Pulsatile LH secretion during the preovulatory surge in the ewe: experimental observations and theoretical...

The secretion of LH during the preovulatory surge has been shown to be pulsatile in several species. We confirmed this phenomenon in intact ewes in two experiments in which we studied the LH surge in detail. In the first experiment, the levels of LH were measured in discrete blood samples taken every 5 min around the onset of oestrus in 5 intact Merino ewes. For 2-5 h immediately prior to the onset of the surge, the median pulse interval was 17.5 min (range 10-55) and the median pulse amplitude was 0.70 ng/ml (range 0.28-1.33 ng/ml). During the ascending phase of the surge, the pulse interval did not change significantly (median 18.3 min) but the median pulse amplitude increased 20 fold to 18.0 ng/ml. During the descending phase, the median pulse interval was 22.5 min and the median amplitude decreased to 8.1 ng/ml (P less than 0.005). During the surge, intervals of 10 min and amplitudes over 30 ng/ml were common. In the second experiment with 2 oestrous Romanov x Préalpes-du-Sud ewes, blood was withdrawn continuously and samples were pooled over 4 min periods for 19-27 h. In the resulting profiles, individual pulses were better defined but the pulse frequencies (median interval 15.2 min during ascending phase) and amplitudes (median 35.0 ng/ml) varied in similar fashion to the Merinos. Using a simple mathematical description of pulsatile secretion, and assuming that LH is only released in pulses, we also found that the high concentrations normally achieved during the surge can only be explained by pulse frequencies and amplitudes well above those normally observed in other physiological situations, including the follicular phase or following ovariectomy. These findings strongly support the experimental data and the combined results of the two studies suggest that the initiation and execution of the LH surge is probably accomplished by changes in the amplitude of pulses. A high frequency is also essential and our data suggest that it may be established some time before the onset of the surge proper, but this requires confirmation. Respectively, the changes in amplitude and frequency reflect the pituitary and hypothalamic components of the positive feedback mechanism. The theoretical study suggested that a change in clearance rate could also contribute to the surge and this awaits further investigation.