Exchangeability of alpha-actinin in living cardiac fibroblasts and muscle cells

We have investigated the exchangeability of alpha-actinin in various structures of cultured chick cardiac fibroblasts and muscle cells using fluorescent analogue cytochemistry in combination with fluorescence recovery after photobleaching. Living cells were microinjected with tetramethylrhodamine-labeled alpha-actinin, which became localized in cellular structures. Small areas of labeled structures were then photobleached with a laser pulse, and the subsequent recovery of fluorescence was monitored with an image intensifier coupled to an image-processing system. In fibroblasts, fluorescence recovery was studied in stress fibers and in adhesion plaques. Bleached spots in adhesion plaques generally attained complete recovery within 20 min; whereas complete recovery in stress fibers occurred within 30 to 60 min. In muscle cells, alpha-actinin became localized in the Z-lines of sarcomeres, in punctate structures, and in apparently continuous bundle-like structures. Fluorescence recovery in Z-lines, punctate structures, and some bundle-like structures was extremely slow. Complete recovery did not occur within the 6- to 7-h observation period. However, some bundle-like structures recovered completely within 60 min, a rate similar to that of stress fibers in fibroblasts. These results indicate that fluorescently labeled alpha-actinin is more stably associated with structures in muscle cells than in fibroblasts. In addition, different structures within the same cell can display different alpha-actinin exchangeabilities which, in muscle cells, could be developmentally related.