Characterization of several invertebrate muscle cell types: A comparison with vertebrate muscles

Ultrastructural classification of invertebrate muscles is complex and not a lways clear. The aim of the present paper was to establish some criteria th at might be useful for classification of invertebrate muscles and for a bet ter understanding of the differences between them. The procedures used were : (1) immunochemical evaluation of those proteins that differentiated stria ted from smooth muscle (troponin, caldesmon, and calponin), and (2) calcula tions of several myofilament parameters to establish differences among musc les. The muscles studied were: striated muscles from the rat, Drosophila, t he crab Callinectes, and the snail Helix (heart); obliquely striated muscle s from the earthworm Eisenia foetida and Helix (mouth); and smooth muscles from the rat, and Helix (retractor, body wall, and intestinal wall). Immuno chemical studies revealed that troponin was only present in the striated mu scles and the obliquely striated muscle from Eisenia, whereas caldesmon and calponin were only present in the smooth muscles and the obliquely striate d muscle from Helix. The highest thick filament/thin filament volume ratio was found in the striated muscles, followed by the obliquely striated muscl es, and the smooth muscles. This suggests the order in which the contractio n strength decreases. The myofilament length is inversely related to the co ntraction speed, which was higher in the striated muscles than in the obliq uely striated muscles. In vertebrates, the smooth muscle seems to be less r apid than the striated muscle because their myofilaments are longer. This a ssertion cannot be generalized for invertebrate smooth muscle, because myof ilament lengths vary widely in both striated and smooth muscles. In smooth muscles, the presence of apparently unordered electron-dense bodies instead of ordered Z lines and the absence of true sarcomeres permit a certain ove rlapping of thin filaments increasing the range of shortening. (C) 2000 Wil ey-Liss, Inc.