Bioscience Reports, Vol. 6, No. 2, 1986
Different Patterns of Protein Turnover in
Skeletal and Gastrointestinal Smooth Muscle
and the Production of N~-Methylhistidine
During Fasting in the Rat
Peter W. Emery, 1 Laura Cotellessa, 2 Mark Hoiness, 2
Christine Egan, 3 and Michael J. Rennie 3'4
Received November 11, 1985
KEY WORDS:
During four days of fasting in rats skeletal muscle protein synthesis fell progressively, whereas skeletal muscle protein breakdown was unchanged until the
third and fourth days when it rose dramatically. In contrast, the synthetic rate of
smooth muscle protein was unchanged during three days of fasting despite a loss of
protein content, indicating an abrupt rise in protein breakdown in this tissue on the first
day of fasting which was sustained thereafter. Urinary excretion of N'-methylhistidine
was significantly increased throughout fasting. The concentration of free N ~methylhistidine in plasma and in muscle tissue was elevated throughout the period of
fasting. This elevation was not caused by reduced renal clearance, but appears to have
been mainly the result of increased breakdown of N'-methylhistidine-containing
proteins in tissues other than skeletal muscle.
INTRODUCTION
For mammals, the relative losses of protein from skeletal and smooth muscle during
fasting, and the mechanisms involved in these losses, are not known in detail. We have
1 Department of Nutrition, Kings College (KQC), London W8 7AH.
2 Department of Medicine, University College, London WC1E 6JJ.
3 Department of Physiology, University of Dundee, Dundee DD 1 4HN.
4 To whom correspondence should be addressed.
143
0144-8463/86/0200-0143505.00/0
~ 1986 Plenum Publishing Corporation
144
Emery,Cotellessa,Holness, Egan,and Rennie
attempted to document these changes in the rat and, in addition, to compare them with
changes in the production of N'-methylhistidine, since excretion of this amino acid is
frequently used as an index of actin and myosin breakdown (Young and Munro, 1978).
We have previously produced evidence suggesting that small but rapidly turningover pooles of protein-bound N~-methylhistidine may contribute substantially to the
whole body production rate in adult rats (Millward er al., 1980). The small rapidly
turning-over pools of N~-methylhistidine are likely to occur in smooth musclecontaining tissues, including the gastrointestinal tract. The contribution of
gastrointestinal serosa has been estimated by various workers as being between
25-50% of whole body production in the adult rat (Millward and Bates, 1983; Wassner
and Li, 1982). However, it is not known how the proportional contributions of skeletal
muscle and gastrointestinal smooth muscle change during starvation.
We have therefore measured the rates of synthesis and degradation of protein in
skeletal and gastrointestinal smooth muscle in fed and fasted rats, together with the
urinary excretion of N'-methylhistidine (Experiment 1). Since we observed an increase
in the concentration of free N'-methylhistidine in the blood plasma of fasted rats, we
have also investigated the renal clearance of this amino acid during fasting (Experiment
2). In Experiment 3 we have quantified the net loss of protein from skin, gut and carcass
during fasting, and we have determined whether the proportion of N'-methylhistidinecontaining proteins in these tissues changes during fasting. Preliminary reports of some
of these experiments have already been published (Cotellessa et al., 1983; Emery et al.,
1983).
METHODS
Weanling male Wistar rats obtained from Hilltop Farms Ltd. were fed on a meatfree diet (20~ casein), until they reached 100 g body weight. Only rats which grew at the
same rate (within 5%) were chosen for study.
Experiment 1: Measurement of Muscle Protein Synthesis and Degradation Rates,
and Urine Production of N~-Methylhistidine
Rats (mean weight 96 + 4 g (SE)) were assigned to seven groups of 8-21 animals of
equal body weight. Group 1 was killed immediately; Group 2 was fed the meat-free diet
ad lib for a further 3 days and Group 3 for 4 days; Groups 4, 5, 6, and 7 were fasted and
killed after 1, 2, 3, and 4 days respectively. Water was provided ad Iibirum. Rats in
Groups 3 and 7 were housed individually in metabolic cages, and urine was collected
daily with 2 N HC1 as preservative.
