Immunoreactive Insulin-Like Growth Factor in Plasma during Pre- and Post-Partum Periods of Thoroughbred Mares from which the Newborn were Removed: its Pattern, Physiological Function and Relation to Other Hormones

Abstract

Plasma concentrations of immunoreactive (ir-) insulin-like growth factor I (IGF-I) were measured in 9 pregnant mares from 30 days before foaling to Day 7 after ovulation of the first post-partum estrus (foal heat) in order to characterize its pattern and investigate its possible role in mares during that time. A marked increase in plasma concentrations of ir-IGF-I was observed during the last month of pregnancy. After foaling, ir-IGF-I sustained its high concentration for the first week after parturition and it gradually decreased thereafter. Throughout the observation period of the current study, a significant positive correlation between ir-IGF-I and FSH (r=0.58, p<0.05) was found. The significant increase in ir-IGF-I and FSH shortly prior to parturition may be responsible for the rapid resumption of ovarian activity in mares and the initiation of foal heat in thoroughbred mares. Statistically significant positive correlation between ir-IGF-I, FSH and ir-inhibin levels observed in the current study confirm the previous suggestion that ir-inhibin secreted during late pregnancy in mares may be biologically inactive.