatment may be related to the relative insensitivity of the assay. Only the MICT assay detected differences in the distribution of bioactive LH isoforms between treatment groups. Based on LH potency estimates determined by the MICT, there was an increase (P < .06) in isoform D (a more basic isoform) and a decrease (P < .05) in isoform I (a more acidic isoform) in the 24 h group as compared to the 0 h group. When comparing the B/I ratios (bioactive-MICT / immunoactive-RIA) for the two treatment groups, significant differences were detected for three LH isoforms. B/I ratios for both isoforms D and G increased (P < .05) in the 24 h group, while the B/I ratio for isoform I decreased (P < .06) in the 24 h group. These results indicate that there is indeed an effect of progestin withdrawal on the bovine LH isoform profile based upon results of the MICT assay. The isoform profile appears to shift toward the more basic isoforms by 24 hours after progestin implant removal and the biopotency of two isoforms (D and G) is increased. Therefore, these results do support our first hypothesis. We must reject our second hypothesis since the CYTO assay was less sensitive and offered a smaller working range as compared to the MICT assay. In conclusion, there is a shift in the LH isoform profile within the bovine pituitary which occurs by 24 hours following the removal of a progestin implant.