Pattern of Plasma Luteinizing Hormone in the Cyclic Cow: Dependence upon the Period of the Cycle*

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Abstract
Docile cows were used to characterize the pattern of plasma LH in systemic circulation during three periods [day 3 (early luteal), day 10 or 11 (midluteal) and day 18 or 19] of the estrous cycle. Blood samples were collected at 10-min intervals for 24 continuous h during each period. Concentration of LH, as measured by RIA, was analyzed by time domain and frequency domain methods of time series analysis. During all periods of the cycle studied, LH fluctuated in a pulsatile manner. During the early luteal period, pulses were classified as low amplitude (∃LH, 0.3-1.8 ng) and high frequency (20-30 pulses/24 h), with each cow exhibiting an inherent rhythmic pattern. However, during the midluteal period, pulses were classified as high amplitude (∆LH, 1.2-7.0 ng) and low frequency (6-8 pulses/24 h), without an inherent rhythmic pattern. Even though patterns differed between the early and midluteal periods in all cows, mean LH concentrations were different (P < 0.05) in only one cow. On day 18 or 19, two cows exhibited a preovulatory surge of LH. Throughout the surge, LH fluctuated in a pulsatile manner, with a frequency more like the early luteal than the midluteal period. Amplitude of the pulses was greater during the ascending than during the descending portion of the surge. These results suggest that LH fluctuates in a pulsatile manner in the plasma of the cyclic cow, and that the pattern is dependent upon the period of the estrous cycle and is probably modulated by ovarian steroids.