These results support the implementation of a preconditioning phase for in vitro WSS studies to establish a physiologic baseline. Our findings complement previous macroscale findings and are consistent with a cellular mechanism involving increased END1 and PDGFB levels, but decreased NOS3 levels, leading to intimal hyperplasia at regions of low magnitude reversing WSS.

Fluorescein-labeled heavy meromyosin subfragment-1 (F-S-l) has been purified by ion exchange chromatography and characterized in terms of its ability to bind specifically to actin. F-S-1 activates the Mg+ § triphosphatase activity of rabbit skeletal muscle actin and decorates actin as shown by negative stains and thin sections of rabbit actin and rat embryo cell microfilament bundles, respectively. Binding of F-S-1 to cellular structures is prevented by pyrophosphate and by competition with excess unlabeled S-1.The F-S-1 is used in light microscope studies to determine the distribution of actin-containing structures in interphase and mitotic rat embryo and rat kangaroo cells. Interphase cells display the familiar pattern of fluorescent stress fibers. Chromosome-to-pole fibers are fluorescent in mitotic cells.The glycerol extraction procedures employed provide an opportunity to examine cells prepared in an identical manner by light and electron microscopy. The latter technique reveals that actin-like microfilaments are identifiable in spindles of glycerinated cells before and after addition of S-1 or HMM. In some cases, microfilaments appear to be closely associated with spindle microtubules. Comparison of the light and electron microscope results aids in the evaluation of the fluorescent myosin fragment technique and provides further evidence for possible structural and functional roles of actin in the mitotic apparatus.KEY WORDS microfilaments HMM subfragment-1 cultured cells actin mitotic spindle fluorescence microscopy Determination of the distribution and organization of actin-like microfilaments in nonmuscle cells is an important approach to discerning the mechanisms of cellular motility (see references 40 and 41 for reviews). Microfilaments are identified in cells fixed and thin-sectioned for electron microscopy by their size (-6 nm diameter) and ability to bind the myosin fragments heavy meromyosin (HMM) and HMM subfragment-1 (S-l), resulting in "arrowhead" configurations or decorated complexes (26,27,18). This reaction is considered to be specific for actin, as HMM and S-1 do not decorate other fibrous structures in the cell (27,19,40 also been employed to study the overall distribution of structures thought to contain actin (5). However, the supramolecular organization of aco tin cannot be determined by these techniques because of the limits of resolution of the light microscope (22). Only a few attempts have been made to correlate directly information from light and electron microscope studies. For instance, the distribution of phase-dense stress fibers (5) and birefringent fibers seen with polarized light microscopy (17) has been shown to correspond to that of microfilament bundles in several cultured animal cells (20,22).Fluorescein-labeled HMM (F-HMM), S-1 (F-S-1), and antiactin antibody in indirect immunofluorescence studies have been shown to stain stress fibers in extensively spread cells which correspond to microfilament bundles (45,47,22,29,20). We have shown that comparison of light an...

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