Corpus luteum regression induced by ultra-low pulses of prostaglandin F2 alpha.

In view of the pulsatile nature of PGF2 alpha secretion from the ovine uterus at the time of luteolysis, experiments were designed to examine the effect of pulsed infusions of PGF2 alpha on luteal function and to re-examine the minimal effective levels of PGF2 alpha required to induce luteolysis. To mimic physiological conditions, hour-long infusions of PGF2 alpha in increasing concentrations were given either 4 times in 19 h or 5 times in 25 h into the arterial supply of the autotransplanted ovary in conscious sheep on day 12 of an induced cycle. Blood flow and progesterone secretion rate from the ovary were used to monitor directly the luteolytic effect of administered PGF2 alpha. The concentration of LH in peripheral plasma was measured throughout each infusion experiment and the presence of a preovulatory peak of LH was used as an indicator of the permanence of luteal regression. Four pulses of PGF2 alpha in 19 h caused complete corpus luteum regression in only 1 of 4 animals whereas the addition of a fifth pulse (5 pulses in 25 h) caused permanent regression in 4 out of 4 animals. Infusion of 5 hour-long pulses of saline or PGF2 alpha at a rate less than 0.04 microgram/h did not induce permanent suppression of progesterone secretion. The average total effective dose of PGF2 alpha required to induce luteal regression when given as 5 pulses was 1/40th of the amount currently regarded as the minimal effective one when given by constant infusion into the ovarian artery. In another series of experiments the luteolytic effect of a single hour-long pulse of 0.1 microgram/h PGF2 alpha given daily for either 3 or 4 days was investigated. A significant fall (ANOVA, F0.01) in progesterone secretion rate, which reached a nadir at 5.3 +/- 2.2 h (means +/- S.D., n = 15), was followed by a recovery of progesterone secretion rate. Permanent luteal regression did not occur with this protracted regimen, suggesting that a relatively short pulse frequency of PGF2 alpha over a minimal period of 24 h is a necessary condition for physiological regression of the corpus luteum in sheep.