Animal Endocrine Systems
Biology 2: Form and Function
Hormones regulate...
• Together with nervous system, responsible for
regulation of body organs
• Secreted by ductless glands into surrounding capillary
network
• Long-term, narrow-to-broad impact mediated by
contact of hormone with receptor molecules at target
site
• Receptor molecules may be internal or external to cell
• Endocrine system is distinct to organ self-regulation,
or autocrine/paracrine system
There are four classes of hormone
• Polypeptides - short chains < 100 amino
acids in sequence, e.g., ADH
• Glycoproteins - long chains (100+) of amino
acids connected to carbohydrate, e.g., FSH
• Amines - derivatives of tyrosine and
tryptophan, e.g., epinephrine, thyroxine
• Steroids - lipid derived
– sex steroids, e.g., testosterone
– corticosteroids, e.g., aldosterone
In mammals, there is a close
association between the endocrine
and neural systems...
• Nervous system-controlled endocrine glands
include:
– Adrenal medulla
– Posterior and anterior pituitary (via
Hypothalamus)
– Pineal gland
• Examples of non-nervous system controlled
endocrine secretion include pancreas
(insulin), and adrenal cortex (aldosterone)
Hormones that enter cells
• Includes all lipophilic (lipid-soluble) hormones
(e.g., steroids, thyroxine)
• Bind to specific receptors in cytoplasm, which
then moves to the nucleus, or binds directly
to receptor proteins in nucleus
• Receptor molecule, once activated, binds to
portions of DNA and stimulates transcription,
ultimately effecting protein production and cell
metabolism
Hormones that do not enter cells...
• Water soluble hormones that bind to the
outside of cell membranes, requiring a
secondary messenger inside cell to complete
message
• Secondary messengers include
– Cyclic AMP
– IP3/Ca2+
Cyclic AMP
• e.g., effects of epinephrine on b-adrenergic centers
• Binding of epinephrine to G-protein receptor causes
G-protein sub-unit to disassociate
• G-protein sub-unit binds with membrane enzyme
adenlyl cyclase
• Adenlyl cyclase, now activated, catalyzes formation of
cAMP from ATP
• cAMP binds to and activates protein kinase-A,
responsible for phosphorylation of certain proteins
specific to tissue/cell
– in liver, stimulates conversion of glycogen to
glucose
– In cardiac muscle, increases speed and force of
heart beat
Inositol triphosphate/Ca2+
• e.g., effects of epinephrine on a-adrenergic centers
• Binding of epinephrine to G-protein receptor causes
G-protein sub-unit to disassociate
• G-protein sub-unit binds with, and activates
membrane enzyme phospholipase C
• Phospholipase C cleaves certain phospholipids to
produce IP3
• IP3 binds with receptors on endoplasmic reticulum,
stimulates release of Ca2+
• Ca2+ binds to calmodulin, which activates different
types of protein kinases, causing phosphorylation of
different cellular proteins
The Posterior Pituitary
• neurally derived: hormones are part of
neuroendocrine reflex.
• Secretes Antidiuretic hormone (ADH) and
Oxytocin (although both are made in the
hypothalamus)
– ADH stimulates water retention by the
kidneys (alcohol inhibits ADH, causing
dehydration)
– Oxytocin stimulates uterine contractions
and milk-ejection reflex
Anterior pituitary
• Epithelially-derived tissue, produces
– GH, growth hormone (somatotropin)
– ACTH, adrenocorticotropic hormone
(corticotropin)
– TSH, thyroid stimulating hormone
(thyrotropin)
– Gonadotropins LH (luteinizing hormone)
and FSH (follicle stimulating hormone)
– Prolactin (PRL)
– MSH, Melanocyte-stimulating hormone
Control of the Anterior Pituitary by the
Hypothalamus is still hormone-mediated
• Releasing and inhibitory hormones are
secreted by the hypothalamus and carried via
a c apillary network connected to a second
caillary network - the hypothalamohypophyseal portal system
• for example, gonadotropin releasing hormone
(GnRH) stimulates the release of FSH and
LH
• In turn, hypothalamus is controlled by
negative feedback inhibition
Other endocrine glands
• Adrenal glands
– The adrenal medulla secrets epinephrine
and norepinephrine, triggering alarm
responses across the body at various
targets, preparing the body for ‘fight or
flight’
– The adrenal cortex secretes cortisol and
other glucocorticoids, an aid in glucose
homeostasis, as well as aldosterone,
responsible for salt balance
The pancreas has both exocrine
and endocrine functions