We have previously shown that an endothelin receptor antagonist can regress medial arterial calcification in a rat model. The aim of this study was to characterize the phenotypic changes of vascular smooth muscle cells during calcification and mineral loss, in order to understand better the underlying mechanisms. Control Wistar rats were compared with rats treated only with warfarin/ vitamin K1 (15 mg/kg per day) for 8 weeks, or in combination with darusentan (30 mg/kg per day) for the final 4 weeks. Vascular smooth muscle cell, bone cell and macrophage phenotypes were evaluated by the local expression of alpha-actin, tartrate-resistant acid phosphatase and ED-1, respectively. Proteins involved in the modulation of bone resorption like osteopontin and osteoprotegerin were also evaluated by immunohistochemistry. The warfarin/vitamin K1 treatment increased medial arterial calcification ninefold (P < 0.05). At sites of calcification, there was a decrease in alpha-actin localization, and an appearance of osteopontin immunostaining. Histochemical and immunostaining for osteoclast and macrophage markers, as well as for osteoprotegerin, were negative. Although the extent of calcification foci was reduced by darusentan, protein localization in the calcified areas was not modified. Thus, the development of medial arterial calcification produces a phenotypic change in vascular smooth muscle cells that does not appear to be normalized in regions remaining calcified during mineral loss.