17
Metabolic Regulation
R.G. Vernon
Hannah Research Institute, Ayr KA6 5HL, UK
Introduction
Ruminants, like other animals, have to meet the nutritional demands of the
many organs and cell types of the body. This has to be done against a
background of a varying, and not always adequate, supply of nutrients. Thus,
once absorbed there are a number of potential fates for a given nutrient and a
plethora of mechanisms and factors, which influence the probability of a given
fate. Such mechanisms operate within cells, between different cells and types
within a tissue, and between organs. Mechanisms may be brought into play to
deal with acute or chronic challenges: the former are important for homoeo-
stasis while the latter are critical for the homoeorrhetic adaptations needed for
different developmental, physiological, nutritional or pathological states. The
nature of these mechanisms and the various types of factors involved are
considered in subsequent sections. It will be obvious to those familiar with the
previous edition of this book that the flavour of the current chapter is very
different from that written by the late Bernard Crabtree. He focused on the
important but still rather specialized field of mathematical modelling of meta-
bolic pathways and their regulation; for those interested in this aspect I strongly
recommend Bernard’s chapter (Crabtree, 1993) and also articles by Brown
(1994), Kacser et al. (1995) and Hofmeyr and Cornish-Bowden (2000).
Levels of Metabolic Control
Within cells
Metabolic pathways
Within cells the fate of a nutrient is determined not only by the activity of relevant
enzymes but, in some cases at least, by: (i) translocases, reflecting the fact that the
ß CAB International 2005. Quantitative Aspects of Ruminant Digestion
and Metabolism, 2nd edition (eds J. Dijkstra, J.M. Forbes and J. France)
443

Control can be complex: protein kinase A and AMP-stimulated kinase phos-
phorylate different serine residues of hormone-lipase which are separated by a
single amino acid; phosphorylation of one serine prevents phosphorylation of
the other (Yeaman et al., 1994). There are many examples of activity being
modulated by small molecules: in some cases a molecule interacts directly with
the catalytic site on the enzyme, in other cases the effector molecule interacts
with a distant site causing a conformational change which results in altered
activity (allosteric regulation). There can be simple product inhibition (e.g.
inhibition of hexokinases I and II by glucose-6-phosphate); inhibition by the
final product of a pathway (e.g. inhibition of ACC by fatty acids); inhibition by a
component of another pathway (e.g. inhibition of carnitine palmitoyl-CoA
transferase-1 by malonyl-CoA and methylmalonyl-CoA, intermediates of fatty
acid synthesis and propionate metabolism, respectively). It is not always inhib-
ition as glycogen synthase, for example, is activated by glucose-6-phosphate.
The complexity of control is illustrated by the fact that phosphofructokinase
is inhibited by both citrate and ATP (substrate) and activated by fructose-
6-phosphate (substrate), ADP (product) and AMP. In general, changes in
444 R.G. Vernon
phosphorylation are due to extracellular stimuli, whereas modulation by small
molecules is a response to intracellular stimuli.
Effective activity can also be modulated by translocation from one part of a
cell to another. For example, activation of hormone-sensitive lipase by cat-
echolamines in adipocytes results not only in increased enzyme activity, but also
a movement of the enzyme from the cytosol to the surface of the fat droplet
(Londos et al., 1999). Stimulation of glucose transport by insulin into adipo-
cytes and muscle cells involve a translocation of Glut 4-containing vesicles from
the interior of the cell to the plasma membrane (Mueckler, 1994).
The effective activity of an enzyme is also determined by the concentration
of the substrates. The importance of this depends on the concentration of
substrate relative to the affinity of the enzyme for the substrate. Thus for both

different promoters showing tissue specificity; PI is the major promoter of
adipocytes whereas PIII is important in lactating mammary tissue (Travers
and Barber, 2001). Most studies of this type have focused on non-ruminant
species, but in the case of ACC-a much of the data comes from work on sheep
tissue. Expression via the different promoters is under distinct physiological and
hormonal control. The decrease in ACC-a expression in sheep adipose tissue
during lactation, for example, is due mostly to a fall in expression via the PI
promoter with only a small decrease in expression via the PII promoter (Travers
and Barber, 2001). Regulation of gene expression via hormones and nutrients
is mediated by transcription factors, which bind to response elements in the
promoter regions of the gene.
Metabolic Regulation 445
Molecular biological approaches have not only revealed the complexity of
promoter systems, they have also shown that many proteins exist in more iso-
forms than previously thought. For example, a novel form of ACC-a was found in
sheep mammary gland, which has a missing sequence of eight amino acids prior
to a key serine that is thought to be important for control of ACC-a activity by
phosphorylation–dephosphorylation (Travers and Barber, 2001). Whether the
altered amino acid sequence influences the phosphorylation of this serine is not
known, but interestingly expression of this isoform of the enzyme in the mam-
mary gland is increased markedly by lactation (Travers and Barber, 2001).
Signal transduction pathways
As many hormones and growth factors have receptors in the plasma mem-
brane, signals have to be transmitted to sites within the cell via signalling
pathways. For some, e.g. catecholamine activation of lipolysis in adipocytes
and its antagonism by adenosine and prostaglandin E, the signalling pathway
appears to be well defined (Fig. 17.2).
However, for many hormones the pathways are only partly resolved. Thus
we know that insulin activates a series of branching pathways which mediate
effects on metabolism, protein synthesis, mitogenesis, etc. (Fig. 17.3), but

