A luteotrophic action of prolactin: suppression of intraluteal prostaglandin production or effect?

Abstract

Adult rats were hypophysectomized and their pituitary glands autotransplanted on day 2 of an estrous cycle (day 1, ovulation); the rats were either hysterectomized or sham hysterectomized immediately before this operation. On day 21, a few days after the beginning of the 3+ month period of regression of progesterone secretion, they were injected sc with 500 micrograms prostaglandin F2 alpha (PGF2 alpha) either on day 21 alone, on days 21 and 22, on days 21, 22, and 23, or on days 21 and 23; controls were injected with saline on days 21, 22, and 23. In the latter the serum progesterone level slowly fell, as it does in untreated rats subjected to pituitary autotransplantation on day 2. PGF2 alpha treatment induced a rapid, drastic fall in the progesterone level, but in each group except the one injected on days 21 and 23, the level returned to, or close to, the control level in about 10 days. Even in the group injected on days 21 and 23, the pattern of serum progesterone resembled the others although the values were much lower. In a similar group of rats subjected to pituitary autotransplantation on day 2 (all of which were also hysterectomized on day 2), a single injection of 500 micrograms PGF2 alpha (which again induced the same changes in serum progesterone as in the first experiment) was combined with either a 6-h or 4-h withdrawal of PRL; this was achieved by injecting 2 bromo-alpha-ergocryptine (CB-154) at zero time to lower PRL secretion, and beef PRL, in a delaying vehicle, 6 or 4 h later, followed by a second dose of PRL 18 h after the first. When combined with such a brief period of PRL withdrawal, the PGF2 alpha treatment induced a very rapid, drastic and permanent fall in serum progesterone, equal to that induced by CB-154 alone, or by removing the pituitary transplant on day 21. The brief periods of PRL withdrawal, by themselves had very little (6 h) or no (4 h) effect on the progesterone level, and treatment with PRL alone was also without effect. Treatment with CB-154, PGF2 alpha, and PRL simultaneously (i.e. without a period of PRL withdrawal) had the same effect as treatment with PGF2 alpha alone.(ABSTRACT TRUNCATED AT 400 WORDS)