This article was downloaded by: [University of Tennessee, Knoxville]On: 22 December 2014, At: 08:55Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Acta Agriculturae Scandinavica, Section A —Animal SciencePublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/saga20

Quantitative Glucose Metabolism inLactating Mink ( Mustela vison ) - Effectsof Dietary Levels of Protein, Fat andCarbohydratesRikke Fink & Christian Friis BørstingPublished online: 05 Nov 2010.

To cite this article: Rikke Fink & Christian Friis Børsting (2002) Quantitative Glucose Metabolism inLactating Mink ( Mustela vison ) - Effects of Dietary Levels of Protein, Fat and Carbohydrates, ActaAgriculturae Scandinavica, Section A — Animal Science, 52:1, 34-42, DOI: 10.1080/09064700252806407

To link to this article: http://dx.doi.org/10.1080/09064700252806407

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to the accuracy, completeness,or suitability for any purpose of the Content. Any opinions and views expressed in thispublication are the opinions and views of the authors, and are not the views of or endorsedby Taylor & Francis. The accuracy of the Content should not be relied upon and should beindependently verified with primary sources of information. Taylor and Francis shall not be liablefor any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, inrelation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and usecan be found at http://www.tandfonline.com/page/terms-and-conditions

Quantitative Glucose Metabolism inLactating Mink (Mustela ×ison) – Effectsof Dietary Levels of Protein, Fat andCarbohydrates

Fink, R. and Børsting, C. F. (Department of Animal Nutrition andPhysiology, Danish Institute of Agricultural Sciences, Research CentreFoulum, P.O. Box 50, DK-8830 Tjele, Denmark). Quantitative glucosemetabolism in lactating mink (Mustela Øison) – Effects of dietary levelsof protein, fat and carbohydrates . Accepted September 30, 2001. ActaAgric. Scand., Sect. A, Animal Sci. 52: 34–42, 2002. © 2002 Taylor &Francis.

Glucose metabolism was measured during two consecutive years, 4weeks postpartum, in a total of 36 yearling female mink, � tted withjugular vein catheters and raising litters of six to seven kits. The damswere fed ad libitum from parturition on diets with different ratios ofmetabolizable energy (ME) derived from protein:fat:carbohydrate s (ex-periment 1: 61:37:2, 46:37:17, 31:37:32; experiment 2: 61:38:1, 47:52:1,33:66:1). After 3 h fasting the dams were fed 210 kJ ME of theexperimental diets. Two hours postprandially a single dose of 50 mCiU-14C- and 2-3H-labelled glucose was administered to each dam andblood samples were drawn 5, 10, 20, 30, 45 and 60 min after thetracer administration. Glucose turnover rates were 4–5% min¼ 1 in alldams, and the approximate daily glucose � ux was 12–17 g day¼ 1;however, these were not signi� cantly affected by dietary treatment. Inconclusion, the mink is able both to synthesize large amounts ofglucose de no×o and to utilize high levels of dietary digestible carbo-hydrates, and thereby to tolerate large variations in dietary carbohy-drate supply.

Rikke Fink* and ChristianFriis Børsting

Department of Animal Nutrition andPhysiology, Danish Institute ofAgricultural Sciences, ResearchCentre Foulum, P.O. Box 50, DK-8830Tjele, Denmark

Key words: energy supply, glucose� ux, glucose turnover rate.

Introduction

A lactating female must provide a continuous supplyof substrates to the mammary gland for the produc-tion of milk. Glucose is an important intermediate inthe metabolism as precursor in the lactose synthesis,and an adequate supply is essential for milk secretion(Davis et al., 1987). This high requirement of glucosefor the mammary gland implies that the rate of entry

or production of this substrate is increased or that itsrate of utilization by other tissues is decreased(Burnol et al., 1983). In species such as rats and pigsthe increased glucose requirement during lactationcan be met by higher dietary intake because theirdiets have a high carbohydrate content. However, themink is a strict carnivore and hence its diet, com-pared with that of omnivores, is rich in animalprotein and fat but low in carbohydrates . Further-more, the digestive tract of the mink is simple, with alow feed passage time of only approximately 3.5 h* Corresponding author.

34

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

Glucose metabolism in lactating mink

(Szymeczko & Skrede, 1990) and low a-amylase (El-nif et al., 1988) and microbial activity (Williams etal., 1998). Therefore the supply of dietarycarbohydrates has to be either monosaccharides orgelatinized starch to obtain a high digestibility. How-ever, the activity of gluconeogenic enzymes and thecapacity of the liver to synthesize glucose is very highin mink (Sørensen et al., 1995), and the feeding habitscomprising many small meals throughout the day arefavourable regarding glucose homoeostasis of theanimals. However, the lactation period places highenergetic demands on the females, and usually thedams are unable to cover their total energy expendi-ture by feed consumption, resulting in mobilizationof body reserves (Tauson, 1997). Weight losses in theorder of 20% frequently occur (Hansen & Berg,1998), and extreme mobilization can in severe caseslead to the lethal disease nursing sickness, biochemi-cally characterized by severe metabolic derangementsincluding hyperglycaemia presumably caused by anuncontrolled gluconeogenesis (Wamberg et al.,1992).

