Hypothalamic-pituitary function in chronically cystic and regularly cycling dairy cows.
Studies were conducted to characterize, in detail, serum luteinizing hormone (LH) profiles 1) before and at various times after ovariectomy (OVX) in chronically cystic (CC) and regularly cycling (control) cows and 2) after injection of luteinizing hormone-releasing hormone (LHRH) and estradiol-17 beta (E2) to OVX CC and control cows. Blood samples were drawn from follicular-phase control cows and CC cows (pre-OVX) for 8 h at 15-min intervals. Cows were bilaterally OVX, and blood samples were collected for 8 h at 15-min intervals at 1, 2, and 4 wk after OVX. Five weeks after OVX, cows were challenged with 20 micrograms LHRH i.m. Six weeks after OVX, cows were challenged with 1 mg E2 i.m. Pulse frequency, pulse amplitude, and interpulse interval were not different (p less than 0.05) before or after OVX in CC and control cows. Peak LH, time to the peak, and area under the curves were not different (p greater than 0.05) after LHRH administration in CC and control cows. After injection of E2, peak LH levels were higher (p less than 0.05) in CC cows than in control cows. However, the time to the peak or the area under the curves were not different (p greater than 0.05) between groups. It appears that the feedback regulation of the hypothalamic-pituitary axis to circulating estrogen levels is altered as a result of the cystic condition.