Muscle protein synthesis was measured in all animals using the large-dose
phenylalanine method (Garlick et al., 1980). Animals were decapitated either 2 min
(three animals from each group) or 10 min (5-18 animals in each group) after the
intravenous injection of 150#mol/100g body weight of phenyl [2,3-3H]alanine
(Amersham). Mixed blood was collected from neck vessels into heparinized pots. The
legs were immediately skinned and the peritoneum opened, and the whole body was
rapidly cooled in ice water. Gastrocnemius, soleus, plantaris, and rectus abdominus
muscles were removed, blotted dry, weighed, and frozen in liquid nitrogen. The whole
Skr
and SmoothMuscleTurnover in Fasting
145
small intestine was also removed, the mucosal layer was removed by scraping between
coo!ed glass plates and the serosal layer was washed with ice cold saline (NaCI 9 g/l),
blotted dry, weighed, and frozen in liquid nitrogen. Protein synthesis was calculated
from the ratio of the specific radioactivities of free and protein-bound phenylalanine,
measured by the specific, enzymatic assay described by Garlick et al. (1980) with
appropriate quench and internal standard corrections. The alkaline soluble protein
content of individual muscles was determined by the method of Lowry et aI. (1951), and
RNA content by the ultraviolet absorption method of Fleck and Munro (1962). N ~methylhistidine was measured in deproteinized (10% sulphosalicylic acid) plasma, in
the soluble supernatant fraction from muscles homogenized in 5~o sulphosalicylic acid,
and in urine hydrolysed for 2 hr at 105~ with 6 N hydrochloric acid, using a Rank
Hilger Chromaspek amino acid analyser (Emery and Rennie, 1982). Formaldehyde (1%
in water) was added to the post-column fluorescent reagent mixture, to eliminate
interference due to histidine. Urinary creatinine was measured by the alkaline picrate
method (Varley, 1967).
Muscle protein breakdown was calculated as the difference between the measured
rate of muscle protein synthesis and the net rate of growth or loss of muscle protein. In
fed rats, breakdown rates at day 0 were determined from the average growth rate over
days 0 3 or 0~4.
Experiment 2: Rates of Disappearance of Exogenous N*-Methylhistidine from Fed
and Fasted Rats
In order to assess the possible effects of fasting on urinary clearance of N ~methylhistidine, a solution of 4.2/~moles N~-methylhistidine dissolved in 1 ml of NaC1
solution (9 g/l) and neutralized to pH 7.4 was injected through cannulae placed in the
tail veins of conscious rats (21 fed and 21 fasted for 3 days) and flushed with 1 ml saline.
The rats were killed by decapitation at intervals (three each, after 15 and 30 rain and
after 1, 2, 4, 6, and 24 hr) and samples of blood, soleus and plantaris muscles and kidney
were taken for the determination of free N'-methylhistidine concentration.
Experiment 3: Analysis of Changes in Excretion and Concentrations of ProteinBound N~-Methylhistidine During Fasting
Twenty-four rats (98 + 4 g) were allocated to three groups. Group 1 was killed
immediately; Group 2 was allowed to feed ad lib for 3 days; Group 3 was given a water
solution of electrolytes only for 3 days (1.2 g NaC1, 1.5 g KC1, 0.25 g K2HPO~, 0.5 g
NaH2PO 4 + 10 ml Addamel trace element solution per litre of water). Groups 2 and 3
were kept in metabolic cages: urine was collected daily using 2 N HC1 as preservative,
and analysed for N~-methylhistidine as described above. The animals were killed by
cervical dislocation. The bodies were skinned and the gastrointestinal tract from
stomach to rectum was removed, the contents were washed out, and the skin, carcass,
and whole gut were weighed and stored frozen for later analysis. The entire skins,
carcasses, and guts were homogenized in 2% (w/v) perchloric acid, and aliquots of the
soluble and insoluble fractions were separated by centrifugation. The precipitates were
re-dissolved in 0.3 N NaOH, and aliquots were taken for analysis of protein by the
Lowry method. Samples of gastrocnemius muscle and of small intestinal serosa were
Emery, Cotellessa, Holness, Egan, and Rennie
146
Table 1.