Most mammals, e.g. dogs (Belo et al., 1976a) andrats (Burnol et al., 1987), have the capacity to adaptto a wide range of protein intakes. However, anotherstrict carnivore, the cat, is unable to decrease theactivities of its nitrogen-metabolizing enzymes, andtherefore has a high obligatory nitrogen loss andthereby a high protein requirement (Rogers et al.,1977). Furthermore, the activities of the lipogenicenzymes do not increase in the cat when a highcarbohydrate diet is fed, suggesting a limited capacityfor de no×o fatty acid synthesis, consistent with thecat’s high dietary fat requirement (Rogers et al.,1977). This lack of hepatic adaptation limits theextent to which the cat can adapt to extremes inprotein, fat and carbohydrate content of the diet(Rogers et al., 1977; Kettlehut et al., 1980).

At present in the Scandinavian countries, the rec-ommended energy ratio [% of metabolizable energy(ME)] during lactation for farmed mink is aminimum of 40% from protein, 40–50% from fat anda maximum of 20% from carbohydrates (Hansen etal., 1991). Glem-Hansen (1979) found that the per-centage of ME derived from protein should constitute43–53% to secure a normal kit performance, andSkrede (1981) found an improved kit performance at21 days postpartum when the fat:carbohydrate ratiowas increased from 32:24 to 50:6, indicating positiveeffects on milk production. However, the physiologi-cal requirement for dietary carbohydrate has beenpaid only limited attention, especially regarding inter-actions with dietary protein intake, and the same istrue regarding the effects of excess amounts of dietarycarbohydrates.

Quantitative measurements of glucose metabolismby the use of glucose labelled simultaneously with 3Hand 14C has been widely used in various species, e.g.dog (Belo et al., 1976b), rats (Katz et al., 1974;Burnol et al., 1983), foxes (Tallas & White, 1988) andmink (Børsting & Damgaard, 1995). Thus, by the useof this method, the objective of the presentexperiments was to elucidate further the effects ofvarying dietary carbohydrate, protein and fat supplyon the quantitative metabolism of glucose in lactatingmink.

Materials and methods

The experiment was carried out in June during twoconsecutive years with a total of 36 yearling femalemink of the standard black colour type raising littersof six to seven kits. In the � rst year (experiment 1) 18females were divided into three groups fed ad libitumwith diets containing 37% of ME from fat, anda decreasing protein:carbohydrat e ratio. The threediets contained the following ratios betweenprotein:fat:carbohydrate s (61:37:2, 46:37:17 and31:37:32) (Table 1). In the second year (experiment 2)another 18 females were divided into three groups fedad libitum with diets of similar protein contents as inthe previous year, but almost without dietarycarbohydrates (1% of ME): (61:38:1, 47:52:1 and33:66:1) (Table 1). All diets were fed from the day ofparturition to the end of the experiment approxi-mately 4 weeks postpartum. The dietary compositionand analysed chemical compositions of the diets aregiven in Table 1. Nutritive values were calculatedfrom the amount of nutrient per kg dry matter (DM)in the diet and by use of table values for digestibilitycoef� cients of individual feedstuffs and the followingME coef� cients: 18.8, 39.8 and 17.6 kJ g¼ 1 digestedprotein, fat and carbohydrates , respectively (Hansenet al., 1991). Three weeks postpartum the dams withtheir litters were transferred from the experimentalfarm to the sampling room, where they were kept inindividual metabolism cages with straw-bedded nestboxes and devices for feeding and drinking water.The experimental procedures were approved by theDanish national legislation for protecting animals,and were in concordance with the guidelinesapproved by the member States of the Council ofEurope for the protection of vertebrate animals usedfor experimental and other scienti� c purposes(Anonymous, 1986).

Catheterization

After an adaptation period of 5–7 days in the sam-pling room, the females were � tted with a jugularvein catheter. Sedation and anaesthesia were induced

35

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

R. Fink and C. F. Børsting

Table 1. Dietary composition and results of chemical analyses of the experimental diets containing differentdietary protein:fat:carbohydrate ratios [% of metabolizable energy (ME)], fed to lactating mink dams (n¾6)

Experiment 1 Experiment 2

61:37:2 46:37:17 31:37:32 61:38:1Group (ME from protein:fat:carbohydrates): 47:52:1 33:66:1

Dietary composition (g kg¼ 1)737 639 478Fish offal 700 661 603120 105 84 150Poultry offal 143 13030 26 21Fish meal 96 91 8320 26 21 20 19Haemoglobin meal 1750 34 28Soya protein and corn gluten (1:1) – – –20 17 14 15Soya-bean hulls 21 3018 30 55Soya-bean and rape-seed oil1 14 58 130– 116Barley and wheat (heat treated) (1:1) 290 – – –2.5 3.2 5.2Salt 2.5 3.2 3.72.5 3.8Vitamin and mineral mixture2 3.8 2.5 3.8 3.3

Chemical analyses301 327 379 293 328 392DM (g kg¼ 1)149 106 76Ash (g kg¼ 1 DM) 142 122 108611 469 331 667Crude protein (g kg¼ 1 DM) 583 478165 162 167Fat (g kg-¼ 1 DM) 171 258 40016.3 16.2Gross energy (MJ kg¼ 1 DM) 16.2 17.2 19.2 22.8

