Regulation of ovarian follicular growth by somatotropin and insulin-like growth factors in cattle.

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.