Evaluation and control of meat quality in pigs

443 Pages·1987·51.457 MB·English

by  P V Tarrant, G Eikelenboom, G Monin, Commission of the European Communities. Coordination of Agricultural Research

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'mE PHYSIOLOGY AND BIOCHEMISTRY OF mE PORCINE STRESS SYNDJO.1E
David Lister
Animal and Grassland Research Institute
Shinfield, Reading RG2 gAQ, U.K.
ABSTRACT
Much of the pertinent biochemistry and physiology of the Porcine
Stress Syndrome has been defined but a number of key problems have yet
to be solved. For example, we do not know why individuals differ in
their threshold of sensitivity to stressors, exactly how muscle
activ i ty is triggered under these circumstances and why some animals
may quickly recover fran such stinruli and others may not. Neither is
it completly clear how sensitivity to stressors and body composition
are linked. There is, however, good evidence that the syrrpathetic
nervous system and the catecholamines of the adrenal R'edulla are
essential elements in all of this.
P. V. Tarrant et al. (eds.), Evaluation and Control of Meat Quality in Pigs
© ECSC, EEC, EAEC, Brussels-Luxembourg 1987
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INI'RODUCTION
The Porcine Stress Syndrome (PSS) has been an important focus in
research on pigs and pigmeat for at least 20 years. The research has
taken on many guises to include topics as different as genetic
polym::>rphisms, animal behaviour, mitochondrial function and the
classical approaches of stress physiology. What began as enquiries
into the nature of aberrations in meat quality, quickly recognised in
the aetiology of the problem the important contribution of genetic
type and the ways in which animals were handled, and drew in expertise
from animal and meat science. The pig was recognised as a model for
certain comparable conditions in man and opened the field for
contributions from human medicine. It is fair to say, however, that
many of the early prognostications of researchers, especially in
regard to associations between the leanness of carcases and the
quality of meat, fell on deaf ears and only relatively recently have
the pig and meat industries begun to ask whether the search for ever
leaner carcases should not be tempered to preserve the vigour of the
live animal and the quality of its meat.
There have been many reviews of PSS in terms of the associated
physiology and biochemistry (Briskey, 1964 ~ Cassens et a1., 1975 ~
Lister et a1., 1981) and little will be served by rehearsing them.
Instead I shall attempt to identify the physiological and biochemical
issues behind the following questions and raise more questions which
require further research.
1. What is PSS?
2. How can PSS be described in objective terms?
3. How is meat quality affected?
4. What makes some pigs stress prone?
WHAT IS PSS?
All animals (and men) will react characteristically to physical,
mental and environmental stressors and, if those stressors are
particularly severe, individuals may die. Within species, stress
prone lines have been identified and perpetuated by breeding. It may
be considered that PSS represents the perpetuation of such proneness
ruoongst pigs which have been selected and bred primarily for, for
example, rapidity and efficiency of growth, or body composition
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and conformation, but which also contain undesirable characteristics
which may be called 'stress proneness'.
Typically Pietrain, Poland China and strains of Landrace pigs
have been noted for their dramatic and often fatal reactions to such
stimuli as transportation, raised environmental temperature and
physical exercise, restraint or general handling (Topel et al., 1968).
This porcine stress syndrome is characterised by muscle treroc>r, an
increased respiratory rate, a systemic acidosis and a rise in body
temperature. After the animal dies, or if it is slaughtered in the
reacting state, rigor roc>rtis develops alroc>st immediately to give rise
to the characteristic pale, soft and exudative (PSE) condition of the
meat. Fatal responses to environmental stress are to be found aIrongst
several species of animals. Capture myopathy is a condition which
develops in wild animals such as antelopes and buffaloes being pursued
and captured for whatever reason and many will die (Harthoom et al.,
1974). Clinical symptoms and post roc>rtem findings are similar to
those of PSS.