4890 5290 6130 5130ME (kJ kg¼ 1)3 6300 8920

Energy ratio (% of ME)60.4 46.1 32.1Protein (%) 62.0 48.6 33.637.9 37.0 37.7 37.5 50.8 66.2Fat (%)1.6 16.7 30.2Carbohydrate (%) 0.4 0.7 0.2

1 In experiment 1: only soya-bean oil; in experiment 2: soya-bean and rape-seed oil (1:1).2 Containing (mg kg¼ 1): retinol 2.8, cholecalciferol 0.28, a-tocopherol 21 840, thiamine 10 000, ribo� avin 4800,pyridoxine 3200, D-pantothenic acid 3200, nicotinic acid 8000, betain anhydrate 33 600, folic acid 240, biotin 80,cyanocobalamin 16, paraaminobensoic acid 800, Fe 19 712, Zn 12 560, Mn 6237, Cu 1025.3 Calculated with use of table values for analysed amounts of the nutrients, coef� cients of digestibility for thefeedstuffs and the following factors for ME: 18.8, 39.8 and 17.6 g¼ 1 digested protein, fat and carbohydrates(Hansen et al., 1991).DM: dry matter.

by intramuscular injection of 0.20 ml ketamine (Ke-taminol® Vet., 50 mg ml¼ 1; Veterinaria AG, Zurich,Switzerland) and 0.20 ml medetomidin hydrochloride(Dormitor® Vet. 1 mg ml¼ 1; Orion Corporation,Espoo, Finland). The animal was then left undis-turbed at room temperature, and deep surgical anaes-thesia, assessed by complete absence of the palpebraland toe pinch re� exes, occurred within 5–10 min.The animal was shaved on the ventral surface of theneck, thorax and the back, and placed on a towel indorsal recumbency. The left jugular vein was dis-sected free and a Silastic® catheter (1.4 mm o.d., 0.6mm i.d.; Dow Corning Co., Michigan, USA) wasinserted, � xed with silk suture and then subcuta-neously extended from the thorax to the dorsal por-tion of the back where it was connected with glue toa tethering system modi� ed from Bonnefond et al.(1988). For recovery the animals were given a sub-cutaneous injection (0.1 ml) of antipamezol hydro-

chloride (Antisedan® Vet. 5 mg ml¼ 1; Orion Corpo-ration, Espoo, Finland) and then returned to theirmetabolic cages. The system used to protect thecatheter consisted of a metal spiral spring withswivels at both ends � xed to the back of the mink byadhesive elastic band and to the wire cage, respec-tively, allowing the animal to move freely in the cage.The catheter inside the spring was of transparentpolyethylene tubing (1.3 mm o.d., 0.85 mm i.d.; Me-dox Surgimed A S, Denmark).

Tracer administration and blood sampling

Four weeks postpartum (3 days after surgery) theexperiment started with a 3 h fasting period. Thedams were then given access to 210 kJ ME (approxi-mately 30 g or 10% of the daily ration) of theexperimental diets for 10 min. Feed residues (if any)were then removed and the amount consumed wasrecorded. Exactly 2 h postprandially a single injection

36

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

Glucose metabolism in lactating mink

(2.0 ml per animal) of 50 mCi of both U-14C- and2-3H-labelled glucose was administered to each dam.The tracer solution per animal consisted of 0.25 mlU-14C-glucose (0.2 mCi ml¼ 1) »0.05 ml 2-3H-glucose(1.0 mCi ml¼ 1) »0.10 ml carrier solution (2.0 mgglucose anhydrate in 100 ml sterile isotonic saline,0.9% NaCl) »1.6 ml sterile isotonic saline. After thetracer injection the catheter was � ushed with 1.0 mlrinse solution (0.8 mg glucose anhydrate in 100 mlsterile isotonic saline), 1.0 ml sterile isotonic salineand 0.5 ml heparinized (25 IU ml¼ 1) isotonic salinesolution. Blood samples (0.6–3.0 ml) were collectedat increasing time intervals after injection (5, 10, 20,30, 45, 60, 90 and 120 min) in heparinized tubes andkept on ice until centrifugation to obtain the plasma.Each blood sample was followed by infusion of anequivalent volume of sterile isotonic saline and � nally0.5 ml heparinized (25 IU ml¼ 1) isotonic saline solu-tion was deposited to � ll the outer catheter. Aftercentrifugation for 15 min at 3300 g plasma wasremoved and stored in plastic tubes at ¼18°C untilassayed. Parallel to this experiment glucose ho-moeostasis and regulation were determined by con-secutive blood samples drawn 10 and 5 min beforefeeding and 30, 60, 90, 120, 150 and 180 min after themeals (Fink et al., 2001a).

Analyses

Diets were analysed for DM by evaporation at 100°Cto constant weight. Ash was determined by combus-tion at 525°C for 6 h, nitrogen (N) was determined bythe micro-Kjeldahl technique and crude protein wascalculated as N * 6.25. Fat was determined afterhydrolysis with hydrochloric acid (HCl) and extrac-tion with petroleum ether (Stoldt, 1952). Carbohy-drates (CHO) were calculated by difference(CHO¾DM¼ash¼CP¼fat). Gross energy (GE)was estimated by use of the energy factors (in kJg¼ 1): protein (23.9), fat (39.8) and carbohydrate(17.6). ME was estimated by the use of estimatedindividual coef� cients of digestibility for the diets, themeasured amount of the digestible nutrient kg¼ 1 dietand ME coef� cients (18.8, 39.8 and 17.6 kJ g¼ 1

digested protein, fat and carbohydrate) (Hansen etal., 1991).

