Smooth Muscle Cells from Rabbit Aorta

Abstract

The rabbit has been often used as an experimental animal in atherosclerosis research [51]. Early, in 1908, Ignatowski [34] studied the “influence of animal food on the organism of rabbits” and found an increase of lipids in artery walls when the rabbit was fed with a diet rich in fat and cholesterol. The production of atheromas by cholesterol rich diets became a standard technique and is still used when the effects of cholesterol on the arterial walls is studied. It is the basis of the lipid theory of atherogenesis [1]. A second theory for atherogenesis, the response-to-injury theory [44], is based upon experiments reported by Baumgartner [2]. He induced a thrombogenic reaction with subsequent fibromuscular intimai proliferates in the aortas of rabbits. Using a balloon catheter the endothelial lining of the aorta was removed and the vessel was distended by inflating the balloon. In 1966, Baumgartner and Studer [3] found a relationship between the distension of the aorta and a subsequent proliferation of cells in the intima. The mechanical stimulus caused smooth muscle cells (SMCs) of the media to migrate into the intima and to proliferate there to form a local intimai fibromuscular cushion. In 1979, another model to generate atherosclerotic proliferations was described by Betz and Schlote [7]. Again the rabbit was used as an experimental animal for local weak electrical stimulation of the carotid artery. Both models showed that in the early stages of atherogenesis, SMCs migrate from the media into the subendothelial space where they proliferate to form a focal thickening of the vessel wall termed a “fibromuscular plaque”. In contrast, SMCs of the normal vessel wall reveal a low proliferation rate [18, 19]. In the normal vessel wall a balance exists between factors inhibiting and factors inducing proliferation of SMCs whereby the inhibitory factors dominate. These factors act in a complex manner in vivo and it is very difficult to study the mechanisms of inhibition or activation of SMC proliferation in detail. Since the rabbit was the first and most frequently used animal in experimental atherosclerosis research [51], in vitro systems consisting of cells from the rabbit may be helpful to analyze the interactions of factors affecting the proliferation of SMCs. This does not only save animal experiments [6] but also enables the study of therapeutic aspects which cannot be solved in in vivo experiments [5]. If, for example, a drug suppresses the development of a stenosing proliferate in an artery by inhibiting the migration or proliferation of SMCs in the intima, it cannot be concluded whether the drug causes this response because of its toxicity; it also cannot be concluded whether the drug inhibits the mitotic process directly or via an indirect mechanism, e.g., by inhibiting the permeability of the endothelium. Finally, the in vivo experiment does not allow the conclusion that the drug acts specifically on the SMCs in the artery walls. Therefore, cultures of various cell types are useful in order to study the action of antiatherogenic drugs. One of the most important cell type in this context is the arterial SMC. Cultures of SMCs enable migration and proliferation to be studied under controlled and standard conditions. In vitro it is even possible to observe either the migration or the proliferation as separate phenomena. It is useful to compare the responses of cultured SMCs of rabbits with the in vivo experiments. Thus, the effective doses of antiatherogenic drugs as well as their toxic concentration can be determined in vitro. Cultures of SMCs from the rabbit aorta can be easily established. The isolation of SMCs from the media can be achieved by enzymatic disaggregation [16] or using the primary expiant technique [44]. In culture, SMCs reveal a characteristic behavior of proliferation, migration, differentiation, and growth pattern. Furthermore, more complex culture systems can be established which represent the in vivo situation better than mass cell cultures or clone cultures of SMCs.