Experiments were carried out to characterize the [3H]CGP12177 binding sites in the microsomal fraction from the guinea pig taenia caecum. 2. The Scatchard plot in the absence of butoxamine (10(-4) M) or bupranolol (10(-4) M) was concave, suggesting two affinity sites: high and low affinity binding sites. 3. The Scatchard plot in the presence of butoxamine gave a straight line, indicating only the low affinity site. 4. The Scatchard plot in the presence of bupranolol was undetectable. 5. These results suggest that the high affinity binding sites for [3H]CGP12177 correspond to beta 2-adrenoceptors in the microsomal fraction from the guinea pig taenia caecum, whereas the low affinity binding sites represent beta 3-adrenoceptors.