Blood samples were analysed for packed cell vol-ume (PCV). Plasma samples were analysed for glu-cose by the combined hexokinase and glucose-6-phosphate dehydrogenase method using an auto-analyser (OpeRA™ Chemistry System; TechniconRA® Systems. Methods Manual, Bayer Corporation,New York, USA). For measurement of speci� c ra-dioactivity of glucose, plasma was deproteinized withBa(OH)2 and ZnSO4 and centrifuged 20 min at 3180g. Samples of supernatant were layered on an ion-ex-

change column containing equal amounts of AG1-X8and AG50W-X8, and glucose was diluted with water,separating the glucose from its radioactively labelledmetabolites. The glucose solutions were evaporatedin a Hetovac vacuum-centrifuge over night anddissolved in H2SO4 and analysed with an electro-chemical detector by high-performance liquidchromatography (HPLC). The collected glucose peakfraction from the HPLC was analysed for 3H and 14Cradioactivity by a liquid scintillation counter (LKB,1219; Wallac, Turku, Finland).

Calculations and statistical analyses

The dilution of 14C-labelled glucose infused into theblood stream represents the � ow (entry orappearance) of new (non-labelled) glucose into theblood or, for practical purposes, glucose productionby all tissues of the body (Bergman, 1983). Thekinetics of 14C-labelled substrates is, however,complicated by the recycling of carbon, which limitsthe interpretation of the experimental data. However,recycling of labelled carbon is avoided by labellingwith a non-recycling tracer [2-3H], which istransformed to water almost as the sole catabolicproduct (Katz & Dunn, 1967).

The natural logarithms of the speci� cradioactivities of 3H- and 14C-glucose versus timewere graphed. Under the assumptions that glucoseturnover rate is of � rst-order kinetics and glucose isonly distributed to one extracellular pool, total entryrate (TER, % min¼ 1), representing the rate of entryof all glucose into the sampled compartment, wasestimated from the decline in speci� c activity of[3H]glucose by linear regression. Glucose irreversibleloss rate (ILR, % min¼ 1) represents the rate ofirreversible loss from the pool of glucose carbon, i.e.carbon that will not return to the pool, calculatedby the decline in speci� c activity of [14C]glucoseaccording to Tallas & White (1988). However, therewas not full linearity during the whole samplingperiod and, therefore, only data from 5 to 60 minafter the tracer administration were used to estimateTER and ILR. Glucose space is de� ned as thevolume of liquid with which the glucose poolequilibrates, expressed as a percentage of body weight(BW, kg), in this study set to 25% (Cherrington &Vranic, 1973; Belo et al., 1976b). Glucose pool size(Q, g) is de� ned as the amount of glucose withwhich the injected dose mixes, calculated as Q¾([GLU] * BW * 25) 100, where [GLU] is the meanconcentration of plasma glucose (g l¼ 1) during thetime while glucose turnover was measured, i.e. 120–180 min postprandially. Accordingly, glucose � uxes(g min¼ 1) were calculated as follows: Total entry,TE¾TER * Q, Irreversible loss, IL¾ILR * Q.

37

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

R. Fink and C. F. Børsting

Diurnal total glucose � ux was estimated by use of theTE (g min¼ 1) * 60 min * 24 h. Glucose recycling rate(R), equal to the difference between total entry andirreversible loss, can be expressed as a percentage ofTE: R¾ (TE¼ IL) TE * 100. Glucose turnover time(TT, min) represents the time required to replace theglucose pool and is calculated as pool size divided bytotal entry: TT¾Q TE.

Statistical analyses were carried out according tothe GLM procedure of the SAS Institute (1985). Thedependent variables were analysed according to amodel comprising the � xed effect of treatment groupwithin each study. Results are given as least-squaresmeans 9standard error of mean (SEM).

Results

The chemical analysis of the experimental diets’protein:fat:carbohydrat e ratios showed values closeto those planned (Table 1). The body weights of thefemales decreased by 15–23% from parturition untilweek 5 postpartum; however, differences were notsigni� cant between treatment groups (Table 2).During the 3 days from insertion of the jugularcatheters until the glucose turnover studies wereperformed, the average daily feed intake wasapproximately 800 kJ day¼ 1, which was lower thanthe recommended levels of approximately 1490 kJday¼ 1 (Hansen et al., 1991). However, practically all

the dams consumed the 210 kJ they were offered 2 hbefore the turnover studies.

In general, the dams appeared calm during theblood sampling through the permanent catheters,lying in a resting posture with their litters in the nestboxes. One female in the 47:52:1 group had to beexcluded from the experiment because her catheterwas disconnected. The PCV values decreased 0.6%units during the 1 h turnover study. From the start ofthe experiment (Fink et al., 2001) to the end of theturnover study 3 h later the decrease in PCV wasapproximately 5% units due to administration ofsaline; however, differences between treatment groupswere not signi� cant. In experiment 1 the postprandialplasma concentration of glucose tended to be highestin the 46:37:17 and 31:37:32 groups fed thecarbohydrate-containing diets. This difference wassigni� cant (PB0.02) between the 31:37:32 (7.1 mmoll¼ 1) and 61:37:2 (5.3 mmol l¼ 1) groups at the onsetof the turnover study 120 min after the meal, and 150min (PB0.07) postprandially (Table 3). Inexperiment 2, where all groups were fedcarbohydrate-fre e diets, no postprandial response inplasma glucose concentrations or differences(PB0.5) between treatment groups were found(Table 3).