Protein content and rates of protein turnover in skeletal muscle (plantar• and small intestinal
smooth muscle in fed and fasted rats. Values are means • SEM for 8-21 rats
Protein
content
(mg/muscle)
Protein
synthesis
(% per day)
Skeletal muscle
Day 0, fed
Day 1, fasted
Day 2, fasted
Day 3, fasted
Day 4, fasted
Day 4, fed
13.1 • 0.8
13.2 • 0.4
13.4 _+ 0.4
12.6 _+ 0.5
10,9 _+ 0.6 a
15.7 • 0.7a
11.4 _+ 1.6
7.6 _+ 0.6 a
6.8 • 0.7 ~
4.8 • 0.7 ~
1.5 • 0.l ~
10.3 • 1.8
Smooth muscle of small intestine
Day 0, fed
Day 1, fasted
Day 2, fasted
Day 3, fasted
Day 3, fed
200
175
152
132
241
_+ 16
• 12
• 4a
_%8~
• 15
81
88
85
84
87
_+ 12
_+ 14
_+ 9
+ 12
__+ 14
Protein
breakdown
(% per day)
6.5
6.6
5.3
10.7
15.1
5.9
74
103
98
97
74
" Significantly different from day O, P < 0.025.
analysed separately. Alkali-soluble protein was separated and measured as above, and
aliquots were hydrolysed for 24 hr at 105~ with 6 M HC1, neutralized and analysed for
N<methylhistidine as described above.
RESULTS
Protein Synthesis and Breakdown in Skeletal Muscle and Urinary N <
Methylhistidine Production (Experiment 1)
Protein synthesis in skeletal muscle showed a progressive fall during 4 days of
fasting. The same pattern of change was observed in four types of skeletal muscle:
soleus, gastrocnemius, plantar• and abdominal muscle. Typical results, for plantaris
muscle, are shown in detail in Table 1; results for the other muscles are summarized in
Fig. 1. The fall in synthesis rate was slightly greater than the rate of loss of protein from
muscle in the first 2 days of fasting, indicating the protein breakdown could not have
increased during this time. On the third day of fasting, protein breakdown in
gastrocnemius and plantaris muscle rose, although in soleus muscle protein
breakdown was still below its pre-fasting value on day 3. On the fourth day there was a
large increase in the rate of protein loss from all leg muscles, which was greater than the
fall in protein synthesis, indicating that skeletal muscle protein breakdown must have
been substantially elevated. In a series of experiments on rats weighing 200-250 g, there
were sma/ler losses of protein, but the pattern of protein loss and depression of protein
synthesis were identical (M. J. Rennie and C. Egan, unpublished results).
These changes in protein synthesis and degradation in skeletal muscles during
fasting are very similar to those reported by Waterlow and Millward (1978) for mixed
quadriceps and gastrocnemius muscles. The presently observed decreases in protein
synthesis were accompanied by reductions in both the content and the activity of RNA
Skeletal and Smooth Muscle Turnover in Fasting
PROTEIN SYNTHESIS
IIO
lOO
8o I
147
~
PROTEINBREAKDOWN
260
220i
~y
A/~
6o
:2~
4o
2o
o
t
I
[
o
1
2
I
60
I
3
L.,.
4
o
I
1
f
[
I
2
3
4
Days
Fig. 1. Relative changes in protein synthesis and protein breakdown
expressed as a percentage of day 0-fed values during fasting in 100-g rats.