242 bp
EXON 2
96 bp
(EXON 3)
EXON 4
47 bp
EXON 5
250 bp
EXON 5A
422 bp
EXON 6
137 bp
P I P II P III
Fig. 17.1. Structure of the regulatory region of the ovine acetyl-CoA carboxylase-a gene
(adapted from Travers and Barber, 2001).
446 R.G. Vernon
kinase B pathway (Fig. 17.3), but recently a new pathway involving the proteins
TC10 and flotillin, which binds to lipid rafts in the plasma membrane, has been
implicated as well (Litherland et al., 2001).
For some important metabolic hormones, e.g. growth hormone, even
less is known. Frustratingly for this key hormone with its important chronic
homoeorrhetic metabolic effects (Bauman and Vernon, 1993; Etherton and
G
s
Catecholamines
G
i
PGE PGE
receptor
Adenosine Adenosine

i
, inhibitory GTP-binding protein.
Insulin
Insulin receptor
Insulin receptor substrate 1,2
shc
Phosphoinositide-3 kinase
MAP kinase
Protein kinase B
Protein kinase C zeta
Metabolic effects
Mitogenic effects
Fig. 17.3. Some of the insulin signal transduction system. MAP kinase, mitogen-activated
protein kinase; shc, src homology collagen-related protein.
Metabolic Regulation 447
Bauman, 1998) most research has focused on systems of questionable
physiological significance (a transient insulin-like effect seen in rodent tissue
after a period of abstinence from growth hormone, and a ‘commitment to
differentiation’ effect observed in a preadipocyte cell line) (Herrington and
Carter-Su, 2001). This reflects a tendency to study what is easy rather than
what is important!
To add to the complexity, we now know that many signal transduction
components exist in several isoforms; for example there are at least three iso-
forms of the b-adrenergic receptor (Carpene et al., 1998), two of the GTP-
binding protein G
s
, at least three isoforms of G
i
(Manning and Woolkalis, 1994)
and nine of adenylate cyclase (Simonds, 1999). The proportion of the different

apparent relationship between lipolysis in adipocytes and blood flow through
the tissue (Vernon and Clegg, 1985), and several locally produced factors
modulate both (Vernon and Houseknecht, 2000; Vernon, 2003).
448 R.G. Vernon
Fatty acids released from adipose tissue are transported in the blood bound
to serum albumin. Albumin has two high-affinity binding sites for fatty acids and
a further five low-affinity binding sites. The concentration of albumin in the
blood is about 0.5 mM, so 1 mM fatty acid will potentially saturate both high-
affinity binding sites; indeed a decreased release of fatty acids has been ob-
served when the concentration exceeded about 1 mM (Vernon and Clegg,
1985). The blood flow through sheep adipose tissue is about 50 ml=min=g
tissue before a meal (Barnes et al., 1983) and this will support a rate of fatty
acid release of about 50 nmol/min/g tissue. The limited amount of data
available suggests a rate of lipolysis of about 5 nmol fatty acid released per
min per g tissue in the fed state, rising to about 15 nmol/min/g tissue on
fasting in sheep (Vernon and Clegg, 1985). A substantial proportion of the
binding sites of albumin entering the tissue will already be occupied by fatty
acids in the fasted state, hence only a limited number will be free to accommo-
date newly released fatty acids. The various estimates come from a number of
different studies, but the general point is that blood flow, or to be precise free-
binding sites, has the potential to limit lipolysis.
Catecholamines both stimulate lipolysis and are vasoactive (Vernon and
Clegg, 1985). In addition, stimulation of lipolysis in sheep adipose tissue in vivo
by catecholamines resulted in a concomitant rise in prostaglandin E
2
(Doris
et al., 1996) which is vasodilatory and which also acts to attenuate lipolysis
(Crandall et al., 1997) (Fig. 17.4). The rise in prostaglandin E
2
production was

