Skeletal muscle performance responds rapidly to changes in use, innervation, and hormone levels. While much is known about the metabolic and histochemical changes that occur, the molecular basis of these adaptive responses is less well understood. A more complete understanding of the physiological basis of skeletal muscle plasticity requires knowledge of tile patterns of coexpression of different contractile protein isoforms, the mechanism by which newly synthesized contractile proteins are incorporated into the myofibrillar lattice, and the effects of the different isoforms on fiber physiology. In these studies, we will follow changes in muscle protein isoforms induced by altering patterns of stimulation or hormone levels. Focussing on three functionally and structurally interactive subsets of contractile proteins, the thin filament calcium regulatory complex, the sarcoplasmic reticulum components of the excitation-contraction coupling system, and the thick filament, we will determine whether the proteins in each of these subsets are coexpressed.