Changes are shown for plantaris (A), gastrocnemius ( I ) , rectus abdominus
(*), and soleus ( 0 ) muscles, and gastrointestinal smooth muscle from small
intestine (A). Also shown is daily production of N<methylhistidine (9
Some values taken from Tables 1 and 3 to emphasize patterns of change.
(Table 2), as was reported by Garlick et al. (1975) for mixed quadriceps and
gastrocnemius muscle.
It has been suggested that skeletal muscles which contain a high proportion of
dark oxidative fibres, such as the soleus, are more resistant to the effect of malnutrition
than are muscles of a more mixed, dark-pale, fibre-type composition, such as the
plantaris (Jefferson et al., 1980). In this experiment we found that, whereas all muscles
initially adapted to fasting by reducing their protein synthesis rate with no apparent
increase in breakdown, and so conserving protein, the soleus maintained this
Table 2. RNA content and activity in skeletal muscle (rectus abdominus) and small
intestinal smooth muscle in fed and fasted rats. Values are mean • SEM for eight rats
RNA content
(mg/g protein)
RNA activity
(g protein synthesized
per day per g RNA)
Skeletal muscle
Day 0, fed
Day 1, fasted
Day 2, fasted
Day 3, fasted
12.5 ___0.3
10.7 • 0.7"
10.6 • 1.1
7.8 • 0.8 a
10,3 _+ 0.9
6.2 _+ 0.6~
5,7 _+ 0.7"
5,1 _+ 0.6"
Smooth muscle
Day 0, fed
Day 1, fasted
Day 2, fasted
Day 3, fasted
72.3
68.1
68.7
62.8
10.3 • 0.9
12.9 +_ 2.8
9.9 -t- 1.4
10.3 _+ 1.0
a Signifies they differ from day 0, P < 0.025.
___5.6
• 4.8
• 7.7
• 8.4
148
Emery, Cotellessa,Holness, Egan, and Rennie
adaptation for 3 days whereas the plantaris and gastrocnemius muscles began to lose
protein rapidly after 2 days. The rate of protein breakdown in these muscles was
roughly doubled and tripled on days 3 and 4 (Fig. 1).
Urinary N*-methylhistidine excretion (Table 3) doubled during the first 3 days of
fasting and fell slightly on the fourth day. The content of the free W-methylhistidine
pool in the bodies of fasted rats also increased due to an increase in the blood and
intracellular pools of N~-methylhistidine. Hence the total production rate of free Wmethylhistidine, calculated as the sum of urinary excretion plus the net change in the
free W-methylhistidine pools, also rose progressively during fasting. Urinary N'methylhistidine excretion is commonly expressed per unit creatinine as a possible index
of fractional breakdown rate of myofibrillar protein (McKeran et aI., 1977): this ratio
also rose during fasting, to almost twice the control value by day 3 (Table 3). Clearly,
neither the total excretion of W-methylhistidine nor the urinary W-methylhistidine/
creatinine ratio is a reliable guide to the changes in skeletal muscle protein breakdown
during fasting.
Protein Synthesis and Breakdown in the Serosal Layer of Gastrointestinal Tract
(Experiment 1)
Whereas the rate of protein synthesis in skeletal muscle fell progressively during 3
days of fasting to 50% of the value in fed rats, protein synthesis in the serosal layer of the
gut did not change significantly throughout this period (Table 1). In a separate series of
experiments on rats weighing 200-250 g identical results were obtained over 3 days of
fasting (M. J. Rennie and C. Egan, unpublished data). There was a significant loss of
tissue protein from the intestinal serosa, of about one third, in 3 days of fasting,
indicating that the rate of protein breakdown in intestinal serosa must have risen. The
maintenance of protein synthesis in the intestinal serosa was also indicated by the
finding that the RNA content of this tissue remained unchanged during the 3-day fast
(Table 2).
