Determination of maximal oxygen consumption in exercising pregnant sheep M. T. JONES, R. E. RAWSON, Department
of Physiology,
AND D. ROBERTSHAW
College of Veterinary
JONES, M.T.,R.E. RAWSON,AND D. ROBERTSHAW. Determioxygen consumption in exercising pregnant nation of maximal sheep. J. Appl. Physiol. 73(l): 234-239, 1992.-Previous work
with pregnant ewes has shown that acute bouts of exercise may cause changes in plasma hormone concentrations, blood flow distribution, and maternal and fetal temperatures. However, most of these studies do not quantify the chosen exercise intensity through measurement of oxygen consumption (VO,). Therefore the purpose of this study was to statistically model the VO, response of pregnant sheep to treadmill (TM) exercise to determine the exercise intensities (% maximal ~oJ of previous studies. Ewes with either single (n = 9) or twin (n = 5) fetuses were studied from 100 to 130 days of gestation. After l-2 wk of TM habituation, maximal TO, (vozm,) was determined by measurements of VO, (open flow-through method) and blood lactate concentration. VO, was measured as a function of TM incline (0, 3, 5, and 7”) and speed (0.8-3.4 m/s). VO, maxaveraged 57 t 7 (SD) ml min-’ kg-‘, and peak lactate concentration during Fxercise averaged 22 ~fr 2 mmol/l. The relationship between VO, (ml min-’ kg-l) and incline (INC) and speed (SP) [Vo, = O.‘IO(INC) + 13.95(SP) + l.O7(INC x SP) - 1.181 was linear (3 = 0.94). Our findings suggest that most previous research used exercise intensities ~60% vozrnax and indicate the need for further research that examines the effect of exercise during pregnancy at levels >60% vo2 m8x. l
l
l
l
blood lactate; gestation; metabolism; treadmill
a well-established model of pregnancy because it develops cardiovascular changes that are quite similar to those that take place in humans and it has a fetus of comparable size (26). This species also has been used to study maternal and fetal physiol .ogical responses to exercise during pregnancy. Previous work using pregnant sheep has shown that acute bouts of locomotory exercise cause reductions in blood flow to the uterus and placenta (5,9,18), increased maternal plasma catecholamines (24), and increas led maternal and fetal temperatures (5, 19), all of which may compromise fetal well- ,being. The majority of previous studies th .at examined the effects of exerci se on pregn .ant sheep reported the tread .mill . speed and incline used during the experiment but did not qu.antify the exercise - intensity through measurement of 0, consumption (Vo2) and determination of maximal 0, consumption (VO, max).Thus it is difficult to compare results from various studies. Bell et al. (5) determined the VO, of pregnant sheep during treadmill exercise from measurements of cardiac output and arteriovenous 0, content difference (indirect Fick method), but Vozmax of the sheep was not deterTHE EWE HAS BECOME
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0161-7567/92
$2.00 Copyright
Medicine,
CorneU University,
Ithaca,
New York 14853
mined. Lotgering and co-workers (18) also used the indirect Fick method to determine VO, in exercising pregnant sheep. Once a point was observed at which 00, reached a plateau, it was defined by the investigators as vo 2m8x,although no measurement of blood lactate concentration was made. VO, does not always reach a plateau at VO, maxbut also has been observed to either decrease or increase with an increase in work rate (30); therefore it is possible to define iTOZrnax inaccurately when VO, is the sole criterion for establishing VO, max. Lactic acid is produced by anaerobic glycolysis and increases in a curvilinear fashion with increasing work rate at exercise intensities >50% . vo 2max(3). During exercise at intensities below VO, max, blood lactate will rise initially and then remain unchanged or decrease as long as the work rate remains constant, but exercise at VO, m8xwill elicit a curvilinear increase in blood lactate concentration (3). Thus, for an accurate definition of VO, max, blood lactate should be measured in conjunction with 60,. The purpose of this study was to determine Vo2max in pregnant sheep through measurements of V.02 and blood lactate and then to statistically model the VO, response of pregnant sheep to treadmill exercise. The development of a regression equation for prediction of V02 from treadmill speed and incline will provide the basis for determining the exercise intensities of previous studies that have examined the effects of exercise on pregnant sheep. METHODS
AnimaZ care and handling. A total of 14 untrained pregnant cross-bred Ramboiullet Columbia ewes (Ovis aries) carrying single (n = 9) or twin (n = 5) fetuses were studied from 100 to 130 days gestation. The gestation period of sheep is -146 days. Sheep were 90 days gestation when they arrived at the laboratory. They were housed in adjacent individual pens in a room with the temperature maintained at 20 t 2°C (SD), and day length was divided into 14 h of light and 10 h of darkness. Sheep with single fetuses were fed a diet of pelleted high-energy hay-grain mixture (1120 g per sheep per day; Early Market Lamb Pellets, Agway, Syracuse, NY) supplemented with alfalfa hay (400 g per sheep per day), whereas ewes with twin fetuses were fed 1500 g of pellets and 500 g of hay each day. Water was provided ad libitum. Sheep, like any ruminant, produce methane (CH,) as a
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Physiological
Society
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EXERCISING
tJo2 DURING
result of the bacterial fermentation of cellulose, with the largest amount of CH, produced after feeding (6). Sheep can produce up to 50 liters of CH, in a 24-h period, but the amount produced is dependent on the type of diet and the amount of food consumed (6). Production of CH, was of concern in this study because we measured 60, with a zirconium oxide 0, analyzer (Applied Electrochemistry S-3A/l) that operated at a temperature of 75OOC. Because CH, is combustible and consumes 0, upon burning, CH, in the animal’s respiratory gases can result in a greater deflection in expired oxygen (FEN,) than the animal achieved (Ametek Instruction Manual, Thermox Instruments Division). With the open flowthrough system, TO, is calculated on the basis of the deflection in FEN,; thus a greater deflection can result in a falsely elevated VO,. To decrease CH, production when VO, was being measured, sheep were not fed until after exercise. There was approximately a 20-h.period between the end of feeding and measurement of VO,. Experimental protocol. Experiments were conducted in accordance with the Guiding Principles in the Care and Use of Animals of the National Institutes of Health. The research protocol was approved by the Institutional Animal Care and Use Committee at Cornell University. Sheep were put into individual pens and allowed 10 days to acclimate to their surroundings. At 100 days gestation, the animals were put on the treadmill for the first time. Sheep ran toward a reflection of themselves in a mirror placed at the end of the treadmill. A l- to 2-wk habituation period followed in which sheep ran on the treadmill frequently and became accustomed to wearing a loose-fitting face mask. After the habituation period, experiments were conducted in which steady-state To2 and CO, production (VCO,) were measured during a series of progressive exercise tests. During each test, the speed was held constant and the incline was increased. The sheep ran -5 min at each incline followed by a 2-min walk (0.8 m/s). Each sheep performed several progressive exercise tests to permit collection of VO, data at several speeds (0.8, 1.1,1.4,1.8,2.2,2.6,3.0,3.2, and 3.4 m/s) and inclines (0, 3, 5, and 7”). Next, experiments were conducted in which animals ran solely at the work rate at which To2 m8xwas presumed to have been attained during the progressive exercise tests. They were also tested at a work rate above and below that of the presumed Vo2 max.Blood samples were drawn from a catheter that previously had been surgically implanted in the jugular vein (Tygon; 0.40 mm ID, 0.70 mm OD). The exercise bout that elicited a curvilinear rise in blood lactate but no further increase in Tjoz was defined as the animal’s VO, max. Measurement of Vo2. Rates of Vog and VCO, were measured concurrently by use of the open-flow method described by Tucker (31) and subsequently modified by Fedak et al. (12). Briefly, expired gases were collected by drawing ambient air past the sheep’s face via a loose-fitting face mask at a rate of 720 l/min by a vacuum pump (Eureka), which exhausted outside of the laboratory. A venturi tube (model 182, BIF, West Warwick, RI) mounted within polyvinylchloride tubing coupled to a
