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Determination of the Affinity of Ractopamine and Clenbuterol for the Beta-Adrenoceptor of the Porcine Adipocyte^1,2,

Purdue University³ West Lafayette, IN 47907

Abstract

Dissociation constants (K_d) of ractopamine and clenbuterol for the swine adipocyte ß-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 K_d values were used and similar estimates were obtained with each method. Ractopamine and clenbuterol showed greater affinity for the ß-receptor than did epinephrine and had similar K_d values of 1 to 2 x 10^–7 M. The low capacity of ractopamine and clenbuterol to stimulate lipolysis in vitro does not result from poor coupling to the ß-receptor. Ractopamine and clenbuterol may be considered partial agonists, possessing high affinity for the ß-adrenoceptor but exhibiting a relative ineffectiveness for adenylate cyclase activation.

¹ Journal paper No. 11918 of the Purdue Univ. Agric. Exp. Sta., W. Lafayette, IN. This work was supported in part by a grant from Eli Lilly and Co.

² The authors thank Boehringer Ingelheim Animal Health, Inc. for supplying clenbuterol and Eli Lilly for supplying ractopamine. The expert assistance of Agnieszka Brzozowska-Prechtl and Melissa Durflinger is gratefully acknowledged.

³ Dept. of Anim. Sci.