Tài liệu Quantitative aspects of ruminant digestion and metabolism - Phần 10 pptx

Mobilization of adipose tissue lipid rich in oleic acid during eitherundernutrition or exercise results in a significant decrease in the saturation ofplasma NEFA and indeed the ratio of

13 Fat Metabolism and Turnover

1School of Veterinary and Biomedical Sciences, Murdoch University,

Murdoch, WA 6150, Australia;2CSIRO, Division of Livestock Industries,

St Lucia, QLD 4067, Australia;3Department of Primary Industries,

Werribee, VIC 3030, Australia

Introduction

This chapter will emphasize the metabolism of non-esterified fatty acids (NEFA)although some discussion will relate to triglyceride (TAG) and ketone bodies.Plasma NEFA are a highly labile form of lipid that are transported between tissues

in the circulation bound to albumin Although plasma NEFA are only a smallproportion (5%) of total plasma lipid they represent an important source ofoxidizable energy, particularly during periods of negative energy balance orwhen there is an acute need for energy such as during exercise This chapter willdeal with the composition and sources of plasma NEFA, the fate of NEFA duringdifferent physiological states as well as the acute and chronic regulation of NEFAmetabolism In addition, this chapter will cover methodology and principles ofNEFA metabolism and also describe adaptations to different physiological states.Although these areas have been covered in several reviews (Lindsay, 1975;Emery, 1979; Annison, 1984; Wiseman, 1984; Chilliard et al., 2000), thischapter will emphasize the quantitative aspects of NEFA metabolism

Composition and Sources of Plasma NEFA and TAG

The metabolically most active pool of long-chain fatty acids is transported andmetabolized as either NEFA (bound to albumin) or TAG In the fed animalNEFA represent less than 5% by weight of total plasma lipid (2 g/l) with theremainder incorporated into various lipoprotein fractions (Kris-Etherton andEtherton, 1982) TAG (<10% of plasma lipid) is present mainly as very low-density lipoproteins (VLDL), with few chylomicrons on most diets Sources ofplasma lipid include the gut, liver and adipose tissue

The gut is limited in its quantitative contribution since the lipid content oftypical forage is less than 3% A 45 kg sheep being fed to maintenance would

ß CAB International 2005 Quantitative Aspects of Ruminant Digestion

and Metabolism, 2nd edition (eds J Dijkstra, J.M Forbes and J France) 345

consume about 700 g dry matter (DM) of good quality forage, including some

21 g of lipid (19 g of fatty acids) per day Dietary lipid is absorbed as NEFA,rapidly esterified to TAG and then packaged into chylomicrons and VLDLwithin the intestinal mucosal cell (Noble, 1981) From here lipid enters thelymph and finally the venous blood Passage through the rumen results insignificant biohydrogenation, which is reflected in a relatively high proportion

of saturated fatty acids in the circulating TAG and NEFA of fed animals (Table13.1) Mobilization of adipose tissue lipid (rich in oleic acid) during eitherundernutrition or exercise results in a significant decrease in the saturation ofplasma NEFA and indeed the ratio of stearic to oleic (S:O) acids and the sum ofstearic and linoleic to oleic (SL:O) acids have been shown to be negativelyrelated to energy balance (Dunshea, 1987)

The dynamic nature of the profile of plasma NEFA is illustrated in Fig 13.1where the chronic and the acute-upon-chronic effects of undernutrition onplasma NEFA and SL:O are shown In this study, Dunshea (1987) fed goats

at either maintenance or 0.25 maintenance in 12 equally spaced meals perday in an attempt to create a quasi-steady state This was certainly achieved forgoats fed at maintenance where total NEFA concentrations and the ratio SL:Owere relatively constant across the 2-h interval between feeding Sub-mainten-ance feeding resulted in a chronic increase in plasma NEFA and a decrease inSL:O However, there were also post-prandial effects on both total NEFA andSL:O consistent with dynamic changes in NEFA mobilization in response to the2-hourly feeding bouts

Lipolysis and Fat Mobilization

The major pathways within and adjacent to the adipocyte are shown inFig 13.2 (see Vernon, 1981) Fat is stored as TAG Glucose is the source

of glycerol while fatty acids can be either synthesized de novo, principallyfrom acetate, or be preformed There is some evidence that intramuscular

Table 13.1 Composition of long-chain fatty acids (molar %) in blood of sheep

(Data from Pethicket al., 1987; Pethick and Parry, unpublished.)

