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How is smooth muscle contraction affected by the drugs called calcium channel blockers? (see p. 101)

Smooth muscle has no troponin. Calcium binds instead to a similar protein called calmodulin13 (cal-MOD-you-lin), associated with the thick filaments. Calmodulin then activates an enzyme called myosin light-chain kinase, which transfers a phosphate group from ATP to the head of the myosin. This activates the myosin ATPase and enables it to bind to actin, but in order to execute a power stroke, the myosin must bind and hydrolyze yet another ATP. It then produces power and recovery strokes like those of skeletal muscle.

As thick filaments pull on the thin ones, the thin filaments pull on intermediate filaments, which in turn pull on the dense bodies of the plasma membrane. This shortens the entire cell. When a smooth muscle cell contracts, it twists in a spiral fashion, somewhat like wringing out a wet towel except that the "towel" wrings itself (fig. 11.24).

In skeletal muscle, there is typically a 2 msec latent period between stimulation and the onset of contraction. In smooth muscle, by contrast, the latent period is 50 to 100 msec long. Tension peaks about 500 msec (0.5 sec) after the stimulus and then declines over a period of 1 to 2 seconds. The effect of all this is that compared to skeletal muscle, smooth muscle is very slow to contract and relax. It is slow to contract because its myosin ATPase is a slow enzyme. It is slow to relax because the pumps that remove Ca2+ from the cell are also slow. As the Ca2+ level falls, myosin is dephosphorylated and is no longer able to hydrolyze ATP and execute power strokes. However, it does not necessarily detach from actin immediately. Its myosin has a latch-bridge mechanism that enables it to remain attached to actin for a prolonged time without consuming more ATP.

Smooth muscle often exhibits tetanus and is very resistant to fatigue. It makes most of its ATP aerobically, but its ATP requirement is small and it has relatively few mitochondria. Skeletal muscle requires 10 to 300 times as much ATP as smooth muscle to maintain the same amount of tension. The fatigue-resistance and latch-bridge mechanism of smooth muscle are important in enabling it to

"acronym for cafcium modulating protein

© The McGraw-Hill Companies, 2003

Chapter 11 Muscular Tissue 435

Chapter 11 Muscular Tissue 435

Dense Bodies Smooth Muscle
Figure 11.24 Smooth Muscle Contraction. (a) Relaxed cells. Actin myofilaments are anchored to dense bodies in the sarcoplasm and on the plasma membrane, rather than to Z discs. (b) Contracted cells. Note the twisting effect.

maintain a state of continual smooth muscle tone (tonic contraction). This tonic contraction keeps the arteries in a state of partial constriction called vasomotor tone. A loss of muscle tone in the arteries can cause a dangerous drop in blood pressure. Smooth muscle tone also keeps the intestines partially contracted. The intestines are much longer in a cadaver than they are in a living person because of the loss of muscle tone at death.

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