Endocrine role

Thyroid gland. One of the endocrine glands, normally situated in the lower part of the front of the neck and consisting two lobes, one on either side of the trachea and joined in front by a narrow isthmus. It excretes, stores, and liberates the thyroid hormones, which play major endocrine roles in regulating the metabolic rate. 

Daughaday WH. The possible autocrine/paracrine and endocrine roles of insulin-ltke growth factors of human tumors. Endocrinology 1990 127 1-4. 

The pancreas is a major exocrine organ that synthesizes and secretes digestive enzymes. It also produces and secretes NaHCOj to neutralize the acidic effluent from the stomach. The pancreas also has an important endocrine role because it synthesizes and secretes the hormones insulin, glucagon, and somatostatin into the bloodstream from cells within its islets of Langerhans. 

An endocrine role has also been postulated in some species (84). Injection of teratocytes from the braconid parasitoid M. croceipes into larvae of the tobacco budworm, Heliothis virescens, caused an elevation of the ecdysteroids, an increase in JH levels and a decrease in the JH esterase activity in the host (85, Dahlman, D. L. Arch. Insect Biochem. Physiol., in press). 

There is growing evidence that several hormones that have endocrine roles also operate at the local tissue level. For example, endotheUal and smooth muscle cells contain all the components of the renin-angiotensin system and UPA axis and the potential to synthesize Angll and cortisol (106-108). 

CJ-Receptors are localized ia the brain stem and limbic stmcture, regions associated with endocrine function (76). In the periphery, CJ-receptors are found in the Hver, heart, ileum, vas deferens, and on lymphocytes and thymocytes. Although there is insufficient evidence to clearly define the functional role of CNS CJ-sites, based on the effects of PCP and the interaction of haloperidol with CJ-sites, CJ-receptor ligands may be antipsychotics or used for the treatment of substance abuse. Several CJ-receptor ligands have shown neuroprotective effects in vivo. Ifenprodil (315) and CNS 1102 (316) are being developed for treatment of stroke (Table 18). 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Interaction of vitamin D and its metaboUtes with sex hormones has been demonstrated, particularly ia birds ia which the egg-laying functions combine calcium needs and reproductive activity. The metaboUtes of vitamin D behave as hormones. As such, they play an active role ia the endocrine system, along with other hormones, to maintain the various body functions. Several biological influences of metaboUtes of vitamin D have been studied, including effects related to cancer (193—197), skin diseases (198—201), immunomodulatory effects (202,203), and Alzheimer s disease (204—206) (Fig. 9). 

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. 

Among vertebrate species, the neuro-endocrine-immime system is responsible for many complex, inter-related physiological processes including neuronal, homeostatic, reproductive and immune functions. There are four main types of hormone polypeptides, eicosanoids, steroids and thyroid hormones. Reflecting the inter-dependency of the neiiro-endocrine and immune systems, hormones, neuropeptides and other neiirotransmitters are known to be produced by some immune cells and play a role in the regulation of the immune system, while endocrine and nervous tissues express receptors for many substances produced by the immune system. The major focus of interest in endocrine disruption has... 

In order to fully appreciate the potential implications of endocrine disruption, it is important to consider the normal role of hormones in controlling the development and functioning of the reproductive systems of the different vertebrate classes. 

In addition to their endocrine disrupting properties, it must be appreciated that many of the chemicals in question possess more general toxic properties, which may be potentiated by metabolism by the organism. Several PAHs, PCBs and PCDDs are carcinogenic, while certain phthalate esters can enhance the excretion of zinc, potentially leading to zinc deficiency. Zinc, an essential element, plays a vital role in spermatogenesis and mature T-cell production. Deficiency may result in abnormalities of the male reproductive system, depletion of spermatogenesis and suppression of the immune system. 

The thyroid gland, located in the base of the neck, exerts i key role on growth and metabolism. In contrast with that of some of the other endocrine glands, this control is effected Ihrough a pair of relatively simple molecules, thyroxine, and its close congener, triiodothyronine. Cases of thyroid deficien-cy (hypothyroidism) are common enough to warrant the production... 

In general, the sterols perform a structural function, for example as components of the lipid layers of membranes. The Cis, C19 and C21 steroids mainly perform an endocrine function. In other words they are hormones. The bile salts (C24-steroids) fulfil a functional role in digestion in animals. 

Additional roles traditionally recognised for white adipose tissue include thermal insulation (as in the blubber of sea mammals) and mechanical protection to internal organs. However, in recent years it has become apparent that the function of white adipose tissue is much more extensive. This follows from the discovery that it is a major endocrine organ, secreting a diverse... 

Trayhurn P (2005) Endocrine and signalling role of adipose tissue new perspectives on fat. Acta Physiol Scand 184 285-293... 

Lesions of the lateral hypothalamic area (LHA) cause anorexia, whereas ablation of the paraventricular nucleus (PVN) cause a hyperphagic obesity syndrome. Consistent with these results, LHA neurons express the orexigenic neuropeptides MCH and orexin. PVN neurons produce several neuropeptides that are anorex-igenic when administered directly into the brain (CRH, TRH, oxytocin), in addition to their better known roles as endocrine regulators. LHA and PVN receive rich inputs from axons of NPY/AgRP and aMSH/CART-producing neurons in the arcuate nucleus. 

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