3H-labelled angiotensin II specfically binds to plasma membranes of rat uterine smooth muscle cells. Two classes of binding sites differing in their affinity for the hormone were demonstrated. The high-affinity binding sites (KD 29 degrees C approximately 2.0 X 10(-8) mol/l) probably correspond to the receptors involved in the biological response. 2. Bilateral nephrectomy significantly increases the concentration of 3H-labelled angiotensin-binding sites, a phenomenon which seems unrelated to the freeing of receptor sites secondary to the suppression of plasma angiotensin. This phenomenon may be responsible for the specific hypersensitivity in vitro to angiotensin of uteri excised in anephric rats as compared with normal rats. 3. Angiotensin II infusion in nephrectomized rats reduced the concentration of 3H-labelled angiotensin-binding sites. 4. It is suggested that the angiotensin receptor concentration is regulated by the concentration of circulating angiotensin.
