section 21.3

Energy Supply

in

Muscle

473

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

17

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

(MLCK);

caldesmon

(CaD); and

calponin

(CaP). Figure 21-14

Jigomsf

receptor

CAMKII

inactive

►MLCK]

fM LC K ]

active

ACTIN

L C 2 0 P

Myosin

ACTIN

Increased

Muscle

Tension

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

2

, phosphatidylcholine 4,5-bisphosphate; PC, phosphatidylcholine; IP

3

, 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).]