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Chapter 48
Mechanisms of Endocrine
Control
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Functions of the Endocrine System
• Growth and development
• Sex differentiation
• Metabolism
• Adaptation to an ever-changing environment
– Regulation of digestion
– Use and storage of nutrients
– Electrolyte and water metabolism
– Reproductive functions
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Mechanisms of Hormones
• Hormones interact with high-affinity receptors.
– These are linked to one or more effector system in
the cell.
• Some receptors are located on the surface of the cell.
– These act through second messenger mechanisms.
• Others are located in the cell.
– They modulate the synthesis of enzymes, transport
proteins, or structural proteins.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Hypothalamic Hormones Regulating the
Secretion of Anterior Pituitary Hormones
• Hypothalamus is the coordinating center of the brain for
endocrine, behavioral, and autonomic nervous system function.
• GH-releasing hormone (GHRH)
• Somatostatin
• Dopamine
• Thyrotropin-releasing hormone (TRH)
• Corticotropin-releasing hormone (CRH)
• Gonadotropin-releasing hormone (GnRH)
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Monitoring Changes in Hormone Levels
• Positive feedback
• Negative feedback
• Level of the substance the hormone regulates
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Hormones
• Function as chemical messengers
– Move through the blood to distant target sites of
action
– Or, act more locally as paracrine or autocrine
messengers that incite more local effects
– Most are present in body fluids at all times in greater
or lesser amounts as needed.
• Characteristics
– A single hormone can exert various effects in
different tissues.
– A single function can be regulated by several
hormones.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Hormone Actions
• Paracrine Actions
– Hormones acting locally on cells other than those
that produced the hormone
– For example, the action of sex steroids on the ovary
• Autocrine Actions
– Hormones exerting action on the cells from which
they were produced
– For example, the release of insulin from pancreatic
beta cells can inhibit its release from the same cells.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Hormone Synthesis and Secretion
• Protein and polypeptide hormones are synthesized and
stored in vesicles in the cytoplasm of the endocrine cell
until secretion is required.
• Steroid hormone are secreted upon synthesis.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Categories of Hormones According
to Structure
• Amines and amino acids
• Peptides, polypeptides, proteins, and glycoproteins
• Steroids
• Fatty acid derivatives
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Which of the following is a characteristic of a hormone?
– A. A single hormone can exert various effects in
different tissues.
– B. A single function can be regulated by several
hormones.
– C. Both
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• C. Both
• Rationale: These are both characteristic of hormones and
are at the heart of the functionality of hormones.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Actions of Hormones Released
into the Bloodstream
• Circulate as free, unbound molecules
– Peptide hormones and protein hormones usually
circulate unbound in the blood.
• Circulate as hormones attached to transport carriers
– Steroid hormones and thyroid hormone are carried
by specific carrier proteins synthesized in the liver.
• Intracellular and extracellular mechanisms participate in
the termination of hormone function.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Factors Affecting Response of a Target
Cell to a Hormone
• Number of receptors present
– Up-regulation
– Down-regulation
• Affinity of these receptors for hormones
– Affected by a number of conditions
– For example, the pH of the body fluids plays an
important role in the affinity of insulin receptors.
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Hormone Receptor Interactions—Second
Messenger
• Glucagon
• Insulin
• Epinephrine
• Parathyroid hormone
• Thyroid-stimulating hormone (TSH)
• Adrenocorticotropic hormone (ACTH)
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Hormone Receptor Interactions—Second
Messenger (cont.)
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Antidiuretic hormone (ADH)
• Secretin
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Hormone Receptor Interactions—
Intracellular Interactions
• Estrogens
• Testosterone
• Progesterone
• Adrenal cortical hormones
• Thyroid hormones
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Control of Hormone Levels
• Affected by diurnal fluctuations that vary with the sleep–
wake cycle
– Growth hormone (GH) and adrenocorticotropic hormone
(ACTH)
• Secreted in a complicated cyclic manner
– Female sex hormones
• Regulated by feedback mechanisms that monitor
substances such as glucose (insulin) and water (ADH) in the
body
• Regulated by feedback mechanisms that involve the
hypothalamic–pituitary target cell system
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Five Cell Types of the Anterior Pituitary
Gland
• Thyrotrophs: produce thyrotropin, also called thyroid
stimulating hormone (TSH)
• Corticotrophs: produce corticotrophin, also called
adrenocorticotropic hormone (ACTH)
• Gonadotrophs: produce the gonadotropins, luteinizing
hormone (LH), and follicle-stimulating hormone (FSH)
• Somatotrophs: produce growth hormone (GH)
• Lactotrophs: produce prolactin
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Function of Hormones Produced by the
Anterior Pituitary
• Body growth and metabolism (GH)
• Function of the thyroid gland (TSH)
• Glucocorticoid hormone levels (ACTH)
• Function of the gonads (FSH and LH)
• Breast growth and milk production (prolactin)
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Is the following statement True or False?
• Most hormones are regulated via positive feedback
mechanism.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• False
• Rationale: In fact, some are regulated via positive
feedback, but the vast majority are regulated by negative
feedback mechanisms.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Which of the following is a mechanism of hormone
regulation?
– A. Affinity of cellular receptors
– B. Production of intracellular second messengers
– C. Number of cell surface receptors
– D. Levels of circulating albumin
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• C. Number of cell surface receptors
• Rationale: Number of cell surface receptors is a direct
cellular response and mechanism of regulation of the
hormone action.
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
Measures for assessing Endocrine
Function and Hormone Levels
• Blood tests
• Urine tests
• Stimulation and suppression tests
• Genetic tests
• Imaging
Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins