Farm Animal Metabolism and Nutrition

ractopamine. This result may indicate

that ractopamine-induced down-regulation

differs between tissues. Likewise, Smith

(1989) observed no down-regulation of

skeletal muscle 
-adrenergic receptors in

cattle or swine fed ractopamine, although

cimaterol did down-regulate skeletal

muscle 
-adrenergic receptors in swine.

These data are consistent with the in vivo

accretion data of Dunsheaet al. (1993)

showing that pigs fed ractopamine had

significantly increased protein accretion

rates but only slightly decreased fat accre-

tion rates (Table 4.8). This differential

response may be a reflection of differences

in receptor down-regulation or desensitiza-

tion between tissues, or due to the involve-

ment of additional receptors.

Potential involvement of other receptors in

phenethanolamine mechanism of action

Pathways or receptors in addition to 
1-

and 
2-adrenergic receptors that elicit

responses to phenethanolamines have not

yet been identified in livestock. However,

the 
3-adrenergic receptor, atypical 
-

adrenergic receptors that currently are

unidentified, interactions among 
-receptors

or between 
-receptors and other physio-

logical pathways, and receptors that do not

belong to the 
-adrenergic receptor family

are possible candidates. Currently, the 
3-

adrenergic receptor is being investigated

for its role in energy expenditure and lipid

metabolism; several authors have suggested

the presence of additional, or atypical, 
-

82 D.E. Moody et al.

-arrestin

P P

P

P

P

Fig. 4.8.Phosphorylation of 
-adrenergic receptors. The 
1- and 
2-adrenergic receptors are

phosphorylated on the carboxy tail by G-protein receptor kinases (GRK; phosphorylation sites shown by

circles) and on the third intracellular loop by protein kinase A (phosphorylation sites shown by squares).

Phosphorylation by GRK results in increased binding of 
-arrestin. Binding of 
-arrestin leads to

uncoupling and internalization of the receptor, resulting in desensitization. (Adapted from Barnes, 1995.)

Table 4.8.Effect of ractopamine on empty body protein and fat accretion of finishing swine. Data are

combined for barrows, boars and gilts that were fed ractopamine beginning at 60 kg and ending at 90 kg

body weight (adapted from Dunshea et al., 1993).

Ractopamine

0 20 % change

Net protein accretion (g day^1 ) 144 188 a 30

Net fat accretion (g day^1 ) 442 414  6

aP< 0.001.

04 Farm Animal Metabolism 04 20/4/00 12:02 pm Page 82