0 200 400 600

Time relative to feed (min)

0.0 1.0 2.0 3.0

4.0

Fig 13.1 Relationships between plasma NEFA concentrations (open symbols), the ratio ofthe molar proportions of stearic plus linoleic to oleic acids (SL:O) in NEFA (closed symbols)and time relative to feeding in a dry goat that was chronically offered either 140 (&, &) or 35(^, ^ ) kJ ME/kg/day divided into 12 equal portions given at 120 min intervals (Dunshea, 1987)

Plasma

Endothel

Extracell

Fatty acid Albumin Fatty acid Fatty acid

Fig 13.2 Triacylglycerol synthesis/degradation cycle in adipose tissue TAG¼ triacylglycerol

1, Acetyl CoA carboxylase; 2, Fatty acid synthase; 3, Esterification; 4, Hormone sensitive lipase;

5, Lipoprotein lipase; 6, Fatty acid equilibration; 7, Membrane transport of fatty acids

adipocytes have a preference for glucose and lactate over acetate but thesignificance of this is still not fully resolved (Pethick et al., 2004) Preformedfatty acids can arise from uptake of plasma NEFA or after hydrolysis ofcirculating VLDL TAG by lipoprotein lipase (LPL) In addition, an intracellularsource of fatty acids can arise during lipolysis, a process regulated by hormone-sensitive lipase (HSL) Fatty acids resulting from lipolysis can be released intothe circulation or else re-esterified into TAG In contrast, the lack of glycerolkinase within the adipocyte ensures that glycerol is quantitatively released intothe circulation Therefore, glycerol and NEFA entry into the plasma poolshould reflect lipolysis and fat mobilization, respectively This is provided thatthe contribution from the adipocyte is much greater than that released into thecirculation as a result of LPL-catalysed hydrolysis of VLDL TAG which, asdiscussed later, may not always be the case.

NEFA Entry Rate

Definition and methodology

Under conditions of constant circulating concentrations and physiological state(steady state) the amount of NEFA entering and leaving the plasma will beequal This is defined as the NEFA entry rate, which is best determined byisotope dilution A potassium soap of radiolabelled fatty acids is dissolved inplasma and infused intravenously at a constant rate After about 1 h the specificradioactivity of plasma NEFA in arterial blood will reach a plateau value andentry rate can be calculated as:

Entry rate of total NEFA (mmol=h) ¼Infusion rate radiolabelled NEFA (dpm=h)

Specific activity total NEFA (dpm=mmol)The site of both infusion and sampling to determine NEFA kinetics has beenthe source of some debate, but the above schedule has been validated (Jensen

et al., 1988)

Plasma NEFA consists of a number of fatty acids of which palmitic,stearic and oleic acids represent some 85% (Table 13.1) Herein lies adifficulty in quantifying NEFA metabolism because not all NEFA behave as

a homogeneous unit Generally, one radioactive fatty acid is used as a tracerfor total NEFA, with the assumption that all NEFA behave similarly Otherworkers have improved the method by infusing mixtures of the three majorradiolabelled fatty acids (Bell and Thompson, 1979; Dunshea et al., 1988).Typically, palmitic acid has a higher entry rate than the other fatty acidswhen compared in a similar concentration (Lindsay, 1975) Stearic acid andoleic acid more commonly show a similar relationship between entry rateand concentration (Pethick et al., 1987) In this chapter (unless otherwisedirected) the tracer fatty acid has been assumed to be representative of allNEFA Finally, determination of NEFA concentration requires care It is bestperformed using either HPLC, GLC or enzymatic methods Non-specific

methods employing titration of copper soaps are prone to overestimationdue to lack of specificity Rapid, enzymatically based micro-assays now existfor the determination of NEFA (Johnson and Peters, 1993) and unlessknowledge about specific fatty acids is required readers are recommended

to use these assays

Plasma NEFA concentrations vs NEFA entry rate

The published data for non-lactating small ruminants, in the fed and fastedstate, are summarized in Fig 13.3 These studies have been grouped togetherbecause in all cases the spectrum of tissues utilizing NEFA is similar Thisextends to pregnancy since the pregnant uterus uses very little NEFA (Pethick

et al., 1983) Secondly, the metabolic rate of tissues is not greatly altered.Large changes in the metabolic rate alter the relationship between concentra-tion and utilization or entry rate (Table 13.2; see also ‘Exercise’ below).When all studies are viewed together (Fig 13.3) a curvilinear relationship

is found, implying a plateau in the entry rate of NEFA This plateau is notdue to peak stimulation of HSL since studies both in vitro (Vernon, 1981)and in vivo (Table 13.6) suggest a much greater capacity However, twofactors limit the extent of fat mobilization as the NEFA concentrationincreases in plasma First, in animals at rest, fat mobilization is associated

