396 Chapter 12
are faster in arterioles than in large arteries, for example—to
provide faster rates of smooth muscle contraction and relax-
ation. Also, both enzymes are subject to regulation within a
given smooth muscle. For example, the myometrium (smooth
muscle of the uterus) must be quiescent during pregnancy but
then must contract forcefully during childbirth.
In addition to being graded, the contractions of smooth
muscle cells are slow and sustained. The slowness of contrac-
tion is related to the fact that myosin ATPase in smooth muscle
is slower in its action (splitting ATP for the cross-bridge cycle)
than it is in striated muscle. The sustained nature of smooth
muscle contraction is explained by the theory that cross bridges
in smooth muscles can enter a latch state.
The latch state allows smooth muscle to maintain its
contraction in a very energy-efficient manner, hydrolyzing
less ATP than would otherwise be required. This ability is
obviously important for smooth muscles, given that they
encircle the walls of hollow organs and must sustain contrac-
tions for long periods of time. The mechanisms by which
the latch state is produced, however, are complex and poorly
understood.
The three muscle types—skeletal, cardiac, and smooth—
are compared in table 12.8.activates myosin light-chain kinase (MLCK), an enzyme that
catalyzes the phosphorylation (addition of phosphate groups) of
myosin light chains, a component of the myosin cross bridges.
In smooth muscle (unlike striated muscle), the phosphorylation
of myosin cross bridges is the regulatory event that permits them
to bind to actin and thereby produce a contraction ( fig. 12.37 ).
The degree of phosphorylation of the myosin light chains largely
determines the smooth muscle contraction strength and duration,
allowing smooth muscles to produce graded contractions.
Relaxation of the smooth muscle follows the closing
of the Ca^2 1 channels and lowering of the cytoplasmic Ca^2 1
concentrations by the action of Ca^2 1 -ATPase active trans-
port pumps. Under these conditions, calmodulin dissociates
from the myosin light-chain kinase, thereby inactivating this
enzyme. The phosphate groups that were added to the myosin
are then removed by a different enzyme, myosin light-chain
phosphatase ( MLCP ). This dephosphorylation inhibits the
cross bridges from binding to actin and is therefore needed for
smooth muscle relaxation ( fig. 12.37 )
Contraction strength and duration depend on the degree
of myosin light-chain phosphorylation, which is determined
by the relative activities of MLCK and MLCP. Some smooth
muscles have faster forms of these enzymes than others—they
Figure 12.37 Excitation-
contraction coupling in smooth
muscle. When Ca^2 1 passes through
voltage-gated channels in the plasma
membrane it enters the cytoplasm and binds
to calmodulin. The calmodulin-Ca^2 1 complex
then activates myosin light-chain kinase
(MLCK) by removing a phosphate group. The
activated MLCK, in turn, phosphorylates
the myosin light chains, thereby activating
the cross bridges to cause contraction.
Contraction is ended when myosin light-chain
phosphatase (MLCP) becomes activated.
Upon its activation, MLCP removes the
phosphates from the myosin light chains and
thereby inactivates the cross bridges.
MLCK
InactiveMLCK
ActiveActiveCalmodulin–Ca2+
complexCross bridge
inactivation
and relaxationAction potentialsThresholdGraded depolarizations
TimeCross bridge
activation
and contractionMyosin light
chainMyosin phosphatase
(MLCP)InactiveMyosin phosphatase
(MLCP)Myosin light
chainPPP Ca2+ + calmodulinDepolarizationVoltage-gated Ca2+
channels open in
plasma membraneMembrane
potential- Dephosphorylation of
cross bridges leads
to relaxation - Phosphorylation of
cross bridges leads
to contraction