Malignant Hyperthermia (MH) which develops in certain pigs (and
people) when they are exposed especially to the halogenated gaseous
anaesthetics, but also spontaneously during stressful encounters, is
another reaction which has been incorporated into the general
condition referred to as PSS (Nelson et al., 1974; Lucke et al.,
1979). MH has been particularly important in defining the
physiological and biochemical events in PSS for it is possible to
precipitate MH under laboratory conditions and thereby allow detailed
roc>nitoring of blood and tissues throughout the reaction.
HOW CAN PSS BE DESCRIBED IN OBJECTIVE TERMS?
The first steps towards characterising PSS were made in the late
1960s when attempts were made to identify the metabolic events in
muscle or roc>re generally associated with the exposure of animals to
various and extreme environmental conditions (Kastenschmidt et al.,
1965; Forrest et al., 1968; Kallweit, 1969). In some studies (Judge
1969) a period of exercise was included in addition to thermal stress.
Results from these experiments paved the way for the roc>re
sophisticated approaches of later studies which for the roc>st part only
confirmed the earlier findings. Temperature stress, it seems, is
likely to lead to respiratory alkalosis which becomes a metabolic
acidosis if exercise is included as a treatment.
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The extent of the reaction to these stressors, and particularly
the ability of animals to maintain or recover homeostasis, was used to
identify so-called stress-susceptibility or -resistance anongst the
experiroontal pigs. This terminology has become comm::mplace in the
literature but generally refers to breeds e.g. Poland China, stress
susceptiblej Chester White, stress-resistant. In recent tiroos this
nomenclature has been refined with the introduction of the terms
Halothane-positive or -negative which may be used to describe animals
within as well as between breeds.
It is generally accepted that the pattern of metabolic,
physiological and biochemical developments in MH is very similar to,
if not the same as, that which occurs as a response to environmental
stress. The MH reaction has been extensively characterised in the
literature (Berman et al.,1970j Hall et al.,1980j Gronert,1980) and
only a short description is necessary here.
Essentially, the characteristic signs of MH (and stressor induced
reactions generally) are increased oxygen consumption and carbon
dioxide production. The developing respiratory and metabolic acidoses
are associated with high plasma lactate levels and falling pH.
Glucose and fat mobilisation (as identified by plasma glycerol
concentration) occur, though plasma free fatty acids may fall. There
are substantial electrolyte changes and some haemoconcentration.
Contrary to the early views, rruscle stirrulation which is the
primary source of heat production in MH (Hall et al., 1976) is fuelled
almost if not wholly via aerobic metabolism and it is only in the
later stages that this contribution declines to less than 50 per cent.
Another early view was that cardio Il¥opathy contributed to the
demise of stress-prone pigs. There is, however, no functional
evidence for this for the hearts of pigs suffering MH appear to
respond adequately to metabolic demand even when there is substantial
acidosis and hyperkalaemia. Only in the later stages does cardio
vascular function become limiting. At this tiroo cyanosis of the skin,
resulting fram peripheral vasoconstriction, is associated with reduced
heat loss, reduction in substrate provision and lactate clearance from
tissues and as the biochemical changes reach extreme values, blood
pressure falls and cardiac arrest occurs.
All these changes are familiar findings in the terminal stages of
the Porcine Stress Syndrome.
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HCM IS MEAT QUALITY AFFECTED?
The consequences of PSS post oortem can be either the accelerat
ion of the nomal biochemical changes which occur during the rigor
process or their curtailment, leading in the first instance to pale,
soft and exudative (PSE) meat and, in the second, to dark, firm and
dry (DFD) meat.
In the living animal, metabolism is sustained through adenosine
triphosphate (ATP) production, predominantly via aerobic means, using
glucose and fatty acids as substrates. At times, demand is such that
aerobiosis is insufficient to meet metabolic needs for ATP. This
occurs in the classical fight/flight reactions when anaerobic
glycolysis provides for an increasing proportion of energy needs.
Though this process is only approximately 5% as efficient in the
production of ATP as aerobic processes, it is nevertheless an
important source of energy for emergency needs and the sole
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mechanism employed in the transition of living IlD.lscle to dead meat.