0.0 0.5 1.0 1.5 2.0

NEFA entry rate (mmol/h/kg)

Contribution to oxygen consumption (%)

Fig 13.3 Relationship between the entry rate of NEFA, contribution of NEFA to oxidation andcirculating concentration in non-lactating animals Key: O, sheep fed;&, sheep pregnant fed; ,goats fed;þ, goats underfed; ~ sheep starved 1 day; ~, sheep starved 3–4 days; &, sheeppregnant starved 3–4 days If x is the NEFA entry rate (mmol/h/kg) and y the NEFA in plasma (mM)then y¼ 0:05  102:14x, r2¼ 0:77(P < 0:001) In calculating the regression each study wasweighted for the number of animals Contribution to oxygen consumption is calculated assumingcomplete oxidation of NEFA and an oxygen consumption of 12.5 mmol/h/kg for all metabolicstates No discrimination on the basis of tracer NEFA was made Sources: Bergman et al (1971);Pethick et al (1983, 1987); Dunshea et al (1988); Pethick and Harman (unpublished); Hall andDunshea (unpublished); plus those cited by Vernon (1981)

with increased rates of ketogenesis (Table 13.5) and subsequent elevation of

D-3-hydroxybutyrate This ketone body tends to reduce NEFA concentrationprobably by increasing the rate of insulin secretion (Heitmann et al., 1987).Secondly, as NEFA concentration increases the plasma albumin approachessaturation and the resultant stimulation of intracellular re-esterification re-duces fat mobilization despite no change in lipolysis (Madsen et al., 1986).These mechanisms regulate fat mobilization and are essential to preventtoxic levels of NEFA in plasma (about 2 mM; Newsholme and Leech,1983) Saturation of tissue uptake could also inhibit further elevation ofentry rate In the liver, NEFA uptake is non-saturable within the physiologicalrange (Bell, 1981), but for skeletal muscle there is evidence for limiteduptake as the concentration of NEFA increases with fasting The fractionalextraction of NEFA in fed sheep was over twice that found for fastedcounterparts (Table 13.2) despite similar blood flow in the hind limb muscle.Muscle can form acetate or esterify NEFA, but it would appear that thesepathways can be saturated within the physiological range of substrate supply.The oxidative pathway for NEFA can only increase to the limit set by oxygenconsumption and this is probably the major limiting aspect of NEFA utiliza-tion at rest (Table 13.2)

The relationship between entry rate of NEFA and concentration in plasmafor lactating animals is shown in Fig 13.4 The relationship is different for non-lactating animals First, the concentration of NEFA is generally lower thanobserved in non-lactating animals, even though the range of NEFA entry rate

is similar or in the case of the fasted goat considerably higher Secondly, there

is no tendency for the entry rate of NEFA to reach a maximum with a linearrelationship being adequate to explain the data This is likely due to themammary gland acting as a non-saturable sink for long-chain fatty acids (seealso ‘lactation’ below)

Table 13.2 Metabolism of NEFA by the hind limb muscle of sheep (Data from Pethick et al.,

1983 (flux rates across muscle halved due to overestimate of blood flow), 1987; Harman,1991.)

Metabolic state

PlasmaNEFA (mM)

Gross utilization

by muscle(mmol/h/kg)

Gross extractionacross muscle(%)

O2uptake

by muscle(mmol/h/kg)

Utilization of NEFA

Tissue uptake

All tissues that utilize long-chain fatty acids also show simultaneousrelease Consequently the terms net and gross utilization have been used todescribe NEFA uptake by tissues Net utilization is derived from the extraction

of NEFA measured as an amount while gross utilization is calculated fromthe extraction of infused radiolabelled NEFA For liver the difference betweenthe two values is not large; however, for the gut and muscle it is not uncommon

to find a net release of NEFA, while the mammary gland shows no netexchange in fed animals Reasons for tissue release of NEFA includelipolysis due to LPL (mammary gland and muscle) and lipolysis from adiposetissue within the tissue bed of interest (muscle and the gut) To discriminatebetween the sources the method of Zierler and Rabinowitz (1964) could beutilized, where a local infusion of insulin is given to inhibit HSL (Capaldo et al.,1994)

