High doses of progesterone can apparently inhibit secretion or release of hypophyseal hormones, including the luteotrophic hormone (lth). Injection of progesterone into non-pregnant and pregnant guinea-pigs led to partial destruction of formed corpora lutea but did not prevent their initial formation.
The data presented suggest that in the guinea-pig lth is released about the time of ovulation, but for a period of less than 3 days. This temporary release of lth is sufficient to initiate formation and function of corpora lutea. No further lth support seems to occur in non-pregnant females.
In pregnant animals, a neurohumoral signalling mechanism initiated by intrauterine events of implantation is assumed to call forth a second release of hypophyseal lth, which is responsible for the maintenance of corpora lutea during the remainder of their functional life span during gestation. Prolactin is not luteotrophic in guinea-pigs.