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Chapter: Anatomy and Physiology for Health Professionals: Endocrine System

Hormones are carried to all parts of the body, in the blood. They are either steroids or amino acid based.


Hormones are carried to all parts of the body, in the blood. They are either steroids or amino acid based. Steroid hormones are made from cholesterol. Non-steroids include amines, glycoproteins, peptides, and proteins, which are made from amino acids. They are more numerous than steroid hormones and have differing sizes of molecules. All types of hormones can stimulate target cell changes, even when they are present in small amounts. The type of target cell influences the precise response that occurs. For example, epinephrine stimulates certain smooth muscle cells to contract but does not cause other types of cells to contract. FIGURE 16-2 shows three classes of hormones and TABLE 16-2 lists vari-ous types of hormones.

Steroid hormones, which are synthesized from cholesterol, are insoluble in water but soluble in lip-ids, which are also known as fats. Therefore, they can easily diffuse into nearly any cell in the body because lipids make up most cell membranes. Only gonadal and adrenocortical hormones are steroids. FIGURE 16-3 depicts the following steps involved in a steroid hor-mone entering a target cell:

1. The steroid hormone diffuses through the cell membrane.

2. It binds to receptors in the nucleus of its tar-get cells.

3. The resulting complex binds inside the nucleus to certain parts of the target cell’s DNA, activating transcription of spe-cific genes into messenger RNA (mRNA) molecules.

4. These mRNA molecules leave the nucleus and enter the cytoplasm.

5. The mRNA molecules are associated with ribosomes, directing the synthesis of cer-tain proteins.

However, most hormones are amino acid based. Molecular sizes of these hormones range from simple amino acid derivatives to peptides to proteins. The nonsteroidal hormones usually bind receptors in tar-get cell membranes by uniting with the binding site of their receptor. The activity site of a hormone is where it exerts its effects. Receptor binding can alter enzyme function and membrane transport mechanisms. The first messenger is the hormone that triggers this chainof biochemical activity.

A third group of hormones, the eicosanoids, include, paracrines, leukotrienes and prostaglandins. These substances are biologically active lipids manufac-tured from arachidonic acid. Nearly all cell membranes­ release them. Leukotrienes release signals that regu-late inflammation and certain allergic reactions.

Prostaglandins are lipids made from arachi-donic acid in cell membranes of the kidneys, heart, liver, lungs, pancreas, brain, reproductive organs, and thymus. They usually act more locally than hormones and are very potent in small quantities. They are made just before release and are rapidly inactivated. They often have diverse or opposite effects. Prostaglandins stimulate hormone secre-tions and influence sodium and water movements in the kidneys helping to regulate blood pressure. They increase uterine contractions during the birth-ing process, and also have effects on blood clotting, inflammation, and pain.

Second Messenger Systems

Second messengers are biochemicals in the cell thatcause changes in response to the hormone’s binding. Signal transduction describes the entire process of direct chemical communication from outside cells to inside them. Cyclic adenosine monophosphate(cAMP) is the second messenger associated with onegroup of hormones. FIGURE 16-4 shows how it works, following the steps listed:

1.A hormone binds its receptor.

2.The resulting complex activates a G protein.

3.This activates the enzyme adenylate cyclase, which is a membrane protein.

4.This catalyzes the circularization of adenos-ine triphosphate into cAMP.

5.This activates enzymes called protein kinases to cause phosphorylation, altering the shapes of and activating substrate molecules. Protein kinase C, when activated, results in phos-phorylation of calcium channel proteins. This opens channels, allows extracellular calcium ions to enter the cells, and begins a positive feedback loop. This rapidly elevates intracellular calcium ion concentrations.

6.The calcium ions act as messengers, usu-ally in combination with the intracellular protein calmodulin. Certain cytoplasmic enzymes are activated, resulting in stimu-latory effects occurring when epinephrine or norepinephrine activates α1 receptors.

The activation of calmodulin is also used in response to oxytocin and to certain regula-tory hormones from the hypothalamus.

Hormones act on their receptors in one of two general ways. The steroid and thyroid hormones, which are lipid soluble, act on receptors inside the cell, directly activating genes. The amino acid–based hormones, except thyroid hormone, are water soluble and act on receptors in the plasma membrane. These receptors are usually coupled to one or more intra-cellular second messengers by regulatory molecules known as G proteins. The intracellular second messen-gers mediate the response of the target cell.

Cellular processes are then altered by these steps to cause the hormone’s effects, based on the kinds of protein substrate molecules present. The enzyme phosphodiesterase­ then inactivates the cAMPquickly. Because of this, a target cell’s continuing response requires a continuing signal from hormone molecules that bind the target cell’s membrane recep-tors. Other second messengers include diacylglycerol and inositol triphosphate.

A cell must have specific receptor proteins on its plasma membrane or interior, to which the hormone can bind, to respond to the hormone. For example, the receptors for adrenocorticotropic hormone (ACTH) are usually found on specific cells of the adrenal cortex. Oppositely, thyroxine receptors are found in almost all body cells because thyroxine is the main hormone that stimulates cellular metabolism. Target­ cell activation also depends on three additional factors:­ the blood levels of the hormone, the amount of receptors for that hor-mone on or in target cells, and the strength or affinity of how the hormone and receptor are bound. Hormones also influence the amount of receptors that respond to other hormones. An example is progesterone, which antagonizes the actions of estrogen by down-regulating estrogen receptors in the uterus. Conversely, estrogen enhances the same cells’ ability to respond to proges-terone by causing them to produce more progesterone receptors. The process of up-regulation involves the absence of a hormone, which triggers an increase in the amount of hormone receptors.

1. List several examples of endocrine and exocrine glands.

2. What are the effects of steroid hormones on a cell?

3. Describe target cells.

4. Describe the types of hormones that consist entirely of lipid-soluble hormones

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