1

*
Department of Equine Sciences, Medicine Section, Faculty of Veterinary Medicine,
Utrecht University, P.O. Box 80.152, 3508 TD Utrecht, the Netherlands, Fax +31-30-
2537990, Email C.M.Westermann@uu.nl
* Corresponding author
ABSTRACT
The aim of the research presented here is to quantify glucose metabolism during moderate exercise in warmblood horses employing the hyperglycemic clamp technique. Seven healthy warmblood mares, all in diestrus, with an average age and weight of 11.6 ± 2.4 years and 569
± 40 kg, were fasted for 12 hours. These horses were subjected to a hyperglycemic clamp in five different intervals of work: At rest 1 (until steady state plasma glucose concentrations were reached with a minimum of 30 minutes), walk (10 minutes, 1.5 m/s), trot (20 minutes,
4.4 m/s), walk (10 minutes, 1.5 m/s), and finally rest 2 (same conditions as first rest). The average amount of glucose metabolized (expressed in M: The amount of glucose taken up per kg bodyweight per minute) was calculated and analyzed by a one-way ANOVA statistical test with post hoc Bonferroni adjustments. M-values were observed of 15.0 ± 2.1 at rest ( 1 ),
25.1 ± 6.2 at walk ( 2 ), 37.4 ± 9.1 at trot ( 3 ), 33.0 ± 13.1 at walk ( 4 ), and 18.7 ± 4.6 µmol/kg
BW/min at rest ( 5 ). The amount of glucose metabolized differed significantly between 1 and 3
(p = 0.000), 1 and 4 (p = 0.002), 3 and 5 (p = 0.001), and 4 and 5 (p = 0.022). In conclusion, glucose metabolism more than doubled in trot compared to rest and shows a lag phase during recovery. These results are compatible with previous work using radio-labeling.
2

INTRODUCTION
Blood glucose is an important source of energy consumed by muscles. Other sources are muscle glycogen and fatty acids. Research of glucose metabolism during exercise has been performed in different species, however, limited knowledge is available of glucose metabolism of horses in exercise (Coggan 1991). Quantification of endogenous glucose production and glucose uptake in dogs and humans during exercise reveals a tight regulation of production and uptake (Berger et al.
1994; Jenkins et al.
1985). In horses, however, there appears to be a mismatch in this balance: Even during moderate exercise plasma glucose concentrations can rise (Farris et al.
1995). Knowledge of exercise physiology of horses depends heavily on the precise quantification of their glucose metabolism. Currently, horses are involved in very different kinds of exercise like dressage, show-jumping, and endurance. High-performance is often demanded of the muscles of these equine athletes. Improvement of muscle strength, endurance or both would not only be of great interest for researchers, but also for trainers of horses. Furthermore, better refined advices for food composition and supplements could be provided.
Geor et al. were the first to report material about glucose metabolism in horses in different circumstances (Geor et al.
2000a; Geor et al.
2000b, c). They utilized radio-active glucose in order to trace the uptake.
The hyperglycemic clamp technique is an alternative non-isotope based method to quantify the glucose metabolism of horses. A critical assumption made in this technique, which was validated by Rijnen and van der Kolk, is that during hyperglycemia after the appropriate corrections the glucose metabolism is equal to the glucose infusion rate (Rijnen and van der
Kolk 2003). This is due to the complete suppression of endogenous glucose production.
The aim of this research is to quantify the amount of glucose metabolized during exercise in horses by means of the hyperglycemic clamp technique. The following hypotheses are tested in the present study: Glucose metabolism increases during exercise and is elevated during the recovery phase in comparison to the pre-exercise phase.
3

