Effect of exogenous estradiol on plasma concentrations of somatotropin, insulin-like growth factor-I, insulin-like growth factor binding protein activity, and metabolites in ovariectomized angus and braham cows

To determine the effect of breed and estradiol-17β on selected hormones and metabolites, ovariectomized (3 mo) Angus (n = 14) and Brahman (n = 12) cows were paired by age and body weight and randomly assigned as either nonimplanted controls (CON) or implanted with estradiol (E2) for 45 d. After Day 7 and through Day 42, plasma concentration of somatotropin was greater for E2 than CON cows (treatment X day, P < 0.05). During an intensive blood sampling on Day 36, E2 cows tended (P < 0.10) to have greater somatotropin pulse amplitudes than CON cows, but other parameters of somatotropin release were not affected (P > 0.10) by E2 treatment. The effect of breed was apparent on Day 36 as Brahman cows had greater (P < 0.05) somatotropin pulse amplitude, basal secretion, and mean concentration than Angus cows. Overall, plasma concentration of IGF-I was greater (P < 0.01) for E2 than CON cows (158.3 vs. 104.2 ng/ml) and was greater for Brahman than Angus cows (164.1 vs. 98.4 ng/ml). However, there was a trend (P < 0.10) for a treatment X breed X day interaction for IGF-I (i.e., the magnitude of increase in IGF-I concentration was greater in E2-Angus than E2-Brahman cows). After Day 7 and through Day 42, total plasma IGF binding protein (IGFBP) activity was greater (P < 0.01) for E2 than CON cows. Ligand blotting revealed at least five forms of IGFBP activity, and E2 cows had greater (P < 0.05) binding activity of IGFBP-3 and the 30- and 32-kDa IGFBP than CON cows. Brahman cows had greater (P < 0.05) IGFBP-3 and the 32-kDa IGFBP than Angus cows. After Day 14 and through Day 42, concentration of urea nitrogen (PUN) was greater (P < 0.001) for CON than E2 cows (treatment X day, P < 0.001). Brahman had greater (P < 0.01) PUN than Angus cows (16.6 vs. 14.2 mg/dl). Plasma concentration of glucose was greater (P < 0.01) for E2 than CON cows (78.9 vs. 76.4 mg/dl) but was not affected (P > 0.10) by breed. In summary, these data suggest that some, but not all, of the positive effects of estradiol on peripheral concentration of IGF-I and IGFBP activity can be attributed to increased somatotropin. Moreover, breed influenced basal and E2-induced secretion of somatotropin and IGF-I such that differences between Brahman and Angus cows in plasma IGF-I concentrations were abated within 3 wk of estradiol implantation. Thus, breed influences the metabolite and hormonal response of cattle to estrogenic implants.