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differential pressure transducer (model P305, Validyne Engineering, Northridge, CA) was used as an indicator of the flow rate through the system. An aliquot of gas was continuously drawn from the mainstream and passed through an H,O absorber (Drierite), and 0, and CO, concentrations were measured [Applied Electrochemistry CD-3A (CO,)]. At l-s intervals, output from gas analyzers was sampled, analyzed, and displayed in real-time by a personal computer (Asyst Software Tech., Rochester, NY). Complete collection of respiratory gases was confirmed when measured 60, values were not significantly different at flow rates 25% lower and 25% higher than the flow rate used during the experiments. The entire system delivered a 95% response in 35 s for a 1% reduction in the 0, content of the air, which was a larger decrement in 0, content than was observed during the actual experiments. The same time response was measured for the determination of CO,. The procedures of Fedak et al. (12) were used to calibrate the open-flow system (OFS) with the use of an N, dilution and CO, infusion procedure. Before and after the experiment, N, and then CO, were bled into the system at known flow rates (15.4 and 3.5 I/min ATPD, respectively), whereas total flow through the system was held at the same rate as during the experiment. Flow rates of N, and CO, during calibration were measured using high:precision flow meters (Matheson Gas, accuracy +l%). VO, was calculated using the formula . vo 2 = 0.26486#~,) - [1.26486(Vc0,) (FI,, - FEN,)] l
where vN2 is the nitrogen equivalent of the fractional 0, concentration, VCO, is the rate of CO, production, FI,, is the fractional concentration of 0, entering the mask, and FEN, is the fractional concentration of 0, leaving the mask. This equation was derived from equations previously published by Fedak and co-workers (12). The coefficients of variation for three repeated VO, measurements in each of three pregnant sheep during treadmill exercise at 1.1 m/s at O” and 1.4 m/sat 7’ averaged 4 and 6%, respectively. Accuracy of the OFS. To check the accuracy of our OFS, we made a comparison between ir0, values measured in exercising human subjects with our system and those measured with a respiratory valve system (RVS). Human subjects were used because it is extremely difficult to measure 00, in animals with an RVS. Animals cannot be trained to satisfactorily hold a respiratory valve in their mouths, and face masks with respiratory valves tend to have a large amount of dead air space (21). A low resistance (dead space 70 ml) respiratory valve was used by the subjects. The volume of inspired air was measured using a calibrated gas meter (RAM-9200, Rayfield Equipment). The subject’s expired air was sampled from a 6-liter mixing chamber, and 0, (Applied Electrochemistry S-3A) and CO, concentration (Beckman LB-2 CO, analyzer) were measured. Minute ventilation, VO,, VCO,, and respiratory exchange ratio values were monitored continuously during exercise by a computerized data acquisition system (REP-200B, Rayfield Equipment) interfaced with the gas meter and analyzers. Output from the interface was sent to a microcomputer that averaged data and then calculated values over 30-s inter-
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TABLE 1. Oxygen consumption measured stationary cycling at the same work rate by use of OFS and RVS Subj
Work Rate, W
DMB DMB DMB THR THR THR
105 175 140 210 245
Vo, DURING
60
during
OFS, ml min-’ . kg-’
RVS, ml min-’ kg-’
% Difference
18 23 34 23 31 37
19 24 35 23 31 37
4 3 0 0 0
l
70
OFS, open-flow system; RVS, respiratory
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l
T
6
0
value system.
0
I I
‘,
PRE
I I
I I
I I
1
0
3
5
7
Treadmill
vals. Before each exercise trial, the 0, and CO, analyzers were calibrated with a known gas mixture, analyzed previously by a mass spectrometer. CH, production. As was mentioned previously, ruminants produce CH, as a result of fermentation processes that take place in the gut. Although a 20-h period elapsed between feeding and exercise, we wanted to be certain that little or no CH, was being produced during exercise. Experiments were conducted in a randomized order on two sheep exercising at a speed of 2.2 m/s on a 3’ incline for 10 min. One sheep was fed its daily food ration 2 h before exercise and the other sheep had not eaten for ~20 h. The same procedure was repeated for 4 d on the same two sheep. An aliquot of gas was continuously drawn from the main stream into an infrared analyzer (model 865, Beckman Instruments) calibrated for measurement of CH, concentration. VO, was measured concurrently. No CH, was detected by the infrared analyzer when the unfed sheep were exercising. However, CH, yas produced when sheep exercised 2 h after feeding, and Vo, averaged 16 t 3% (SD) higher in exercising fed sheep. From this information, we concluded that Tjo, measured during exercise is not affected when the sheep are fed 20 h before exercise. Measurement of blood lactate. All blood samples were taken from the jugular vein while the animal was on the treadmill at preexercise, after 1 and 5 min of walking, and at each minute during exercise. Samples were drawn into syringes, immediately emptied into tubes (82 X 10.25 mm) containing sodium fluoride and potassium oxalate (Vacutainer Systems, Becton Dickinson, Rutherford, NJ), and placed in ice. After completion of the experiment, blood lactate concentration was determined using a lactate analyzer (model 2300 Stat, Yellow Springs Instrument, Yellow Springs, OH). Data analysis. Blood lactate concentration and Vo2 m8x TABLE 2. Maximal lactate concentration for pregnant ewes
FIG. 1. Oxygen consumption (\io,) values for 1 pregnant sheep during progressive exercise test. PRE, standing on treadmill before exercise. Treadmill speed was held constant at 2.6 m/s while grade was increased after 6 min of exercise at each incline. Animal exercised at 0, 3, 5, and 7’.
of ewes with single fetuses and those carrying twins were compared using a t test. VO, was plotted as a function of treadmill speed and incline. Three multiple-regression analyses were performed to determine the slopes and regression coefficients for data collected from the sheep with single fetuses, the sheep carrying twins, and for all sheep combined into one group. An F test was used to compare the residual sums of squares from the three regression analyses (32). An a! level of 0.05 was used for all comparisons. Statistical analyses were performed on a personal computer with SAS software (SAS Institute, Cary, NC). RESULTS Accuracy of the OFS. To compare VO, values measured from the OFS and RVS, two human subjects performed two submaximal cycling (Monark cycle ergometer) trials 48 h apart. The cycle ergometer was calibrated before each trial. On one occasion 60, was measured using the OFS, and on the other day it was measured by the RVS. The results from the two tests are presented in Table 1.
