Page 654 - Fundamentals of anatomy physiology

Spotlight Figure 18–3
G Proteins and Second Messengers

MECHANISM OF HORMONE ACTION Watch the > ™ on

First and Second Messengers Mechanism of Hormone Action: Second Messenger cAMP

A hormone that binds to receptors in the plasma The first messenger A G protein is an
membrane cannot directly affect the activities inside the (a peptide hormone, enzyme complex
target cell. For example, it cannot begin building a protein catecholamine, or coupled to a
or catalyzing a specific reaction. Instead, the hormone uses eicosanoid) binds to a membrane receptor
an intracellular intermediary to bring about its effects. The membrane receptor and that serves as a link
hormone, or first messenger, does something that leads to activates a G protein. between the first and
the appearance of a second messenger in the cytoplasm. second messenger.
The second messenger may act as an enzyme activator, Hormone
inhibitor, or cofactor. The net result is a change in the rates Hormone
of various metabolic reactions. The two most important Protein
second messengers are cyclic AMP (cAMP), a derivative of receptor Protein
ATP, and calcium ions (Ca2+). receptor
G protein
(inactive) G protein
activated

Effects on cAMP Levels Effects on Ca2+ Levels

Many G proteins, once activated, exert their effects by changing the concentration Some G proteins use Ca2+ as a Ca2+
of cyclic AMP, which acts as the second messenger within the cell. second messenger.

Hormone Hormone Hormone

Protein Protein Protein
receptor receptor receptor

G protein Increased G protein Enhanced G protein
activated production activated breakdown activated PLC,
Opening of
Acts as of cAMP PDE of cAMP DAG, Ca2+ channels
second cAMP adenylate AMP and IP3 Ca2+
cyclase Release of
ATP cAMP stored Ca2+ Ca2+
from ER or SER
messenger

kinase

Opens ion Activates Reduced Ca2+ acts as
channels enzymes enzyme second messenger Ca2+
activity
Calmodulin

If levels of cAMP increase, In some instances, G protein Activates
enzymes may be activated or activation results in decreased enzymes
ion channels may be opened, levels of cAMP in the
accelerating the metabolic cytoplasm. This decrease has
activity of the cell. an inhibitory effect on the cell.

First Messenger Examples First Messenger Examples First Messenger Examples
• Epinephrine and norepinephrine • Epinephrine and norepinephrine • Epinephrine and norepinephrine (α1 receptors)
• Oxytocin
(β receptors) (α2 receptors) • Regulatory hormones of hypothalamus
• Calcitonin • Several eicosanoids
• Parathyroid hormone
• ADH, ACTH, FSH, LH, TSH

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