CULTURED RAT SKELETAL-MUSCLE CELLS TREATED WITH CYTOCHALASIN EXHIBIT NORMAL DYSTROPHIN EXPRESSION AND INTRACELLULAR FREE CALCIUM CONTROL

Many studies performed to elucidate the molecular and cellular process es involved in muscular dystrophies have led to the working hypothesis of a key role for the cytoskeleton elements linking the extracellular matrix to myofibrils. It was recently suggested that cytochalasin B t reatment of mouse soleus muscle promoted cell damage mediated by a cyt osolic increase in free calcium concentration. Since intracellular cal cium overload may be a primary event resulting from the alteration of cytoskeletal structure, this study was intended to evaluate whether or not the integrity of the F-actin microfilament network is necessary f or calcium homeostasis. The developmental establishment of the normal cytoarchitecture was altered by treatment of myoblasts with the actin- disrupting agents cytochalasin B and D, and the effects were compared with those in myoblasts treated with colchicine. These drugs modified the morphogenesis in that they prevented the formation of elongated my otubes by myoblast fusion, but did not prevent the maturation of contr actile myogenic cells. The subcellular organisation of actin filaments visualised by confocal fluorescence microscopy was modified by colchi cine and cytochalasins, but appearance of contractile apparatus and me chanical activity were not precluded. Sarcolemmal addressing of dystro phin, the subsarcolemmal protein lacking in Duchenne muscular dystroph y, was not prevented by cytochalasin. The evaluation of the basal acti vity of cytosolic calcium measured with indo-1 suggested that the disr uption of actin or microtubules did not prevent developing muscle cell s to maintain a low basal calcium activity. We propose that the global integrity of the cytoskeleton network is not crucial for the maintena nce of calcium homeostasis in muscle cells developing in vitro. These results are discussed with regard to current theories attempting to un derstand the functional consequences of an abnormal expression of the dystrophin-glycoprotein complex interacting with the extracellular mat rix and the cytoskeleton.