In experiment 1 the total glucose entry rate (TER)was 5.2% min¼ 1 in all groups, whereas irreversibleloss rates (ILR) were 4.4, 4.6 and 4.7% min¼ 1,

Table 2. Live weight of the dams, and glucose pool size (Q), total entry (TE), irreversible loss (IL), turnover time(TT) and recycling (R) in the treatment groups fed different dietary protein:fat:carbohydrate ratios (% ofmetabolizable energy)

Experiment 1

Group: 61:37:2 46:37:17 31:37:32 P 1

886 915924912951 99Weight (g) 0.33Q (mg) 215 98 257924 246 97 0.17TE (mg min¼ 1) 11.290.5 12.290.8 12.190.9 0.61IL (mg min¼ 1) 9.390.4 10.890.7 10.990.7 0.14

11.391.1 9.091.3R (%) 0.00117.191.021.091.7TT (min) 21.292.0 0.6319.290.8

Experiment 2

Group: 61:38:1 47:52:1 33:66:1 P 1

Weight (g) 968 918 1072 921 965 912 0.15251 916Q (mg) 239923 212 915 0.30

TE (mg min¼ 1) 9.790.9 9.490.8 8.490.7 0.498.090.69.090.89.390.9IL (mg min¼ 1) 0.51

R (%) 4.690.9 4.790.9 0.964.391.1TT (min) 25.591.426.792.4 25.591.3 0.85

Data are means9SEM.1 Differences between treatment groups within each sampling time.

38

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

Glucose metabolism in lactating mink

Table 3. Plasma glucose concentration (mmol l¼ 1) inexperiments 1 and 2, 30–180 min after feedingin the treatment groups fed different dietaryprotein:fat:carbohydrate ratios (% of metabolizableenergy)

Experiment 1

61:37:2 46:37:17Group: 31:37:32 P 1

Minutes after feeding4.990.2 5.690.4 5.590.3 0.20305.490.3 6.290.760 6.990.4 0.145.790.2 6.891.090 7.490.4 0.125.390.2 6.890.7120 7.190.3 0.024.890.2 5.790.4 5.790.3 0.071505.190.2 5.490.6 5.990.4180 0.13

Experiment 2

61:38:1 47:52:1 33:66:1Group: P 1

Minutes after feeding5.490.530 5.290.2 5.590.3 0.755.290.3 5.090.360 5.090.6 0.855.390.4 5.290.6 4.990.590 0.845.790.5 5.190.5120 5.090.4 0.505.690.4 4.890.5150 4.790.3 0.246.290.7 5.090.6 5.090.3 0.22180

Data are means9SEM.1 Differences between treatment groups within eachsampling time.

Fig. 1. Glucose turnover rate 4 weeks postpartum in lactatingmink fed different ratios of protein:fat:carbohydrates in (a) experi-ment 1 and (b) experiment 2. TER: rate of entry of all glucose intothe sampled compartment estimated from the decline in speci� cactivity of [3H]glucose. ILR: rate of irreversible loss from the poolof glucose calculated by the decline in speci� c activity of[14C]glucose.

Fig. 2. Approximate daily glucose � ux (9SEM) 4 weeks post-partum in lactating mink fed different ratios ofprotein:fat:carbohydrates in (a) experiment 1 and (b) experiment 2,calculated by the use of total entry (TE) and irreversible loss (IL)of glucose.

respectively, in the 61:37:2, 46:37:17 and 31:37:32groups, differences being non-signi� cant betweentreatment groups (Fig. 1a). Likewise treatment groupdid not signi� cantly affect the TE and IL of glucose(Table 2). The glucose pool size was 13% lower in the61:37:2 group than in the 31:37:32 group fedthe carbohydrate-containin g diet. However, thepercentage of glucose recycling (R) was signi� cantly(PB0.01) lower in the groups fed the intermediateand high carbohydrate-containin g diets (46:37:17 and31:37:32) than in the 61:37:2 group (Table 2). The TTof glucose was very similar (approximately 20 min) inall groups in experiment 1.

In experiment 2 the total entry rates (TER) were3.9–4.0% min¼ 1 and the irreversible loss rates (ILR)3.7–3.8% min¼ 1 (Fig. 1b). The differences in TE, ILand R of glucose between treatment groups were notsigni� cant (Table 2). However, the values for TE andIL were 13% and 14% lower in the 33:66:1 groupthan in the 61:38:1 group. Glucose pool sizedecreased with decreasing dietary protein supply andwas 16% lower in the 33:66:1 group than in the61:38:1 group. The TT of glucose was approximately26 min in experiment 2, and not signi� cantly affected

by treatment group. The approximate daily glucose� uxes (TE) in the 61:37:2, 46:37:17 and 31:37:32groups in experiment 1 were 16.2, 17.6 and 17.4 gday¼ 1, respectively (Fig. 2a). In the 61:38:1, 47:52:1

39

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

R. Fink and C. F. Børsting

and 33:66:1 groups in experiment 2 the daily glucose� uxes (TE) were 13.9, 13.6 and 12.0 g day¼ 1,respectively (Fig. 2b).