Clearance of N~-Methylhistidine During Fasting (Experiment 2)
We were surprised at the large increase in the free N~-methylhistidine in blood and
muscle tissues during fasting and initially interpreted this as being due to a possible
failure of the elimination of amino acid from the body (Table 3). It is known that in
fasting rats urine production may fall partly due to adjustment of mechanisms designed
to limit salt loss. In Experiment 3 therefore we substituted a weak electrolyte solution in
place of pure water for drinking in an effort to keep urine flow high. Urine flow
increased but this made no difference to the retention of N~-methylhistidine within the
free pool of the rats (data not shown).
There was no difference between fed and fasted rats in the rate of clearance of
exogenous N~-methylhistidine, which was removed from the blood with a half time of
4,5 hr (Fig. 2). The changes in plasma N~-methylhistidine could be described by the
equation Yt = (Y0 --Yass) e-~176
+ Y. . . . where y is the plasma concentrations at
times t and nought and Y,s~ is the asymptotic value in fed and fasted rats. The
distribution space of W-methylhistidine was 40% of body weight, i.e. greater than
blood volume (8%) and smaller than total body water (60%). The distribution ratio (i.e.
Skeletal and Smooth Muscle Turnover in Fasting
149
+l+l+l+l+l
I
O
+1+1+1+1+1
'6
+1+1+1+1+1
"F-.
"r9
r
~
q;:~ q'~ i-~ e--.I
O
Z
o
=
,'1::1
y.~o
+1+1+1+11
+l
a
Z
v
+1 +l +l +l +l +1
"~o
v
~4
z~
150
,
Emery, Cotellessa, Holness, Egan, and Rennie
4.0
E
3.0
"S
E
:z, 2,[1
9
-x
L
-i-
o
,
1.0
o
0.5
I
O
rr~
i
0
I
[
I
3
I
J
T
6
Hours
Fig. 2. Changes in blood N~-methylhistidine
in fed (0) and fasted (9 rats after injection of
4.2 #tool
NZ-methylhistidine/100 g
body
weight. Values are means _+ SD. The rate of fall
is identical in each group of animals.
concentration in tissue:plasma) of the amino acid in muscle was 1.3 + 0.2 and in kidney
2.8 + 0.4. The rate of fall of the N~-methyIhistidine concentration of muscle and kidney
was also identical in fed and fasted rats.
Comparison of the rate of elimination from the body of exogenously administered
N*-methylhistidine-containing proteins. Since the rate of protein breakdown in
disappearance rate between fed and fasted rats and that the accumulation of N 'methylhistidine in the tissues of the fasted rats may have been caused by an increase of
the rate of production of the free amino acid as a result of breakdown of actin and other
N'-methylhistidine-containing proteins. Since the rate of protein breakdown in
skeletal muscle did not rise during the first 2 days of fasting (Table 1) we are forced to
the conclusion that the rise in the skeletal muscle free pool was secondary to changes in
the plasma pool which was being supplied by protein breakdown in other tissues, e.g.
gastrointestinal serosa and other smooth muscle-containing tissues. We now have
evidence (H. Hundal and M.J. Rennie, unpublished) that N~-methylhistidine may be
transported into muscle by the amino acid carrier system, L. Free NT-methylhistidine
shows a tissue:blood distribution ratio some 300o greater than unity, presumably due
to counter transport with amino acids such as alanine, whose transport is linked to the
Na gradient in the A system, but which are also substrates for the L system.
Changes in Body Mass and Tissue Composition During Fasting (Experiment 3)
In the rats allowed access to food, growth continued at about 5~o per day and a
similar increase occurred in carcass weight and protein content (Table 4). The gut did
not increase significantly in weight or protein content. Skin weight increased by 17%
over the 3 days, but skin protein content was too variable to show a significant change.
