In a series of experiments on female miniature pigs, the pattern of plasma LH and progesterone levels during the oestrous cycle, late pregnancy and lactation and after ovariectomy were characterized, and the effect of pentobarbitone treatment was tested.
The preovulatory surge of LH occurred in seven out of eight animals between 00.00 and 12.00 h on day 0 of the oestrous cycle (day 1 of standing heat). Plasma progesterone started to decline 8 days before oestrus and reached its lowest value 5 days before the preovulatory LH peak. Increases in progesterone concentration were already noticeable 48 h after the LH surge. During late pregnancy, parturition and lactation, plasma LH was low and showed only minor fluctuations, while plasma progesterone declined 4 to 5 days before parturition. Both hormones remained at low levels throughout lactation. Three weeks before parturition increases in LH were always followed by an increase in progesterone. This dependency was greatly diminished immediately before delivery. Four to 12 days after weaning the animals came into oestrus which was followed by an increase in LH and later an increase in progesterone concentrations.
Ovariectomy during dioestrus resulted in a steady increase in plasma LH levels for 35–39 days. Ovariectomy caused abortion if performed on day 100 of pregnancy. It was followed by a rapid increase of plasma LH concentration. Normal parturition (around day 115) and lactation took place when animals were spayed on day 112 of pregnancy. In this case, plasma LH levels remained even lower than before ovariectomy as long as lactation was maintained. Immediately after weaning a rapid increase in the normal postovariectomy pattern of LH secretion was observed.
Pentobarbitone anaesthesia (30–35 mg/kg body wt, initial dose), during pro-oestrus– oestrus, for less than 5 h had no effect on the preovulatory LH increase. However, pentobarbitone anaesthesia for more than 6 h inhibited the LH peak and ovulation if the animal was under deep anaesthesia before 24.00 h on the day before oestrus. Pentobarbitone treatment of ovariectomized pigs resulted in a clear decrease in LH levels 40 min after a single i.v. dose.