Discussion

When blood glucose concentration is constant andthe animal is in a steady state, the utilization ofglucose will equal its production and the combinedprocesses are termed turnover rates. The validity ofthe equations used to determine rates of glucoseutilization depends on ful� lment of the steady-stateassumption. During the two experiments the groupsfed the carbohydrate-fre e diets maintained a constantplasma glucose concentration, hence ful� lling thisassumption. The glucose concentration in the46:37:17 and 31:37:32 groups fed the carbohydrate-containing diets declined by 21% and 17% during the1 h turnover measurements, respectively, implyingthat the steady-state assumption was not quiteful� lled during the turnover study in these twogroups. However, the turnover times were 21 min, i.e.that during the experimental hour total glucose � uxwas 286% of the original pool and therefore the 21%decline in plasma glucose concentration is considerednegligible.

The decline in speci� c activity approximated asimple exponential curve showing that glucosemetabolism followed � rst-order kinetics during the� rst hour after tracer administration. Theoretically,glucose pool size can be calculated by extrapolationof the speci� c activity curve back to the time ofadministration. However, in this experiment pool sizebased on this extrapolation overestimated the size ofthe total glucose pool, probably because the slopeimmediately after injection was higher than estimatedhere by the simpli� ed single-pool model. Thus, sincethe study did not succeed in estimating the glucosepool size, estimates for glucose space from dogs(Cherrington & Vranic, 1973; Belo et al., 1976b) andsheep (Judson & Leng, 1972) were used to esitmatethe glucose pool size (25% of body weight).

The approximate daily glucose � uxes were basedon the average plasma glucose concentration duringthe turnover study, which resulted in no signi� cantdifferences in � uxes between treatment groups in anyof the experiments. However, if the daily glucose� uxes instead were based on plasma glucoseconcentrations from 60 to 120 min postprandially inexperiment 1, it would have been signi� cantly(PB0.02) higher in the 46:37:17 (21 g day¼ 1) and31:37:32 (21 g day¼ 1) groups fed carbohydrate-containing diets than in the 61:37:2 (17 g day¼ 1)group. Since the feeding habits of the mink comprisemany small meals throughout the day a constantly

higher plasma glucose concentration, and thereby ahigher daily glucose � ux in dams fed the 46:37:17 and31:37:32 diets, may exist under normal conditionswith constant access to feed. Therefore, the presentresults indicate that daily glucose � ux may be lowerin dams fed carbohydrate-free diets compared withdams fed at least 17% of ME from carbohydrates .Furthermore, in experiment 2, where all groups werefed carbohydrate-free diets, the glucose � ux tended tobe lower in the 33:66:1 group, indicating that withoutdietary carbohydrate supply, 33% of ME fromprotein yielded insuf� cient amounts of amino acids asprecursors for the gluconeogenesis to maintainnormal glucose � ux. The difference in glucose � uxbetween the 61:37:2 group in experiment 1 and the61:38:1 group in experiment 2 fed very similar dietswas only small and can be ascribed to smalldifferences in diets, animals and years.

High dietary carbohydrate supply did not result inextreme elevations of plasma glucose concentrations,indicating that the mink has a high glycolyticcapacity, corresponding to results by Sørensen et al.(1995) and Børsting & Gade (2000). Likewise, TEwas not signi� cantly affected by increased dietarycarbohydrate supply in experiment 1, stronglyindicating a lower de no×o synthesis of glucose, whichwas possibly caused by the lower supply of precursorsin the form of amino acids or a decreased activity ofthe gluconeogenic enzymes. In the same study (Finket al., 2001a) the plasma glucagon concentration was,in contrast to the situation in omnivores (Ganong,1993), signi� cantly higher in the 31:37:32 and 33:66:1groups fed the low-protein diets, suggesting that themink, irrespective of carbohydrate supply, tries tokeep a high gluconeogenic enzyme activity byincreasing glucagon secretion when amino acidsupply is decreased.

In mink dams with six kits in the 5th week oflactation consuming the recommended 1490 kJday¼ 1 (non-lactating dam: 830 kJ»110 kJ per kitfor milk production; Hansen et al., 1991) the averageintake of digestible carbohydrates would be onlyapproximately 0.5 g per female per day in mink fedthe carbohydrate-free diets and 14 g and 25 g,respectively, in mink fed the 46:37:17 and 31:37:32diets. It can be assumed that the amount of absorbedglucose equals the amount of digestiblecarbohydrates because only starch is digested(yielding glucose units) and because none of thecarbohydrates disappear from the digestive intestinaltract owing to fermentation. Compared with the totalglucose � uxes (TE: Fig. 2a, b), dietary supplyaccounts for only 3% of the daily glucose � ux in thegroups fed the carbohydrate-free diets and 80% and145%, respectively, in the 46:37:17 and 31:37:32groups. Total glucose � ux was measured 2–3 h

40

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

Glucose metabolism in lactating mink

postprandially and was therefore possibly lower thanthe diurnal mean value during farm conditions, wherethe dams have free access to feed. The decreased PCVvalues could not affect the turnover rates; however, itcould have had a dilution effect on the plasma glu-cose concentration. Thus, underestimation of TE,because of an underestimated plasma glucose concen-tration, feed consumption, loss of glucose in the gutetc., may explain why the estimated daily glucose � uxwas only approximately 70% of the glucose consump-tion. In the groups fed the carbohydrate-free diets,the daily TE of approximately 13 g per day repre-sented the amount of glucose synthesized by gluco-neogenesis, which is at same level as daily glucoseuptake in dams fed 17% of ME from carbohydrates .