E‘T s-r
15 -
t
oxygen consumption and peak blood elicited by maximal exercise PRE
Sheep
Twins Singles Combined
Incline
(degrees)
~%nax,
ml min-’ l
l
kg-’
53k6 (4) 6Ok6 (8) 57t7 (12)
Values are means _+ SD for no. of sheep in parentheses. maximal oxygen IV consumntion. a
vo2 -,
WS
Ml
M2
Ex3
Ex4
MS
Time of Blood Sample (minutes)
Peak [Lactate], mmol/l 23k-2 (3) 2123 (7) 22t2 (10)
Wl
. FIG. 2. Blood lactate values during exercise at maximal VO, ) for same pregnant sheep as in Fig. 1. PRE, standing on treadmil12byfore exercise. Wl and W5, samples taken after minutes 1 and 5 of walking at 0.8 m/s on level treadmill. Exl-Ex5, samples taken after each minute of exercise at speed (2.6 m/s) and incline (7”) that elicited VO 2max* wo
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EXERCISING
\io,
DURING
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SPEED Im-sec’ll FIG. 3. Predicted TO, vs. treadmill speed by incline response surface according to equation 13.95 (speed) + 1.07 (incline X speed) - 1.18.
There was a mean difference of 2.2 t 1% (SE) between VO, values measured with the OFS and RVS. Thus it was concluded our. OFS provided reasonably accurate measurements of VO, in exercising humans. The VO, values measured with the OFS were always lower than, or the same as, those measured using the RVS. These results are consistent with those previously reported by Norton and co-Yorkers (21), who observed the OFS to underestimate VO, by a mean of 6.2% when compared with the RVS. vo2 and blood lactate concentration. The 14 sheep used in this study ran well and needed little or no encouragement to perform the majority of treadmill speeds and TABLE
h,
= 0.70 (incline)
+
inclines. However, two sheep were not able to exercise at a work load that elicited VO, max.VO, values for 12 of the 14 sheep reached a plateau after an increase in work load. Because the vascular catheters of two sheep failed to remain patent during the experiment, blood lactate values were obtained from only 10 of the 12 sheep that were able to. exercise at an intensity that elicited VO, m8x. The vo 2maxvalues and peak exercise lactate concentrations are listed in Table 2 for both groups of sheep. There was no significant difference in bo2max or peak lactate between the ewes carrying a single fetus and those carrying twins. Thus the data from the two groups were combined. Body mass of the sheep at 129 t 1 days gestation aver-
3. Responses of pregnant sheep to treadmill exercise Reference
Bell et al. (4) Bell et al. (5) Chandler and Bell (7)
Chandler et al. (8) Clapp (9) Curet et al. (10) Emmanouilides et al. (11) Leury et al. (16) Leury et al. (17) Lotgering et al. (18)
Lotgering et al. (19) Orr et al. (23) Palmer et al. (24)
Speed, m/s
Incline,
0.70 0.70 0.40 0.70 0.70 0.65 1.34
10 10 10 10 10 0
0.90 1.12 0.70 0.70 0.57 1.65 0.57 1.65 1.34 1.12
Time, min
Estimated VO, , ml. min-’ . kg-’
Estimated %Vo2 -
60 30 60 60 60 60-180 45-60 30-60
23 23 16 23 23 8 30 11 14 23 23 7 47 7 47 30 14
41 41 28 41 41 14 52 20 25 41 41 12 82 12 82 52 25
degrees
5.7
0 0 10
60 60
10 0 10 0 10
10 10 10 10
5.7
34 45
0
Estimated O2 consumption (VOW) was determined on the basis of the equation 1.18, Estimated %VO, mu was determined from the estimated VO, and the mean
VO, 90,
= 0.70 (incline) + 13.95 (speed) + 1.07 (incline x speed) mar (57 ml. min-’ kg-‘) measured in the present study. l
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\io,
DURING
aged 54 t 10 kg for ewes with one fetus and 63 t 10 kg for those carrying twins. When the two groups were combined, average body weight was 57 t 10 kg. VO, values for one sheep during a progressive exercise test are shown in Fig. 1. In this test, the speed was held constant at 2.6 m/s while the grade was increased approximately every 6 min. Because the animals’ VO, did not increase when the treadmill incline was changed from 5 to 7”, it was assumed VO, maxwas attained. Thus a second test was conducted at 2.6 m/s and 7O in which both To2 and blood lactate were measured. Blood lactate concentrations during exercise at VO, IIlLu are shown in Fig. 2 for the same sheep as in Fig. 1. Values represented are from preexercise, walking at 0.8 m/s on a level treadmill, and running at 2.6 m/s on a 7’ incline. Actual vo2 values (n = 436) were collected from 14 pregnant sheep exercising at a range of speeds and inclines. Initially, the VO, data were grouped according to ewes with single fetuses, ewes with twins, and all pregnant ewes, and three separate multiple regression analyses were performed on the data. Because no statistical difference was observed among the three multiple regression equations, all of the Vo2 data were combined into one group. The linear (? = 0.94) relationship between 60, (ml mine1 . kg-l) and treadmill incline and speed is shown in Fig. 3 [Oo, = 0.70 (incline) + 13.95 (speed) + 1.07 (incline X speed) - 1.181. l
DISCUSSION
The purpose of this study was to determine v02max in pregnant sheep through measurements of VO, and blood lactate concentration during treadmill exercise. Blood lactate concentration at VOW-. Of the 14 sheep used in the study, 12 were able to attain Vo2mar as indicated by the fact that an increase in work rate was accompanied by a curvilinear increase in blood lactate (n = 10) but no change in VO,. At VO, max,blood lactate increased - 15 times over preexercise values. An increase of similar magnitude has also been observed in rats (13), pigs (1, 20), and horses (15). The peak blood lactate concentration observed during exercise in the present study is comparable with data obtained previously from several mammalian species exercising at vo2 mm(27,29). In two studies, blood lactate was measured in pregnant sheep before as well as during exercise (4, 7), and preexercise blood lactate concentration was 1.0 mmol/l, followed by an increase to 2.5 mmol/l during exercise at 0.7 m/s on a 10’ grade. This exercise intensity can be classified as mild to moderate on the basis of an estimated vo2 of 23 ml min-l kg-l, derived from the regression equation developed in this study. Determination of yo2 -. The sheep in the present study had an average VO, maxof 57 t 7 ml min-l kg-‘, an increase - H-fold above preexercise 60,. An increase of this magnitude is consistent with measurements of . vo 2m8xmade on pigs (l), steers (14), and a variety of wild mammals (29). Only two previous studies (18,29) reported i702 m8xvalues for sheep. Taylor et al. (29) used an open-flow method of respirometry and measured a VO, m8xof 46 t 6 (SD) ml min-l kg-l in two nonpregnant African sheep l
l
l
l
l