Nutrients need to be apportioned appropriately between the various organs
and tissues of the body. Key factors are blood flow, metabolic capacity of cells
and hormonal and nervous signals.
Blood flow varies considerably from tissue to tissue (Table 17.2) and there
is even marked variation within some tissues such as skin (Bell et al., 1983;
Gregory and Christopherson, 1986). Differences in blood flow between organs
in general reflect the differences in metabolic activity (Table 17.3) (Rolfe and
Brown, 1997). A relationship between blood flow and metabolic activity within
an organ has been demonstrated for the mammary gland in lactating goats
(Linzell, 1974) and portal-drained viscera in sheep and cattle (see Chapter 12).
Blood flow, and hence nutrient supply, to a tissue varies with physiological and
nutritional state. For example, on feeding in sheep, blood flow increased to the
rumen epithelium and salivary glands, decreased to abdominal adipose tissue,
but did not change to heart, kidney and subcutaneous adipose tissue (Barnes
et al., 1983). The onset of lactation in goats results in a fivefold increase
compared to pregnancy in blood flow to the mammary gland (Linzell, 1974).
Exercise or stress induces marked changes in blood flow with a much greater
proportion of cardiac output going to skeletal muscle (Bell et al., 1983).
Blood flow is under complex control, involving paracrine and autocrine
factors (e.g. Fig. 17.4), hormones and the nervous system. Catecholamines are
vasoactive and can both accentuate and attenuate blood flow, depending on
which receptors are activated. Increased sympathetic activity during exercise,
for example, causes increased release of adrenalin from the adrenal medulla,
which increases blood flow through skeletal muscle. In adipose tissue increased
sympathetic activity can lead to initial vasoconstriction due to activation of
Nerve endings
Noradrenaline
(acute)
Noradrenaline
(chronic)

(prostacyclin); 20:4, arachidonic acid.
450 R.G. Vernon
a-adrenergic receptors, followed by vasodilatation due to activation of b-
adrenergic receptors (Vernon and Clegg, 1985).
Access by nutrients to most cells requires their passage from the blood to
the extracellular space. Endothelial cell permeability thus provides another
means of manipulating nutrient fate (Vernon and Peaker, 1983). The liver in
particular has a very ‘leaky’ endothelium, reflecting the important role of the
liver in the uptake and degradation of proteins and even larger structures such
Table 17.2. Blood flow of various tissues in sheep (data from Barnes
et al., 1983; Bell et al., 1983; Gregory and Christopherson, 1986;
Weaver et al., 1990).
Tissue Blood flow (ml/min/100 g)
Brain 69, 70
Heart 62, 95, 110, 154
Kidney 460, 550, 650
Lactating mammary gland 50
Gastrointestinal tract
Rumen 24, 112
Abomasum 67, 105, 204
Small intestine 60, 62, 130
Large intestine 48, 64, 80, 105
Liver – hepatic artery 4, 8, 13
Liver – hepatic portal vein 285
Skeletal muscle 2, 9, 10–65
Adipose tissue 0.3, 4, 8–23
Skin 1–13, 3, 2–20
Table 17.3. Tissue oxygen use as percentage of whole body oxygen use and blood flow as
percentage of cardiac output in sheep (data also from A.W. Bell, unpublished observations).
Tissue

Hocquette et al., 1996) and new ones continue to be discovered. The Glut-4
transporter is insulin-sensitive and is found in adipocytes and myocytes – cells
with a high capacity for glucose metabolism (Mueckler, 1994; Hocquette et al.,
1996). Thus, if plasma glucose is increased, for example after a meal, the
concomitant rise in serum insulin will cause a preferential uptake of glucose by
cell types expressing Glut-4. Even in ruminants, which are thought to be less
responsive to insulin than most non-ruminants, insulin-infusion induced a six-
fold increase in glucose uptake across the hind limb of mature sheep (Fig. 17.5).
The corollary, of course, is that when serum insulin and glucose concentrations
are low as during fasting, utilization of glucose by other tissues (e.g. brain) will
be favoured.
Fatty acids are mostly supplied to tissues either as non-esterified fatty acids
(NEFA) bound to albumin or as a part of triacylglycerols, which are transported
as part of very low-density lipoproteins (VLDL) secreted by the liver, and
chylomicrons secreted by the gastrointestinal cells. VLDL and chylomicrons
are too large to cross the endothelial cell barrier, so triacylglycerols are hydro-
lysed by the action of lipoprotein lipase, an enzyme secreted by a variety of cells
including adipocytes, myocytes and mammary epithelial cells (Barber et al.,
1997). Following secretion it is transported to the luminal surface of the
0 2 10 20 30 406
0.4
0.8
1.2
0
Blood insulin (µg/I)
Glucose uptake (mM)
Fig. 17.5. Effect of insulin on glucose arteriovenous difference across the hind limb of lactating
(*) and non-lactating (*) sheep (data from Vernon et al., 1990).
452 R.G. Vernon