The two systems in muscle cells which control these changes are the
glycolytic system which converts glycogen to lactate and those
processes which lead to the dephosphorylation of ATP. Acidification
occurs as a consequence of glycolysis: rigor, as a result of the loss
of ATP. The series of reactions which regulate all this is described
by Bendall (1973).
It is important to recognise this interdependence of glycolysis
and the production of lactate and the enzymic processes in living
muscle whose function is to synthesis ATP from adenosine diphosphate
(ADP) and inorganic phosphate in anaerobic conditions. In fast
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muscles, the production of sufficient pyruvate for mitochondrial
oxidation requires only a small fraction of the glycolytic enzymes
present; the large excess of these enzymes exists for anaerobic
circumstances. The freely reacting adenine oononucleotides exist
alm::>st entirely as ATP and glycolysis can occur only as long as ADP is
being produced by an ATPase system. Ultimately, therefore, the rate
of glycolysis is determined by ATPase activity and not directly by the
activities or amounts of glycolytic enzymes or hormones.
In the ini tial stages of a surge in demand for ATP or in the post
mortem period, phosphocreatine, which is present at high levels
initially, provides for the re-conversion of ADP (produced by ATPase
activity) to ATP. Glycolytic phosphorylation of ADP becomes increas-
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ingly ~rtant as the concentration of phosphocreatine falls. The
first reactions of glycolysis produce enough substrate to remove ADP
at the rate it is formed but there is no suggestion that the
concentration of ADP is rate limiting. Control is achieved via feed
back on both phosphorylase and phosphofructokinase activity by the
adenosine monophosphate (AMP) level which in turn is regulated by AMP
deaminase. This enzyme is also primarily responsible for the
simultaneous accumulation of inosine monophosphate (IMP) and depletion
of AMP especially at pH<6.5. The action of myokinase restores the
lost AMP via ADP which in turn is restored by ATPases and a consequent
reduction in ATP concentration.
Glycolysis stops and the ultimate pH is reached when there is no
freely reacting adenine mononucleotide in the muscle. Glycolysis will
also stop at an abnormally high pH (DFD meat) when all glycogen has
been depleted from muscle. This implies inadequate stores of glycogen
at death for normal rigor will continue to the usual ultimate pH (N5.5
at which there may be 'residual' glycogen in muscle. In this case the
exact determinant of ultimate pH Le. the point at which glycolysis
stops is not clear. It seems likely however that AMP deaminase is
primarily responsible since it represents the sole cause of loss of
adenine nucleotides.
From this it can be concluded that variations in the rate and
extent of pH fall are attributable to the controls exercised before
and after death by ATPases and AMP deaminase. The problems arise in
identifying the physiological basis for these activities.
The practical consequences of these biochemical changes are the
PSE and DFD conditions of pigmeat. Rigor mortis takes place in pig
muscle over a period of about 10 hours during which the pH has fallen
fran about 7.3 to 5.5. On occasion this change may occur wi thin a few
minutes of death and the meat develops the pale, soft and exudative
characteristics. The classical explanation for this has been
• thenroprotonic stress (Scopes, 1964) which occurs when the pH of
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muscle falls below 6.0 and its temperature is >300C. Under these
conditions extensive denaturation of the soluble and structural
proteins occurs which leads to a loss of their water binding abilities
and the precipitated prot~ins interfere with the optical properties of
the muscle to cause more incident light to be reflected and a paler
appearance.
Penny (1975) suggested that during normal rigor, drip came from
the extracellular water compartment which became enlarged as the
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et al.'s (1984) finding that rabbit l.dorsi fibres swelled to two to
three times their usual diameter when inroorsed in hypertonic salt
solutions and, like Offer & Trinick's (1983) observations, was
explained by myofibrillar swelling. Wilding et ale (1984) also
concluded that myofibrillar swelling was constrained by the endomysial
sheaths around fibres.
