Characteristics of Photorelaxation in Vascular Smooth Muscle: Evidence Supporting the Hypothesis of Drug-Receptor Equilibrium Disturbance

The affinity of norepinephrine for the α-adrenergic receptor in rabbit thoracic aorta is determined by three different methods. The first method assumes that the effect is directly proportional to receptor occupancy from which affinity is computed from the A
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. The second method is a null method and employs a prior partial blockade, assuming only that equal effects imply equal receptor occupancy without making the direct proportion assumption. The third method is a new ``relaxation'' method in which the equilibrium is upset by a transient perturbation with UV radiation, and the kinetics of the restoration process are analyzed, from which not only affinity, but both forward and reverse rate constants are determined. Affinity constants determined by the ``null'' and ``relaxation'' methods are in agreement, while the affinity constant determined by A
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is low. The nature of the photo-induced relaxation is discussed along with the basis for extending the relaxation method to other drug-effector combinations.