section 21.3
Energy Supply
control similar to vascular smooth muscle. Therefore, gut
smooth muscle can contract tonically or phasically, and
depolarization can be directly transmitted from one cell to
the next.
As the name implies, smooth muscle lacks the highly or-
dered sarcomere structure of striated muscle, having thick
and thin filaments in less orderly arrays with relatively
less myosin (one fifth as much) than in striated muscle.
Smooth muscle thin filaments have tropomyosin but gen-
erally lack troponin. Myosin in smooth muscle is found
in monomeric form as well as small thick filaments, and
phosphorylation is almost essential for condensation of
monomeric myosin into filaments. Thus, the amount of
myosin available to cross-bridge with actin may be physi-
ologically adjustable. Like other myosin II types, smooth
muscle myosin is a hexamer, and several isoforms of the
heavy chains and both light chains are known. The SM-1
isoform (M.W. 204,000) has an unusually long COOH-
terminal tail not found in SM-2 (M.W. 200,000). Some
smooth muscle myosins have an extra 7 amino-acid seg-
ment in the head near the aminoterminal which is associ-
ated with higher ATPase activity and velocity of shorten-
ing. The light chains are called LCn and LC
2 0
, based on
their molecular weight, for each of which two forms have
been described. The LC
isoform particularly seems to
influence speed of shortening and tension development,
the LCi7a form being associated with higher speed.
In skeletal muscle, disinhibition of actin is necessary
for contraction to occur, and control of contraction is said
to be actin-linked. In smooth muscle, phosphorylation
of myosin light chains (MLCs) is required for contrac-
tion. Several mechanisms alter MLC phosphorylation, and
so in smooth muscle, control of contraction is primarily
myosin-linked. Three control proteins have been identified
in smooth muscle:
myosin light chain kinase
(CaD); and
(CaP). Figure 21-14
fM LC K ]
L C 2 0 P
F IG U R E 2 1 -1 4
Pathways activating contraction in vascular smooth muscle. As in other muscle types, increased cytosolic Ca2+
is the most important signal, but control of cytosolic [Ca2+] and the mode of Ca2+ action is more complicated in
smooth muscle, and other mechanisms may contribute. Key: G. GTP-binding protein; PLC, phospholipase C;
PLD, phospholipase D; PIP
, phosphatidylcholine 4,5-bisphosphate; PC, phosphatidylcholine; IP
, inositol
1,4,5-triphosphate; PA, phosphatidic acid; DAG, diacylglycérol; SR, sarcoplasmic reticulum; CaK calmodulin; PKC,
protein kinase C; MLCK, myosin light chain kinase; LC
2 0
, 20-kDa myosin light chain; CaD, caldesmon; CaP. calponin;
CaMKII, Ca/calmodulin protein kinase II; MAPK, mitogen-activated protein kinase; MEK, MAP/ERK kinase; Gx,
small GTP-binding protein; CaBP, calcium-binding protein; RyR, ryanodine receptor. Dashed lines indicate pathways
not fully defined or that are speculative. The role of membrane potential is not indicated in this figure. [Reproduced with
permission from A. Horowitz, et al., Mechanisms of smooth muscle contraction.
Physiol. Rev.
76(4), 967-1003 (1996).]
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