Farm Animal Metabolism and Nutrition

adrenergic receptor (Pietri-Rouxel et al.,

1995) and porcine 
1- and 
2-adrenergic

receptors (Mills, personal communication)

have been expressed in CHO cells. However,

the current classification of ligands tested in

livestock is based on data obtained from

rodent or human receptors.

Response to specific 
-adrenergic

receptor ligands varies across species,

presenting a significant limitation to the

extrapolation of data from one species to

another (Pietri-Rouxel et al., 1995). For

example, bupranolol acts as an antagonist

in both humans and rodents, but as a

partial agonist at bovine 
3-receptors,

while propranolol is an antagonist at the

rodent 
3-receptor but a partial agonist at

the human and bovine homologues of this

receptor (see Strosberg, 1997). These find-

ings demonstrate the need to characterize


-adrenergic receptor ligand activity in the

species of interest.

The receptor subtype selectivity of

phenethanolamines used in livestock has

been evaluated only with regard to the 
1-

and 
2-adrenergic receptors. Salbutamol,

clenbuterol and cimaterol were all

developed as 
2-selective bronchodilators

(Brittain et al., 1976; Asato et al., 1984;

Engelhardt, 1984). Studies in human heart

(Hall et al., 1989) and guinea pig trachea

(O’Donnell and Wanstall, 1978; Colbert et

al., 1991) with salbutamol and in rat

jugular vein with clenbuterol (Cohen et al.,

1982) have confirmed this 
2-receptor

selectivity. The 
-adrenergic receptor

profile of ractopamine-HCl was determined

in isolated smooth and cardiac muscle

tissues of rat and guinea pig (Colbert et al.,

1991). Ractopamine possessed a 100-fold

higher affinity than salbutamol at the 
1-

adrenergic receptor but only one-tenth the

affinity of salbutamol at the 
2-adrenergic

receptor. The selectivity of ractopamine for

the 
1- and 
2-adrenergic receptor sub-

types was evaluated by competition

binding analysis with crude membrane

preparations from C6 rat glioma cells

(Smith et al., 1990) and by nuclear

magnetic resonance (NMR) evaluation of

the 
-adrenergic receptor peptide binding

(Schmidt et al., 1993). Ractopamine

possessed higher binding affinity for crude

membranes prepared from rat heart (
1)

than from rat lung (
2). Thus, ractopamine

has been shown to be selective for the 
1-

adrenergic receptor, in contrast to other

phenethanolamine repartitioning agents

that are 
2-selective.

Receptor desensitization and down-regulation

Many membrane-bound receptors, includ-

ing 
1- and 
2-adrenergic receptors,

undergo decreased sensitivity to agonists

following prolonged exposure. This may

occur by uncoupling of the receptors from

Gs proteins (desensitization), as well as by

receptor sequestration and internalization

(down-regulation; see Benovic et al., 1988;

Barnes, 1995).

Desensitization of receptors occurs

through phosphorylation (Fig. 4.8).

Phosphorylation of 
1- and 
2-adrenergic

receptors occurs in the C-terminal region by

the activity of a family of serine-threonine

kinases known as G-protein-coupled

receptor kinases (GRKs; Benovic et al., 1986;

Inglese et al., 1993). The 
-adrenergic

receptor kinase (
ARK) is one member of

the GRK family. Phosphorylation in the C-

terminal region facilitates the binding of

arrestin proteins that uncouple the inter-

actions between receptor and Gs proteins,

resulting in receptor desensitization (Pippig

et al., 1993). Desensitization of 
-adrenergic

receptors by members of the GRK family

has been demonstrated for the 
1-

(Freedman et al., 1995) and 
2- (Inglese et

al., 1993) receptors, but not for the 
3-

adrenergic receptor (Liggett and Raymond,

1993). The 
3-adrenergic receptor lacks

phosphorylation sites in its C-terminal

region which may make it resistant to GRK-

mediated desensitization.

Down-regulation of receptors, resulting

in decreased numbers of receptors on the

cell membrane, also decreases responsive-

ness to receptor agonists (Benovic et al.,

1988). Spurlock et al.(1994) studied the

effect of ractopamine on 
-adrenergic

receptor density in skeletal muscle and adi-

pose tissue of pigs. These authors reported

decreased receptor density in adipose, but

not skeletal muscle tissue, in response to

Phenethanolamine Repartitioning Agents 81

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