Structural diversity among phene-
thanolamines and differences in their oral
potencies provide ample opportunity for
other -adrenergic receptor agonists to be
safely developed for use in livestock
(Smith, 1998). Clenbuterol and other
phenethanolamines have been developed
for use in humans for bronchodilation and
tocolytic purposes, and extreme differences
in oral dosage are required to achieve
therapeutic effects. For example, a total
dose of only 10 μg of clenbuterol is an
effective bronchodilator while an oral dose
of 10,000 μg preceded by intravenous
infusion is required for effectiveness from
ritodrine (see Smith, 1998). Because of
these large differences in minimum dosage
needed to observe effects in humans, it is
reasonable to expect that the maximum
dosage with no observable effects
(NOEL) in humans differs greatly among
phenethanolamines. Therefore, the
problems that arose from the illegal treat-
ment of animals with clenbuterol are not
representative of the appropriate use of all
phenethanolamines.Industry Applications
Phenethanolamines will probably be
administered as a feed additive during
the final phase of finishing. Because
phenethanolamines are short acting and
efficacious over limited time periods, their
use may be limited to intensive production
systems. Currently, the only phenethanol-
amines available for use as repartitioning
agents are zilpaterol (Zilmax®), which is
manufactured by Hoechst Roussel and cur-
rently is on the market in Mexico and
South Africa, and ractopamine, an Elanco
Animal Health product that is cleared for
use in swine in the US.
Additionally, a novel approach to
activate the -adrenergic receptors is being
investigated by researchers in Australia
(Sillence, 1996; Hill et al., 1998). The long-
term objective of these researchers is the
development of a vaccine to stimulate the
production of antibodies that will interact
with the 2-adrenergic receptor and mimicthe anabolic effects of -agonist drugs.
Using this approach, one or two doses of
the vaccine could be administered to
stimulate the production of antibodies,
allowing the system to be used in non-
intensive systems and grazing animals (see
Sillence, 1996). Although the efficacy of
this approach remains to be demonstrated
in vivo, the successful production of rabbit
antibodies with functional activity at the
bovine 2-adrenergic receptor in vitrohas
been described (Hill et al., 1998).
The development of phenethano-
lamines initially was targeted to decrease
fatness so producers could provide more
efficiently the leaner meat products
demanded by consumers. Since the initial
work with phenethanolamines was com-
pleted, genetic selection has resulted in
leaner animals, particularly in swine (PIC
Technical Update, 1993). The use of
phenethanolamines may allow slaughter at
heavier weights and maintenance of the
same level of leanness as with current
slaughter weights. Increased lean produced
per day in a grow-out facility will remain
important to the producer. Additionally,
the repartitioning effects of phenethanol-
amines may be useful as a management
tool to increase lean muscle and decrease
fatness in lines of animals selected for
criteria other than carcass composition,
such as reproductive efficiency, improved
meat quality or increased longevity. Such a
strategy would enable selection pressure to
be applied to traits that would result in
livestock that are better able to reproduce
and raise offspring in an efficient manner.
The development and use of phene-
thanolamine repartitioning agents has the
potential to impact all aspects of the meat
and livestock industries (Fig. 4.9). Producers
will be able to raise livestock more
efficiently, meat packers will have higher
yielding carcasses, and meat processors will
have the opportunity to develop new low-
fat meat products more efficiently. In addi-
tion, consumers will benefit from products
with reduced cholesterol (Perkins et al.,
1992) and reduced calories. Finally, the use
of phenethanolamine repartitioning agents
will provide environmental benefits. Less86 D.E. Moody et al.04 Farm Animal Metabolism 04 20/4/00 12:02 pm Page 86