Prolactin levels in ovariectomized Brahman, Brahman x Hereford and Hereford cows following a 20 mg dose of estradiol-17beta.
Six Brahman (B), six Brahman x Hereford (BxH) and six Hereford (H) chronically ovariectomized cows were injected intramuscularly with 20 mg of estradiol-17beta (E2). The cows were bled via coccygeal vessel puncture immediately before E2 injection, every 2 hr from 0 to 8 hr post-injection, every hr from 9 to 24 hr post-injection and every 2 hr from 26 to 36 hr post-injection. Serum prolactin (PRL) concentrations were quantitated by a validated radioimmunoassay. All cows exhibited a PRL surge following the E2 injection. A PRL surge was defined as a sustained elevation in PRL of at least one standard deviation above the level of PRL before the rise. Nadir levels of PRL prior to the surge did not differ significantly between breeds. Time (hr) to the onset of the E2-induced PRL surge was 5.0, 5.0 and 6.2 in B, BxH and H, respectively (P<.10). Elapsed time (hr) from E2 injection to the PRL peak level varied (P<.01) between B (10.8) and H (17.8) and BxH (11.8) and H. Peak PRL levels (ng/ml) varied (P<.10) between breeds (B, 70.6; BxH, 123.9; H, 49.4). Area under the PRL curve (sq cm) varied (P<.05) between BxH (45.2) and H (24.7) but not between BxH and B (34.3; P>.10) or B and H (P>.10). Duration (hr) of the PRL surge did not differ significantly between breeds (B, 19.3; BxH, 20.5; H, 21.2). Overall, bleeding period effects (P<.01), breed effects (P<.10), and breed x period interactions (P<.01) were found.
Rhodes
R C
RC
Texas A&M University Agricultural Research and Extension Center, Overton 75684 USA; Department of Animal Science, College Station 77843 USA.
Randel
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RD
Harms
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PG