Variations in the isoform composition of equine chorionic gonadotrophin across early gestation in the horse

Abstract

Equine chorionic gonadotrophin (eCG) is a heterodimeric glycoprotein hormone secreted by the placental endometrial cups during the first third of gestation in the horse. Previous research has shown that eCG is a highly heterogeneous molecule with significant differences in bioactivity between isoforms. The aim of this study was to investigate the eCG isoform composition across early gestation in the horse. Iso-electric focusing (liquid phase) was performed to fractionate plasma samples into 10 pH ranges between 3 and 10. Blood samples were collected from mares (n=33) weekly between 50 and 120 days of gestation and divided into 3 groups 50-60, 60-65 and greater than 75 days. A competitive ELISA was used to determine the immunoactivity in the plasma and fractionated samples. Data from the 10 fractions were grouped into acidic (pH 3.0-5.1), intermediate (pH 5.2-7.9), or basic (pH 8.0-10.0) isoform categories for analysis. In the period prior to and during the eCG peak around day 60 of gestation, there was no substantial change in acidic, intermediate, or basic isoform groups of eCG. However, after 75 days of gestation, the percentage of acidic isoforms of eCG decreased markedly with a concurrent increase in intermediate isoforms. Basic isoform composition did not differ to a significant degree in the specified timeframes. Isoform quantity in the relatively narrow pH range of 4.5-5.1 varied significantly between the three periods with the highest concentration in 60-75 gestation period which coincides with the eCG concentration. These results support previous work on hCG, the human analog of eCG that isoform composition changes across gestation. It has been shown that acidic isoforms of LH and FSH have slower clearance rates than the more basic isoforms which supports our finding that acid isoforms are dominant at the same time eCG concentrations peak in plasma.