Co-distribution of tropomyosin and alpha-actinin with actin in Psammobatis extenta electrocytes brings out their similarity with muscle fiber cytoplasm.

Electric organs of Psammobatis extenta (Rajiformes) electric fish derive from myoblasts of the caudal region (16). Here we study the presence of muscle proteins, actin and the actin-binding proteins, alpha-actinin and tropomyosin, in the electrocytes by means of biochemical approaches, scanning electron microscopy and immunocytochemical methods. NBD-phallacidin is employed to detect the filamentous form of actin (F-actin). Immunoblots of actin and alpha-actinin from P. extenta skeletal and smooth muscle show that the electric organ forms of actin and alpha-actinin correspond to muscle types. Scanning electron microscopy shows that P. extenta electrocytes are highly polarized cells, semicircular in shape, with an anterior, concave innervated face and a posterior, convex, non-innervated face. The immunofluorescence patterns of alpha-actinin and tropomyosin distribution are similar to those of actin, in that these epitopes appear to occur throughout the entire electrocyte cytoplasm. F-actin, as revealed by NBD-phallacidin fluorescence, was also found throughout the cytoplasm. This is the first time that evidence is presented to demonstrate the existence of muscle actin in this weak electric fish species electrocyte. The close evolutionary connection to that of muscle cells is discussed.