Calculated from Fink et al. (2001b), the carbohy-drate fraction in mink milk is 7.8% and the milkproduction in the 4th week of lactation in damsnursing six kits is approximately 150 g day¼ 1, i.e. theoutput of carbohydrate s in the milk is approximately12 g day¼ 1. This is within the same range as the totalabsorption of glucose (10 g), when fed a conventionallactation diet with 12% of ME from carbohydrateand a feed consumption of the recommended 1490 kJday¼ 1, or as the present experiments showed, theamount of glucose synthesized by gluconeogenesis.Thus, evidently, gluconeogenesis plays a very impor-tant role in the mink, especially during lactation.

Differences in glucose turnover rate between di-etary treatment groups were not signi� cant andthereby in contrast to results in dogs by Romsos et al.(1976) who found that the glucose turnover ratetended to be 20–30% lower in bitches fed carbohy-drate-free diets. In lactating rats Burnol et al. (1987)found signi� cantly lower glucose turnover rates whenthe animals were fed a high-fat diet compared with ahigh-carbohydrat e diet.

Recycling of glucose in dogs and other carnivoressuch as the cat (Kettlehut et al., 1980) and fox (Tallas& White, 1988) tends to increase postprandially,whereas it decreases in humans and rodents. Thecanine liver releases lactate postprandially, unlikehumans and rodents, and does not take up glucose toany signi� cant extent. Therefore, the intrahepaticsource for glycogen and fat synthesis seems to begluconeogenic amino acids, probably derived fromboth exogenous and endogenous hepatic protein(Davis et al., 1987). This corresponds well to thesigni� cantly higher recycling found in experiment 1 indams fed the carbohydrate-free diet (61:37:2) com-pared with the 46:37:2 and 31:37:32 groups.

In conclusion, the mink is capable of synthesizingsuf� cient glucose to support a normal glucose � uxeven when fed carbohydrate-free diets; however, deno×o synthesis depends on the availability of suf� cient

gluconeogenic precursors in the form of amino acids.Simultaneously, it is also capable of utilizing highamounts of dietary carbohydrates , without criticallyelevated plasma glucose concentrations. These � nd-ings demonstrate that the mink has a high activity ofthe gluconeogenic enzymes and also a large glycolyticcapacity, and thereby a high ability to adapt tovariations in dietary protein and carbohydratesupply.

Acknowledgements

The Danish Ministry of Food, Agriculture and Fish-eries supported this study. The authors wish to thankBirthe M. Damgaard for performing the catheteriza-tions of the mink, and Anne K. Rosted for skilledtechnical assistance throughout the experiment andfor performing the speci� c glucose activity analyses.We also wish to thank Annette Gade and Mette B.Pedersen for assisting during collection of the bloodsamples.

References

Anonymous. 1986. European Convention for the Protection ofVertebrate Animals Used for European Treaty Series No. 123.Council of Europe, Strasbourg.

Belo, P. S., Romsos, D. R. & Leveille, G. A. 1976a. In� uence ofdiet on glucose tolerance, on the rate of glucose utilization andon gluconeogenic enzyme activities in the dog. J. Nutr. 106,1465–1474.

Belo, P. S., Romsos, D. R. & Leveille, G. A. 1976b. Determinationof glucose utilization in the dog with [2-3H], [6-3H]-, and [U-14C]glucose. Proc. Soc. Exp. Biol. Med. 152, 475–479.

Bergman, E. N. 1983. The pools of cellular nutrients: glucose. In:Riis, P. M. (ed.) Dynamic Biochemistry of Animal Production.World Animal Science A3. Elsevier, Amsterdam, pp. 173–194.

Bonnefond, C., Foushe, L. G. & Martinet, L. 1988. A chronicjugular catheterization for remote blood sampling in freely mov-ing mink. Physiol. Behav. 44, 141–146.

Børsting, C. F. & Damgaard, B. M. 1995. The intermediateglucose metabolism in the nursing period of the mink. NJF-Sem-inar No. 253, Gothenburg, Sweden; NJF-Report 106, 104–111.

Børsting, C. F. & Gade, A. 2000. Glucose homeostasis in mink(Mustela ×ison ). A review based on interspecies comparisons.Scientifur 24 (1), 9–18.

Burnol, A.-F., Leturque, A., Ferre, P. & Girard, J. 1983. Glucosemetabolism during lactation in the rat: quantitative and regula-tory aspects. Am. J. Physiol. 245 (Endocrinol. Metab. 8), E351–E358.

Burnol, A.-F., Leturque, A., De Saintaurin, M.-A., Penicaud, L. &Girard, J. 1987. Glucose turnover rate in the lactating rat: effectof feeding a high fat diet. J. Nutr. 117, 1275–1279.