MATERIALS AND METHODS
Animals – This study was approved by the Committee on Animal Welfare of the Faculty of
Veterinary Medicine, University of Utrecht. Seven Dutch warmblood mares, aged 8 to 14 years ( AVG 11±2) were included in this study, all in diestrus at the time of the experiment.
The horses were used to frequent handling and tredmill exercise. Bodyweight ranged from
522 to 648 kg (AVG 569 ±40 kg). The horses were housed on pasture, but during the experiment they were kept in individual stables for 3 days. Food was withheld for 12 hours prior to the experiment to rule out glucose uptake from the small intestines. Water was supplied ad libitum. After the experiment the horses were fed hay and pellets, monitored for one night and sent back to pasture.
Hyperglycemic clamp technique – To get familiar with the method, a pilot of the experiment was performed two months prior to the definite experiments. Quantification of glucose metabolism using this technique is described by Rijnen and van der Kolk (Rijnen and van der
Kolk 2003).On the day before each experiment, two 13 cm MILA © catheters were inserted in both jugular veins and covered with Vetrap
©
and Leucoplast
©
during nighttime. On the day of the experiment the horses were weighed to calculate the priming dose of glucose. The horses were positioned on the tredmill a . An ECG device b was installed and turned on. Heartrates were monitored during the whole experiment. Two bloodsamples were taken for determination of basal plasma glucose-concentration. Hereafter, the bodyweight-dependentpriming dose (BPD) of glucose (as a 50% solution) was given IV within 10 minutes. The priming dose varied from 125 to 156 ml (AVG 136.9
±9.7 ml). It was calculated by the following equation: BPD (ml) = 0,24 x BW .
After this, the continuous infusion started at a bodyweight-dependend rate (AVG 0.321
±0.035 ml/h/kgBW) with a calibrated infusion pump c .
Every 10 minutes a venous bloodsample (heparinized) was taken for immediate determination of glucose concentration in an automated analyzer d . Blood glucose concentrations were considered hyperglycemic at ≥ 10 mM. A steady concentration was defined as a plasma glucose concentration within narrow ranges (± 0.2) for at least 30 minutes (4 samples) and no earlier than 90 minutes after the start of the clamp. Also, sodium,
4

potassium, chloride, lactate and pH were measured in venous blood. During steady state plasma glucose concentrations, additional venous blood samples (heparin vacuettes) were taken for determination of plasma insulin-concentration. These samples were centrifuged for
5 minutes at 6000 X g. Plasma was separated and stored at 20ºC until analysis of insulin could be performed. Insulin concentration was measured with a radioimmunoassay kit e validated for samples of horses (van der Kolk et al.
1995). If this steady state (‘rest 1’) continued for ≥30 minutes, the exercise procedure was started. This comprised 10 minutes of walking (‘walk 1’), 20 minutes of trotting (‘trot’) and another 10 minutes of walking (‘walk 2’).
Tredmill speeds ranged from 1.4 to 1.7 m/s during walk and 3.9 to 4.6 m/s during trot. The chosen speed depended on size and stride length of each individual horse.
During the exercise procedure we adjusted the infusion rate of glucose, in order to achieve steady concentrations of plasma glucose (>10 mM). During exercise, bloodsamples were taken every 5 minutes. After the protocol the horses rested again and the experiment ended when concentrations of blood glucose were steady for ≥30 minutes (at ‘rest 2’). At the end of each experiment, urine samples were collected to calculate the loss of glucose in urine.
Calculations – In this study, we assumed that endogenous production of glucose during the experiment is totally suppressed due to the hyperglycemia. Therefore, the amount of glucose infused (INF) equals the amount of glucose metabolized (M). The INF (50% glucose-solution; mmol/kgBW/min) was calculated by use of the following equation:
INF=(mL/h X 8.333)/(BW X 180) where BW stands for bodyweight in kilograms. This method needs two correction factors. During hyperglycemia, glucose is lost in urine and therefore a correction is made for the amount of glucose loss in urine (UC). The UC was calculated by the following equation: UC=29.37/(BW X 180).
During the hyperglycemic clamp test, glucose-concentrations are not maintained perfectly constant. Therefore a correction must be made, called the space correction. It adjusts for the amount of glucose, which has been added or removed from the glucose space. The formula is based on human values, with the assumption that 0.028 mmol of glucose is removed from each deciliter of glucose space in a 20-minute period (DeFronzo et al.
1979). The glucose space in L is given by 0.19 X BW. The SC was calculated by use of the following equation:
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SC=((G1-G2) X (0.19 X BW) / 10 X BW which is reduced to the following:
SC=(G1-G2) X 0.019
The M (mmol/kg/min) was calculated by use of the following equation: M=INF-UC-SC
The insulin concentration I (pmol/L) during the steady state in the experiment is a measure of bétacell response to hyperglycemia. The M/I ratio is an index of tissue-sensitivity to insulin, because it reflects the quantity of glucose metabolized per unit of insulin.
Assays and data analysis – all M values were calculated and a one way ANOVA with post hoc Bonferroni statistical test was performed with SPSS f . Differences were considered significant at values of p<0.05.
6

