Interaction between somatotropin, insulin-like growth factor I, insulin-like growth factor binding proteins and reproduction in female beef cattl

Female beef cattle actively immunized against growth hormone releasing factor (GRF) were used in three experiments to investigate the relationship between somatotropin (ST), insulin-like growth factor-I (IGF-I), insulin-like growth factor binding proteins (IGFBP) and reproduction function. In the first study, prepubertal Angus, Charolais, and Simmental heifers that were actively immunized against GRF or restricted in feed intake exhibited decreased body weight and serum concentration of IGF-I. Attainment of puberty was delayed by GRF-immunization in Angus and Charolais, but not in Simmental heifers. Restriction of feed intake did not affect reproductive performance in Angus, Charolais or Simmental heifers.;In the second study, replacement therapy with recombinantly derived bovine somatotropin (rbST) was administered to initiate puberty in two groups of prepubertal, GRF-immunized Angus and Charolais heifers for 56 d. Daily injections of rbST increased serum ST, IGF-I, insulin, and plasma glucose; however, decreased serum IGFBP-2. During the 56-d treatment period heifers initiated puberty at similar rates, regardless of rbST treatment.;To clarify interactions of ST, IGF-I, IGFBP and follicular development we administered rbST and (or) pregnant mare serum gonadotropin (PMSG) to cyclic, normal- and GRF-immunized beef cows in the third study. Estrous cycles were synchronized with prostaglandin (PGF$sb{rm 2a})$ and treatment administration (rbST, PMSG, rbST + PMSG, or vehicle) occurred at 0 and 24 h following last PGF$sb{rm 2a}.$ Serum, ovary bearing largest follicle (DOM), and follicular fluid (FFL) from the DOM and second largest follicle (SUB) were collected 40 h after PGF$sb{rm 2a}.$ Administration of PMSG had no effect on any parameter examined; therefore, data were combined and two treatments resulted (rbST and vehicle). Immunization against GRF decreased serum and FFL concentrations of IGF-I and increased concentrations of IGFBP-2 in serum and FFL from SUB, but not DOM. Treatment with rbST increased serum and FFL IGF-I, and increased both serum and FFL IGFBP-2 (both DOM and SUB).