Biomedical Engineering Reference
In-Depth Information
The ratio between actin and myosin is greater in smooth myocytes (15/1) than
striated myocytes (7/1). This ratio is very large in non-myocytes.
α
-Smooth muscle
actin (
α
smA), localized in microfilament bundles, is typical of smooth myocytes.
8.1.3.1
Myosin Light Chain Kinase and Phosphatase
The myosin regulatory system is more important in smooth muscles than striated
muscles. The myosin light chain (MLC) phosphorylation state that determines actin
polymerization associated with smooth muscle contraction is tightly controlled by
the relative activities of antagonist enzymes myosin light chain kinase (MLCK) and
phosphatase (MLCP; Fig.
8.1
).
Myosin light chain kinase activated by calcium-calmodulin increases sensitivity
of myosin to calcium and regulates actin-myosin interactions. Phosphorylation of
MLC leads to cross-bridge formation between myosin heads and actin filaments,
and hence to smooth muscle contraction. Dephosphorylation of myosin light chains
by myosin light chain phosphatase causes smooth muscle relaxation.
8.1.3.2
Small RhoA GTPase and RoCK Kinase
The RhoA-RoCK pathway inhibits myosin phosphatase. Stimulation of GPCRs
generates smooth myocyte contraction via Ca
2
+
-calmodulin-dependent activation
of MLCK and subsequent myosin phosphorylation. Kinase RoCK also regu-
lates MLC phosphorylation. Myosin phosphatase- and RhoA-interacting protein
(MPRIP)
5
is detected with protein kinase-G and myosin phosphatase and colocalize
with myofibrils in smooth myocytes.
8.1.4
Calmodulin
Calmodulin (i.e., calcium-modulated protein) is an ubiquitous, multifunctional
regulator and calcium sensor. It controls numerous cell functions, such as me-
tabolism (calcium, cyclic nucleotide, and glycogen), secretion, smooth muscle
contraction, intracellular transport, and gene expression.
5
A.k.a. Rho-interacting protein RhoIP3.
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