Energy Balance In Smooth Muscle

For smooth muscle as for striated muscle (Chapter 7), ATP is required for many processes—for example, activation of the contractile proteins, crossbridge cycling, calcium sequestration and removal, membrane electrical activity, and synthesis of structural components. Although in many instances the processes are the same in both smooth and striated muscles, both qualitative and quantitative differences exist.

One ATP-consuming process is essential in smooth muscle but relatively unimportant in striated muscle. As described previously, activation of the contractile proteins in smooth muscle, but not in striated muscle, involves phosphorylation of one of the myosin light chains. This requires ATP. Once phosphorylated, each crossbridge can cycle many times, with each cycle also consuming ATP. At first glance, it would seem that more ATP would be required during smooth muscle contraction, but the opposite is true: Less ATP is used to generate contractions with forces equal to those developed by striated muscle. This economy is due partly to the myosin isoforms found in smooth muscle. These isoforms have a low ATPase activity that imparts low velocities of contraction but somehow enables smooth muscle to develop force with low ATP consumption. Also, many smooth muscles have the ability to maintain a tonic contraction with even lower ATP use than that needed during the initial stages of the contraction. During the early stages of force

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