AbstractAdult rat arterial smooth muscle cells were shown to express mRNA for the platelet-derived growth factor (PDGF)α- andβ-receptors and to bind radioiodinated PDGF-AA and PDGF-BB in a phenotype-dependent and growth state-dependent manner. PDGFα-receptor mRNA was not detected in the intact aortic media, but appeared as the cells converted from a contractile to a synthetic phenotype during serum-free primary culture. PDGFβ-receptor mRNA was expressed already in vivo, and increased further as the cells were isolated and cultured in vitro. Exposure of the cells to human platelet PDGF resulted in increased PDGFα-receptor mRNA levels, decreased PDGFβ-receptor mRNA levels, and decreased binding of both PDGF-AA and PDGF-BB. Following removal of the exogenous mitogen, the content of PDGFα- andβ-receptor mRNA increased, as did the binding of PDGF-AA and PDGF-BB. Subsequently, the content of PDGF A-chain mRNA started to rise, and the cells retained a high rate of DNA synthesis in a serum-free medium. As a result of this autocrine stimulation, the PDGF receptors were down-regulated. Although smooth muscle cells in serum-free primary cultures bound the different PDGF isoforms to a varying extent (AA