Monocyte adhesion and changes in endothelial cell number, morphology, and F-actin distribution elicited by low shear stress in vivo.

Left common carotid arteries of New Zealand white rabbits were ligated rostral to origin of the thyroid artery to reduce flow in the carotid upstream of this branch, and the vessels were examined 5 days later. Estimates of mean shear stress in the upstream carotid artery indicated a decrease of 73% (from 12.1 +/- 1.6 dynes/cm2 to 3.26 +/- 0.58 dynes/cm2). The contralateral common carotid artery carried collateral flow and experienced a 170% increase in shear stress (from 11.3 +/- 1.6 dynes/cm2 to 30.5 +/- 4.6 dynes/cm2). There was an adaptive reduction in the diameter in the left common carotid artery (low shear) from 2.07 +/- 0.06 mm to 1.75 +/- 0.12 mm, but the diameter of the right carotid was unchanged. Fluorescence microscopy and scanning electron microscopy of endothelium exposed to low shear revealed attachment of leukocytes (5.02 +/- 1.59 cells/mm2, mean +/- SE) that were identified as monocytes using the monoclonal antibody HAM 56. Laser confocal microscopy demonstrated that they were migrating across the endothelial cell monolayer. Fluorescence microscopy and scanning electron microscopy of left common carotid artery (low shear) also revealed cell morphology suggestive of endothelial cell desquamation. Endothelial cell loss was confirmed by morphometric determination of cell number (1.29 +/- 0.13 x 10(4) cells/mm length in experimental animals versus 1.71 +/- 0.08 x 10(4) cells/mm length in sham-operated animals). This endothelial cell loss may be an adaptation to a narrowing of carotid arteries exposed to low shear, which reduces luminal surface area of the vessel. Staining of F-actin with rhodamine phalloidin showed that endothelial cells exposed to low shear were less elongated and had fewer stress fibers than normal cells. By contrast, increasing shear stress by two- to threefold caused an increase in the number of stress fibers and a reduction in peripheral actin staining. Distal carotid ligation provided a consistent and well-defined in vivo technique for manipulating shear stresses imposed on a large population of endothelial cells.