RESULTS
At first we determined the basal plasma glucose concentration after 12 hours of withholding food. The mean basal plasma glucose concentrations ranged from 4.8 to 5.5 mM (AVG
5.1
±0.27 mM).
The mean time to reach a steady blood glucose concentration within hyperglycemic ranges
(≥10 mM) was 116±21 min (‘rest 1’). At rest 1, the average blood glucose concentration was
13.29
±1.14 mM. For walk 1, trot, walk 2 and rest 2 these values were 13.12±1.21 mM;
12.55
±1.25 mM; 11.81±1.09 mM and 11.34±1.19 mM respectively. The infusion rates ranged from 0.28 ml/h/kg BW to 0.38 ml/h/kg BW (AVG 0.32±0.03 ml/h/kg BW) during different gaits.
In table 1, all M (metabolism) values are shown. In table 2 ranges and average M values are shown.
Urine glucose-concentrations ranged from 8.1 to 67.9 mM ( AVG 24.2 ± 22.3 mM). M values were 15.0±2.1 µmol/kg BW/min; 25.1±6.20 µmol/kg BW/min; 37.4±9.15 µmol/kg BW/min;
33 .0±13.09 µmol/kg BW/min; 18.7±11.35 µmol/kg BW/min at rest 1, walk 1, trot, walk 2 and at rest 2 respectively.
In figure 1, average glucose metabolism values (M) with standard deviations are shown.
In figure 2, the average M values were tested with an one way ANOVA and post hoc
Bonferroni statistical test. Significant differences (p<0.05) were calculated for rest 1 vs. trot and walk 2; trot vs. rest 2 and walk 2 vs. rest 2 and vice versa.
In figure 3 is visible that lactate blood concentrations were far below the aerobic lactate threshold concentration of 4 mM.
The speed of the tredmill was not the same for each horse, but the heart rates were rather similar as can be seen in figure 4.
In figure 5, mean pH values are shown. Significant differences (p<0.05) were calculated between rest 1 vs. trot and walk 2; walk 1 vs. trot; trot vs. rest 2 and vice versa.
Figure 6 shows increasing plasma potassium concentrations during exercise. Maximal values are reached during trot and lower values are determined at rest 2 than at rest 1. Significant
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differences (p<0.05) were calculated between rest 1 vs. rest 2 and trot; walk 1 vs. trot and rest
2; trot vs. rest 2 and walk 2; walk 2 vs. rest 2 and vice versa.
Sodium and chloride were also determined and figures 7 and 8 show that plasma sodium and chloride concentrations remain constant throughout the experiment.
8

