Download 2. Steroid Hormones

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A hormone is a chemical released by a
cell or a gland in one part of the body
that sends out messages that affect
cells in other parts of the organism.
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Endocrine hormone molecules are
secreted (released) directly into the blood
stream.
Exocrine hormones (or ectohormones) are
secreted directly into a duct, and, from the
duct, they flow either into the bloodstream
or from cell to cell by diffusion in a process
known as paracrine signalling.
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Video
(AP1 EPITHELIA EXOCRINE &
ENDOCRINE GLANDS)
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Chemically, hormones are of four types:
1. Hormonal amine.
2. Peptide, protein, or glycoprotein.
3. Steroid.
4. Eicosanoid.
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1. Hormonal Amines
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2. Steroid Hormones
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Four kinds of steroid hormones differ in
structure and action; they are the
androgens (C19), the estrogens (C18), the
progestins (C21), and the corticosteroids
(C21).
All are synthesized from cholesterol .
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3. Peptide, Protein, and Glycoprotein
Hormones
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Mechanism of Hormone Action
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Hormone receptors can be classified into
three types on the basis of their locations
in the cell and the types of hormone they
bind:
Nuclear receptors, which bind
triiodothyronine (T3) after it enters the cell.
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Cytosolic receptors, which bind steroid
hormones as they diffuse into the cell.
Cell surface receptors, which detect watersoluble hormones that do not enter the
cell (peptides, proteins, glycoproteins,
catecholamines).
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Nuclear receptors of
thyroid hormones
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Video
(Thyroxine _ Mechanism of Action
of Thyroid Hormones)
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Steroid Hormone Receptors
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Video
(Steroid Hormones _ Mechanism
of Action)
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Cell Surface Receptors
intracellular second messenger is utilized to implement
the hormonal action and involves:
1. G-protein-coupled adenylate cyclase cAMP system.
2. G-protein-coupled Phosphatidylinositol Ca2+
pathway.
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G-Protein-Coupled Adenylate CyclasecAMP System
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G-Protein-Coupled PhosphatidylinositolCa2+ Pathway
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Video
(How Hormones Use G-protein
Signaling Pathways_ A Video
Review of the Basics)
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Hypothalamus
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Hypophysiotropic Peptides released by
hypothalamus
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Thyrotropin-releasing hormone (TRH)
Thyrotropin-releasing hormone is a tripeptide amide.
TRH principally stimulates the synthesis and release
of thyroid-stimulating hormone (TSH, thyrotropin) in
the anterior pituitary, and also stimulates the
release of prolactin (PRL).
Both effects are mediated by membrane receptors
coupled to the GQα-phospholipase C-β-calciumprotein kinase C second-messenger system.
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2.Gonadotropin-releasing hormone (GnRH) or
luteinizing hormone-releasing hormone (LHRH)
Gonadotropin-releasing hormone (GnRH) is a
decapeptide which, like TRH, has a pyroglutamic
acid residue in its N terminus.
GnRH stimulates the release of luteinizing
hormone (LH) and follicle-stimulating hormone
(FSH), the two gonadotropic hormones
produced by the pituitary.
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3. Corticotropin-releasing hormone (CRH)
Corticotropin-releasing hormone is a 41-aminoacid polypeptide.
CRH stimulates the release of ACTH and βendorphin by the anterior pituitary corticotrophs.
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4. Growth hormone-releasing hormone
(GHRH)
Growth hormone-releasing hormone
(GHRH) is a 44-amino-acid polypeptide.
GHRH stimulates the synthesis and release
of GH in anterior pituitary somatotrophs by
a cAMP-mediated mechanism.
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5.Somatostatin
Somatostatin (growth hormone release-inhibiting
hormone) is a tetradecapeptide [14 amino acids]
with an intrachain disulfide bridge.
Somatostatin inhibits the synthesis and release of
GH from the somatotropes by neutralizing the
effect of GHRH, an effect mediated by Giα
inhibition of adenylyl cyclase.
Within the hypothalamus, somatostatin inhibits
the release of GHRH by the same mechanism and
therefore exerts an inhibitory effect at two levels
of GH control.
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6.Prolactin-inhibiting hormone (PIH)
It is known as dopamine.
It functions as a neurotransmitter in the CNS
and as a precursor of norepinephrine and
epinephrine in the adrenal medulla.
Dopamine is a potent inhibitor of PRL release by
the lactotropes (and mammosomatotropes) of
the anterior pituitary, and this effect is mediated
by D2 receptors that are coupled to Giα inhibition
of adenylate cyclase.
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Neurohypophyseal Peptides
ADH
(Vasopressin)
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Video
(GTAC - Chemical Signalling_
Antidiuretic hormone (ADH)
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Oxytocin
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The principal action of oxytocin is ejection of milk
from the lactating mammary gland ("milk letdown"), and it also participates in parturition.
The mechanism of action of oxytocin does not
involve cAMP but may involve regulation of
increased intracellular Ca2+.
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