In fasted animals the body weight fell by 25% which was due to a 17~ fall in carcass
weight, 35~ fall in gut weight, and 27~ fall in skin weight. Much of the fall in carcass
weight must have been due to losses of water and fat, however, since there was no
statistically significant loss of protein from the carcass. In contrast, the gut lost 31~ of
its protein in 3 days of fasting. The small (8~) loss of skin protein is difficult to evaluate
Skeletal and Smooth Muscle Turnover in Fasting
151
Table 4. Body composition in young rats during feeding and fasting
Content
N~-methylhistidine
(nmole/mg protein)
Wet weight
(g)
Protein content
(g)
56.8 • 0.8
66.6 +_ 1.4"
47.0 • 0.7"
9.66 • 0.26
12.00 _+0.5"
9.21 • 0.53
8.80 _+0.23
9.15 • 0.40
5.70 • 0.7a
0.98 • 0.05
1.07 • 0.08
0.68 +_0.02"
17.2 • 0.9
20.2 • 0.8~
12.6 _+0.7"
1.55 • 0.14
1.82 • 0.13
1.42 • 0.11
0.365 _+0.006
0.433 _+0.006"
0.302 • 0.011"
0.055 • 0.001
0.064 • 0.001"
0.050 • 0.002
2.00 • 0.10
1.99 _+0.14
2.09 _+0.08
2.20 • 0.14
2.31 • 0.06
1.57 • 0.06"
0.204 _+0.010
0.206 • 0.010
0.138 • 0.006a
1.32 • 0.14
1.28 _+0.16
1.02 • 0.16
Carcass
Do
Da, fed
D3, fasted
Gut
Do
D 3, fed
D3, fasted
Skin
Do
D 3, fed
D3, fasted
m
Gastrocnemius muscle
Do
D 3, fed
D 3, fasted
Small intestine serosa
Do
D3, fed
D3, fasted
" Significantly different from day O, P < 0.025.
because of analytical difficulties in separating alkali soluble a n d insoluble p r o t e i n s in
this tissue.
M u c h of the p r o t e i n in the carcass is present in skeletal muscle, so t h a t these
changes in carcass c o m p o s i t i o n are b r o u g h t a b o u t by the a l t e r a t i o n s in skeletal muscle
p r o t e i n m e t a b o l i s m described earlier. T h u s fasting causes an i m m e d i a t e cessation of
muscle g r o w t h b y a r e d u c t i o n in the rate of skeletal muscle p r o t e i n synthesis, b u t there
is very little net loss of p r o t e i n b e c a u s e the rate of skeletal muscle p r o t e i n b r e a k d o w n
also r e m a i n s low. In the gut, however, fasting causes an i m m e d i a t e increase in the rate
of b r e a k d o w n of s m o o t h muscle protein, leading to a r a p i d net loss of p r o t e i n of m o r e
t h a n 10~o p e r day. T h e figures in T a b l e 4 also imply that m u c o s a l a n d serosal p r o t e i n s
are lost at a p p r o x i m a t e l y equal rates. T a b l e 4 also shows that the p r o p o r t i o n of N ~m e t h y l h i s t i d i n e in p r o t e i n did n o t c h a n g e in response to feeding a n d fasting in either
skeletal o r s m o o t h muscle. It is therefore r e a s o n a b l e to s u p p o s e t h a t the t u r n o v e r of the
m y o f i b r i l l a r p r o t e i n s which c o n t a i n N~-methylhistidine was altered in the same
direction as total muscle p r o t e i n turnover, which is i n d i c a t e d b y the results of
E x p e r i m e n t 1. It is n o t likely that the increased excretion of N~-methylhistidine could
have c o m e from increased b r e a k d o w n of skeletal muscle act• while b r e a k d o w n of
o t h e r skeletal muscle p r o t e i n s decreased.
DISCUSSION
T h e present results indicate t h a t fasting in y o u n g rats causes m a r k e d differences in
152
Emery, Cotellessa, Holness, Egan, and Rennie
the pattern and mechanism of loss of protein from skeletal muscle and gastrointestinal
smooth muscle. In skeletal muscle a fall in muscle protein synthesls appears to be the
major cause of the relatively small loss of tissue protein over the first 2 days of fasting,
although beyond this increased protein breakdown makes a substantial contribution,
and the net rate of loss of protein increases. On the other hand, in gastrointestinal
muscle there appears to be no significant change in protein synthesis as a result of
fasting so that all the losses of tissue protein appear to be due to an immediate increase
in protein breakdown.
