512. COMMERCIAL EQUINE CHORIONIC GONADOTROPHIN ISOFORM COMPOSITION, IMMUNOACTIVITY, AND BIOACTIVITY IN A MURINE MODEL USING OVARIAN AUGMENTATION AND TESTICULAR INTERSTITIAL CELL BIOASSAYS

Abstract

Equine chorionic gonadotrophin (eCG) is a placental glycoprotein hormone that is harvested from the plasma of pregnant mares for the formulation of commercial products which are used in a variety of assisted reproductive procedures in livestock. It is well documented that the bioactivity of gonadotrophin products is highly variable. The aim of this study was therefore to determine how different eCG products affected target tissues and cells. Isoforms of eCG were separated with iso-electric focusing and immunoactivity measured with RIA. For in vivo bioassay, dose-response eCG treatments were administered subcutaneously to immature female mice with follow-up treatment on day 2. On day 3 the mice were asphyxiated and ovaries dissected and weighed relative to standard. For in vitro bioassay, adult male mice were asphyxiated, testes removed, decapsulated, dispersed, strained and washed with DMEM:F12 0.1% BSA. The cell stock was counted and diluted for culture at 20,000 cells/well. Cell viability was determined using trypan blue. Five doses were tested for each eCG product. The cells were incubated at 32ºC for 3 hours in 5.0% CO₂ in humidified conditions. Media was collected after 3 hours and immediately assayed for testosterone with RIA. Product eCG with ~90% isoforms with pH 3.0–3.7 and ~10% with pH 3.8–4.4, showed 29% greater biologically activity in the ovarian augmentation assay and 44.8% more immunoactivity then stated product bioactivity. Product eCG with ~70% isoforms with pH 3.0–3.7 and ~30% with pH 3.8–7.4 was 3.0% less effective in vivo and had 7.9% less immunoactivity then stated bioactivity, however, in vitro testosterone production was more effectively stimulated then with the previous more acidic eCG product. Our study shows a selective difference in commercial eCG biological activity that appears to depend on subtle isoform heterogeneity. Future studies aim to determine the specific actions of eCG isoforms in target tissues.