Dynamics of actin in cardiac myofibrils and fibroblast stress fibers.

The exchangeability of actin in cardiac myofibrils and fibroblast stress fibers was investigated using fluorescent analogue cytochemistry in combination with fluorescence recovery (FR) after photobleaching. Living embryonic chicken cardiac myocytes and fibroblasts were microinjected with rhodamine (rh)-labeled muscle and nonmuscle actins. After incorporation of the fluorescent actin analogue into cellular structures, small areas of labeled structures were photobleached with a laser pulse. In cardiac myofibrils, FR in their proximal striated portions occurred at a slower rate than that in their proximal nonstriated and distal terminal portions with each rh-isoactin injected. Thus, nascent myofibrils at different developmental stages display different actin exchangeabilities. Further, in all portions of myofibrils, FR of rh-muscle actin was faster than that of rh-nonmuscle actin. This indicates that actin molecules in cardiac myofibrils cannot be readily exchanged by heterotypic nonmuscle actin. In fibroblasts, photobleaching of stress fibers yielded similar results in both their proximal mid-points and distal terminal portions, and the FR rate was consistently faster than that observed in any part of the myofibrils. This result seems to be related to the dynamic properties of actin filaments in stress fibers at all portions. Further, the fact that stress fibers possessed a similar exchange rate with muscle and nonmuscle actins appears to be related to a more primitive nature of stress fibers than myofibrils.