Oral Contraceptives

Inhibitors of ovulation. Negative feedback control of gonadotropin release can be utilized to inhibit the ovarian cycle. Administration of exogenous estrogens (ethinylestradiol or mestranol) during the first half of the cycle permits FSH production to be suppressed (as it is by administration of progestins alone). Due to the reduced FSH stimulation of tertiary follicles, maturation of follicles and, hence, ovulation are prevented. In effect, the regulatory brain centers are deceived, as it were, by the elevated estrogen blood level, which signals normal follicular growth and a decreased requirement for FSH stimulation. If estrogens alone are given during the first half of the cycle, endometrial and cervical responses, as well as other functional changes, would occur in the normal fashion. By adding a progestin (p. 254) during the second half of the cycle, the secretory phase of the endome-trium and associated effects can be elicited. Discontinuance of hormone administration would be followed by menstruation.

The physiological time course of estrogen-progesterone release is simulated in the so-called biphasic (sequential) preparations (A). In monophasic preparations, estrogen and progestin are taken concurrently. Early administration of progestin reinforces the inhibition of CNS regulatory mechanisms, prevents both normal endometrial growth and conditions for ovum implantation, and decreases penetrability of cervical mucus for sperm cells. The two latter effects also act to prevent conception. According to the staging of progestin administration, one distinguishes (A): one-, two-, and three-stage preparations. In all cases, "withdrawal-bleeding" occurs when hormone intake is discontinued (if necessary, by substituting dummy tablets).

Unwanted effects: An increased incidence of thrombosis and embolism is attributed to the estrogen component in particular. Hypertension, fluid reten tion, cholestasis, benign liver tumors, nausea, chest pain, etc. may occur. Apparently there is no increased overall risk of malignant tumors.

Minipill. Continuous low-dose administration of progestin alone can prevent conception. Ovulations are not suppressed regularly; the effect is then due to progestin-induced alterations in cervical and endometrial function. Because of the need for constant intake at the same time of day, a lower success rate, and relatively frequent bleeding anomalies, these preparations are now rarely employed.

"Morning-after" pill. This refers to administration of a high dose of estrogen and progestin, preferably within 12 to 24 h, but no later than 72 h after coitus. Menstrual bleeding ensues, which prevents implantation of the fertilized ovum (normally on the 7th day after fertilization, p. 74). Similarly, implantation can be inhibited by mifepristone, which is an antagonist at both progesterone and glucocorticoid receptors and which also offers a noninvasive means of inducing therapeutic abortion in early pregnancy.

Stimulation of ovulation. Gonadotropin secretion can be increased by pulsatile delivery of GnRH (p. 242). The estrogen antagonists clomiphene and cy-clofenil block receptors mediating feedback inhibition of central neuroendocrine circuits and thereby disinhibit gonadotropin release. Gonadotropins can be given in the form of HMG and HCG (p. 252).

Hypophysis I FSH I I LH

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