Cytoskeletal filaments in embryonic chick myocardial cells as revealed by the quick-freeze deep-etch method combined with immunocytochemistry

Cytochalasin D reduces Ca2+ sensitivity and maximum tension via interactions with myofilaments in skinned rat cardiac myocytes

The F-actin disrupter cytochalasin D depresses cardiac contractility, an effect previously ascribed to the interaction of cytochalasin D with cytoskeletal actin. We have investigated the possibility that this negative inotropic effect is due to the interaction of cytochalasin D with sarcomeric actin of the thin filament. Confocal images of Triton X-100-skinned myocytes incubated with a fluorescent conjugate of cytochalasin D revealed a longitudinally striated pattern of binding, consistent with a myofibrillar rather than cytoskeletal structure.Tension-pCa relationships were determined at sarcomere lengths (SLs) of 2.0 and 2.3 [mu]m following 2 min incubation with 1 [mu]M cytochalasin D. Cytochalasin D significantly reduced the pCa for half-maximal activation (pCa50) at both SLs. The shift in pCa50 was significantly greater at a SL of 2.3 [mu]m compared with that at a SL of 2.0 [mu]m. Cytochalasin D had no effect on the Hill co-efficient at either SL. Cytochalasin D significantly reduced the maximum tension at both SLs. We suggest that the length-dependent decrease in myofilament Ca2+ sensitivity in response to cytochalasin D is due to a decrease in the affinity of troponin C for Ca2+. Cytochalasin D has been used for many years as the agent of choice for disruption of cytoskeletal actin. However, we have demonstrated for the first time an interaction of cytochalasin D with sarcomeric actin of the thin filament, which can account for the effects of cytochalasin D on cardiac contractility.

Immunocytochemical colocalization of desmin and vimentin in human fetal skeletal muscle cells

BACKGROUND

Desmin and vimentin are the major intermediate filaments in muscle. They have been extensively studied in animal experiments. This study is the first to identify the distribution and to analyse the correlation of desmin and vimentin in human fetal skeletal muscle. Vimentin might be replaced by or transformed into desmin during myogenesis in chick embryo, although the precise process remains to be elucidated. The aim of this report is to evaluate the ratio of desmin to vimentin in human fetal muscle.

METHODS

By double-labeling immunoelectron microscopy, desmin and vimentin intermediate filaments were localized in developing skeletal muscles of 20-29-week-old human fetuses. The ratio of desmin and vimentin was analyzed statistically.

RESULTS

Two sizes of colloidal gold particles, 5 nm (vimentin) and 10 nm (desmin), were distributed along the intermediate filaments. The commonest distance between gold particles was approximately 40-50 nm. Desmin and vimentin labeled with gold particles were arranged very close together in the same intermediate filament. The ratio of vimentin to desmin varied but the amount of vimentin decreased progressively from the undifferentiated myoblast to the differentiated myocytes. As the fetuses developed, desmin increased and vimentin decreased. Desmin and vimentin intermediate filaments were identified in the intermyofibrils of differentiated myocytes, in subsarcolemmal space, and in myoblast.

CONCLUSIONS

Desmin and vimentin were colocalized in the same intermediate filaments. More vimentin existed in the less differentiated myocytes, although a small number of desmin filaments were already found in undifferentiated myoblasts. These intermediate filaments may not only connect myofibril bundles, cell organelles, and cell membrane but also provide a basis for myofibrillogenesis that is similar to relation between connective fibers and parenchymal cells.