Endocrine system estrogens

This section describes effects of exposure to PCBs on the thyroid and other non-reproductive endocrine systems. Estrogenic, anti-estrogenic, and anti-androgenic effects of PCBs are discussed in Sections 3.2.5 (Reproductive Effects), 3.5.2 (Mechanisms of Toxicity), and 3.6 (Endocrine Disruption). 

The liver plays an important role in the endocrine system. The concentrations of hormones in plasma, and the activity of the glands which secrete them, are determined by the rate at which they are deactivated by the liver. The liver also has a major function in female reproduction since it is the target tissue of ovarian estrogen, to which it responds by producing the yolk protein vitellogenin. " Xenobiotics that affect either of these functions can therefore be considered to be potential endocrine disrupters. 

Endocrine disrupters Chemicals that cause disturbances of the endocrine system (e.g., by acting as agonists or antagonists of the estrogen receptor). 

Depending upon the circumstance and desired effects, endocrine-disrupting chemicals can be either good or bad. The endocrine system is a finely balanced system responsible for fertility and many of the feminine and masculine traits we are all familiar with. Endocrine disruptors are used by millions of women in the form of the pill to control fertility. Chemicals in birth control pills subtly manipulate the endocrine system to reduce fertility. Unfortunately, we now know that many chemicals are capable of influencing the endocrine systems. When these chemicals, such as DDT and TCDD, are released into the environment, they reduce the fertility of wildlife. Exposure to endocrine disruptors is linked to decreased fertility in shellfish, fish, birds, and mammals. Endocrine disruptors such as nonylphenol have been shown to feminize male fish, interfering with reproduction. Some studies have also linked exposure to endocrine disruptors to decreases in human male sperm count. Ironically, urinary metabolites of the birth control pill as well as the female hormone estrogen pass through waste treatment plants and are released into the aquatic environment, where even small concentrations cause feminization of male fish. 

Polybrominated Biphenyls. PBBs have the potential to interact with the endocrine system based on effects that mainly include changes in levels of thyroid and female reproductive hormones. No studies were located that investigated the estrogenic and antiestrogenic activity of PBBs in vitro or in vivo at the level of the estrogen receptor. 

There are also a few examples of positive feedback mechanisms in the endocrine system.25 43 In a positive feedback loop, rising concentrations of one hormone cause an increase in other hormones, which, in turn, facilitates increased production of the first hormone. The primary example of this type of feedback occurs in the female reproductive system, where low levels of estrogen production increase the release of pituitary hormones (LH, FSH).10 43 Increased LH and FSH then facilitate further estrogen production, which further increases pituitary hormone secretion, and so on (see Chapter 30). Positive feedback mechanisms are relatively rare, however, compared with negative feedback controls in the endocrine system. 

Use of Hormones to Alter Normal Endocrine Function. Because of the intrinsic control mechanisms in the endocrine system, administration of exogenous hormones can often affect the normal release of hormones. This fact can be exploited in certain situations to cause a desired change in normal endocrine function. For instance, oral contraceptives containing estrogen and progesterone inhibit ovulation by inhibiting the release of LH and FSH from the anterior pituitary. 

Therefore, when it became clearly understood that atrazine is neither estrogenic nor a genotoxic, direct-acting carcinogen - and that the atrazine-associated tumor responses appeared only in female SD rats, a strain with a high, normally occurring incidence of mammary tumors - it became important to study the effect of high doses of atrazine on the SD animal model s own endocrine system and hormonal milieu. 

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