Endotoxin induces delayed ovulation following endocrine aberration during the proestrous phase in Holstein heifers

The effect of endotoxin on follicular growth and on secretion of LH, estradiol-17β, progesterone and cortisol during the proestrous phase in cattle was investigated. Holstein heifers were treated with PGF₂ at 11–13 d after ovulation to induce luteolysis. At 42 hr after PGF₂ treatment, heifers were administered either lipopolysaccharide (LPS; Escherichia coli, O111:B4, 5 μg/kg, n = 6) or saline (control; n = 6) by i.v. bolus injection. Ovarian structures were monitored daily by transrectal ultrasonography, and blood samples were collected at various times for hormonal analysis. The duration from PGF₂ treatment to ovulation was significantly longer in the LPS group (8.0 ± 1.3 d) than in the control group (4.2 ± 0.2 d). LPS significantly reduced the pulse frequency of LH for 6 hr after the administration, and increased the mean concentration and pulse amplitude of LH from 3 to 6 hr after the administration. The plasma concentrations of progesterone and cortisol were transiently increased after LPS administration. The plasma concentration of estradiol-17β was significantly decreased at 24 hr after LPS administration compared to that in the controls. Five of six LPS-treated heifers exhibited no preovulatory LH surge until 120 hr after PGF₂ treatment and the remaining heifer exhibited the surge at 108 hr after PGF₂ treatment, while the LH surge was observed at 54–78 hr after PGF₂ treatment in control heifers. These results suggest that endotoxin disrupts progression of the proestrous phase of cattle, interrupting the preovulatory estradiol rise and thus delaying the LH surge and the subsequent ovulation.