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 BRAHMAN COWS

To determine the effect of breed and estradiol-17 beta on selected hor mones and metabolites, ovariectomized (greater than or equal to 3 mo) Angus (n = 14) and Brahman (n = 12) cows were paired by age and body w eight and randomly assigned as either nonimplanted controls (CON) or i mplanted 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 Da y 36, E2 cows tended (P < 0.10) to have greater somatotropin pulse amp litudes than CON cows, but other parameters of somatotropin release we re not affected (P > 0.10) by E2 treatment. The effect of breed was ap parent on Day 36 as Brahman cows had greater (P < 0.05) somatotropin p ulse amplitude, basal secretion, and mean concentration than Angus cow s. Overall, plasma concentration of IGF-I was greater (P < 0.01) for E 2 than CON cows (158.3 vs. 104.2 ng/ml) and was greater for Brahman th an 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 m agnitude of increase in IGF-I concentration was greater in E2-Angus th an 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 CO N 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 t he 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 th rough 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 gr eater (P < 0.01) PUN than Angus cows (16.6 vs. 14.2 mg/dl). Plasma con centration 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 o f estradiol on peripheral concentration of IGF-I and IGFBP activity ca n be attributed to increased somatotropin. Moreover, breed influenced basal and E2-induced secretion of somatotropin and IGF-I such that dif ferences between Brahman and Angus cows in plasma IGF-I concentrations were abated within 3 wk of estradiol implantation. Thus, breed influe nces the metabolite and hormonal response of cattle to estrogenic impl ants. (C) Elsevier Science Inc. 1997.