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with an average body weight of 23 t 0.2 kg. Peak blood lactate averaged 23 mmol/l. These sheep attained VO, max on a level treadmill at an average speed of 3.7 m/s. Our regression equation predicts a VO, value of 50 ml. min-l kg-’ at a speed of 3.7 m/s, which is within one standard deviation of the mean V02max (57 t 7) determined for sheep in our study. Although our data are similar to the data of Taylor et al. (29), another study (18) that determined VO 2maxin sheep is not consistent with our findings. Lotgering et al. (18) determined 60, in exercising pregnant sheep from measurements of cardiac output and arteriovenous 0, content difference and reported VO, m8xto be 32 t 10 (SD) ml min. kg-l, which was an increase of approximately six times over rest. Sheep in the present study experienced an &fold increase in VO, from preexercise to V02 mm.Lotgering et al. (18) defined TO, max as the point at which VO, reached a plateau, but no measurements of blood lactate concentration were made. At the speed (1.65 m/s) and incline (10”) at which sheep in the study of Lotgering et al. (18) attained VO 2mm, our regression equation would predict a vo2 of 47 ml mine1 . kg-‘, which is -32% greater than the measured value of 32 ml min-l kg-l. Venous catheter placement may explain some of the difference. Lotgering and co-workers determined VO, from measurements of cardiac output (thermodilution) and differences in 0, content of arterial (common internal iliac artery) and venous (right atrium) blood. We previously have made measurements of VO, in exercising sheep using both respirometry and the indirect Fick method concurrently and observed, on average, 27% higher measurements of Vo2 when comparing values from respirometry with those obtained from the Fick method (25). Because the catheter was in the right atrium rather than the pulmonary artery, it is possible that a representative sample of mixed venous blood was not obtained in our study (25) or the one by Lotgering et al. (18). Consistent with this hypothesis is the fact that measurements of irO, made in exercising ponies with the use of both respirometry and the indirect Fick method (pulmonary arterial blood) yielded less than a 1% difference between VO, values (28). Relationship of Vo2 to treadmill speed and incline. Our findings demonstrate that the relationship of Vo2 to treadmill speed and incline is a linear one. This linear relationship between Vo2 and work rate has been shown previously in rats (2, 22), pigs (20), humans (3), and a variety of other mammals (27). Themdevelopment of a regression equation for prediction of VO, from treadmill speed and incline has permitted us to estimate the relative exercise intensities of other studies that used exercising pregnant sheep (Table 3). We would estimate on the basis of our regression equation that exercise intensity varied from mild to moderate in the majority of these studies. In fact, only two studies (18,19) appeared to use exercise intensities > 60% i702 max.Not only have most of the animal studies selected mild or moderate exercise intensities, but the same is also true of studies in which pregnant humans have served as subjects. Clearly, research that examines the effects of high-intensity exercise during pregnancy is needed before conclusions of its advisability can be drawn . l
l
l
l
l
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EXERCISING
Vo, DURING
In conclusion, VO, and blood lactate in sheep during exercise respond similarly to those of other animals, and most pregnant sheep- can be taught to exercise at an intensity that elicits VO, max. The authors thank Drs. Peter W. Nathanielsz and Gary Sforzo for contributions to the study and Lute Guanzini and Shawnee Riplog for technical assistance. This research was supported by a grant from the American Heart Association, New York Affiliate, and by National Institute of Child Health and Human Development Grant HD-21350. Address for reprint requests: M. T. Jones, Dept. of Physiology, D-123 Schurman Hall, Cornell University, Ithaca, NY 14853-6401. Received 28 May 1991; accepted in final form 28 January
of rats running
up and down an incline. J. Appl. Phys-
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Textbook of Work Physiology (3rd ed.). New York: McGraw-Hill, 1986. BEU, A. W., J. M. BASSETT, K. D. CHANDLER, AND R. C. BOSTON. Fetal and maternal endocrine responses to exercise in the pregnant ewe. J. Deu. Physiol. 5: 129-141, 1983. BELL, A. W., J. R. S. HALES, A. A. FAWCETT, AND R. B. KING. Effects of exercise and heat stress on regional blood flow in pregnant sheep. J. Appl. Physiol. 60: 1759-1764,1986. (2nd ed.). BLAXTER, K. L. The Energy Metabolism of Ruminants London: Hutchinson Scientific and Technical, 1967. CHANDLER, K. D., AND A. W. BELL. Effects of maternal exercise on fetal and maternal respiration and nutrient metabolism in the pregnant ewe. J. Deu. Physiol. 3: 161-176, 1981. CHANDLER, K. D., B. J. LEURY, A. R. BIRD, AND A. W. BELL. Effects of undernutrition and exercise during late pregnancy on uterine, fetal and uteroplacental metabolism in the ewe. Br. J. Nutr. 53:
625-635,1985. 9. CLAPP, J. F. Acute exercise stress in the pregnant ewe. Am. J. Obstet. Gynecol. 136: 489-494, 1980. 10. CURET, L. B., J. A. ORR, J. H. G. RANKIN, AND T. UNGERER. Effect
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cal changes in thoroughbred racehorses following submaximal and maximal exercise. In: Equine Exercise Physiology, edited by D. H. Snow, S. G. B. Persson, and R. J. Rose. Cambridge, UK: Granta Editions, 1983, p. 408-415. LEURY, B. J., A. R. BIRD, K. D. CHANDLER, AND A. W. BELL. Glucose partitioning in the pregnant ewe: effects of undernutrition and exercise. Br. J. Nutr. 64: 449-462, 1990. LEIJRY, B. J., K. D. CHANDLER, A. R. BIRD, AND A. W. BELL. Effects of maternal undernutrition and exercise on glucose kinetics in fetal sheep. Br. J. Nutr. 64: 463-472, 1990. LOTGERING, F. K., R. D. GILBERT, AND L. D. LONGO. Exercise responses in pregnant sheep: oxygen consumption, uterine blood flow, and blood volume. J. AppZ. Physiol. 55: 834-841, 1983. LOTGERING, F. K., R. D. GILBERT, AND L. D. LONGO. Exercise responses in pregnant sheep: blood gases, temperatures, and fetal cardiovascular system. J. AppZ. Physiol. 55: 842-850, 1983. NORTON, K. I., M. D. DELP, M. T. JONES, C. DUAN, D. R. DENGEL, of blood flow during exercise AND R. B. ARMSTRONG. Distribution after blood volume expansion in swine. J. Appl. Physiol. 69: 1578-
1586,199O. 21. NORTON, K. I., M. D. DELP, W. K. PRUSACZYK, AND R. B. ARMSTRONG. A comparison of methods used to determine 60, of exercising humans and animals. Med. Sci. Sports Exercise 21: 480-486, 1989. 22. NORTON, K. I., M. T. JONES, AND R. B. ARMSTRONG. Oxygen con-
sumption and distribution of blood flow in rats climbing a laddermill. J. Appl. Physiol. 68: 241-247, 1990. 23. ORR, J., T. UNGERER, J. WILL, K. WERNICKE, AND L. B. CURET. Effect of exercise stress on carotid, uterine, and iliac blood flow in pregnant and nonpregnant ewes. Am. J. Obstet. Gynecol. 114: 213271,1972. 24. PALMER, S. M., G. K. OAKES, J. A. CHAMPION, D. A. FISHER, AND C. K. HOBEL. Catecholamine physiology in the ovine fetus. Am. J. Obstet. Gynecol. 149: 426-434, 1984. 25. RAWSON, R. E., M. T. JONES, K. COLLIER, AND D. ROBERTSHAW. A
comparison of two methods of measuring oxygen consumption in sheep (Abstract). FASEB J. 5: A765, 1991. 26. ROSENFELD, C. R. Changes in uterine blood flow during pregnancy. In: The Uterine Circulation, edited by C. R. Rosenfeld. Ithaca, NY: Perinatology, 1989, p. 135-156. 27. SEEHERMAN, H. J., C. R. TAYLOR, G. M. 0. MALOIY, AND R. B. respiratory system. II. ARMSTRONG. Design of the mammalian Measuring maximum aerobic capacity. Respir. Physiol. 44: 11-23, 1981. 28. STANEK, K. A., F. J. NAGLE, G. E. BISGARD, AND W. C. BYRNES.
