Myosin isoforms in anuran skeletal muscle: Their influence on contractile properties and in vivo muscle function

Functional studies on isolated single anuran skeletal muscle cells represen t classic experiments from which much of our understanding of muscle contra ction mechanisms have been derived. Because of their superb mechanical stab ility when isolated, single anuran fibers provide a uniquely powerful model system that can be exploited to understand the relationship between myosin heavy chain (MHC) and myosin light chain (MLC) composition and muscle fibe r function. In this review, we summarize historic and recent studies of MHC and MLC expression patterns in the fiber types of anuran species. We exten d the traditional classification scheme, using data from recent reports in which frog MHCs have been cloned, to reveal the molecular basis of frog mus cle fiber types. The influence of MHC and MLC isoforms on contractile kinet ics of single intact fibers is reviewed. In addition, we discuss more subtl e questions such as variability of myosin coexpression along a single cell, and its potential influence on contractile function. The frog jump is used as a model system to elucidate principles of muscular system design, inclu ding the role of MHC isoforms on in vivo muscle function. Sequence informat ion is used from cloned frog MHCs to understand the role of specific region s of the myosin motor domain in regulating contractile function and the evo lutionary origins of fast and slow amphibian MHCs. Finally, we offer promis ing future possibilities that combine molecular methods (such as recombinan t gene transfer) with single cell contractile measurements to address quest ions regarding myosin structure/function and gene regulation. (C) 2000 Wile y-Liss, Inc.