Slow type muscle cells in the earthworm gizzard with a distinct, Ca2+-regulated myosin isoform

Abstract

Histochemical staining of the gizzard from the earthworm,Lumbricus terrestris, reveals low ATPase and high succinic dehydrogenase activity for all muscle cells as compared to the main part of the body wall. In accordance with the presence of slow type muscle cells in the gizzard, isolated actomyosin shows an ATPase activity three times lower than the body wall actomyosin. Gizzard myosin represents an isoform, distinct from those of the body wall muscle, by comparison of the light chain pattern in isoelectric focusing. No difference was observed in the Ca2+-regulatory properties between gizzard and body wall actomyosin. Gizzard actomyosin is dual-regulated, and the myosin contains a regulatory light chain which is reversibly dissociated by EDTA. Isolated gizzard binds two molecules of Ca2+ per molecule, in the same range of free Ca2+ concentrations over which actomyosin is activated, suggesting that the myosin-linked regulatory system is mediated by direct binding of Ca2+. The molar ratios of the major contractile proteins of body wall and gizzard actomyosins differ considerably, indicating a structural diversity of fast and slow type muscle cells.