29.
30.
31. 32.
Effect of hyperoxia on oxygen consumption in exercising ponies. J. AppZ. Physiol. 46: 1115-1118, 1979. TAYLOR, C. R., G. M. 0. MALOIY, E. R. WEIBEL, V. A. LANGMAN, J. M. 2. KAMAU, H. J. SEEHERMAN, AND N. C. HEGLUND. Design of the mammalian respiratory system. III. Scaling maximum aerobic capacity to body mass: wild and domestic mammals. Respir. Physiol. 44: 25-37, 1980. TAYLOR, H. L., E. R. BUSKIRK, AND A. HENSCHEL. Maximal oxygen intake as an objective measure of cardiorespiratory performance. J. Appl. Physiol. 8: 73-80, 1955. TIJCKER, V. A. Respiratory exchange and evaporative water loss in the flying budgerigar. J. Exp. BioZ. 48: 67-87, 1968. ZAR, J. H. Biostatistical AnaZysis (2nd ed.). Englewood Cliffs, NJ: Prentice-Hall, 1984.
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of Physiology,
AND D. ROBERTSHAW
College of Veterinary
JONES, M.T.,R.E. RAWSON,AND D. ROBERTSHAW. Determioxygen consumption in exercising pregnant nation of maximal sheep. J. Appl. Physiol. 73(l): 234-239, 1992.-Previous work
with pregnant ewes has shown that acute bouts of exercise may cause changes in plasma hormone concentrations, blood flow distribution, and maternal and fetal temperatures. However, most of these studies do not quantify the chosen exercise intensity through measurement of oxygen consumption (VO,). Therefore the purpose of this study was to statistically model the VO, response of pregnant sheep to treadmill (TM) exercise to determine the exercise intensities (% maximal ~oJ of previous studies. Ewes with either single (n = 9) or twin (n = 5) fetuses were studied from 100 to 130 days of gestation. After l-2 wk of TM habituation, maximal TO, (vozm,) was determined by measurements of VO, (open flow-through method) and blood lactate concentration. VO, was measured as a function of TM incline (0, 3, 5, and 7”) and speed (0.8-3.4 m/s). VO, maxaveraged 57 t 7 (SD) ml min-’ kg-‘, and peak lactate concentration during Fxercise averaged 22 ~fr 2 mmol/l. The relationship between VO, (ml min-’ kg-l) and incline (INC) and speed (SP) [Vo, = O.‘IO(INC) + 13.95(SP) + l.O7(INC x SP) - 1.181 was linear (3 = 0.94). Our findings suggest that most previous research used exercise intensities ~60% vozrnax and indicate the need for further research that examines the effect of exercise during pregnancy at levels >60% vo2 m8x. l
l
l
l
blood lactate; gestation; metabolism; treadmill
a well-established model of pregnancy because it develops cardiovascular changes that are quite similar to those that take place in humans and it has a fetus of comparable size (26). This species also has been used to study maternal and fetal physiol .ogical responses to exercise during pregnancy. Previous work using pregnant sheep has shown that acute bouts of locomotory exercise cause reductions in blood flow to the uterus and placenta (5,9,18), increased maternal plasma catecholamines (24), and increas led maternal and fetal temperatures (5, 19), all of which may compromise fetal well- ,being. The majority of previous studies th .at examined the effects of exerci se on pregn .ant sheep reported the tread .mill . speed and incline used during the experiment but did not qu.antify the exercise - intensity through measurement of 0, consumption (Vo2) and determination of maximal 0, consumption (VO, max).Thus it is difficult to compare results from various studies. Bell et al. (5) determined the VO, of pregnant sheep during treadmill exercise from measurements of cardiac output and arteriovenous 0, content difference (indirect Fick method), but Vozmax of the sheep was not deterTHE EWE HAS BECOME
234
0161-7567/92
$2.00 Copyright
Medicine,
CorneU University,
Ithaca,
New York 14853
mined. Lotgering and co-workers (18) also used the indirect Fick method to determine VO, in exercising pregnant sheep. Once a point was observed at which 00, reached a plateau, it was defined by the investigators as vo 2m8x,although no measurement of blood lactate concentration was made. VO, does not always reach a plateau at VO, maxbut also has been observed to either decrease or increase with an increase in work rate (30); therefore it is possible to define iTOZrnax inaccurately when VO, is the sole criterion for establishing VO, max. Lactic acid is produced by anaerobic glycolysis and increases in a curvilinear fashion with increasing work rate at exercise intensities >50% . vo 2max(3). During exercise at intensities below VO, max, blood lactate will rise initially and then remain unchanged or decrease as long as the work rate remains constant, but exercise at VO, m8xwill elicit a curvilinear increase in blood lactate concentration (3). Thus, for an accurate definition of VO, max, blood lactate should be measured in conjunction with 60,. The purpose of this study was to determine Vo2max in pregnant sheep through measurements of V.02 and blood lactate and then to statistically model the VO, response of pregnant sheep to treadmill exercise. The development of a regression equation for prediction of V02 from treadmill speed and incline will provide the basis for determining the exercise intensities of previous studies that have examined the effects of exercise on pregnant sheep. METHODS
AnimaZ care and handling. A total of 14 untrained pregnant cross-bred Ramboiullet Columbia ewes (Ovis aries) carrying single (n = 9) or twin (n = 5) fetuses were studied from 100 to 130 days gestation. The gestation period of sheep is -146 days. Sheep were 90 days gestation when they arrived at the laboratory. They were housed in adjacent individual pens in a room with the temperature maintained at 20 t 2°C (SD), and day length was divided into 14 h of light and 10 h of darkness. Sheep with single fetuses were fed a diet of pelleted high-energy hay-grain mixture (1120 g per sheep per day; Early Market Lamb Pellets, Agway, Syracuse, NY) supplemented with alfalfa hay (400 g per sheep per day), whereas ewes with twin fetuses were fed 1500 g of pellets and 500 g of hay each day. Water was provided ad libitum. Sheep, like any ruminant, produce methane (CH,) as a
0 1992 the American
Physiological
Society
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EXERCISING
tJo2 DURING
