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Regulation of blood pressure by direct action of corticosteroids on blood vessel walls was first hypothesized in 1952 . The presence of receptors specific for these hormones within the peripheral vessel walls is a pre-requisite of this hypothesis. This report documents specific binding of both aldosterone (AL) and dexamethasone (DM) in cultured human arterial smooth muscle cells. After the arterial smooth muscle cultures were incubated with tritiated AL or DM at 37CC for 30-60 min, the cells were sonicated and the protein bound steroid fraction isolated on a Sephadex G25 column. Using ion exchange chromatography of this fraction, each corticosteroid receptor complex displayed a distinct, reproducible elution pattern. Aldosterone showed a single peak at 0.096 ± 0.005 mol/l sodium phosphate and DM had two peaks at 0.029 ± 0.003 and 0.050 ± 0.004 mol/l sodium phosphate. The Scatchard plots of specific binding from AL saturation curves are linear and revealed mean ± s.d. steady state binding parameters of dissociation constant (Kd)=0.35 ±0.15 nmol/l and maximum binding capacity (Bmax)=98 ± 53 x 10~18 mol/jj-g DNA. Similarly, the mean ± s.d. steady state binding parameters for DM are Kd=4.4 ± 2.0 nmol and Bmax=3031 ± 1385 x 10~18 mol/(jig DNA. Therefore, there are approximately 1150 AL binding sites and 30 000 DM binding sites per cell. The Kd and Bmax values are similar to those previously described for corticoid receptors in rat aortic smooth muscle cells and are consistent with physiological steroid concentrations. The presence of specific corticosteroid receptors in arterial smooth muscle cells supports the hypothesis that these steroids are directly involved in the regulation of blood pressure in humans.