In vitro model systems for evaluation of smooth muscle cell response to cryoplasty

Restenosis is a major health care problem, with approximately 40% of angioplasties resulting in restenosis. Mechanisms related to elastic recoil, cell proliferation, and extracellular matrix (ECM) synthesis are implicated. In vivo studies have demonstrated the potential for cryotherapy to combat the process of restenosis, but the mechanisms whereby freezing and/or cooling can reduce or eliminate smooth muscle cell (SMC) proliferation and ECM synthesis are not well known. While in vivo testing is ultimately necessary, in vitro models can provide important information on thermal parameters and mechanisms of injury. However, it is important to carefully choose the model system for in vitro work on cryoinjury characterization to adequately reflect the clinical situation. In this study, we examined the differences in response to cryoinjury by SMCs from different species (rat, pig, and human) and in different cellular environments (suspension vs. tissue equivalent). Tissue equivalents, composed of cells embedded in collagen or fibrin gel, provide a 3-D tissue-like environment, while allowing for controlled composition. As reported here, all SMCs showed similar trends, but rat cells appeared less sensitive to cooling at faster cooling rates in suspension, while human SMCs were less sensitive to temperatures just above freezing when embedded in collagen. In addition, the SMCs were less sensitive in suspension than they were in collagen. Cells in suspension exhibited 70% viability at -11°C, whereas cells in the tissue equivalent model showed only 30% survival. Future studies will aim to more adequately represent the conditions in restenosis by providing inflammatory and proliferative cues to the cells.