result of the bacterial fermentation of cellulose, with the largest amount of CH, produced after feeding (6). Sheep can produce up to 50 liters of CH, in a 24-h period, but the amount produced is dependent on the type of diet and the amount of food consumed (6). Production of CH, was of concern in this study because we measured 60, with a zirconium oxide 0, analyzer (Applied Electrochemistry S-3A/l) that operated at a temperature of 75OOC. Because CH, is combustible and consumes 0, upon burning, CH, in the animal’s respiratory gases can result in a greater deflection in expired oxygen (FEN,) than the animal achieved (Ametek Instruction Manual, Thermox Instruments Division). With the open flowthrough system, TO, is calculated on the basis of the deflection in FEN,; thus a greater deflection can result in a falsely elevated VO,. To decrease CH, production when VO, was being measured, sheep were not fed until after exercise. There was approximately a 20-h.period between the end of feeding and measurement of VO,. Experimental protocol. Experiments were conducted in accordance with the Guiding Principles in the Care and Use of Animals of the National Institutes of Health. The research protocol was approved by the Institutional Animal Care and Use Committee at Cornell University. Sheep were put into individual pens and allowed 10 days to acclimate to their surroundings. At 100 days gestation, the animals were put on the treadmill for the first time. Sheep ran toward a reflection of themselves in a mirror placed at the end of the treadmill. A l- to 2-wk habituation period followed in which sheep ran on the treadmill frequently and became accustomed to wearing a loose-fitting face mask. After the habituation period, experiments were conducted in which steady-state To2 and CO, production (VCO,) were measured during a series of progressive exercise tests. During each test, the speed was held constant and the incline was increased. The sheep ran -5 min at each incline followed by a 2-min walk (0.8 m/s). Each sheep performed several progressive exercise tests to permit collection of VO, data at several speeds (0.8, 1.1,1.4,1.8,2.2,2.6,3.0,3.2, and 3.4 m/s) and inclines (0, 3, 5, and 7”). Next, experiments were conducted in which animals ran solely at the work rate at which To2 m8xwas presumed to have been attained during the progressive exercise tests. They were also tested at a work rate above and below that of the presumed Vo2 max.Blood samples were drawn from a catheter that previously had been surgically implanted in the jugular vein (Tygon; 0.40 mm ID, 0.70 mm OD). The exercise bout that elicited a curvilinear rise in blood lactate but no further increase in Tjoz was defined as the animal’s VO, max. Measurement of Vo2. Rates of Vog and VCO, were measured concurrently by use of the open-flow method described by Tucker (31) and subsequently modified by Fedak et al. (12). Briefly, expired gases were collected by drawing ambient air past the sheep’s face via a loose-fitting face mask at a rate of 720 l/min by a vacuum pump (Eureka), which exhausted outside of the laboratory. A venturi tube (model 182, BIF, West Warwick, RI) mounted within polyvinylchloride tubing coupled to a
235
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differential pressure transducer (model P305, Validyne Engineering, Northridge, CA) was used as an indicator of the flow rate through the system. An aliquot of gas was continuously drawn from the mainstream and passed through an H,O absorber (Drierite), and 0, and CO, concentrations were measured [Applied Electrochemistry CD-3A (CO,)]. At l-s intervals, output from gas analyzers was sampled, analyzed, and displayed in real-time by a personal computer (Asyst Software Tech., Rochester, NY). Complete collection of respiratory gases was confirmed when measured 60, values were not significantly different at flow rates 25% lower and 25% higher than the flow rate used during the experiments. The entire system delivered a 95% response in 35 s for a 1% reduction in the 0, content of the air, which was a larger decrement in 0, content than was observed during the actual experiments. The same time response was measured for the determination of CO,. The procedures of Fedak et al. (12) were used to calibrate the open-flow system (OFS) with the use of an N, dilution and CO, infusion procedure. Before and after the experiment, N, and then CO, were bled into the system at known flow rates (15.4 and 3.5 I/min ATPD, respectively), whereas total flow through the system was held at the same rate as during the experiment. Flow rates of N, and CO, during calibration were measured using high:precision flow meters (Matheson Gas, accuracy +l%). VO, was calculated using the formula . vo 2 = 0.26486#~,) - [1.26486(Vc0,) (FI,, - FEN,)] l
where vN2 is the nitrogen equivalent of the fractional 0, concentration, VCO, is the rate of CO, production, FI,, is the fractional concentration of 0, entering the mask, and FEN, is the fractional concentration of 0, leaving the mask. This equation was derived from equations previously published by Fedak and co-workers (12). The coefficients of variation for three repeated VO, measurements in each of three pregnant sheep during treadmill exercise at 1.1 m/s at O” and 1.4 m/sat 7’ averaged 4 and 6%, respectively. Accuracy of the OFS. To check the accuracy of our OFS, we made a comparison between ir0, values measured in exercising human subjects with our system and those measured with a respiratory valve system (RVS). Human subjects were used because it is extremely difficult to measure 00, in animals with an RVS. Animals cannot be trained to satisfactorily hold a respiratory valve in their mouths, and face masks with respiratory valves tend to have a large amount of dead air space (21). A low resistance (dead space 70 ml) respiratory valve was used by the subjects. The volume of inspired air was measured using a calibrated gas meter (RAM-9200, Rayfield Equipment). The subject’s expired air was sampled from a 6-liter mixing chamber, and 0, (Applied Electrochemistry S-3A) and CO, concentration (Beckman LB-2 CO, analyzer) were measured. Minute ventilation, VO,, VCO,, and respiratory exchange ratio values were monitored continuously during exercise by a computerized data acquisition system (REP-200B, Rayfield Equipment) interfaced with the gas meter and analyzers. Output from the interface was sent to a microcomputer that averaged data and then calculated values over 30-s inter-
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EXERCISING
TABLE 1. Oxygen consumption measured stationary cycling at the same work rate by use of OFS and RVS Subj
Work Rate, W
DMB DMB DMB THR THR THR
105 175 140 210 245
Vo, DURING
60
during
OFS, ml min-’ . kg-’
RVS, ml min-’ kg-’
% Difference
18 23 34 23 31 37
19 24 35 23 31 37
4 3 0 0 0
l
70
OFS, open-flow system; RVS, respiratory
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l
T
6
0
value system.
