Abstract
Ultrastructural classification of invertebrate muscles is complex and not always clear. The aim of the present paper was to establish some criteria that might be useful for classification of invertebrate muscles and for a better understanding of the differences between them. The procedures used were: (1) immunochemical evaluation of those proteins that differentiated striated from smooth muscle (troponin, caldesmon, and calponin), and (2) calculations of several myofilament parameters to establish differences among muscles. The muscles studied were: striated muscles from the rat, Drosophila, the crab Callinectes, and the snail Helix (heart); obliquely striated muscles from the earthworm Eisenia foetida and Helix (mouth); and smooth muscles from the rat, and Helix (retractor, body wall, and intestinal wall). Immunochemical studies revealed that troponin was only present in the striated muscles and the obliquely striated muscle from Eisenia, whereas caldesmon and calponin were only present in the smooth muscles and the obliquely striated muscle from Helix. The highest thick filament/thin filament volume ratio was found in the striated muscles, followed by the obliquely striated muscles, and the smooth muscles. This suggests the order in which the contraction strength decreases. The myofilament length is inversely related to the contraction speed, which was higher in the striated muscles than in the obliquely striated muscles. In vertebrates, the smooth muscle seems to be less rapid than the striated muscle because their myofilaments are longer. This assertion cannot be generalized for invertebrate smooth muscle, because myofilament 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 overlapping of thin filaments increasing the range of shortening.
