Energetics, mechanics and molecular engineering of calcium cycling in skeletal muscle.

During muscle contraction and relaxation, Ca2+ moves through a cycle. About 20 to 40% of the ATP utilized in a twitch or a tetanus is utilized by the SR Ca2+ pump to sequester Ca2+. Parvalbumin is a soluble Ca2+ binding protein that functions in parallel with the SR Ca2+ pump to promote relaxation in rapidly contracting and relaxing skeletal muscles, especially at low temperatures. The rate of Ca2+ dissociation from troponin C, once thought to be much more rapid than the rate of relaxation, is likely to be similar to the rate of cross-bridge detachment and to the rate of muscle relaxation under some conditions. During the past fifty years, great progress has been made in understanding the Ca2+ cycle during skeletal muscle contraction and relaxation. Nonetheless, there are still mysteries waiting to be unraveled.