Temporal relationship of the pulsatile fluctuation of luteinizing hormone and progesterone in cattle: a time series cross-correlation analysis.

The temporal relationship between the pulsatile patterns of plasma luteinizing hormone (LH) and progesterone (P4) was studied in mid-luteal (ML) and early-pregnant (EP) dairy cows. Blood samples were collected (via external jugular vein cannulae) at 10-min intervals for 16 h in 5 ML cows (d 10 to 12 of the cycle) and for 10 h in 5 EP cows (d 52 to 56 of gestation). Concentrations of LH and P4 were determined by radioimmunoassays and a time series cross-correlation analysis was utilized to evaluate the temporal relationship between them. A pulsatile pattern was found for both hormones in both groups, and in all animals LH peaks were uniformly followed by P4 peaks. In 80% of the cows in both groups the highest cross-correlation occurred between samples LH(n) and P4 (n + 1) (n = sample number), suggesting that a lag time of about 10 min is necessary for luteal stimulation. Results from both groups demonstrate that P4 is released from the corpus luteum in a pulsatile manner and that its release is at least partially dependent upon the pulsatile pattern of plasma LH. Correlation coefficients between LH and P4 mean level, basal levels, peak frequencies and peak amplitudes obtained in both groups indicate that the conceptus alters the hypothalamic-pituitary-ovarian relationship found in the ML cows, suggesting the existence of another factor(s) acting along with LH to release P4 in EP cows.