calpain requires 3–50 μMcalcium, and the
mMcalpain requires 400–800 μM calcium
for one-half maximal activity (Goll et al.,
1998). Thus, μMcalpain is the physiologic-
ally active isoform. For a review of the
calpain system and skeletal muscle growth,
including the effects of various
phenethanolamines on the calpain system,
see Goll et al.(1998). In general, there is
good correlation between enzyme or
inhibitor activity and mRNA expression
(Bardsley et al., 1992). An increase in
calpastatin expression and activity com-
bined with a decrease in μM calpain,
suggesting a net decrease in protein
degradation, has been observed in cattle
treated with cimaterol (Parr et al., 1992)
and in sheep treated with L-644,969
(Kretchmar et al., 1990; Koohmarie et al.,
1991; Pringle et al., 1993). In contrast, Ji
(1992) observed an increase in mMcalpain
activity in pigs treated with ractopamine,
but detected no significant differences in
μMcalpain or calpastatin activity. Similarly,
Bergen et al. (1989) reported increased
activity of cathepsin L, but not of cathepsin
B and H, nor the calcium-dependent
proteases following ractopamine treatment
in pigs. Consistent with the in vivoprotein
synthesis and degradation studies, this
suggests that ractopamine does not affect
protein degradation. One possible explana-
tion for the conflicting results on muscle
protein degradation in response to
phenethanolamine treatment may relate to
the differences in -adrenergic receptor
specificity of ractopamine (1) compared
with cimaterol and L-644,969 (2). It may
be postulated that activation of the 2-
adrenergic receptor stimulates both an
increase in protein synthesis and a
decrease in protein degradation, while
activation of the 1-adrenergic receptor
affects only protein synthesis.
Protein synthesis and degradation in
response to phenethanolamine treatment
have also been evaluated in vitro using
skeletal muscle cell cultures, but with
varying results. Some authors concluded
that phenethanolamines do not have a
direct effect on cultured muscle cells,
while others reported increased proteinsynthesis or increased protein synthesis
combined with decreased degradation as
direct responses to phenethanolamine
treatment (see Mersmann, 1998). As with
all in vitrostudies, these results must be
interpreted with caution as they do not
represent the true muscle cell environment
found in vivo. Bridge et al. (1998)
suggested that some of the variation among
in vitrostudies may be attributed to differ-
ences in the densities and sub-populations
of -adrenergic receptors present in
cultured muscle cells.Endocrine effectsIt has been suggested that the repartition-
ing effects of phenethanolamines may be
due in part to opposing effects of
phenethanolamines on insulin sensitivity
in adipose tissue versus skeletal muscle
(Anderson et al., 1991). Decreased sensi-
tivity to insulin in response to -adrenergic
agonists has been demonstrated in
adipocytes from rats (Hausman et al., 1989)
and pigs (Liu and Mills, 1990), while
increased sensitivity to insulin following
prolonged treatment with -adrenergic
agonists has been observed from soleus
muscle of rats (Budohoski et al., 1987).
However, adipocytes isolated from mice
treated with clenbuterol did not differ from
controls in sensitivity to insulin (Orcutt et
al., 1989), and insulin binding to mouse
adipocytes following in vivo administra-
tion of ractopamine and clenbuterol was
not significantly affected (Dubrovin et al.,
1990). Studies using the hyperinsulinaemic,
euglycaemic clamp technique in cattle
identified only a transient decrease in
insulin sensitivity following clenbuterol
administration (Sternbauer et al., 1998), and
no change in insulin sensitivity or
responsiveness following ractopamine
administration (Eisemann and Bristol, 1998).
Therefore, the significance of changes in
insulin sensitivity or responsiveness identi-
fied in vitroto the overall in vivoeffects of
phenethanolamines is unclear.
The effects of phenethanolamines on
circulating hormones known to influencePhenethanolamine Repartitioning Agents 7704 Farm Animal Metabolism 04 20/4/00 12:02 pm Page 77