Cherrington, A. D. & Vranic, M. 1973. Effects of arginine onglucose turnover and plasma free fatty acids in normal dog.Diabetes 22, 537–543.

Davis, M. A., Williams, P. E. & Cherrington, A. D. 1987. Nethepatic lactate balance following mixed meal feeding in thefour-day fasted conscious dog. Metabolism 36, 856–862.

Elnif, J., Hansen, N. E., Mortensen, K. & Sørensen, H. 1988.Production of digestive enzymes in mink kits. In: Murphy, B. D.

41

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014

R. Fink and C. F. Børsting

& Hunter, D. B. (eds) Biology, Pathology and Genetics of FurBearing Animals. Proceeding of the IV International Congress onFur Animals Production. Aug. 21–24, 1998, Canada, pp. 320–326.

Fink, R., Børsting, C. F. & Damgaard, B. M. 2001a. Glucosehomeostasis and regulation in lactating mink (Mustela ×ison ) –effects of dietary protein, fat and carbohydrate supply. ActaAgric. Scan. (submitted.

Fink, R., Tauson, A-H., Hansen, K.B., Wamberg, S. and Kris-tensen, N. B. 2001b. Energy intake and milk production in mink(Mustela ×ison ) – Effect of litter size. Arch. Anim. Nutr. (ac-cepted).

Ganong, W. F. 1993. Review of Medical Physiology, 16th edn.Prentice-Hall, Connecticut, 572 pp.

Glem-Hansen, N. 1979. Protein requirement for mink in the lacta-tion period. Methods for evaluation of protein requirement duringlactation. Acta Agric. Scand. 29, 129–139.

Hansen, B. K. & Berg, P. 1998. Mink dam weight changes during thelactation period. I. Genetic and environmental effects. Acta Agric.Scand, Sect. A. Animal. Sci. 48, 49–57.

Hansen, N. E., Finne, L., Skrede, A. & Tauson, A.-H. 1991.Energiforsyningen hos mink og ræv. NJF-utredning rapport 63.DSR Forlag Landbohøjskolen, Copenhagen, 59 pp.

Judson, G. J. & Leng, R. A. 1972. Estimation of the total entry rateand resynthesis of glucose in sheep using glucose uniformlylabelled with 14C and variously labelled with 3H. J. Biol. Sci. 25,1313 –1332.

Katz, J. & Dunn, A. 1967. Glucose-2-t as a tracer for glucosemetabolism. Biochem. 6, 1–5.

Katz, J., Dunn, A., Chenoweth, M. & Golden, S. 1974. Determina-tion of synthesis, recycling and body mass of glucose in rats andrabbits in ×i×o with 3H- and 1 4C-labelled glucose. Biochem. J. 142,171–183.

Kettlehut, I. C., Foss, M. C. & Migliorini, R. H. 1980. Glu-cose homeostasis in a carnivorous animal (cat) and in rats

fed a high-protein diet. Am. J. Physiol. 239, R437–R444.

Rogers, Q. R., Morris, J. G. & Freedland, R. A. 1977. Lack ofhepatic enzymatic adaptation to low and high levels of dietaryprotein in the adult cat. Enzyme 22, 348–356.

Romsos, D. R., Belo, P. S., Bennink, M. R., Bergen, W. G. &Leveille, G. A. 1976. Effects of dietary carbohydrate, fat andprotein on growth, body composition and blood metabolite levelsin dog. J. Nutr. 106, 1452–1464.

SAS Institute. 1985. SAS® User’s Guide: Statistics. 5th edn. Cary,NC: SAS Institute, 956 pp.

Skrede, A. 1981. Varying fat:carbohydrate ratios in mink diets. I.Effects on reproduction, early kit growth, viability and bodycomposition. Scienti� c Reports of the Agricultural University ofNorway 60(16), 2–18.

Sørensen, P. G., Petersen, I. M. & Sand, O. 1995. Activities ofcarbohydrate and amino acid metabolizing enzymes from liver ofmink (Mustela ×ison ) and preliminary observations on steady statekinetics of the enzymes. Comp. Biochem. Physiol. B112, 59–64.

Stoldt, W. 1952. Vorslag zur Vereinheitlichung der Fettbestimmungin Lebensmitteln. Fette u. Seifen. 54, 206–207.

Szymeczko, R. & Skrede, A. 1990. Protein digestion in mink. Acta.Agric. Scand. 40, 189–200.

Tallas, P. G. & White, R. G. 1988. Glucose turnover and defence ofblood glucose levels in arctic fox (Alopex lagopus). Comp.Biochem. Physiol. A91, 493–498.

Tauson, A.-H. 1997. Prolactin pro� les of pregnant, lactating andnon-mated female mink (Mustela ×ison ). J. Reprod. Fert. 51(Suppl.), 195–201.

Wamberg, S., Clausen, T. N., Olesen, C. R. & Hansen, O. 1992.Nursing sickness in lactating mink (Mustela ×ison ) II. Pathophys-iology and changes in body � uid composition. Can. J. Vet. Res.56, 95–101.

Williams, C., Elnif, J. & Buddington, R. K. 1998. The gastrointesti-nal bacteria of mink (Mustela ×ison L): in� uence of age and diet.Acta Vet. Scand. 39, 473–482.

42

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 08:

55 2

2 D

ecem

ber

2014