FRACTAL ANALYSIS OF CYTOSKELETON REARRANGEMENT IN CARDIAC-MUSCLE DURING HEAD-DOWN TILT

Head-down tilt by tail suspension of the rat produces a volume, but no t pressure, load on the heart. One response of the heart is cytoskelet on rearrangement, a phenomenon commonly referred to as disruption. In these experiments, we used fractal analysis as a means to measure comp lexity of the microtubule structures at 8 and 18 h after imposition of head-down tilt. Microtubules in whole tissue cardiac myocytes were st ained with fluorescein colchicine and were visualized by confocal micr oscopy. The fractal dimensions (D) of the structures were calculated b y the dilation method, which involves successively dilating the outlin e perimeter of the microtubule structures and measuring the area enclo sed. The head-down tilt resulted in a progressive decrease in D (decre ased complexity) when measured at small dilations of the perimeter, bu t the maximum D (maximum complexity) of the microtubule structures did not change with treatment. Analysis of the fold change in complexity as a function of the dilation indicates an almost twofold decrease in microtubule complexity at small kernel dilations. This decrease in com plexity is associated with a more Gaussian distribution of microtubule diameters, indicating a less structured microtubule cytoskeleton. We interpret these data as a microtubule rearrangement, rather than erosi on, because total tubulin fluorescence was not different between group s. This conclusion is supported by F-actin fluorescence data indicatin g a dispersed structure without loss of actin.