Recruitment and dynamics of leukocytes in the formation of arterial intimal thickening--a comparative study with normo- and hypercholesterolemic rabbits.

Leukocyte involvement in intimal thickening was investigated as a function of time and diet. Fibromuscular or foam cell-rich thickings were induced by electrical stimulation (ES) of carotid arteries in rabbits either on a normal or a high (1%) cholesterol diet. Under both dietary conditions granulocytes (predominantly neutrophils), monocytes and lymphocytes migrated through and accumulated beneath a continuous, yet structurally altered endothelium already after 1 day of ES. This preceded the occurrence of smooth muscle cells (SMCs) in the intima. Under normocholesterolemia, leukocyte attachment to the endothelium decreased with continued ES, which coincided with the re-establishment of a normal endothelial cell pattern. Neutrophils ceased to invade the stimulated intima and disappeared from the lesion after 14 days. The proportion of mononuclear leukocytes was also reduced in the thickened intima, finally amounting to 5.5 +/- 5.9% in the 4-week-old fibromuscular lesion where SMCs prevailed. Hypercholesterolemia did not affect neutrophil involvement in response to ES. However, it provoked lipid deposition first in macrophages, then in SMCs and resulted in elevated amounts of mononuclear leukocytes both within the foam cell-rich thickening and in association with the overlying endothelium. These data indicate adaptive behavior of leukocytic infiltration in the development of fibromuscular thickening, and a shift to a chronic inflammatory response under additional hypercholesterolemia.

Enhanced endothelial permeability and invasion of leukocytes into the artery wall as initial events in experimental arteriosclerosis.

The temporal sequence of the very early events in arteriosclerosis, as induced by electrical stimulation in carotid arteries of rabbits, was examined by combined light microscopy and transmission electron microscopy. After one session (30 min) of DC impulses, the endothelial permeability to horse-radish peroxidase was increased mainly beneath the anode, judging from the massive accumulation of reaction products of peroxidase in the subendothelium. After a further stimulation period, some of the endothelial cells displayed alterations in pattern and size and heavy cytoplasmic deposition of silver salt. However, the endothelium was maintained as a continuous lining. During this initial phase, a considerable number of granulocytes and monocytes was found adhering to the endothelium of the stimulated region and also within the subendothelial space. The invasion of the leukocytes preceded the migration of smooth muscle cells from the media into the intima, a process which began after two days of the electrical stimulation schedule. These initial phases of plaque development may represent a special form of an inflammatory response.

Temporal sequence of morphological alterations in artery walls during experimental atherogenesis--occurrence of leukocytes.

In rabbit carotid arteries arteriosclerotic lesions were induced by repeated local transmural electrical stimulations. The sequence of early morphological alterations in the vessel wall and especially the kinetics of leukocytes were examined by transmission electron microscopy. After a stimulation period of only 1 day monocytes and heterophilic granulocytes adhered to the endothelial surface. In the subendothelium mainly beneath the anode, focal amorphous insudates were present together with mononuclear and granulocytic cells. Thereby, the endothelium was maintained as a continuous lining as shown by surface staining with silver nitrate. However, both pattern and size of the endothelial cells were altered in comparison to the controls. Some of the endothelial cells displayed a heavy cytoplasmic silver salt deposition. After 2 days of the electrical stimulation schedule, the first myocytes occurred in the subendothelial space. The mediamyocytes sending pseudopods through the internal elastic lamina still appeared to be in a contractile phenotype. In the 7-day-old proliferative lesion modulated smooth muscle cells were the predominant cell type; only 10%-20% of the subendothelial cells were identified as macrophages and heterophils. This proportion decreased further, and after a stimulation period of 28 days the granulocytes disappeared completely. At this stage of plaque development, the intimal myocyte population mainly consisted of contractile smooth muscle cells and intermediary states between the contractile and modulated phenotype. The insudation, immigration of white blood cells, and subsequent migration and proliferation of myocytes reinforces the view that the initial phases of arteriosclerotic lesions may represent a special form of an inflammatory response.