It is now proposed (see Offer, 1984) that drip formation during
rigor mortis represents the redistribution of extra and intra
cellular water and the expulsion of part of the extracellular
component out of the cut ends of meat close to the perimysium,
probably as a result of internal pressure exerted by the connective
tissue network and disruption of cell membranes (Currie & w)lfe,
1983) •
This explanation does not require there to be a change in the
total water content of muscle prior to an animal's death. It is,
however, quite possible, indeed likely, that quite sizeable changes
occur in the fluid compartments in the body prior to death, especially
if there are profound metabolic and physiological disturbances
happening simultaneously (Guyton et al., 1975). Lister et ale (1985 -
in preparation) recently examined this proposition in halothane
sensitive pigs of the Lacombe breed.
During MH reactions precipitated by halothane and suxamethonium
administration, the typical changes in muscle and rectal temperature
occurred, the haematocrit rose and plasma volume fell. The pH of
biopsy samples of muscle fell and amounts of expressible juice and the
turbidity of extracts of muscle increased. Moreover, the water
content of the muscle samples at death was higher in reacting pigs
than in non-reacting control animals. Thus at least a proportion of
the drip released by PSE muscle may well be attributable to the
increase in intracellular fluid volume which accompanies the
development of MH and, probably, PSS.
DFD meat is the condition which affects meat which remains at
pH>6.0 - 6.3 when there is insufficient glycogen available in muscle
at death to allow the full development of acidification to occur. The
premature depletion of muscle glycogen in stressed animals is not
easily explained. It is not induced simply by fasting, exercise or a
combination of the two; mixing groups of animals even for a short
period may, however, bring it about. Tentative explanations may be
found by examining the mechanisms which allow muscle LO function under
a variety of phvsioloqical states.
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The concentration of glucose in the blood of animals is
maintained within fairly constant limits even though there might be
fluctuations in the storage and nobilisation of nutrients which are
associated with feeding and activity. Even under resting conditions,
however, activity can only be maintained for relatively short periods
if carbohydrates (derived from liver glycogen in the main) are the
sole source of energy. For longer periods of activ i ty, the long chain
free fatty acids (FFA) nobilised fran adipose tissue are employed
(Newsholme, 1980) which progressively, as activity intensifies, cause
a reduction in glucose utilisation by muscle. The control over this
balance is thought to be exercised by fatty acid oxidation and due in
part to the inhibitory effect of citrate on the enzyme
phosphofructokinase. A further increase in the workload will reduce
the concentration ratio ATP/ADP and glycolysis is stimulated at the
expense of stores of muscle glycogen. A continuing ability to
mobilise fatty acids and the maintenance of aerobiosis is thus of the
utmost importance for sustained activity.
Hall et ale (1980) showed that in MH reactions, the continued use
of free fatty acids for energy purposes can be prejudiced. Lipolysis
is stimulated during MH under the influence of catecholamines, but
plasma FFA concentrations may fall as a consequence of their re
esterification during the developing acidosis. Because the utilis
ation of FFA is determined by their concentration in plasma, the
continued energy demand of the reaction must be met ultimately by
muscle glycogen.
Thus one might conclude that the loss of muscle glycogen and
predisposi tion to Dm by stressed animals is a natural consequence of
continued muscular stimulation when the supply of other energy
substrates, notably FFA, is reduced.
WHAT MAKES PIGS STRESS PRONE?
It can be concluded from much of the above that many of the
symptoms of PSS are the consequences of profound muscular stimulation.
It may well be also that such stimulation not only provides the
symptoms of PSS but also the ease with which this occurs, conditions
whether pigs become stress susceptible or resistant.
The rate of glycolysis in muscle, occurring either before death
or during rigor nortis, is determined by ATPase dctivity and not
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directly by the activities or amounts of glycolytic enzymes or
hormones. Stress proneness, therefore, amounts to the ease with which
IIllsc1e is stiIru1ated and how readily recovery can be established.
There seems to be general agreement, though a dearth of evidence,
that the ultimate triggering mechanism resides in nusc1e itself.
However, no major rrorpho1ogica1 differences have been identified in
muscle except evidence of myofibri11ar damage and regeneration (Muir,
1970; Venable, 1973). Swat1and and Cassens (1972) described various
abnormalities in the peripheral innervation of striated nusc1e though
no functional abnormalities have been recognised.
