Microinjection of skeletal actin labeled with rhodamine into cultured cardiac myocytes was followed by rapid incorporation of fluorescence into myofibrils of the cells. Myocytes examined as shortly as 5 min postinjection displayed fluorescent bands corresponding to the sarcomeres. By 10 min, distinct alternating wide and narrow bands of fluorescence were observed. The wide bands appeared to correspond to the full breadth of the I-bands, whereas the narrow bands of fluorescence corresponded to the M-lines. This pattern of fluorescence remained essentially unchanged for at least 15 h postinjection. The myofibrils of cardiac myocytes were functional after rhodamine-actin incorporation as judged by their ability to contract. The results of this study suggest that cardiac myofibrils are morphologically stable structures which, nonetheless, exhibit extensive exchange of actin subunits.

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