There is no evidence that individual NEFA are utilized at different rates

by muscle, but there is conflicting evidence as to the rate of the stearic acidutilization by liver Bell (1981) reviewed the data and suggested minimalhepatic utilization of stearic acid; in contrast there is substantial incorporation

of radiolabelled stearic acid into ketones, suggesting no impairment to uptake(Pethick et al., 1983) In resting fed (and probably fasted) sheep about half ofthe NEFA entry rate is accounted for by the gut, liver and muscle (Table 13.3).Alternative sites might include the heart, kidneys and spleen Assuming the

0.0 0.2 0.4 0.6 0.8

NEFA entry rate (mmol/h/kg)

Fig 13.4 Relationship between the entry rate of NEFA and circulating concentration inlactating animals Key: O, cow fed;~, goat fed, &, sheep fed, þ, goat fasted If x is the NEFAentry rate (mmol/h/kg) and y the NEFA in plasma (mM) of fed animals, then y¼ 0:27xþ0:17, r2¼ 0:36 (P < 0:001) If the value for the fasted goat is included, the slope is significantlyreduced to 0.15, r2¼ 0:45 (P < 0:001) In calculating the regression each study was weighted forthe number of animals Sources: Annison et al (1967a, 1968); Yamadagni and Schultz (1969);Bickerstaffe et al (1972, 1974); Konig et al (1979, 1984); King (1983); Emmanuel and Kennelly(1984); McDowell et al (1987, 1988); Bauman et al (1988); Sandles et al (1988); Dunshea et al.(1989, 1990); Pullen et al (1989); Sechen et al (1989)

gross extraction of NEFA by the heart is 55% as in man (Wisneski et al., 1987),the heart could utilize 19% of the NEFA entry rate Finally, adipose tissue,which receives 16% of the cardiac output at rest (Bell and Hales, 1985), couldalso utilize significant NEFA During exercise the muscle becomes a relativelymore important sink for NEFA.

Oxidation of NEFA

Whole body

Two methods are commonly employed to measure the oxidation rate of NEFA.Either the entry rate of NEFA and CO2 are determined along with the contri-bution of NEFA to the blood bicarbonate pool at equilibrium, or alternatively anopen circuit calorimeter is utilized and the amount of radioactivity infused iscompared to that excreted as respiratory14CO2either at equilibrium or during

a 12 to 24-h period For accurate results, long infusion times are preferredsince there is a delay (5 h) in the accumulation of14C in CO2(Annison et al.,1967a) During ketosis this time extends to 15 h due to catabolism of NEFA viaketones (Pethick et al., 1983) Another factor of concern is CO2 fixation,which amounts to 17% and 7% of CO2 entry rate in fed and fasted sheep,respectively (Annison et al., 1967a) The common NEFA in plasma are oxi-dized at similar rates However, due to ruminal biohydrogenation of fatty acidsthe absorption of linoleate is low and hence there is limited oxidation of thisessential fatty acid (Lindsay and Leat, 1977) Estimates of prompt NEFAoxidation are shown in Table 13.4 Inter-laboratory comparisons are difficultbut, whether fed or fasted, NEFA oxidation is relatively low and generally onlyabout half that of other fatty acids, such as acetate or ketones Oxidationincreases during late gestation but there still remain substantial amounts ofNEFA, which enter non-oxidative pathways

Table 13.3 Gross utilization of NEFA by different tissues in sheep (Data from Bergman et al.,1971; Pethicket al., 1983, 1987; Harman, 1991.)

