Regulation of Ovarian Follicular Growth by Somatotropin and Insulin-Like Growth Factors in Cattle

Abstract

Somatotropin (ST), insulin-like growth factor (IGF)-I, and IGF-II affect animal growth and lactation as well as animal reproduction. Understanding the effects of ST and the IGF on reproduction is important because ST and IGF-I undergo dynamic changes prior to the postpartum breeding period. In addition, administration of recombinant bovine somatotropin (rbST) to lactating cows is a common practice that increases blood concentrations of ST and IGF-I during the breeding period. In vivo, administration of rbST caused greater ovarian follicular development. The effects of rbST may represent direct actions of ST because ST receptors are found within granulosa cells as well as oocytes. Alternatively, the actions of ST may be indirectly mediated by increased IGF-I and (or) nutrient partitioning that occurs after rbST. Both IGF-I and IGF-II are synthesized within the ovary. Ovarian IGF are, therefore, a composite of IGF from both endocrine (liver) and autocrine and paracrine (ovary) sources. The IGF stimulate ovarian function by acting synergistically with gonadotropins to promote growth and steroidogenesis of ovarian cells. Actions of IGF-I and -II are restrained by a series of IGF binding proteins (IGFBP) that either originate from the blood or are synthesized locally within the follicle. Degradation and differential synthesis of IGFBP are important mechanisms regulating IGFBP amounts. The relative amounts of IGFBP may ultimately determine ovarian IGF action. Future studies of ST and IGFs should focus on the hormones, receptors, and binding proteins as well as the metabolic requirements for normal ovarian function in dairy cattle.

Metabolic changes in early lactation and impaired reproductive performance in dairy cows

“…In fact, growth hormone (GH) stimulates in cows the systemic production, and not the ovarian production, of IGF-I. GH concentration itself is, to some extent, positively affected by elevated NPY concentrations [62]. Indeed, a study in cows showed that injection of NPY in the third cerebroventricle tended to increase pituitary secretion of GH [94].…”

“…How insulin exactly affects the steroid production is still not completely elucidated, and there might be differences among species with regard to the mechanism [69]. Another important effect of IGF-I (and IGF-II) observed in vitro, is the synergistic action of IGF-I (and IGF-II) with both LH and FSH [62], which is similar to the effect of insulin on the sensitivity of the pituitary to GnRH and the strengthening effect of gonadotrophins on steroid production in humans [69].…”

“…The effect of IGF-I in vivo in both follicular fluid and blood is moderated by binding proteins (IGFBP) [62,87]. The resulting effect of a certain IGF-I concentration together with a certain concentration and composition of IGFBPs is however not known [62].…”

“…The resulting effect of a certain IGF-I concentration together with a certain concentration and composition of IGFBPs is however not known [62]. It is also unclear, whether a high systemic IGF-I concentration always favors reproduction, because studies in mice revealed that instead of systemically produced IGF-I, locally produced IGF-I may be more important for the stimulation of the ovary [62]. This may be important, because the size of the follicle is important for the extent to which follicular IGF-I reflects the systemic concentration of IGF-I [31].…”

The effect of polymorphism in gene of insulin-like growth factor-I on the serum periparturient concentration in Holstein dairy cows

“…It can change reproductive activity by affecting on neural pathway which control production of GnRH or effect on the secretion of pituitary gonadotropin; it has also direct effects on the ovary and influences its susceptibility to FSH and LH [5] . In addition, IGF-I can influence on the proliferation and differentiation of granulosa cell, postpartum follicular growth and consequently first ovulation of the dominant follicle, development of corpus luteum and preimplantation embryo development [6,7] .…”