DISCUSSION
A quantitative measurement of glucose metabolism in horses during rest and different grades of exercise has been described.
Glucose as a substrate for ATP production comes from various sources: Glucose can be taken up in the small intestine, it can be produced by the liver in gluconeogenesis, or by glycogenolysis in the liver and muscles. Muscle glycogen is the most important resource for glucose in the exercise phase. A relatively small amount of glucose is withdrawn from the blood (Hinchcliff K.W. 2004). In a state of hyperglycemia, more glucose is taken from the blood circulation, but the absolute amount of muscle glycogen consumed remains constant
(Geor et al.
2000c). The contribution of plasma glucose to total carbohydrate oxidation
(plasma glucose, muscle glycogen and lactate) is approximately 26% during induced hyperglycemia and 17% without exogenous glucose infusion (Geor et al.
2000c).
The calculated glucose metabolism (M) values reveal interesting differences between all gaits. The glucose metabolism during trot more than doubles compared to the rest 1 phase.
Apparently, muscles take up a large amount of glucose from the hyperglycemic blood circulation during exercise. In comparison the results of Geor et al are: Glucose uptake values
(Rd) were 8.3±0.9 µmol/kg BW/min at rest, 29.1±3.1 µmol/kg BW/min at 45 minutes of exercise (34±1% of VO
2 max) and 40.4±2.9 µmol/kg BW/min between 60 and 90 minutes of exercise (Geor et al.
2000c). These results are in good agreement with our findings.
Geor et al (Geor et al.
2000c) compared glucose uptake rates between a group with glucose infusion and a control group during exercise. They assumed that there would be a large mismatch between hepatic glucose production (HGP) and glucose uptake (Rd) in the control group, i.e. a hyperglycemia would occur during exercise. In contrast, they found a close match between HGP and Rd during low-intensity exercise. In the other group, HGP was only partially suppressed during the whole exercise procedure. However, HGP increased mostly after 45 minutes of exercise. The assumption of HGP suppression by exogenous glucose
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infusion can be defended, because measurements were done during 20 minutes of lowintensity exercise.
There must be an explanation for the (small) mismatch in HGP and Rd in horses. Apparently,
HGP is not only influenced by metabolic feedback but also by a feed forward regulation. In the horse, it is possibly due to higher sympathoadrenal activation during low intensity exercise, compared to humans and dogs.
Another important finding is that the glucose uptake is enhanced by hyperglycemia. In a state of (induced) hyperglycemia, muscles rely on glucose more than with normal blood glucose concentrations (Geor et al.
2000a). During hyperglycemia, also hyperinsulinemia occurs and this facilitates glucose uptake in the muscles and the whole body.
An important difference between the method used by Geor et al and the hyperglycemic clamp technique is that with the current method glucose metabolism is quantified by achieving steady concentrations of blood glucose above 10 mM at a certain infusion rate (Geor et al.
2000c).
Jager et al determined a decrease in blood glucose concentration on the onset of exercise, followed by an increase after 15 minutes (Graaf-Roelfsema 2007). This increase is probably due to enhanced glycogenolysis after 15 minutes of exercise.
We observed a lag phase in glucose metabolism during walk 2 and rest 2. During the hyperglycemic clamp technique, we expect plasma insulin levels to be elevated. According to
Jager et al and Freestone et al, plasma insulin is low during exercise and a hyperinsulinemia rebound occurs after exercise (Freestone et al.
1991; Graaf-Roelfsema 2007). This rebound can be caused by withdrawal of sympathetic inhibition of the pancreatic beta-cells or by a rise of endogenous opioid peptides that enhance insulin release (Freestone et al.
1992). During hyperglycemia and exercise, plasma insulin is possibly less elevated than during rest, and increases again in rest 2. This can explain the elevated glucose metabolism: insulin-mediated glycogenogenesis and glucose uptake in the muscle by GLUT 4.
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Plasma catecholamine levels are decreased during recovery and cause lower rates of lipolysis and NEFA’s in the blood circulation. Glucose is therefore the most important substrate for energy repletion in the muscle cell (Hyyppa et al.
1997). These mechanisms explain the increased metabolism of glucose during recovery.
Geor et al found that muscle glycogen consumption is not affected by plasma glucose concentrations, therefore we considered muscle biopsies unnecessary (Geor et al.
2000c).
Plasma lactate concentrations are low during all gaits and never exceeded concentrations > 1 mM (figure 3). The threshold level of lactate is 4 mM, because it is exported out of the cell until this value is reached. After exceeding 4 mM, lactate rises exponentially in the cytoplasm.
The values for lactate might be inaccurate, as these have been determined in whole blood samples. In the horse, RBCs function as ‘lactate sinks’. Up to 50% of the lactate may be in
RBC (Poso A.R. 1995; Vaihkonen L.K. 1999). Large differences of this characteristic have been reported interindividually (Vaihkonen L.K. 1999; Vaihkonen L.K. 1998). However, plasma lactate concentrations are sufficiently low for aerobic exercise.
Instead of collecting all urine during the experiment, we estimated the average glucose concentration in urine (UC) from the measurement of one sample collected at the end of the experiment. This approach can be justified by the fact that the UCs do not influence the M values to a great extent. Because our clamptime is longer than 2 hours (3-5 h), the total amount of urine is different from the assumption of the formula to calculate the UC. The increased production due to glucosuria should also be considered. Urine glucose concentrations were very different for each horse.
The formula used for the space correction is made for blood sample intervals of 10 minutes.
During exercise, blood samples were taken with a 5 minute interval.
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The amount of effort delivered by the horses has been monitored by an ECG device.
Comparable heart rates indicate that All horses exercised at approximately the same level
(figure 4).
Plasma potassium increases during exercise and decreases in the recovery phase. This is due to two opposite factors: (i) Release of potassium from contacting muscle cells and (ii) increased uptake of potassium by inactive muscle cells (Cleroux et al.
1987). During moderate exercise, release overrules uptake and plasma potassium rises.
The pH has been determined to monitor the acid/base balance. Average pH values are presented in figure 5. The pH shows a significant elevation during exercise. This can be explained by the release of potassium to the blood circulation with constant plasma sodium concentrations. Hydrogen ions exchange for potassium and H + rises in the muscle cells. In the blood circulation the pH decreases due to lower H + concentrations.
In conclusion, glucose metabolism more than doubles during trot compared to rest 1.
Furthermore, metabolism during walk 2 is significantly higher than during rest 1 and rest 2.
There is no significant difference between rest 1 vs. rest 2 and walk 1 vs. walk 2.
Further recommendations for research are quantification of glucose metabolism during longer moderate exercise or during high intensity and anaerobic exercise.
Footnotes a Treadmill, Kagra, Graber A, Fahrwagen, Switzerland b Televet 100 version 4.0, Veterinairy Telemetric ECG Systems, Frankfurt am Main, Germany c Volumetric pump, Model Argus 414, Argus Medical AG, Heimberg, Switzerland d ABL-605, Radiometer Copenhagen, Westlake, Ohio e Coat-A-Count TKIN2 836, Diagnostic Products Corp, Los Angeles, California f SPSS 16.0 for Windows, Chicago, Illinois
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APPENDIX
Minimum
Maximum
Average
SD
Table 1: mean metabolism values of all horses based on 3 measurements (μmol/kgBW/min)
Faldo
Princess
Nocturne
Nanda
Opium
Truffel
Rolindra
Rest 1
18.7±3.1
13.9±3.9
13.8±2.7
14.8±2.4
12.5±1.6
16.8±3.2
14.8±3.2
Walk 1
25.7±0.0
19.1±3.8
20.9±1.0
22.4±0.0
21.0±0.0
30.7±1.0
36.1±7.6
Trot
34.5±0.0
33.4±0.0
33.7±3.8
25.8±4.8
33.3±3.8
51.7±0.1
48.1±5.4
Walk 2
27.9±11.1
24.8±1.9
23.9±0.0
19.4±1.0
32.6±1.0
50.9±1.0
51.6±1.9
Rest 2
18.1±5.4
16.0±3.9
17.1±2.4
14.8±3.2
15.3±0.9
27.7±4.7
22.0±2.4
Table 2: Ranges and average M values (μmol/kg/min)
Rest 1
12.5
18.7
15.0
2.1
Walk 1
19.1
36.1
25.1
6.2
Trot
25.8
51.7
37.4
9.1
Walk 2
19.4
51.6
33.0
13.1
Rest 2
14.8
27.7
18.7
4.6
13

