Mouse oocytes matured in vitro in chemically defined medium were not penetrated by spermatozoa. The time required for dissolution of the zona pellucida of such oocytes by alpha-chymotrypsin was much longer than that for ovulated oocytes. Addition of fetal calf serum to the medium for maturation of oocytes improved the incidence of sperm penetration and shortened the time of enzymic dissolution of the zona pellucida. These results suggest that the low rate of fertilization of oocytes matured in vitro is mainly due to qualitative changes of the zona pellucida, which could be overcome by a factor or factors in fetal calf serum.

Regulation of mammary prolactin receptors by steroid hormones was investigated in ovariectomized mid-pregnant mice. Ovariectomy increased the number of mammary prolactin receptors per cell with no effect or a slight decrease in dissociation constant (Kd). The simultaneous removal of adrenals prevented this increase in numbers. A single injection of glucocorticoid (corticosterone or cortisol) in ovariectomized-adrenalectomized mice restored the number of prolactin receptors in mammary glands to the same level as that in ovariectomized controls without changing the Kd. Aldosterone, deoxycorticosterone and oestradiol did not affect the number of mammary prolactin receptors after ovariectomy-adrenalectomy. Serum concentration of prolactin was not influenced by the hormone manipulation except with injections of oestradiol or cortisol and apparently did not correlate with the number of prolactin receptors. These results indicated that glucocorticoids are required for the increase in the number of mammary prolactin receptors induced by ovariectomy in mid-pregnant mice.

No abstract

Correlations between numbers and dissociation constants of mammary prolactin receptors of lactating mice and litter weight were examined. The apparent numbers and dissociation constants of mammary prolactin receptors were obtained from Scatchard plots with inhibition of iodine-125 prolactin binding by various concentrations of unlabeled prolactin. Litter weight measured 5 h after separation from the mother showed a nearly quadruple, almost linear increase during the first 15 d after birth. The dissociation constant for prolactin binding was fairly constant through lactation and did not correlate with either number of prolactin receptors or litter weight. The number of prolactin receptors in mammary cells increased rapidly in early lactation, reached a maximum at mid-lactation, began to decrease thereafter, and was correlated closely with litter weights on d 5 (.70) and 10 (.64) postpartum, suggesting that number of prolactin receptors represents the lactational potential of the lactating mouse.