Utilization as % of entry rate

Tissue Dry, fedaat rest Exerciseb30–60% VO2max Fasted pregnantc

Tissue oxidation

Measurement of tissue oxidation compares the rate of14C-NEFA uptake with

14CO2 release Both long infusion and/or collection times are required forreliable results This is due to very slow equilibration of CO2, especially inresting muscle (Pethick et al., 1983)

Direct oxidation of NEFA by resting muscle is low (Table 13.4) with valuesreflecting that found in the whole animal However, if fixation of CO2 isallowed for (possibly as high as 20%; Pell et al., 1986) then 50–60% of theNEFA is probably oxidized in sheep Even lower rates of oxidation were foundfor the steer hind limb These lower rates are perhaps surprising but they arenot related to poor methodology since short-chain fatty acids are extensivelyoxidized in similar experiments (Pethick et al., 1981) Intramuscular fat mightact as a significant site of esterification; alternatively NEFA could pass through

an intramuscular pool of TAG before being directly oxidized (Dagenais et al.,1976) A similar mechanism is implicated for adipose tissue (Ookhtens et al.,1987) This may represent an adaptation to maintain intracellular NEFAconcentration below toxic levels During exercise NEFA are directed more

Table 13.4 Oxidation of NEFA in ruminants.a

Experimental

method

Metabolicstate/species

% NEFA promptly

Whole-body Fed animals

Wilson (1984)

Pullenet al (1989)Fasted animals

Dry sheep (3 days) 45, 35a Lindsay and Leat (1977);

Leat and Ford (1966)Pregnant sheep (3 days) 63, 46a Pethicket al (1983);

Wilson (1984)

Oxidation by Sheep

Exercise, 30% VO2max 87 Pethicket al (1987)Fasted pregnant (3 days) 52 Pethick (1980)Steer

a

Values corrected for CO 2 fixation using data from Annison et al (1967a) of 1.2 and 1.07 for fed and fasted animals, respectively Experiments where 14 C-linoleic acid was used as a tracer have not been included Pregnant animals in last month of pregnancy.

readily to oxidation (Table 13.4) either directly or due to a higher rate ofesterification and lipolysis in muscle.

NEFA Metabolism in Different Physiological States

Fed animals at rest

Magnitude

Estimates of NEFA entry rate in maintenance-fed small ruminants range from0.08 to 0.32 mmol/h/kg (21–121 g/day, see Fig 13.3) Much of the vari-ation in these measurements relates to the pattern of feeding, with the lowervalues being observed in animals fed semi-continuously Plasma NEFA levelsand entry rates are highest before and lowest after feeding (Dunshea et al.,1988) A further source of variation occurs because plasma NEFA are highlylabile (t1=2< 2 min) and also very stress-sensitive (Holmes and Lambourne,1970; Boisclair et al., 1997) In this context, Boisclair et al (1997) foundthat excitement around feeding or other minor animal handling procedureswere sufficient to elevate plasma NEFA, particularly in young cattle treated withbovine somatotropin (bST) Therefore, it is imperative for animals to be accus-tomed to handling before commencing studies This has not always been thecase The value of about 35 g/day appears to be a good estimate of the NEFAentry rate in a 45 kg ruminant fed to maintenance, although lower values havebeen measured (Pethick et al., 1987)

Sources of plasma NEFA and TAG

Estimates of lipid absorption in sheep fed roughage-based diets are in the order

of 12 to 16 g/day of TAG (Harrison and Leat, 1972; Pullen et al., 1988).NEFA content of intestinal lymph is low such that only some 0.5 g/day enterthe circulating pool directly However, the absorbed TAG also makes acontribution towards the NEFA entry rate through the action of LPL and to asmall extent hepatic lipase The work of Bergman et al (1971) demonstratedrelease of NEFA into the circulation as chylomicron TAG was hydrolysed byliver, gut and hindquarters The extent to which fatty acids liberated by LPLpass through the circulating NEFA pool before intracellular metabolism isquestionable (Fig 13.2); however, in Fig 13.5 it is assumed that completeequilibration occurs A further source of TAG and NEFA is VLDL TAG formed

in the liver While little de novo lipogenesis occurs in the liver of ruminants(Bell, 1981), there is extraction of NEFA from the circulation withsubsequent release of about 4–5 g/day (double this in pregnant animals) ofVLDL TAG (Pullen et al., 1988; Freetly and Ferrell, 2000) Overall TAGderived from the diet and liver might account for 50% of the NEFA entry rate(Fig 13.5)