Defects in the ability of nusc1e components to handle ea++ have
++
long been suspected but rarely demonstrated, though the role of ea
in the stinu1ation of myofibrillar ATPases and phosphorylase kinase
nust be of potential importance. The sarcoplasmic reticulum was
thought to have impaired function though it is likely that this could
be explained by experimental artefact (Greaser et a1., 1969) (though
see Heffron - this volume). Halothane sensitive pigs appear to have
higher sarcoplasmic ea++ levels than normal (Cheah & Cheah, 1984) and
this has been linked with significantly higher activity of Ca ++
activated phospholipase ~(EC 3.1.1.4) (Cheah & Cheah, 1981). This in
turn leads to the enhanced release of mitochondrial Ca++. More
recently Cheah & Cheah (1984) suggested that the enhanced
phospholipase ~ activity was due to the higher concentrations of
endogenous calrrodu1in in sensitive pigs.
Even via these postulated mechanisms, a means has yet to be
identified of switching on the aberrant behaviour of IIllsc1e in MH or
PSS. Monnede and Lentzer (this volume) propose that behavioural
responses to stressful environmental factors in intensive pig
husbandry are prime factors in the aetiology of PSS. It is certainly
true that environmental stressors affect the incidence of PSS and PSE
meat but anaesthesia or tranquillisation substantially rrodifies the
incidence of PSE only arrongst those animals which are not genetically
prone to the PSS condition (Briskey & Lister, 1969). \'hat matters
then, is the inherent mechanism which sets the threshold of
sensitivity in pigs to make them more or less liable to develop PSS.
The role which the sympathetic nervous system and the adrenal
gland play in stress responses generally (see Usdin, Kvetnansky and
Kopin, 1976) and MH in particular (Hall, 1976) has been very well
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documented • In MH, catecholarnines appear primarily to prarote the
developing reaction via ct + 8 adrenergic responses, but pre-treatrrent
with ~ adrenergic blockers or agonists can prevent or initiate a fatal
MH response (Lister et al., 1976: Hall et al., 1977). The combination
of effects Irediated by the main adrenoreceptors can account for the
majority of the metabolic events seen in MH and PSS. 8 receptors, for
example, are associated, inter alia, with increased liver glycogen
olysis and vasoconstriction, 8 receptors with heart rate and fat
1
mobilisation, and 82 receptors with muscle glycogenolysis and vaso
dilation.
The use of receptor specific drugs has allowed us to clarify the
particular mechanisms of relevance within PSS and MH. ct blockade
clearly prevents the development of MH in sensitive pigs whereas 8
blockade with propranolol (Lister et al., 1976) or carazolol (S.Lens -
personal comnnmication) does not. The development of PSE meat can be
impeded by ct blockade, though not substantially, and treatrrent with
propranolol may also have a slight beneficial effect (Lister, 1974).
Warriss & Lister (1982) have provided clear evidence, however, of the
effectiveness of carazolol in reducing the incidence of PSE in stress
susceptible Pietrains. Carazolol, like propranolol, is described as a
non-specific 8 blocker but Warriss & Lister's findings allow a clear
distinction to be drawn between MH and aspects of PSS such as PSE
meat. rhatever the specific mechanism is which triggers MH in the
living pig,it is not the same as the trigger for the production of PSE
meat' although that might represent a further part of the continuum of
adrenergic reactions which constitute PSS.
The contribution identified for the sympathetic nervous system in
MH and PSS suggests mechanisms which link up with the other notable
feature of the syndromes i.e. the leanness of susceptible individuals.
Pietrain pigs are thought to develop their leanness through enhanced
responsiveness to the fat mobilising effect of noradrenaline (Wbod
et al., 1977). For such a mechanism to contribute to leanness in the
growing animal, the adipose tissue of lean pigs would have to be
exposed to levels of noradrenaline which were higher than those found
in fatter types, or the appropriate adrenergic receptors present in
greater concentrations. Gregory & Lister (1981), using a Valsalva-