Determination of the Affinity of Ractopamine and Clenbuterol for the Beta-Adrenoceptor of the Porcine Adipocyte2

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Abstract
Dissociation constants (Kd) of ractopamine and clenbuterol for the swine adipocyte β-adrenergic receptor were estimated from competition studies with epinephrine for the stimulation of lipolysis. Both compounds competitively inhibited epinephrine-stimulated lipolysis in the absence of adenosine deaminase. Three methods for estimating Kd values were used and similar estimates were obtained with each method. Ractopamine and clenbuterol showed greater affinity for the β-receptor than did epinephrine and had similar Kd values of 1 to 2 × 10−7M. The low capacity of ractopamine and clenbuterol to stimulate lipolysis in vitro does not result from poor coupling to the β-receptor. Ractopamine and clenbuterol may be considered partial agonists, possessing high affinity for the β-adrenoceptor but exhibiting a relative ineffectiveness for adenylate cyclase activation.

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