ABSTRACT
The purpose of these experiments was to investigate the part played by the pituitary gland and the uterus in the control of the corpus luteum in the sheep. Six experiments were carried out, as follows:
Hypophysectomy early and late in the oestrous cycle. Hypophysectomy on days 2–5 allowed the corpus luteum to continue secreting normal amounts of progesterone for up to 9 days, but thereafter the secretion declined. Hypophysectomy on days 9–10 caused the progesterone secretion to fall within 4 days. Thus it seems that the corpus luteum has a limited functional life span, and it normally requires the presence of a pituitary luteotrophin during the second half of the oestrous cycle if it is to secrete normally for 15 days. Pituitary stalk section early and late in the oestrous cycle. Regardless of the time of stalk section, the corpus luteum behaves as it would during a normal cycle. Thus pituitary stalk section does not produce the same effect as hypophysectomy, and seems to allow the severed pituitary gland to continue secreting luteotrophin. Hypophysectomy and hysterectomy carried out simultaneously early in the cycle. The results were similar to those in 1, progesterone secretion having declined significantly by days 12–15. This confirms that the declining secretory activity is due to a deficiency of pituitary luteotrophin, and not to any uterine luteolytic effect in this experiment. Pituitary stalk section and hysterectomy carried out simultaneously early in the cycle. In striking contrast to 3, some corpora lutea were still
secreting progesterone normally on day 18. This shows once again that the isolated pituitary gland can continue to secrete luteotrophin. The different responses in this experiment and 2 emphasise the fact that the uterine luteolytic effect is normally dominant to the pituitary luteotrophic stimulus. Thus it would be impossible to demonstrate luteotrophic activity if the uterus were still present. Hysterectomy carried out at mid cycle, followed by hypophysectomy 20–30 days later. Whilst hysterectomy alone prolongs the secretory activity of the corpus luteum, subsequent hypophysectomy results in a rapid decline in progesterone secretion, commencing 48 hours after the operation. Thus the corpora lutea prolonged by hysterectomy, unlike those of the normal cycle, require daily pituitary luteotrophin secretion for their continued existence. Hysterectomy carried out at mid cycle, followed by pituitary stalk section 20–30 days later. Unlike 5, stalk section allows the corpora lutea to continue to secrete progesterone in large amounts for at least 15 days after the operation. This experiment, together with 2 and 4, once again emphasises that the stalk-sectioned pituitary gland can continue to secrete luteotrophin, at least for a time.
These experiments therefore support the view that the cyclical corpus luteum of the sheep is under a dual control. There is a pituitary luteotrophin, whose secretion continues after stalk section, and a uterine luteolysin that is dominant to the luteotrophic stimulus, and can still function normally after pituitary stalk section.
The fact that the corpus luteum of the hysterectomised animal cannot function for more than about 15 days after stalk section suggests that the luteotrophic stimulus may be complex, possibly envolving more than one hormone.
