Chapter 16

An ATP analog, \(\beta, \gamma\) -methylene-ATP, in which a \(-\mathrm{CH}_{2}-\) group replaces the oxygen atom between the \(\beta\) - and \(\gamma\) -phosphorus atoms, is a potent inhibitor of muscle contraction. At which step in the contraction cycle would you expect \(\beta, \gamma\) -methylene-ATP to block contraction?

ATP stores in muscle are augmented or supplemented by stores of phosphocreatine. During periods of contraction, phosphocreatine is hydrolyzed to drive the synthesis of needed ATP in the creatine kinase reaction: Phosphocreatine \(+\mathrm{ADP} \longrightarrow\) creatine \(+\mathrm{ATP}\) Muscle cells contain two different isozymes of creatine kinase, one in the mitochondria and one in the sarcoplasm. Explain.

Rigor is a muscle condition in which muscle fibers, depleted of ATP and phosphocreatine, develop a state of extreme rigidity and cannot be easily extended. (In death, this state is called rigor mortis, the rigor of death.) From what you have learned about muscle contraction, explain the state of rigor in molecular terms.

Use the equations of Chapter 9 to determine the free energy difference represented by a \(\mathrm{Ca}^{2+}\) gradient across the sarcoplasmic reticulum membrane if the luminal (inside) concentration of \(\mathrm{Ca}^{2+}\) is \(1 \mathrm{m} M\) and the concentration of \(\mathrm{Ca}^{2+}\) in the solution bathing the muscle fibers is \(1 \mu M\).

In which of the following tissues would you expect to find smooth muscle? a. Arteries b. Stomach c. Urinary bladder d. Diaphragm e. Uterus f. The gums in your mouth

When an action potential (nerve impulse) arrives at a muscle membrane (sarcolemma), in what order do the following events occur? a. Release of \(\mathrm{Ca}^{2+}\) ions from the sarcoplasmic reticulum b. Hydrolysis of ATP, with release of energy c. Detachment of myosin from actin d. Sliding of myosin along actin filament e. Opening of switch 1 and switch 2 on myosin head

When athletes overexert themselves on hot days, they often suffer immobility from painful muscle cramps. Which of the following is a reasonable hypothesis to explain such cramps? a. Muscle cells do not have enough ATP for normal muscle relaxation. b. Excessive sweating has affected the salt balance within the muscles. c. Prolonged contractions have temporarily interrupted blood flow to parts of the muscle. d. All of the above.

Duchenne muscular dystrophy is a sex-linked recessive disorder associated with severe deterioration of muscle tissue. The gene for the disease: a. is inherited by males from their mothers. b. should be more common in females than in males. c. both a and b. d. neither a nor b.