We have previously identified α-defensin in association with medial smooth muscle cells (SMCs) in human coronary arteries. In the present paper we report that α-defensin, at concentrations below those found in pathological conditions, inhibits phenylephrine (PE)–induced contraction of rat aortic rings. Addition of 1 μM α-defensin increased the half-maximal effective concentration (EC₅₀) of PE on denuded aortic rings from 32 to 630 nM. The effect of α-defensin was dose dependent and saturable, with a half-maximal effect at 1 μM. α-Defensin binds to human umbilical vein SMCs in a specific manner. The presence of 1 μM α-defensin inhibited the PE-mediated Ca⁺⁺ mobilization in SMCs by more than 80%. The inhibitory effect of α-defensin on contraction of aortic rings and Ca⁺⁺ mobilization was completely abolished by anti–low-density lipoprotein receptor–related protein/α_2-macroglobulin receptor (LRP) antibodies as well as by the antagonist receptor-associated protein (RAP). α-Defensin binds directly to isolated LRP in a specific and dose-dependent manner; the binding was inhibited by RAP as well as by anti-LRP antibodies. α-Defensin is internalized by SMCs and interacts with 2 intracellular subtypes of protein kinase C (PKC) involved in muscle contraction, α and β. RAP and anti-LRP antibodies inhibited the binding and internalization of α-defensin by SMCs and its interaction with intracellular PKCs. These observations suggest that binding of α-defensin to LRP expressed in SMCs leads to its internalization; internalized α-defensin binds to PKC and inhibits its enzymatic activity, leading to decreased Ca⁺⁺mobilization and SMC contraction in response to PE.