Imaging remodeling of the actin cytoskeleton in vascular smooth muscle cells after mechanosensitive arteriolar constriction

Experiments were performed to determine whether remodeling of the actin cytoskeleton contributes to arteriolar constriction. Mouse tail arterioles were mounted on cannulae in a myograph and superfused with buffer solution. The α₁-adrenergic agonist phenylephrine (0.1-1 μmol/1) caused constriction that was unaffected by cytochalasin D (300 nmol/1) or latrunculin A (100 nmol/1), inhibitors of actin polymerization. In contrast, each compound abolished the mechanosensitive constriction (myogenic response) evoked by elevation in transmural pressure (P_TM; 10-60 or 90 mmHg). Arterioles were fixed, permeabilized, and stained with Alexa-568 phalloidin and Alexa-488 DNAse I to visualize F-actin and G-actin, respectively, using a Zeiss 510 laser scanning microscope. Elevation in P_TM, but not phenylephrine (1 μmol/1), significantly increased the intensity of F-actin and significantly decreased the intensity of G-actin staining in arteriolar vascular smooth muscle cells (VSMCs). The increase in F-actin staining caused by an elevation in P_TM was inhibited by cytochalasin D. In VSMCs at 10 mmHg, prominent F-actin staining was restricted to the cell periphery, whereas after elevation in PTM, transcytoplasmic F-actin fibers were localized through the cell interior, running parallel to the long axis of the cells. Phenylephrine (1 μmol/1) did not alter the architecture of the actin cytoskeleton. In contrast to VSMCs, the actin cytoskeleton of endothelial or adventitial cells was not altered by an elevation in P_TM. Therefore, the actin cytoskeleton of VSMCs undergoes dramatic alteration after elevation in P_TM of arterioles and plays a selective and essential role in mechanosensitive myogenic constriction.