The remaining component of NEFA entry rate probably arises from lysis within adipose tissue Again, some released fatty acids would emanatefrom the action of LPL on circulating TAG but this contribution remainsunknown The alternative source is due to the action of HSL Further experi-

lipo-ments are required perhaps utilizing the fat-tailed sheep or an inguinal fat pad

as a model

The relative contribution of adipose and non-adipose sources towardsplasma NEFA in the sheep can be derived from the work of Petterson et al.(1994) In that study, insulin (a potent inhibitor of adipose tissue lipolysis) wasinfused while maintaining euglycaemia Maximally inhibited NEFA concentra-tions are assumed to be the result of insulin-independent, LPL-catalysed hy-drolysis of circulating TAGs In the study of Petterson et al (1994) insulininfusion decreased plasma NEFA concentration from 0.15 to 0.07 mM insheep fed to maintenance Given that at low concentrations of NEFA therelationship between plasma NEFA concentration and entry rate is essentiallylinear (Fig 13.3), then lipolysis (via HSL) would account for about 50% of theNEFA entry rate Hyperinsulinaemia caused similar plasma NEFA concentra-tions to the basal values observed by Pethick et al (1987) where entry rate ofNEFA was 21 g/day This value can be largely accounted for by the estimates

of NEFA derived via TAG metabolism from the diet (13 g/day) and hepaticproduction (5 g/day) Therefore, it appears that in resting undisturbed sheep,minimal NEFA are released into the circulation as a result of adipose tissue

lipolysis Given these assumptions it seems that all TAG-bound fatty acidsinitially pass through the NEFA pool during metabolism Although there issome evidence for this in the mammary gland (Annison, 1984), there is alsoevidence for preferential uptake of NEFA arising from TAG hydrolysis (path-way 7, Fig 13.2; Hamosh and Hamosh, 1983; Raclot and Oudart, 2000).The most striking feature of the model shown in Fig 13.5 is the roughequivalence between the amount of NEFA being absorbed (as TAG) and thatoxidized Most of the NEFA turnover seems to involve recycling of fatty acidsbetween plasma TAG and adipose tissue.

Undernutrition

Fasting

Fuel homoeostasis during fasting requires both a new source of energy and themaintenance of euglycaemia so that tissues with an absolute glucose require-ment retain normal function Mobilization of NEFA from adipose tissue helpssatisfy both needs Estimates of NEFA entry rate during fasting range from 0.32

to 0.64 mmol/h/kg (120–240 g/day, Fig 13.3) which is sufficient to accountfor over 100% of the oxygen consumption assuming complete oxidation

To maximize NEFA oxidation the liver partially oxidizes NEFA into ketones(Table 13.5) and to a lesser extent acetate Ketogenesis is stimulated due to acombination of several events Increased hepatic delivery of NEFA is crucial butchanges to intrahepatic metabolism are also triggered by the declining ratio ofinsulin to glucagon and reduced food intake (Brindle et al., 1985), ensuring thatmuch of the NEFA enter into the mitochondrion for subsequent beta oxidationand formation of ketones Overall about 50% of the NEFA carbon is processedvia ketogenesis, an adaptation that increases the use of NEFA since ketones,being water-soluble and more diffusible, are readily oxidized Thus ketone bodyaccumulation in plasma should be thought of as a normal physiological adap-tation However, pathological cases can occur, particularly during pregnancy

Table 13.5 Concentration of circulating fatty acids and rates of ketogenesis in the blood ofsheep AcAc¼ acetoacetate; 3-HB ¼D-3-hydroxybutyrate Pregnant animals in last month ofpregnancy.a

Metabolite concentration (mM) Ketone body

synthesis(mmol/h/kg)Metabolic state Blood [AcAc] Blood [3-HB] Plasmaa[NEFA]

(see pregnancy below) Utilization of NEFA and ketones by the extra hepatictissues results in the sparing of glucose Both the uptake and oxidation ofglucose decline Reduced oxidation is augmented by regulation at pyruvatedehydrogenase with much of the glucose recycled via lactate or glycogenicamino acids Increased NEFA and ketone body utilization is an importantcomponent of decreasing the proportion of pyruvate dehydrogenase in theactive state (Randle, 1986) The overall effect of a prolonged fast is for nearcessation of glucose oxidation by skeletal muscle with NEFA and ketones(roughly 50:50) becoming the predominant fuel (Pethick et al., 1983).Chronic undernutrition

