DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

58 markedly stimulates Ca 2+ flux, the IP 3
channel is regulated by Ca 2+
and by the activities of PKA and PKG. Ca 2+ concentrations in the
100-300 nM range enhance Ca 2+ release, but concentrations near 1
μM inhibit release, which can create the oscillatory patterns of Ca 2+
release seen in certain cells. Phosphorylation of the IP 3
receptor by
PKA enhances Ca 2+ release, but phosphorylation of an accessory
protein, IRAG, by PKG inhibits Ca 2+ release. In smooth muscle,
this effect of PKG represents part of the mechanism by which cyclic
GMP relaxes vessel tone. In skeletal and cardiac muscle, Ca 2+
release from intracellular stores (the sarcoplasmic reticulum) occurs
through a process termed Ca 2+ -induced Ca 2+ release, and is primarily
mediated by the ryanodine receptor (RyR). Ca 2+ entry into a
skeletal or cardiac myocyte through L-type Ca 2+ channels causes
conformational changes in the ryanodine receptor that induce the
release of large quantities of Ca 2+ into the sarcoplasm. Drugs that
activate the RyR include caffeine; drugs that inhibit the RyR include
dantrolene.
Calcium released into the cytoplasm from the ER is rapidly
removed by plasma membrane Ca 2+ pumps, and the ER pool of Ca 2+
is refilled with extracellular Ca 2+ flowing through store-operated
Ca 2+ channels (SOC) in the plasma membrane. These currents are
termed Ca 2+ release-activated currents, or I CRAC
. The mechanism by
which ER store depletion opens the store-operated channels requires
two proteins, the channel itself, termed Orai1, and an ER sensor
termed STIM1. The Orai1 channel is a 33-kDa protein with four
membrane-spanning helices and no homology with other Ca 2+ channels
(Prakriya, 2009). Orai1 is highly selective for Ca 2+ . The C terminal
end of the channel contains coiled-coil domains thought to
interact with the STIM1 sensor. STIM1 is a 77-kDa protein containing
a Ca 2+ sensor domain termed an EF hand. This domain is located
at the N-terminus of the protein on the inside of the ER membrane
before the single membrane-spanning domain. There are multiple
protein-protein interaction motifs in the middle and C-terminal end
of the molecule. Specifically, there are two coiled-coil domains on
the C-terminal side of the transmembrane domain in STIM1 that
may interact with coiled-coil domains in the Orai1 channel. Under
resting conditions, the STIM1 protein is uniformly distributed on the
ER membrane. Release of Ca 2+ from the ER stores results in dimerization
of STIM1 and movement to the plasma membrane where
STIM1 and Orai1 form clusters, opening the Ca 2+ pore of Orai1 and
refilling of the ER Ca 2+ pool (Fahrner et al., 2009).
SECTION I
GENERAL PRINCIPLES
Ion Channels
The lipid bilayer of the plasma membrane is impermeable
to anions and cations, yet changes in the flux of
ions across the plasma membrane are critical regulatory
events in both excitable and non-excitable cells. To
establish and maintain the electrochemical gradients
required to maintain a membrane potential, all cells
express ion transporters for Na + , K + , Ca 2+ , and Cl − . For
example, the Na + ,K + -ATPase pump expends cellular
ATP to pump Na + out of the cell and K + into the cell.
The electrochemical gradients thus established are used
by excitable tissues such as nerve and muscle to generate
and transmit electrical impulses, by non-excitable
cells to trigger biochemical and secretory events, and
by all cells to support a variety of secondary symport
and antiport processes (Chapter 5).
Passive ion fluxes down cellular electrochemical
gradients are regulated by a large family of ion channels
located in the membrane. Humans express ~232 distinct
ion channels to precisely regulate the flow of Na + ,
K + , Ca 2+ , and Cl − across the cell membrane (Jegla et al.,
2009). Because of their roles as regulators of cell function,
these proteins are important drug targets. The
diverse ion channel family can be divided into subfamilies
based on the mechanisms that open the channels,
their architecture, and the ions they conduct. They can
also be classified as voltage-activated, ligand-activated,
store-activated, stretch-activated, and temperature-activated
channels. Examples of channels that are major
drug targets are detailed next.
Voltage-Gated Channels. Humans express multiple isoforms
of voltage-gated channels for Na + , K + , Ca 2+ , and
Cl − ions. In nerve and muscle cells, voltage-gated Na +
channels are responsible for the generation of robust
action potentials that depolarize the membrane from its
resting potential of −70 mV up to a potential of +20 mV
within a few milliseconds. These Na + channels are composed
of three subunits, a pore-forming α subunit and
two regulatory β subunits. The α subunit is a 260 kDa
protein containing four domains that form a Na + ionselective
pore by arranging into a pseudo-tetramer
shape. The β subunits are ~36 kDa proteins that span
the membrane once (Figure 3–9A). Each domain of the
α subunit contains six membrane-spanning helices
(S1-S6) with an extracellular loop between S5 and S6,
termed the pore-forming or P loop; the P loop dips back
into the pore and, combined with residues from the corresponding
P loops from the other domains, provides a
selectivity filter for the Na + ion. Four other helices surrounding
the pore (one S4 helix from each of the
domains) contain a set of charged amino acids that form
the voltage sensor and cause a conformational change
in the pore at more positive voltages leading to opening
of the pore and depolarization of the membrane (Purves
and McNamara, 2008). The voltage-activated Na + channels
in pain neurons are targets for local anesthetics
such as lidocaine and tetracaine, which block the pore,
inhibit depolarization, and thus block the sensation of
pain. They are also the targets of the naturally occurring
marine toxins, tetrodotoxin and saxitoxin (Chapter 20).
Voltage-activated Na + channels are also important
targets of many drugs used to treat cardiac arrhythmias
(Chapter 29).