Introduction to the Endocrine System

General Features and Definitions

Types of Hormones

Functions of the Endocrine System

Components of the Endocrine System

Chemical Structure of Hormones

Homeostasis

Control of Endocrine Activity

Regulation of Hormone Secretion

Transport and Distribution of Hormones

Mechanism of Hormone Action

Regulation of Hormone Receptors

General Features of the Endocrine System

1. Endocrine glands are ductless
2. Endocrine glands have a rich supply of blood.
3. Hormones, produced by the endocrine glands are secreted into the bloodstream.
4. Hormones travel in the blood to target cells close by or far away from point of secretion.
5. Hormones receptors are specific binding sites on the target cell.

Important Definitions:�Endocrine System

• Endocrine--endo means within. This is a system which controls body function through hormones.
• Endocrine System is composed of a number of glands.
• Glands are specialized tissues that produce a hormone.

Endocrine Glands

Important Definitions

• What are hormones?

Hormones are organic chemical messengers produced and secreted by endocrine cells into the bloodstream. Hormones regulate, integrate and control a wide range of physiologic functions.

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Silverthorn, Human Physiology, 3rd edition Figure 6-1&2

Important Definitions

• What are endocrine glands?

Endocrine glands are ductless glands comprised of endocrine cells. This means that these glands do not have ducts that lead to the outside of the body. For example, sweat glands are NOT endocrine glands (they are instead exocrine glands) because sweat glands have ducts that lead to the outside surface of your skin (that’s how the sweat gets out). The fact that endocrine glands are ductless means that these glands secrete hormones directly into the blood stream (instead of to the outside of your body).

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Important Definitions

• What are target cells?

Target cells refer to cells that contain specific receptors (binding sites) for a particular hormone. Once a hormone binds to receptors on a target cell, a series of cellular events unfold that eventually impact gene expression and protein synthesis.

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Silverthorn, Human Physiology, 3rd edition Figure 6-1&2

Important Definitions

• What are hormone receptors?

Hormone receptors are binding sites on the target cell (either on the surface or in the cytoplasm or nucleus of the target cell) that are activated only when specific hormones bind to them. If a hormone does not/cannot bind to it’s receptor, then no physiologic effect results.

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See next slide for a picture of a hormone bound to its receptor

Growth hormone regulates cell growth by binding to growth hormone receptors on target cells.

Types of Hormones

Steroid Hormones

• These are all derived from cholesterol.
• Examples: testosterone, estrogen, progesterone, mineralicoids, glucocorticoids.
• Steroids can cross the plasma membrane!

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Protein Hormones

• These are made of amino acids.
• Examples: Insulin, hypothalmus-signaling hormones.
• Protein hormones cannot cross the plasma membrane!

Chemical Structure of Hormones

• Two general classes of hormones: water soluble and lipid soluble.
• Water soluble (polar): proteins, glycoproteins, polypeptides, amino acid derivatives.
• Lipid soluble (nonpolar): steroids, amino acid derivatives, fatty acids.
• Different classes have different mechanisms of action, different modes of transport through the body, and differing stability in the circulation.

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Examples of Water Soluble Hormones

• Proteins: growth hormone, prolactin, insulin
• Glycoproteins: follicle-stimulating hormone (FSH), luteinizing hormone (LH) , thyroid-stimulating hormone (TSH)
• Polypeptides: arginine vasopressin, oxytocin, somatostatin
• Amino acid derivatives:epinephrine, melatonin

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Examples of Lipid-Soluble Hormones

• Steroids: estrogen, progesterone, testosterone, glucocorticoids, mineralocorticoids

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• Amino acid derivatives: Thyroid hormones (T3, T4)

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• Fatty acids: prostaglandins, thromboxanes

Classes of Hormones

• Steroids vs. Peptide Hormones

Hormones fall into 2 general classes based on their molecular structure and synthesis.

• All steroid hormones are made initially from the precursor (precursor = first step in biosynthetic pathway) cholesterol.

