Purine nucleotides have important roles in the metabolic function of most cells. Contractile cells are, however, especially vulnerable to depletion of ATP, for energy for myofibril shortening comes from myosin ATPase-catalyzed hydrolysis of ATP. In addition, ATP provides energy for the Na+/K+ ATPase pump, which sustains sarcolemmal (plasma) membrane gradients of sodium and potassium essential to myocyte electrical activity, and for uptake of calcium by the sarcoplasmic reticulum membrane. ATP is normally maintained at 5 to 6 mM in the myocyte cytoplasm.