Activity of calcium activated protease in skeletal muscles and its changes in atrophy and stretch.

The reduction of protein content in skeletal muscle undergoing disuse-induced atrophy is correlated with accelerated rates of protein degradation and reduced rates of protein synthesis. It is not known in what manner myofibers are partially disassembled during disuse atrophy to fibers of smaller diameter; nor is it known which proteases are responsible for this morphological change in contractile protein mass. Dayton and colleagues have suggested that the Ca(2+)-activated protease (CaP) may initiate myofibril degradation. The discovery of a form of CaP that is activatable by nanomolar concentrations of Ca2+ indicates that CaP activity may be regulated by physiological concentrations of Ca2+. The enhancement of proteolysis by the Ca2+ ionophore A23187, reported by Etlinger, is consistent with a significant role for CaP in protein degradation. It was of interest, therefore, to measure the levels of CaP activity and the CaP inhibitor in extracts obtained from skeletal muscles of rat and chicken limbs undergoing disuse atrophy or stretch hypertrophy, respectively.