The present results also show that fasting in young rats causes a marked increase
in whole-body N~-methylhistidine production at a time (day 0-2) when the rate of
breakdown of skeletal muscle protein remains constant. This strongly suggests that the
extra N~-methylhistidine produced early in fasting is not due to a rise in skeletal muscle
protein breakdown. The use of the total urinary N'-methylhistidine production or the
N~-methylhistidine:creatinine ratio as an index of skeletal muscle breakdown rate is
thus clearly invalid in fasting. How far this applies to other situations is presently
unknown, but we have evidence from studies in man that the increased urinary
excretion of N~-methylhistidine observed immediately after surgery does not come
from increased breakdown of skeletal muscle tissue (Rennie et al., 1984).
Some of the N~-methylhistidine excreted appears to be accounted for by losses
from gastrointestinal serosa and skin. For gastrointestine at least it appears that the
mechanism of the loss includes an increase in protein breakdown since there was no
reduction in the rate of serosal protein synthesis.
If it is assumed that the rate of synthesis of N~-methylhistidine-containing proteins
in the gut remained constant during 3 days of fasting (as demonstrated for mixed
proteins), then the rate of degradation of these proteins must have increased by
approximately 10~ per day to cause the observed fall of one third in the total amount of
protein-bound N'-methylhistidine in the gut. Since the total N~-methylhistidine
content of the entire gut protein was originally about 700 nmoles this would result in an
increase in production of 70 nmoles of N~-methylhistidine per day from the gut. This is
only about 10~ of the increase in whole body N~-methylhistidine production of
2.0 #moles over 3 days, or 660 nmoles per day. Clearly there must have been increases
in the breakdown rates of other N~-methylhistidine-containing proteins, presumably
including smooth muscle in other tissues, which made a substantial contribution to the
increase in whole-body N'-methylhistidine production observed during fasting.
Moreover, although we cannot calculate precisely the quantitative contributions of
each tissue to urinary N~-methylhistidine excretion because this depends on the relative
turnover rates of actin and other proteins in the tissues, it appears that skeletal muscle
contributes an even smaller proportion to urinary N~-methylhistidine excretion in the
fasted state than it does in the fed state. It would be of great interest to identify the other
tissues which contribute substantially to N~-methylhistidine excretion in other
circumstances of disease and malnutrition.
The role of smooth muscle as a possible source of amino acid carbon to be used for
gluconeogenesis, and as a primary fuel under normal circumstances, and additionally
for the production of acute phase protein or for healing after trauma or sepsis, is not
well understood. The present results suggest that, in the immediate response to fasting
at least, smooth muscle may make a substantial contribution to the availability of
Skeletal and Smooth Muscle Turnover in Fasting
153
amino acids with skeletal muscle protein apparently being conserved for at least 2 days.
The control of mechanisms in skeletal and smooth muscle which adapt in opposite
directions is difficult to contemplate. If the well-known changes in h o r m o n e secretion
which a c c o m p a n y fasting, i.e. a fall in insulin and rises in corticosterone and glucagon,
have a controlling influence then they must affect different tissues differently. O u r
detailed understanding of the control of protein mass in tissues other than skeletal
muscle is very poor. The present results suggest that investigation of the control of
protein synthesis and b r e a k d o w n in s m o o t h muscle would be valuable.
ACKNOWLEDGMENTS
This work was supported by The Cancer Research Campaign, The Wellcome
Trust, The Medical Research Council, and The Nuffield Foundation. L.C. was a
Fellow of the advanced training p r o g r a m of the Italian L a b o u r D e p a r t m e n t and the
EEC. We are grateful for the enthusiastic support of Professor Richard Edwards and
for helpful discussions and encouragement from Dr. D. J. Millward, who provided us
with generous access to results of unpublished work.
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