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Fiber type-specific expression of major proteolytic systems in fast- to slow-transforming rabbit muscle. Am J Physiol Cell Physiol 280: C239-C247, 2001.--The present study investigates the role of two major proteolytic systems in transforming rabbit and rat muscles. The fast-to-slow transformation of rabbit muscle by chronic low-frequency stimulation (CLFS) induces fast-to-slow transitions of intact, mature fibers and replacement of degenerating fibers by newly formed slow fibers. Ubiquitination, an indicator of the ATP-dependent proteasome system, and calpain activity were measured in homogenates of control and stimulated extensor digitorum longus muscles. Calpain activity increased similarly (~2-fold) in stimulated rat and rabbit muscles. CLFS had no effect on protein ubiquitination in rat muscle but led to elevations in ubiquitin protein conjugates in rabbit muscle. Immunohistochemistry was used to study the distribution of [micro]-calpain and m-calpain and of ubiquitinated proteins in myosin heavy chain-based fiber types. The findings suggest that both proteolytic systems are involved in fiber transformation and replacement. Transforming mature fibers displayed increases in [micro]-calpain and accumulation of ubiquitin protein conjugates. The majority of these fibers were identified as type IIA. Enhanced ubiquitination was also observed in degenerating and necrotic fibers. Such fibers additionally displayed elevated m-calpain levels. Conversely, p94, the skeletal muscle-specific calpain, decayed rapidly after stimulation onset and was hardly detectable after 4 days of CLFS. calpains; chronic low-frequency stimulation; fiber type; ubiquitination

1. The cytosolic and mitochondrial isozymes of malate dehydrogenase were purified from rabbit heart. Antisera against the two isoenzymes were obtained from sheep. The antiserum against mitochondrial malate dehydrogenase showed no cross-reaction with the cytosolic isozyme and vice versa. 2. Apparent turnover rates of the two isozymes were determined in rabbit liver and heart by means of single pulse labeling with [14C]leucine and isolation of the enzymes labelled in vivo by the immunochemical method. The apparent half-lives of the cytosolic and mitochondrial isozyme are 8.6 and 21.9 h in liver and 21.6 and 33.9 h in heart. These values are shorter than the apparent half-lives of total soluble protein of liver and heart. 3. Turnover of mitochondrial malate dehydrogenase in rabbit liver was also calculated from the kinetics of enzyme accumulation and regression during a starvation-refeeding cycle. The half-life during starvation is 15.4 h and in the refeeding period 19.6 h. the approximately two-fold increase in mitochondrial malate dehydrogenase content of liver during starvation appears thus to be brought about by an increased rate of synthesis. [ABSTRACT FROM AUTHOR]