A β-Actin Isotype Is Present in Rat Cardiac Endothelial Cells But Not in Cardiac Myocytes

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Abstract
Objective: Active constriction in cardiac capillary endothelial cells (CCECs) is controversial. It is thought by many researchers that CCECs are not actively involved in constriction; others believe that these cells do contribute some of the force required for capillary constriction. Because actin is a major component of most contractile mechanisms responsible for changing cell shape, we compared two probes as potential monitors of actin distribution in CCECs in situ. Methods: We used SDS‐PAGE, Western blotting, electron microscopy, conventional epifluorescence and confocal microscopy to evaluate an antibody against non‐muscle β‐actin and fluorescently labeled phalloidin to detect actin in CCECs. Results: Fluorescently labeled phalloidin detected actin in cardiac muscle and faintly in CCECs. The antibody probe detected non‐muscle β‐actin in CCECs. However, this actin isotype did not occur in cardiac myocytes. In endothelial cells non‐muscle β‐actin was concentrated at the cell periphery, and arranged as bundles of fibers, not as typical stress fibers. Conclusions: Phalloidin is not suitable as a probe to monitor actin distribution in CCEC's in situ. The antibody is a potentially useful tool in monitoring the actin cytoskeleton and determining the capacity of CCECs for active vasoconstriction in vivo. Non‐muscle β‐actin in CCECs is arranged in filament bundles which are not typical stress fibers.

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