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Mechanisms of Hormonal
Regulation
Chapter 20
1
Hormones

General characteristics

Specific rates and rhythms of secretion
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Diurnal, pulsatile and cyclic, and patterns depending
on circulating substances
Operate within feedback systems
Affect only cells with appropriate receptors
The liver inactivates hormones, rendering the
hormones more water soluble for renal excretion
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The Endocrine System
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Regulation of Hormone Release

Hormones are released:

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In response to an alteration in the cellular
environment
To maintain a regulated level of certain
substances or other hormones
Hormones are regulated by chemical,
hormonal, or neural factors
Negative feedback
Positive feedback
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Feedback
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Hormone Transport

Hormones are released into the circulatory
system by endocrine glands
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
Water-soluble hormones circulate in free,
unbound forms
Lipid soluble hormones are primarily circulating
bound to a carrier
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Cellular Mechanism of Hormone
Action


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Target cell
Up-regulation
Down-regulation
Hormone effects
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
Direct effects
Permissive effects
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Cellular Mechanism of Hormone
Action

Hormone receptors
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
Water-soluble hormones
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
Located in or on the plasma membrane or in the
intracellular compartment of the target cell
High molecular weight
Cannot diffuse across the plasma membrane
Lipid-soluble hormones

Easily diffuse across the plasma membrane and
bind to cytosolic or nuclear receptors
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Cellular Mechanism of Hormone
Action

Water-soluble hormones



First messenger
Signal transduction
Second-messenger molecules

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
Calcium
Cyclic adenosine monophosphate (cAMP)
Cyclic guanosine monophosphate (cGMP)
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Cellular Mechanism of Hormone
Action
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Cellular Mechanism of Hormone
Action

Lipid-soluble hormones

Steroid hormones


Diffuse across the plasma membrane
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
Androgens, estrogens, progestins, glucocorticoids,
mineralocorticoids, and thyroid hormones
Bind to cytoplasmic or nuclear receptors
Activate


RNA polymerase
DNA transcription and translation
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Lipid-Soluble Hormones
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Structure and Function of the
Endocrine Glands

Hypothalamic-pituitary axis
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
Hypothalamus
Pituitary gland
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
Anterior pituitary (adenohypophysis)
Posterior pituitary (neurohypophysis)
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The Pituitary Gland
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Posterior Pituitary Hormones


Synthesized with their binding proteins in the
supraoptic and paraventricular nuclei of the
hypothalamus
Secreted by the posterior pituitary

Antidiuretic hormone (ADH)
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
Controls plasma osmolality
Oxytocin

Uterine contractions and milk ejection in lactating
women
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Posterior Pituitary Hormones
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Anterior Pituitary Hormones
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Adrenocorticotropic hormone
Melanocyte-stimulating hormone
Growth hormone
Prolactin
Thyroid-stimulating hormone
Luteinizing hormone
Follicle-stimulating hormone
β-lipotropin
β-endorphins
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Anterior Pituitary Hormones
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Thyroid and Parathyroid Glands

Thyroid gland
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
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Two lobes lateral to the trachea
Isthmus
Follicles (follicle cells surrounding colloid)
Parafollicular cells (C cells)


Secrete calcitonin
Regulation of thyroid hormone secretion

Thyrotropin-releasing hormone and thyroid
stimulating hormone
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Thyroid and Parathyroid Glands

Thyroid hormones
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90% T4 and 10% T3
Bound to thyroxine-binding globulin, thyroxinebinding prealbumin, or albumin
Affect growth and maturation of tissues, cell
metabolism, heat production, and oxygen
consumption
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Thyroid and Parathyroid Glands
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Thyroid and Parathyroid Glands

Parathyroid glands

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Small glands located behind the upper and lower
poles of the thyroid gland
Produce parathyroid hormone


Regulator of serum calcium
Antagonist of calcitonin
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Parathyroid Glands
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Parathyroid Glands
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Endocrine Pancreas


The pancreas is both an endocrine and an
exocrine gland
Houses the islets of Langerhans


Secretion of glucagon and insulin
Cells
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Alpha—glucagon
Beta—insulin
Delta—somatostatin and gastrin
F cells—pancreatic polypeptide
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Endocrine Pancreas
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Endocrine Pancreas

Insulin

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

Synthesized from proinsulin
Secretion is promoted by increased blood glucose
levels
Facilitates the rate of glucose uptake into the cells
of the body
Anabolic hormone

Synthesis of proteins, lipids, and nucleic acids
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Endocrine Pancreas

Glucagon



Secretion is promoted by decreased blood glucose
levels
Stimulates glycogenolysis, gluconeogenesis, and
lipolysis
Somatostatin

Possible involvement in regulating alpha and beta
cell secretions
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Endocrine Pancreas
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Adrenal Glands

Adrenal cortex
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80% of an adrenal gland’s total weight
Zona glomerulosa
Zona fasciculata
Zona reticularis
Adrenal medulla

Innervated by the sympathetic and
parasympathetic nervous systems
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Endocrine Pancreas
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Adrenal Glands

Adrenal cortex


Stimulated by adrenocorticotropic hormone
(ACTH)
Glucocorticoid hormones

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Direct effects on carbohydrate metabolism
Anti-inflammatory and growth-suppressing effects
Influence awareness and sleep habits
Most potent naturally occurring glucocorticoid is
cortisol
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Adrenal Glands

Adrenal cortex

Mineralocorticoid hormones

Affect ion transport by epithelial cells
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Increase the activity of the sodium pump of the epithelial
cells
Cause sodium retention and potassium and hydrogen loss
Most potent naturally occurring mineralocorticoid is
aldosterone

Regulated by the renin-angiotensin system
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Aldosterone
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Adrenal Glands

Adrenal cortex

Adrenal estrogens and androgens

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Estrogen secretion by the adrenal cortex is minimal
The adrenal cortex secretes weak androgens

Androgens are converted by peripheral tissues to stronger
androgens such as testosterone
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Adrenal Glands

Adrenal medulla

Chromaffin cells (pheochromocytes)
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Chromaffin cells secrete the catecholamines
epinephrine (majority) and norepinephrine
Release of catecholamines has been characterized
as a “fight or flight” response
Catecholamines promote hyperglycemia
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Catecholamines
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Neuroendocrine Response to Stress



The endocrine system reacts with the nervous
system to respond to stressors
The stress response also involves the immune
system
Influenced by corticotropin-releasing
hormone from the hypothalamus
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Tests of Endocrine Function



Radioimmunoassay
Enzyme-linked immunosorbent assay
(ELISA)
Bioassay
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Aging and the Endocrine System

Thyroid gland


Glandular atrophy, fibrosis, nodularity, and
increased inflammatory infiltrates
Parathyroid glands

Related to alterations in calcium balance


Inadequate intake, malabsorption, or renal changes
Adrenal glands

Decreased clearance of cortisol
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