Heat begins when the pituitary gland releases follicle stimulating hormone (FSH), which causes the ovaries to develop egg follicles and begin to produce estrogen. Estrogen causes the vulva to swell and the lining of the uterus to shed blood. This is responsible for the bloody discharge of proestrus.
During proestrus there is a steady increase in estrogen concentration, which peaks one to two days before the beginning of estrus and then drops precipitously (see the chart on page 438). LH follows a similar course, but peaks a day or two after estrogen and then drops precipitously. The surge and drop in LH signals the onset of estrus and triggers ovulation two days later. Fertile matings can occur three days before the LH peak through six to seven days after the peak.
LH also ensures that the ovulatory follicles convert to corpora luteal cysts and begin to produce progesterone. The serum progesterone level parallels the rise in LH. It begins to rise during the last two days of proestrus, rises above 2 ng/ml as the LH peaks, continues to rise during estrus, remains elevated for 8 to 10 weeks during diestrus, and returns to baseline levels dur- ing anestrus. Progesterone is essential for keeping the uterine lining receptive to the implantation and growth of embryos. Removal of the ovaries during early pregnancy, or failure of the corpora luteal cysts to produce progesterone, results in spontaneous abortion.
The hormonal effects of the estrous cycle have implications for preventing pregnancy (see Contraceptive Drugs, page 457). If progesterone is given during the first three days of proestrus, it blocks the release of pituitary FSH and aborts the heat cycle. Testosterone works by blocking the release of LH from the pituitary gland. It must be given 30 days before estrous to abort the heat cycle.