13.4 G Protein-Coupled
Receptors That Regulate Ion
Channels
By: Meredith Clement
G Protein Receptors
Many Neurotransmitters receptors are
ligand gate ion channels ( Ch. 7) but some
are G- protein-coupled receptors. The
effector protein for some of these types of
reactions are Na+ or K+ ion channels. The
binding of the neurotransmitter to one of
these receptors causes the channel to
open or close.
G Protein Receptors
Other Neurotransmitters and Odorant and
photoreceptors, are G protein receptors
that indirectly modulate the activity of ion
channels by the actions of second
messengers.
Cardiac Muscarinic Acetylcholine
Receptors Activate a G Protein that
Opens K+ Channels
The Muscarinic Acetylcholine Receptors in
cardiac muscle are inhibitory. The activation of
this receptor is coupled to a Gi protein that leads
to the opening of the associated K+ channels.
The influx of the ions causes hyperpolarization
of the plasma membrane. The signal from the
receptor is transduced to the effector protein by
the released Gβγ subunit rather than the Gα·GTP.
The Gβγ directly activates the ion channel.
Figure 13-21
Gt-Coupled Receptors are
Activated by Light
Rods and Cones are the primary
recipients for visual stimulation.
– Rods are Stimulated by weak light like
moonlight.
– Cones are involved in color vision.
– All of these signals are interpreted by the
visual cortex in the brain.
Gt-Coupled Receptors are
Activated by Light
Rhodopsin is a G protein that is stimulated by light. It is
coupled to a trimeric G protein called Transducin (Gt).
Rhodopsin consists of the seven spanning protein opsin
to which 11-cis-retinal( light absorbing protein) is
bonded.
Upon absorption of light, Rhodopsin is rapidly converted
to all trans isomers which causes a conformational
change in the opsin protein that activates it.
This is equivalent the conformation change that occurs
upon ligand binding by other G protein –coupled
receptors.
The resulting form of opsin bound to all trans-retinal is
called Meta-rhodopsin II or Activated opsin.
Gt-Coupled Receptors are
Activated by Light
Activated Opsin is unstable and disassociates
back into component parts, releasing opsin and
all trans- retinal. In the dark this is converted
back to 11-cis –retinal which can rebind with
opsin in order to reform rhodopsin.
Rod cells in the dark are constantly secreting
neurotransmitters. The depolarized state of the
membrane cells is due the presence of open
non-selective ion channels
Gt-Coupled Receptors are
Activated by Light
Adsorption of light by rhodopsin causes
these channels to close.
The more light absorbed, the more
channels that close. This results in a more
negative environment and fewer
neurotransmitters being released.
The human eye is able to see as few as
five photons.
Figure 13-23
Rod Cells Adapt to Varying Levels
of Ambient Light
Visual Adaptation
– Allows contrasting light to be measure instead of
absolute amounts of light when going from daylight to
a dimly lighted room.
– This involves the phosphorylation of activated opsin
by rhodopsin kinase. The more sites that are
phosphorylated, the less able opsin is to activate Gt
and thus induce the closing of the cGMP-gated cation
channels.
– When the level of ambient light is reduced, the opsins
become dephosphorylated and the ability to activate
Gt increases. Resulting in fewer additional photons
being necessary to generate a visual signal.
Rod Cells Adapt to Varying Levels
of Ambient Light
At high levels of ambient light the level of
opsin phosphorylation is such that the
protein β-arrestin binds to the C-terminal
segment of opsin. This prevents the
interaction of Gt with activated opsin which
totally blocks the formation of the active
Gtα·GTP complex. This results in the in the
shutdown of all rod cell activity.
Figure 13-26
Rod Cells Adapt to Varying Levels
of Ambient Light
In dark adapted cells the Gtα and Gβγ
subunits are in the outer segments but
exposure for 10 minutes to daytime light
causes 80% of the subunits to migrate into
other cellular compartments.
– this results in transducin protein not being able to bind to activated
opsin.
Figure 13-27