See next slide for a diagram of the biosynthetic pathway of steroid hormones

from cholesterol.

Steroid Hormones

• Steroid hormones are produced by the gonads and adrenal cortex.
• Thyroid hormones are not steroids, but will be categorized with steroids for simplicity.
• Steroid hormones are made from cholesterol in the smooth endoplasmic reticulum and mitochondria of endocrine cells.

Steroid Hormones

• Steroid hormones cannot be stored in vesicles in the endocrine cells that produce them. As soon as steroid hormones are produced, they diffuse out of the endocrine cell and enter the bloodstream.
• Steroid hormones are lipid soluble and their receptors are located inside their target cell.

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Peptide Hormones

• Peptide hormones are comprised of chains of amino acids.
• Like most proteins, peptide hormones are synthesized on ribosomes of the (rough) endoplasmic reticulum of endocrine cells.
• Peptide hormones can be stored in vesicles in endocrine cells until they are needed at some later point.

Peptide Hormones

• Peptide hormones do not readily pass through cell membranes (lipid bilayers) and they are referred to as water soluble.
• Receptors for peptide hormones are found on the cell surface of their target cells.

Some General Actions of Hormones

• Hormones cause cells to change.
• Hormones can result in changes in gene expression (DNA-RNA-Protein).
• Hormones can result in enzyme cascades which control our metabolism.
• Hormones drive our reproductive systems.

ACTION OF A NONSTEROID HORMONE�

Some Specific Actions of Hormones

• Fetal development and differentiation
• Cell growth and cancer
• Metabolism
• Cardiovascular function
• Renal function
• Skeletal function
• Reproductive function
• Immune function
• Central nervous system function

Homeostasis

• Definition: the maintenance of a constant environment (internal).
• Parameters regulated: Temperature, osmolarity, pH, nutrient levels, hormone levels, etc.
• Homeostasis is critical for cell viability and proper functioning.
• Loss of homeostasis results in disease/death.
• Homeostasis is maintained by feedback mechanisms (primarily negative feedback).

Another Example: Regulation of LH Release in the Male

• LH increases production of testosterone from the testis.
• Testosterone feeds back upon the pituitary to inhibit LH release.

testosterone

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Endocrine overview

• Hormones are released by glands.
• Hormones are released by feedback.
• Our body works to carefully regulate hormone levels.
• Negative feedback usually controls hormone secretion.

Principal functions of the endocrine system

• Maintenance of the internal environment in the body (maintaining the optimum biochemical environment).
• Integration and regulation of growth and development.
• Control, maintenance and instigation of sexual reproduction, including gametogenesis, coitus, fertilization, fetal growth and development and nourishment of the newborn.

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Response vs. distance traveled

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Endocrine action: the hormone is distributed in blood and binds to distant target cells.�Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.�Autocrine action: the hormone acts on the same cell that produced it.

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What determines the size of hormone effects?

1) The amount of hormone in the circulation (reaching the target tissue)

- the more hormone, the greater the effect

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2) The presence and number of receptors for that hormone on the target tissue.

- no receptor, no response

- some receptors, some response

- many receptors, higher response

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How do you regulate hormone levels?

• Hormones are generally not secreted at a constant rate.
• Regulation of hormone levels involves:

- regulation of hormone production

- regulation of hormone secretion (often a separate step)

- sometimes, regulation of hormone metabolism

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Patterns of Hormone Secretion

• There are three basic patterns of secretion: pulsatile, acute, and cyclic.
• Pulsatile: relatively constant level of hormone, over a long period
• Acute: rapid increase in hormone level for a short time in response to a stimulus
• Cyclic: hormone increases and decreases in a constant pattern

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Mechanism of Hormone Action: Receptors

• For hormones to act on a cell, that cell must have a receptor for that hormone.
• Receptors bind the hormone, resulting in a biological response.
• Receptors are found only in target tissues for that hormone.
• Receptors are very specific (they only bind a specific hormone, not all hormones)
• Receptors have high affinity for their hormone (bind hormone at very low hormone concentration).

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