0
I I
‘,
PRE
I I
I I
I I
1
0
3
5
7
Treadmill
vals. Before each exercise trial, the 0, and CO, analyzers were calibrated with a known gas mixture, analyzed previously by a mass spectrometer. CH, production. As was mentioned previously, ruminants produce CH, as a result of fermentation processes that take place in the gut. Although a 20-h period elapsed between feeding and exercise, we wanted to be certain that little or no CH, was being produced during exercise. Experiments were conducted in a randomized order on two sheep exercising at a speed of 2.2 m/s on a 3’ incline for 10 min. One sheep was fed its daily food ration 2 h before exercise and the other sheep had not eaten for ~20 h. The same procedure was repeated for 4 d on the same two sheep. An aliquot of gas was continuously drawn from the main stream into an infrared analyzer (model 865, Beckman Instruments) calibrated for measurement of CH, concentration. VO, was measured concurrently. No CH, was detected by the infrared analyzer when the unfed sheep were exercising. However, CH, yas produced when sheep exercised 2 h after feeding, and Vo, averaged 16 t 3% (SD) higher in exercising fed sheep. From this information, we concluded that Tjo, measured during exercise is not affected when the sheep are fed 20 h before exercise. Measurement of blood lactate. All blood samples were taken from the jugular vein while the animal was on the treadmill at preexercise, after 1 and 5 min of walking, and at each minute during exercise. Samples were drawn into syringes, immediately emptied into tubes (82 X 10.25 mm) containing sodium fluoride and potassium oxalate (Vacutainer Systems, Becton Dickinson, Rutherford, NJ), and placed in ice. After completion of the experiment, blood lactate concentration was determined using a lactate analyzer (model 2300 Stat, Yellow Springs Instrument, Yellow Springs, OH). Data analysis. Blood lactate concentration and Vo2 m8x TABLE 2. Maximal lactate concentration for pregnant ewes
FIG. 1. Oxygen consumption (\io,) values for 1 pregnant sheep during progressive exercise test. PRE, standing on treadmill before exercise. Treadmill speed was held constant at 2.6 m/s while grade was increased after 6 min of exercise at each incline. Animal exercised at 0, 3, 5, and 7’.
of ewes with single fetuses and those carrying twins were compared using a t test. VO, was plotted as a function of treadmill speed and incline. Three multiple-regression analyses were performed to determine the slopes and regression coefficients for data collected from the sheep with single fetuses, the sheep carrying twins, and for all sheep combined into one group. An F test was used to compare the residual sums of squares from the three regression analyses (32). An a! level of 0.05 was used for all comparisons. Statistical analyses were performed on a personal computer with SAS software (SAS Institute, Cary, NC). RESULTS Accuracy of the OFS. To compare VO, values measured from the OFS and RVS, two human subjects performed two submaximal cycling (Monark cycle ergometer) trials 48 h apart. The cycle ergometer was calibrated before each trial. On one occasion 60, was measured using the OFS, and on the other day it was measured by the RVS. The results from the two tests are presented in Table 1.
E‘T s-r
15 -
t
oxygen consumption and peak blood elicited by maximal exercise PRE
Sheep
Twins Singles Combined
Incline
(degrees)
~%nax,
ml min-’ l
l
kg-’
53k6 (4) 6Ok6 (8) 57t7 (12)
Values are means _+ SD for no. of sheep in parentheses. maximal oxygen IV consumntion. a
vo2 -,
WS
Ml
M2
Ex3
Ex4
MS
Time of Blood Sample (minutes)
Peak [Lactate], mmol/l 23k-2 (3) 2123 (7) 22t2 (10)
Wl
. FIG. 2. Blood lactate values during exercise at maximal VO, ) for same pregnant sheep as in Fig. 1. PRE, standing on treadmil12byfore exercise. Wl and W5, samples taken after minutes 1 and 5 of walking at 0.8 m/s on level treadmill. Exl-Ex5, samples taken after each minute of exercise at speed (2.6 m/s) and incline (7”) that elicited VO 2max* wo
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EXERCISING
\io,
DURING
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PREGNANCY
SPEED Im-sec’ll FIG. 3. Predicted TO, vs. treadmill speed by incline response surface according to equation 13.95 (speed) + 1.07 (incline X speed) - 1.18.
There was a mean difference of 2.2 t 1% (SE) between VO, values measured with the OFS and RVS. Thus it was concluded our. OFS provided reasonably accurate measurements of VO, in exercising humans. The VO, values measured with the OFS were always lower than, or the same as, those measured using the RVS. These results are consistent with those previously reported by Norton and co-Yorkers (21), who observed the OFS to underestimate VO, by a mean of 6.2% when compared with the RVS. vo2 and blood lactate concentration. The 14 sheep used in this study ran well and needed little or no encouragement to perform the majority of treadmill speeds and TABLE
h,
= 0.70 (incline)
+
inclines. However, two sheep were not able to exercise at a work load that elicited VO, max.VO, values for 12 of the 14 sheep reached a plateau after an increase in work load. Because the vascular catheters of two sheep failed to remain patent during the experiment, blood lactate values were obtained from only 10 of the 12 sheep that were able to. exercise at an intensity that elicited VO, m8x. The vo 2maxvalues and peak exercise lactate concentrations are listed in Table 2 for both groups of sheep. There was no significant difference in bo2max or peak lactate between the ewes carrying a single fetus and those carrying twins. Thus the data from the two groups were combined. Body mass of the sheep at 129 t 1 days gestation aver-
3. Responses of pregnant sheep to treadmill exercise Reference
Bell et al. (4) Bell et al. (5) Chandler and Bell (7)
Chandler et al. (8) Clapp (9) Curet et al. (10) Emmanouilides et al. (11) Leury et al. (16) Leury et al. (17) Lotgering et al. (18)
Lotgering et al. (19) Orr et al. (23) Palmer et al. (24)
Speed, m/s
Incline,
0.70 0.70 0.40 0.70 0.70 0.65 1.34
10 10 10 10 10 0
0.90 1.12 0.70 0.70 0.57 1.65 0.57 1.65 1.34 1.12
Time, min
Estimated VO, , ml. min-’ . kg-’
Estimated %Vo2 -
60 30 60 60 60 60-180 45-60 30-60
23 23 16 23 23 8 30 11 14 23 23 7 47 7 47 30 14
41 41 28 41 41 14 52 20 25 41 41 12 82 12 82 52 25
degrees
5.7
0 0 10
60 60
10 0 10 0 10
10 10 10 10
5.7
34 45
0
Estimated O2 consumption (VOW) was determined on the basis of the equation 1.18, Estimated %VO, mu was determined from the estimated VO, and the mean
VO, 90,
= 0.70 (incline) + 13.95 (speed) + 1.07 (incline x speed) mar (57 ml. min-’ kg-‘) measured in the present study. l
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EXERCISING
\io,
DURING
