section 21.2
Mechanism of Muscle Contraction: Overview
465
ligand is moderate [Ca2+];. This led to the hypothesis
of
calcium-induced calcium release,
or
CICR,
as the
explanation for E-C coupling: Ca2+ entering through the
DHPR would trigger opening of the RyR, thus serving
as “trigger calcium.” However, since skeletal muscle
contracts in Ca2+-free media, entry of external Ca2+
through DHPR is not necessary, and the CICR hypothesis
requires elaborate modification in skeletal muscle. It is
now known that the skeletal muscle DHPR is a good
voltage sensor, but a poor calcium channel, with a low
conductance and slow kinetics.
It is now widely believed that in skeletal muscle a
depolarization-induced change in the structure of the
DHPR
a
i subunit directly influences the RyR in such a
way as to markedly increase its conductance for Ca2+.
Since the resulting increase in [Ca2+]; can open other RyR
channels, this produces a surge of Ca2+ release. In con-
trast, cardiac muscle expresses a different
a
1 DHPR sub-
unit than skeletal muscle. The cardiac subunit has much
higher channel conductance and faster kinetics than the
skeletal muscle type, and so admits much more extracellu-
lar calcium during the action potential. Thus, CICR does
play an important role in cardiac muscle. In both cases,
high [Ca2+]j reduces RyR conductance, by direct binding
of Ca2+ (or Ca2+-calmodulin) to RyR, by activation of a
kinase that phosphorylates RyR, and probably both. At the
same time, Ca-calmodulin (Ca-CaM) activates a protein
kinase that phosphorylates the SR Ca-ATPase, which in-
creases its activity 10- to 100-fold. These two mechanisms
combine to terminate the Ca2+ spike.
As mentioned above, DHPR are named for their bind-
ing by dihydropyridines, which include the drugs nifedip-
ine and nimodipine, which block the DHPR (and other
closely related) Ca2+ channels. These channels are also
blocked by phenylalkylamines (e.g., verapamil) and ben-
zothiazipines (e.g., diltiazem). Since Ca2+ entry through
these channels is not required for E-C coupling in skele-
tal muscle, these drugs have little effect in this tissue.
Vascular smooth muscle, however, is almost completely
dependent upon entry of extracellular Ca2+ for contrac-
tion, and in this tissue these drugs significantly reduce
tension. Myocardium, which depends partially on CICR
for E-C coupling, is intermediate in sensitivity to these
drugs.
Three RyR genes are expressed in humans. RyRl
predominates in skeletal muscle and in Purkinje cells of
the cerebellum. RyR2 is very predominant in heart and
is the most abundant form in brain. About 2-5% of the
RyR in skeletal muscle is RyR3. Smooth muscle also
has RyR3 in small amounts, with more abundant RyRl
and RyR2. The sequence homology between any two
of these is about 70%. Ryanodine at low concentration
(<10 mM) binds to RyR in the open state, and holds
these channels open, producing sustained contracture,
but at high concentration ryanodine blocks the channels.
Dantrolene sodium is a drug that blocks RyRl channels
in skeletal muscle and is a direct-acting muscle relaxant,
while producing little effect on the RyR2 channels in
cardiac muscle. Neomycin and other aminoglycosides
also inhibit skeletal muscle SR Ca2+ release by binding
to the same part of RyRl as ryanodine.
SR contains two other types of protein important to
normal function: calcium-binding proteins and structural
proteins.
Calsequestrin
is a Ca2+-binding protein (M.W.
44,000), especially abundant at the terminal cisterns. It is
highly acidic, with glutamate and aspartate accounting for
about 37% of the amino acids. One calsequestrin binds up
to 43 Ca2+ ions, with a mean
Km
of about 1 mM. There is
evidence indicating that calsequestrin is required for nor-
mal RyR activity. This protein has an FT (chromosome
lq21) and an ST/cardiac form (lpl 1—p i3.3). Another
protein, called
high-affinity Ca2+-binding protein,
has
a lower capacity but a higher affinity for Ca2+ than calse-
questrin. Both of these buffer the [Ca2+] in the SR lumen,
increasing the SR Ca2+ capacity and limiting the [Ca2+]
gradient against which the ATPase has to work.
Triadin
(M.W. 94,000) is a transmembrane glycoprotein that ex-
ists as a disulfide-linked multimer of uncertain structure
with most of its mass inside the SR, where it is believed
to bind to RyR and probably calsequestrin. The extracel-
lular portion links to the
a
i-subunit of the DHPR. Thus,
triadin may functionally link the DHPR to the RyR, or
it may simply serve to colocalize the DHPR, RyR, and
calsequestrin.
Junctin
(M.W. 26,000) is another protein
that binds calsequestrin. Its 210 amino acids include a
short cytoplasmic domain and a large luminal domain that
binds calsequestrin, anchoring it to the T-tubule/SR junc-
tional region of the SR membrane.
Methyl xanthines, especially caffeine, enhance Ca2+ re-
lease from SR, increasing the size and duration of a twitch.
If the caffeine concentration is high enough, Ca2+ release
can occur without depolarization and cause calcium con-
tractures. This is the mechanism of muscle spasm in severe
caffeine toxicity. Electrically silent contractures also oc-
cur in a rare disorder called
malignant hyperthermia,
in
which exposure to halogenated anesthetics and/or muscle
relaxants such as succinylcholine causes a sustained in-
crease in sarcoplasmic [Ca2+], with resulting increases in
SR Ca2+-ATPase activity, muscle contracture, and both
aerobic and anaerobic metabolism. The ensuing increase
in body temperature, hyperkalemia, and acidosis can be
lethal. In about 50% of humans so affected, there is a mu-
tation in RyRl that results in a reduced threshold [Ca2+]
for CICR, among other abnormalities.