Mean metabolism with SD
15
10
5
0
30
25
20
50
45
40
35
1 2 3 gait
4
Figure 1: Glucose metabolism values with standard deviations
5
30
25
20
15
50
45
40
35
10
5
0
*
■
*
▲
1 2 3 gait
1-4 ■ p=0,002 4-5 ● p=0,022 3-5 ▲ p=0,001 1-3
* p=0,000
Figure 2: Mean metabolism profile
●
■
4
●
▲
5
14

Mean plasma lactate concentrations
140
120
100
80
60
40
20
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1 2 3 gait
4
Figure 3: Mean plasma lactate concentrations
Mean heart rate values
1 2 3 gait
Figure 4: Mean heart rates
4
5
5
15

Mean pH values
7.43
7.42
7.41
7.4
7.39
7.48
7.47
7.46
7.45
7.44
■ ●
▲
■
*
▲
●
*
1 2 3 gait
4 5
14 ■ p = 0.000 1-4 ● p = 0.039
Figure 5: Mean pH values
2-3 ▲ p = 0.003
3-5 * p = 0.000
3.9
3.7
3.5
3.3
3.1
2.9
● ■
▲
▲
□
■
▼
2.7
2.5
1 2 3 gait
4
1-3 ■ p = 0.000
3-4 º p = 0.000
1-5 ● p = 0.048
3-5 □ p = 0.000
2-3 ▲ p = 0.001
4-5 ▼ p = 0.006
Figure 6: Mean plasma potassium concentrations
□
▼
●
5
2-5 * p = 0.001
16