aged 54 t 10 kg for ewes with one fetus and 63 t 10 kg for those carrying twins. When the two groups were combined, average body weight was 57 t 10 kg. VO, values for one sheep during a progressive exercise test are shown in Fig. 1. In this test, the speed was held constant at 2.6 m/s while the grade was increased approximately every 6 min. Because the animals’ VO, did not increase when the treadmill incline was changed from 5 to 7”, it was assumed VO, maxwas attained. Thus a second test was conducted at 2.6 m/s and 7O in which both To2 and blood lactate were measured. Blood lactate concentrations during exercise at VO, IIlLu are shown in Fig. 2 for the same sheep as in Fig. 1. Values represented are from preexercise, walking at 0.8 m/s on a level treadmill, and running at 2.6 m/s on a 7’ incline. Actual vo2 values (n = 436) were collected from 14 pregnant sheep exercising at a range of speeds and inclines. Initially, the VO, data were grouped according to ewes with single fetuses, ewes with twins, and all pregnant ewes, and three separate multiple regression analyses were performed on the data. Because no statistical difference was observed among the three multiple regression equations, all of the Vo2 data were combined into one group. The linear (? = 0.94) relationship between 60, (ml mine1 . kg-l) and treadmill incline and speed is shown in Fig. 3 [Oo, = 0.70 (incline) + 13.95 (speed) + 1.07 (incline X speed) - 1.181. l
DISCUSSION
The purpose of this study was to determine v02max in pregnant sheep through measurements of VO, and blood lactate concentration during treadmill exercise. Blood lactate concentration at VOW-. Of the 14 sheep used in the study, 12 were able to attain Vo2mar as indicated by the fact that an increase in work rate was accompanied by a curvilinear increase in blood lactate (n = 10) but no change in VO,. At VO, max,blood lactate increased - 15 times over preexercise values. An increase of similar magnitude has also been observed in rats (13), pigs (1, 20), and horses (15). The peak blood lactate concentration observed during exercise in the present study is comparable with data obtained previously from several mammalian species exercising at vo2 mm(27,29). In two studies, blood lactate was measured in pregnant sheep before as well as during exercise (4, 7), and preexercise blood lactate concentration was 1.0 mmol/l, followed by an increase to 2.5 mmol/l during exercise at 0.7 m/s on a 10’ grade. This exercise intensity can be classified as mild to moderate on the basis of an estimated vo2 of 23 ml min-l kg-l, derived from the regression equation developed in this study. Determination of yo2 -. The sheep in the present study had an average VO, maxof 57 t 7 ml min-l kg-‘, an increase - H-fold above preexercise 60,. An increase of this magnitude is consistent with measurements of . vo 2m8xmade on pigs (l), steers (14), and a variety of wild mammals (29). Only two previous studies (18,29) reported i702 m8xvalues for sheep. Taylor et al. (29) used an open-flow method of respirometry and measured a VO, m8xof 46 t 6 (SD) ml min-l kg-l in two nonpregnant African sheep l
l
l
l
l
l
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with an average body weight of 23 t 0.2 kg. Peak blood lactate averaged 23 mmol/l. These sheep attained VO, max on a level treadmill at an average speed of 3.7 m/s. Our regression equation predicts a VO, value of 50 ml. min-l kg-’ at a speed of 3.7 m/s, which is within one standard deviation of the mean V02max (57 t 7) determined for sheep in our study. Although our data are similar to the data of Taylor et al. (29), another study (18) that determined VO 2maxin sheep is not consistent with our findings. Lotgering et al. (18) determined 60, in exercising pregnant sheep from measurements of cardiac output and arteriovenous 0, content difference and reported VO, m8xto be 32 t 10 (SD) ml min. kg-l, which was an increase of approximately six times over rest. Sheep in the present study experienced an &fold increase in VO, from preexercise to V02 mm.Lotgering et al. (18) defined TO, max as the point at which VO, reached a plateau, but no measurements of blood lactate concentration were made. At the speed (1.65 m/s) and incline (10”) at which sheep in the study of Lotgering et al. (18) attained VO 2mm, our regression equation would predict a vo2 of 47 ml mine1 . kg-‘, which is -32% greater than the measured value of 32 ml min-l kg-l. Venous catheter placement may explain some of the difference. Lotgering and co-workers determined VO, from measurements of cardiac output (thermodilution) and differences in 0, content of arterial (common internal iliac artery) and venous (right atrium) blood. We previously have made measurements of VO, in exercising sheep using both respirometry and the indirect Fick method concurrently and observed, on average, 27% higher measurements of Vo2 when comparing values from respirometry with those obtained from the Fick method (25). Because the catheter was in the right atrium rather than the pulmonary artery, it is possible that a representative sample of mixed venous blood was not obtained in our study (25) or the one by Lotgering et al. (18). Consistent with this hypothesis is the fact that measurements of irO, made in exercising ponies with the use of both respirometry and the indirect Fick method (pulmonary arterial blood) yielded less than a 1% difference between VO, values (28). Relationship of Vo2 to treadmill speed and incline. Our findings demonstrate that the relationship of Vo2 to treadmill speed and incline is a linear one. This linear relationship between Vo2 and work rate has been shown previously in rats (2, 22), pigs (20), humans (3), and a variety of other mammals (27). Themdevelopment of a regression equation for prediction of VO, from treadmill speed and incline has permitted us to estimate the relative exercise intensities of other studies that used exercising pregnant sheep (Table 3). We would estimate on the basis of our regression equation that exercise intensity varied from mild to moderate in the majority of these studies. In fact, only two studies (18,19) appeared to use exercise intensities > 60% i702 max.Not only have most of the animal studies selected mild or moderate exercise intensities, but the same is also true of studies in which pregnant humans have served as subjects. Clearly, research that examines the effects of high-intensity exercise during pregnancy is needed before conclusions of its advisability can be drawn . l
l
l
l
l
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EXERCISING
Vo, DURING
In conclusion, VO, and blood lactate in sheep during exercise respond similarly to those of other animals, and most pregnant sheep- can be taught to exercise at an intensity that elicits VO, max. The authors thank Drs. Peter W. Nathanielsz and Gary Sforzo for contributions to the study and Lute Guanzini and Shawnee Riplog for technical assistance. This research was supported by a grant from the American Heart Association, New York Affiliate, and by National Institute of Child Health and Human Development Grant HD-21350. Address for reprint requests: M. T. Jones, Dept. of Physiology, D-123 Schurman Hall, Cornell University, Ithaca, NY 14853-6401. Received 28 May 1991; accepted in final form 28 January
of rats running
up and down an incline. J. Appl. Phys-
iol. 55: 518-521,1983. 3. ASTRAND, P.-O., AND K. RODAHL.
5.
6. 7.
8.
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