The situation most often facing a grazing ruminant during food shortage is one

of chronic undernutrition rather than acute fasting With this in mind Dunshea

et al (1988) investigated the effect of chronic food restriction on fat mobilization

in goats Both plasma NEFA concentration and entry rate were increased duringchronic undernutrition and these increases were related to the severity of feedrestriction Indeed, plasma NEFA concentration and entry rate were closely andnegatively related to energy balance Importantly, the increases in NEFA entryrate were highly correlated with body fat mobilization Thus, NEFA entry rateincreased from 0.12 at maintenance to 0.14 and 0.36 mmol/h/kg at 0.5 and0.25 times maintenance energy intake, respectively (Dunshea et al., 1988).One of the reasons for an increase in plasma NEFA and entry rate duringchronic undernutrition is the reduction in plasma insulin, a potent antilipolytichormone In this context, Petterson et al (1994) found that plasma NEFAdoubled whereas plasma insulin decreased by 25% in sheep that were fed at0.5 times maintenance compared to maintenance-fed controls However,infusion of insulin during an euglycaemic clamp decreased plasma NEFA tosimilar concentrations in both groups of animals Importantly, the nadir inplasma NEFA was achieved within the physiological range of plasma insulin(see section on ‘Acute homoeostatic regulation’ for further discussion)

Thomp-Heat stress and its effects on NEFA metabolism are less well investigated inruminants Recent studies in our laboratories have investigated the effects ofheat stress on NEFA metabolism (Beatty et al., 2004) Bos taurus (Angus)heifers (six treated vs six controls of 350 kg liveweight) housed in climaterooms were offered feed at 2.25% of body weight and had ad libitum access

to water The wet bulb temperature was increased (over 6 days) to 328C,

maintained at this temperature for 6 days and then lowered over the next 6 days.The main effect was virtual cessation of feed intake (2.0% vs 0.15% BW,P<0.001) and a fivefold increase in plasma NEFA as the temperature increasedabove 28–308C Pair feeding the same Bos taurus cattle at a level observedduring heat stress but with the cattle kept at thermoneutral temperatures caused

a similar increase in plasma NEFA This suggests that a reduced feed intake andnot an extra effect of heat stress drove the increased mobilization of adiposetissue (measured as elevated plasma NEFA) per se To further support thisfinding, Bos indicus cattle subjected to similar wet bulb extremes maintainedfeed intake and showed no rise in plasma NEFA

Pregnancy

Most studies have shown that NEFA concentration and entry rate are elevated

in late pregnancy, even in ewes that are apparently well fed (Table 13.5) Theobserved entry rates vary between 0.35 and 0.62 mmol/h/kg, a range similar

to that seen in fasted dry sheep There are two reasons to explain these highvalues First, twin pregnant ewes have difficulty in consuming enough feed tomeet energy requirements; this is particularly so on poor quality roughage diets(Foot and Russell, 1979) Secondly, there is a tendency for fat mobilization due

to insulin resistance (Petterson et al., 1994) The function of increased fatmobilization is presumably to maintain euglycaemia in the face of an enormousglucose drain by the pregnant uterus

A special adaptation of pregnancy is utilization ofD-3-hydroxybutyrate bythe pregnant uterus sufficient to account for up to 25% of the oxygen con-sumption (Pethick et al., 1983) A resultant net 18% reduction in glucoserequirement would be a great benefit to a twin-pregnant ewe where some70% of the glucose synthesized is consumed by the pregnant uterus (seeChapter 20) Neither acetoacetate nor NEFA are utilized as a fuel by thepregnant uterus (Pethick et al., 1983)

During fasting, NEFA entry rates increase up to 0.9 mmol/h/kg (Fig 13.3)

A puzzling problem is the hyperketonaemia seen in fasted pregnant animalscompared to the fasted dry animals, despite similar rates of ketogenesis (Table13.5) These data point to limitations in ketone uptake in the pregnant animals.This has been shown for skeletal muscle, whereD-3-hydroxybutyrate is used lessrapidly as the concentration increases, while acetoacetate uptake remains moreproportional to concentration (Pethick and Lindsay, 1982) It is likely thatincreased NEFA uptake and oxidation inhibit theD-3-hydroxybutyrate dehydro-genase reaction due to a more reduced state of the pyridine nucleotides Thus wehave a mechanism for pathological ketoacidosis seen in pregnancy toxaemia.Another finding in ketotic animals (pregnant or lactating) is the development

of fatty liver The aetiology of this accumulation seems to be associated with liveruptake of NEFA in proportion to concentration, attainment of maximal rates ofketogenesis and therefore a substrate-regulated increase in the rate of esterifica-tion Normally, the esterified lipid is released as VLDL TAG, but when the NEFAentry rate is chronically elevated VLDL synthesis is not increased sufficiently to