SECTION 21.3
Energy Supply
in
Muscle
475
binding to actin/tropomyosin. In some species, another
calcium-binding protein called caltropin (M.W. ~ 1 1,000)
has also been shown to reverse the CaD inhibition of
actin-myosin interaction in a Ca2+-dependent manner.
Phosphorylation is another such mechanism. CaD can be
phosphorylated by CaMKII, PKC, PKA, casein kinase II,
cdc2 kinase, and mitogen-activated protein kinase (MAP
kinase). Mammalian CaD is phosphorylated at two sites by
MAP kinase, which is probably the physiologically rele-
vant kinase in mammals. The extent to which actin-myosin
interaction is normally inhibited by CaD is not clear, and
so the importance of these mechanisms regulating CaD
in the control of smooth muscle contraction is debated. It
has been argued that CaD phosphorylation is required to
permit formation of latch bridges.
Calponin is
another polypeptide monomer (M.W.
~32,000) that can inhibit actin-activated myosin ATPase
activity. In contrast to CaD, CaP exerts its effect in the ab-
sence of tropomyosin and completely inhibits motility in a
2/3 ratio with actin. CaP inhibits myosin binding to actin,
but does so by reducing the affinity of actin for myosin
rather than competing for the same site. CaP can be phos-
phorylated by PKC and CaMKII, both of which reverse
CaP’s inhibitory activity. As with caldesmon, many ques-
tions remain. The ratio of CaP to actin actually observed
in smooth muscle is in the range
1 : 1 0
to 1:16, far from
the 2/3 ratio found to produce near-complete inhibition of
motility. Therefore, the importance of CaP and its regula-
tion by phosphorylation is still debatable.
Relaxation of smooth muscle (especially airway smooth
muscle) by /3
2
agonists remains a puzzle. It had been
thought that G-protein-mediated cAMP production stim-
ulated cAMP-dependent kinases such as PKA, eliciting
phosphorylation of MLCK, which prevented Ca-CaM
binding and subsequently inhibited MLCK. This would
result in decreasing LC
2 0
phosphorylation and loss of
tension. It is clear that /3
2
stimulation does indeed in-
crease [cAMP] and activity of cAMP-dependent kinases
in smooth muscle. However, /3-agonists that relax tracheal
smooth muscle can reduce the sensitivity of LC
2 0
phospho-
rylation to [Ca2+]; without reducing the Ca2+ sensitivity
of MLCK activity, which is not consistent with a mecha-
nism based on inhibition of Ca-CaM binding to MLCK.
/32-agonists generally elicit hyperpolarization by in-
creasing K+ conductance, particularly in a K+ chan-
nel population called large-conductance calcium- and
voltage-activated K+ (KCa) channels, also called maxi-K
channels. The probability of these channels being open
is increased by phosphorylation by cAMP-dependent
kinases,
and a subpopulation may
also be directly
opened by /32-rcccptor activation via a G-protein interac-
tion. Hyperpolarization reduces the open probability of
voltage-dependent Ca2+ channels and reduces [Ca
2
+]j,
which reduces MLCK activity. There are several other
mechanisms by which
/3
stimulation can alter [Ca2+];, and
there are circumstances in which /3-adrenergic relaxation
occurs without hyperpolarization. It has also been claimed
that the light chain phosphatase, PP-I, is stimulated by
/3-adrenergic pathways. Given the diversity of smooth
muscle, it is quite possible that the relative importance
of various activating and relaxing mechanisms varies with
the specific smooth muscle type and, perhaps, with the
recent electrical and chemical history of the cell.
Control of cardiac muscle is somewhat similar to gut
smooth muscle while its mechanical properties are similar
to skeletal muscle. Myocardial cells are stimulated to con-
tract by action potentials generated by a combination of
fast Na+ and Ca2+ voltage-dependent channels, and which
propagate from cell to cell via gap junctions, as in gut
smooth muscle. As in smooth muscle, various transmitters
and hormones can alter extracellular Ca2+ entry, but in my-
ocardium these effects are largely confined to the plateau
phase of the action potential. Myocardial cells have large
T-tubules and moderately extensive SR, with DHPR and
RyR directly apposed to one another as in skeletal muscle.
Myocardial DHPR is a better Ca2+ channel than skeletal
DHPR, and calcium-induced calcium release triggered by
Ca2+ entry during the plateau phase plays a significant
role in E-C coupling in myocardium, but a fully activating
[Ca2+]; is not normally attained in the absence of sympa-
thetic stimulation. In the absence of action potentials, no
normally occurring chemical stimulus induces sufficient
Ca2+ entry to initiate contraction, and so myocardium is
entirely phasic. Phosphorylation of myosin light chains
is not required for contraction in myocardium but signif-
icantly increases cross-bridge cycling rate. Sympathetic
(norepinephrine) stimulation of the heart increases both
Ca2+ entry and light chain phosphorylation, thus increas-
ing both force and speed of contraction. Ca2+ channel
blockers reduce force of contraction and oxygen demand
in myocardium, although the reflex increase in sympa-
thetic drive to the heart (in response to the decrease in
blood pressure induced by these agents) may largely off-
set these effects.
It has recently become possible to alter the relation be-
tween force and [Ca2+]i in myocardium, a useful trick in
congestive heart failure.
Pimobendan
modestly increases
the affinity of cardiac Tn C (cTn) for Ca2+, thereby in-
creasing the activation of contraction at any given [Ca2+];.
The more potent
levosimendan
binds to the N-terminal re-
gion of cTn C in a Ca2+-dependent manner, and amplifies
the effect of Ca2+, perhaps by increasing the stability of
the Ca2+-induced conformational change in cTn C or by
enhancing cooperativity in the thin filament.
