Conversion of the myometrial oxytocin receptor from low to high affinity state by cholesterol.

Reconstitution experiments with the myometrial oxytocin receptor from guinea pig showed a strong requirement of the oxytocin receptor binding function for the presence of cholesterol in preformed liposomes. To investigate the effect of cholesterol on the oxytocin receptor in the plasma membrane, a new method to modify the membrane cholesterol content was developed. With substituted cyclodextrins we were able to selectively deplete the myometrial plasma membrane of cholesterol. Vice versa, incubation of cholesterol-depleted membranes with a soluble cholesterol cyclodextrin complex restored the cholesterol content of the plasma membrane. Binding experiments showed, that with the removal of cholesterol from the membrane, the oxytocin receptor was converted from high to low affinity state. Increasing the cholesterol content of the membrane again restored the high binding affinity. Substitution of membrane cholesterol with other steroids showed a strong dependence of the oxytocin receptor function on the structure of the cholesterol molecule. Experiments with the detergent solubilized oxytocin receptor and with oxytocin receptors expressed in the Baculovirus/Sf9 cell system provided further evidence for a direct interaction of the oxytocin receptor with cholesterol.