Growth Hormone

Melatonin Serotonin

Fibroblast growth factor (FGF)

" Appendix A provides a comprehensive summary of most human hormones.

Fibroblast growth factor (FGF)

" Appendix A provides a comprehensive summary of most human hormones.

ent anatomical relationship between point A and point B.

1. Systemic

The classic systemic endocrine system is described in Figure 1-1. The hormone is biosynthesized and stored within specific cells associated with an anatomi cally defined endocrine gland. The hormone is not released until receipt of an appropriate physiological signal, which may take the form of either a change in the concentration of some component in the blood (e.g., [Ca2+], [glucose], [K+]) or the delivery of a neural signal. Once released into the bloodstream, the hormone can travel to a distant cellular target, frequently the hormone (particularly if it is a hydrophobic molecule like hypothalamic hormone release hypothalamic hormone release

Hormones Released Into The Bloodstream
FIGURE 1-2 Example of an endocrine system that involves a closed portal system that effects conservation of hormone concentrations between the source secreting system and the target receptor-containing cells.

2. Paracrine signal signal

prostaglandin receptor

FIGURE 1-3 Example of an autocrine system operating on the same cell that released it. The active hormone also could reach its receptor from the cell interior without having been secreted first.

prostaglandin receptor

FIGURE 1-3 Example of an autocrine system operating on the same cell that released it. The active hormone also could reach its receptor from the cell interior without having been secreted first.

the steroid hormones) may be carried in the bloodstream by a specific transport protein [e.g., transcortin for Cortisol, the vitamin D-binding protein for l,25(OH)2D3]. The target cell for hormone X is by definition a cell that possesses specific high-affinity receptors for hormone X. Then, as a consequence of accumulation of the hormone by the target cell, a signal transduction process ensues and a specific set of biological responses is generated. Frequently some aspect of the biological responses will result in a change in the concentration of some blood component so that a "feedback" signal is sent to the originating endocrine gland to diminish the biosynthesis and secretion of the hormone. Thus, the endocrine B cell of the pancreas secretes insulin in response to elevated blood glucose levels. The glucagon then acts upon distal liver cells to decrease the blood levels of glucose, and the decreased blood glucose then feeds back upon the pancreatic B cell to diminish the secretion of the insulin.

Classic endocrine systems involve secretory glands that release their hormone products into the open or general circulatory system (the B-cell/liver system described earlier), as well as closed circulatory systems. An example of the latter is the secretion of the hypothalamic releasing hormones into a closed portal system, which ensures that most of these hormones will be delivered to the anterior pituitary, which contains the target cells for these releasing hormones; see Figure 1-2.

In the paracrine system, the distance from secretion point A to target cell point B is sharply reduced. Here cell A biosynthesizes and secretes hormone X. Hormone X then diffuses from the secreting cell A to immediately adjacent target cells B. An example of a paracrine system is the production of testosterone by the interstitial Leydig cells of the testes, which then diffuses into the immediately adjacent seminiferous tubules.

3. Autocrine

A variation of the paracrine system is when the hormone-secreting cell and the target receptor-containing cell are one and the same; this is an autocrine system. Figure 1-3 represents a model of a paracrine and an autocrine endocrine system. Examples of autocrine hormones would be the prostaglandins and some of their relatives, such as the thromboxanes, leu-kotrienes, and lipoxins.

4. Neurotransmitters

Over the past decade, it has been increasingly accepted as a consequence of mechanistic studies that neurotransmitters are really hormones, i.e., they are

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