conclusive evidence linking activation of
-adrenergic receptors to the anabolic effects
of phenethanolamines in livestock has yet to
be presented. In rodents, antagonists of
-adrenergic receptors have been used
in combination with agonists in efforts
to block the anabolic effects of
phenethanolamines. Reeds et al. (1988)
used propranolol, a non-selective 1- and
2-antagonist, and atenolol, a 2-specific
antagonist, in combination with
clenbuterol in rats. The antagonists were
administered orally at dosages 10–100
times greater than those of clenbuterol. The
effects of clenbuterol on energy expendi-
ture, cardiac growth and fat deposition
were attenuated by the -adrenergic
receptor antagonists, but the effect of
clenbuterol on muscle growth was not
affected. Thus, these authors questioned
whether the anabolic effects of clenbuterol
were mediated by -adrenergic receptor
activation. The results of Reeds et al.
(1988) were confirmed by MacLennan and
Edwards (1989) when similar dosages of
clenbuterol and propranolol were adminis-
tered orally to rats. However, MacLennan
and Edwards (1989) also administered
propranolol by intraperitoneal injection,
followed by subcutaneous injection of
clenbuterol. Using this protocol, the ability
of clenbuterol to induce muscle hyper-
trophy was blocked by propranolol,
suggesting the involvement of -adrenergic
receptors. Likewise, Choo et al. (1992)
showed that the anabolic effects of
clenbuterol were inhibited by the 2-specific
antagonist ICI-118,551, as well as by high
dosages of propranolol administered orally.
These authors also observed that when the
anabolic effects of clenbuterol were
inhibited by -receptor antagonists, the
lipolytic effects of clenbuterol were not
affected. Thus, although the anabolic
effects of clenbuterol were blocked by -
adrenergic receptor antagonists, they may
not be facilitated by the same mechanisms
that elicit changes in adipose tissue. It
must also be recognized that these results
represent the effects of specific agonists
and antagonists in rats and have not been
confirmed in livestock species to date.Mechanisms responsible for the
anabolic effects of phenethanolamines in
livestock were investigated initially by
measuring fractional protein synthesis and
degradation rates in vivo. Individual
studies reported increased, decreased and
unchanged rates of protein synthesis and
degradation following treatment with
phenethanolamines (see Moloney et al.,
1991). The variability across studies is
likely to be due in part to differences in the
phenethanolamine used, its receptor
specificity, the species studied, and the
dose and duration of treatment. In general,
results from these studies support the
hypotheses that protein synthesis is
enhanced and/or protein degradation is
inhibited by treatment with the 2-agonists
cimaterol, clenbuterol and L-644,969 (see
Moloney et al., 1991), whereas only protein
synthesis is affected by the 1-agonist
ractopamine (Bergen et al., 1989).
Alternatives to measuring total protein
synthesis or degradation are to measure
total RNA produced in skeletal muscle,
mRNA expression of specific genes or the
activity of specific enzymes important in
protein synthesis or degradation. Increased
protein synthesis in response to
phenethanolamines has been suggested
based on increased total RNA and mRNA
of -actin in skeletal muscle of pigs treated
with ractopamine (Bergen et al., 1989;
Helferich et al., 1990) and sheep treated
with L-644,969 (Koohmaraie et al., 1991).
Smith et al.(1989) reported an increase in
the mRNA abundance of myosin in cattle
treated with ractopamine, and Helferich et
al.(1990) showed increased expression of
additional unidentified muscle proteins by
ractopamine-treated pigs based on the
results of an in vitrotranslation assay.
Protein degradation rates have been
predicted based on the expression and
activity of proteolytic degradative enzymes.
The calpains (μMand mM) are intracellular
non-lysosomal calcium-dependent neutral
proteases that contribute to protein degrada-
tion, and calpastatin is an endogenous
inhibitor of the calpains. The calpain
isoforms are named after their sensitivities
to calcium required for activation. The μM76 D.E. Moody et al.04 Farm Animal Metabolism 04 20/4/00 12:02 pm Page 76