The affinity of ractopamine, clenbuterol, and L-644,969 for the beta-adrenergic receptor population in porcine adipose tissue and skeletal muscle membrane.

The affinities (Ki) with which ractopamine (RA), clenbuterol (CB), and L-644,969 (L6) bind the beta-adrenoceptor populations of adipose tissue (middle and outer subcutaneous [SQ] layers) and longissimus (LM) and semitendinosus (ST) muscle were determined. Within a given agonist, Ki values (nanomolar) were similar among tissues, except for RA, which had a higher (P < or = .05) affinity (lower Ki) for middle SQ adipose tissue than for outer SQ adipose and both muscles. For all tissues, binding affinities were greatest for CB (126), followed by L6 (350) and RA (856, exclusive of middle SQ adipose). The data indicate that both adipose and muscle tissues are targets of CB, RA, and L6 in vivo, and that tissue preferences of the agonists cannot be established from affinity data alone. The relatively constant affinity of the agonists for the various tissues examined implies that if tissue preferences exist, the efficiency with which postreceptor events are invoked by these agonists is the determining factor.