Mean plasma sodium concentrations
140
139
138
137
136
135
134
143
142
141
1 2 3 gait
4
Figure 7: Mean plasma sodium concentrations
Mean plasma chloride concentrations
105
104
103
102
101
100
99
98
97
96
95
1 2
Figure 8: Mean plasma chloride concentrations
3 gait
4
5
5
17

REFERENCES
Berger, C.M., Sharis, P.J., Bracy, D.P., Lacy, D.B. and Wasserman, D.H. (1994)
Sensitivity of exercise-induced increase in hepatic glucose production to glucose supply and demand. The American journal of physiology 267 , E411-
421.
Cleroux, J., Peterson, M. and Leenen, F.H. (1987) Exercise-induced hyperkalaemia: effects of beta-adrenoceptor blocker vs diuretic. British journal of clinical
pharmacology 24 , 225-229.
Coggan, A.R. (1991) Plasma glucose metabolism during exercise in humans. Sports
medicine (Auckland, N.Z 11 , 102-124.
DeFronzo, R.A., Tobin, J.D. and Andres, R. (1979) Glucose clamp technique: a method for quantifying insulin secretion and resistance. The American journal
of physiology 237 , E214-223.
Farris, J.W., Hinchcliff, K.W., McKeever, K.H. and Lamb, D.R. (1995) Glucose infusion increases maximal duration of prolonged treadmill exercise in
Standardbred horses. Equine Veterinairy Journal Supplements 18 , 357-361.
Freestone, J.F., Beadle, R., Shoemaker, K., Bessin, R.T., Wolfsheimer, K.J. and
Church, C. (1992) Improved insulin sensitivity in hyperinsulinaemic ponies through physical conditioning and controlled feed intake. Equine Vet J 24 ,
187-190.
Freestone, J.F., Wolfsheimer, K.J., Kamerling, S.G., Church, G., Hamra, J. and
Bagwell, C. (1991) Exercise induced hormonal and metabolic changes in
Thoroughbred horses: effects of conditioning and acepromazine. Equine Vet J
23 , 219-223.
Geor, R.J., Hinchcliff, K.W., McCutcheon, L.J. and Sams, R.A. (2000a) Epinephrine inhibits exogenous glucose utilization in exercising horses. J Appl Physiol 88 ,
1777-1790.
Geor, R.J., Hinchcliff, K.W. and Sams, R.A. (2000b) beta-adrenergic blockade augments glucose utilization in horses during graded exercise. J Appl Physiol
89 , 1086-1098.
Geor, R.J., Hinchcliff, K.W. and Sams, R.A. (2000c) Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise. J Appl Physiol 88 , 1765-1776.
Graaf-Roelfsema, E. (2007) Endocrinological and behavioural adaptations to experimentally induced physical stress in horses , Utrecht University, Utrecht.
Hinchcliff K.W., G., R.J. (2004) Integrative physiology of exercise , Saunders,
Edinburgh. p 6.
18

Hyyppa, S., Rasanen, L.A. and Poso, A.R. (1997) Resynthesis of glycogen in skeletal muscle from standardbred trotters after repeated bouts of exercise. American
journal of veterinary research 58 , 162-166.
Jenkins, A.B., Chisholm, D.J., James, D.E., Ho, K.Y. and Kraegen, E.W. (1985)
Exercise-induced hepatic glucose output is precisely sensitive to the rate of systemic glucose supply. Metabolism: clinical and experimental 34 , 431-436.
Poso A.R., L.H.J., Rasanen L.A. (1995) Distribution of lactate between red blood cells and plasma after exercise. Equine Veterinary Journal Supplements 18 .
Rijnen, K.E. and van der Kolk, J.H. (2003) Determination of reference range values indicative of glucose metabolism and insulin resistance by use of glucose clamp techniques in horses and ponies. American journal of veterinary
research 64 , 1260-1264.
Vaihkonen L.K., H.S., Poso A.R. (1999) Factors affecting accumulation of lactate in red blood cells. Equine Veterinary Journal Supplements 30 .
Vaihkonen L.K., P.A.R. (1998) Interindividual differences in total and carrier mediated lactate influx into red blood cells. Am J Physiol Reg Integr Comp
Physiol 274 . van der Kolk, J.H., Wensing, T., Kalsbeek, H.C. and Breukink, H.J. (1995)
Laboratory diagnosis of equine pituitary pars intermedia adenoma. Domestic
animal endocrinology 12 , 35-39.
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