PROPERTIES OF THE BINDING-SITES OF [H-3] 9-METHYL-7-BROMOEUDISTOMIN-DIN BOVINE AORTIC SMOOTH-MUSCLE MICROSOMES

[H-3]9-Methyl-7-bromoeudistomin D ([H-3]MBED), a powerful caffeine-lik e Ca2+ releaser, binds to the caffeine binding site of terminal cister nae of skeletal muscle sarcoplasmic reticulum and activates Ca2+-induc ed Ca2+ release. Properties of the binding site of [H-3]MBED were inve stigated in aortic smooth muscle. The specific activity was higher in microsomes than in other fractions. [H-3]MBED binding sites in smooth muscle microsomes were of a single class with a high affinity (K-D 50 nm), comparable with that in skeletal muscle sarcoplasmic reticulum. C affeine competitively inhibited [H-3]MBED binding, indicating MBED sha res the same binding site with caffeine. Solubilization and fractionat ion of the microsomes gave two fractions of [H-3]MBED binding activiti es. These results suggest that, in smooth muscle, there are multiple b inding sites of [H-3]MBED and caffeine, which might correspond to diff erent pharmacological actions of caffeine on smooth muscle. Therefore, [H-3]MBED, which binds to the different binding sites of caffeine, is useful as a probe for investigation of the actions of caffeine at the molecular level.