Insulin-like growth factor binding protein-2 (IGFBP-2) mRNA in equine follicles during vernal transition and the breeding season

摘要

The period of vemal transition between anestrus and cyclicity is characterized by a resurgence of follicular activity, irregular exhibition of estrous behavior and resumption of secretion of gonadotropins and ovarian steroids. In many mares, during vernal transition, large dominant follicles grow and regress, but do not ovulate [1]. These follicles appear to be steroidogenically incompetent. It is known that the insulin-like factor (IGF) system, which is composed of IGF-I and -II, IGF receptors, IGF-binding proteins (IGFBPs), and IGFBP proteases play an essential role in modulating the response of follicles to gonadotropins. The IGFs stimulate granulosa cell proliferation and interact with FSH to control estradiol production by granulosa cells [2]. IGFBPs are involved in regulating this process by virtue of their ability to bind IGFs with high affinity and so regulate IGF bioavailability. Changes in the levels of IGFBPs in follicular fluid correlate with functional changes in the ovarian follicles. In many species, follicular growth and atresia are characterized by dramatic changes in intrafollicular IGFBP concentrations, reflecting changes in expression and in intrafollicular proteolytic activity which degrades IGFBP-2, - 4, and -5. A recent study has identified IGFBPs in equine serum and follicular fluid and has shown that they are clearly related to the steroidogenic capacity and physiological status of equine follicles [3]. IGFBP-2 has been postulated to be involved in regulating the bioavailability of IGF during the final stages of follicle growth in ruminants [4]. We aim to test the hypothesis that the expression of IGFBP-2 in large (>30 mm) follicles of mares during vernal transition will be higher than in cycling mares.