Pituitary Ovarian Relationships in the Cow

Abstract

Ovulation in the cow, as in other species, appears to be accomplished by increasing secretion rates of follicle-stimulating hormone and luteinzing hormone during proestrus and estrus. Plasma levels of LH, as measured by a recently developed solid phase radioimmunoassay, rise markedly at the beginning of estrus and decline to quite low levels during the first three days of the cycle. Luteinizing hormone appears to be the major luteotrophin responsible for corpus luteum growth and progesterone secretion. One or more coincident peaks in plasma levels of progesterone and luteinizing hormone were noted during the luteal phase of the cycle (between Days 7 and 18) in some but not in all animals. These peaks were individually variable but usually occurred at about 4- to 5-day intervals. Plasma progesterone levels declined precipitously between Days 16 and 19, after which plasma LH levels rose sharply. Progesterone appears to dominate the events that occur during the cycle; it is only after the pituitary is released from the apparent progesterone inhibition that the blood levels of luteinizing hormone necessary for estrus and ovulation are attained. The role of estrogens in luteinizing hormone release in the cow have not yet been studied. Conversely, corpus luteum regression is not from inhibition of LH secretion; it occurs at a time when plasma luteinizing hormone levels are rising rapidly. The mechanism by which this rapid regression of luteal function occurs is incompletely understood; it may not involve anterior pituitary hormones at all and probably involves a mechanism requiring the presence of the uterus.The roles of prolactin, follicle-stimulating hormone, and estrogens in regulation of corpus luteum function are inadequately understood. Exogenous estrogen is luteolytic, and levels of follicle-stimulating and luteinizing hormones that promote increased estrogen secretion may promote luteolysis. Stress may increase plasma progesterone, which could conceivably interfere with reproductive functions.