Effect of regular voluntary exercise on resting cardiovascular responses in SHR and WKY pregnant rats M. T. JONES, K. I. NORTON, Department of Physiology, Exercise Biochemistry Laboratory, Human Life Sciences, The
D. M. BLACK,
R. E. GRAHAM, AND R. B. ARMSTRONG Cornell University, Ithaca, New York 14853; Georgia 30602; and Department of New South Wales 2308, Australia
animal model for JONES,M.T.,K.I. NORTON,D.M. BLACK, R.E. GRAHAM, ent time there is not a well-established AND R. B. ARMSTRONG.Effect of regular voluntary exercise on most of the hypertensive disorders in pregnancy because resting cardiovascular responses in SHR and WKY pregnant rats. J. Appl. Physiol. 73(2): 713-720, 1992.-The purpose of this study was to assess the influence of regular voluntary exercise in pregnant normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats on 1) uteroplacental perfusion and mean arterial pressure in the resting conscious condition and 2) fetal number, fetal weight, and number of fetal resorptions. WKYs and SHRs were randomly assigned to standard cages [CWKY (n = 10); CSHR (n = 6)] or cages with activity wheels [EWKY (n = 7); ESHR (n = S)]. EWKYs and ESHRs exercised for 12 wk, and then all rats were bred and experiments were conducted on gestational day 17. Resting blood flow (microspheres), heart rate (HR), and mean arterial pressure (Pa) were measured. No significant difference was found in Pa, HR, uterine blood flow (ESHRs 52 t 8 ml min-’ al00 g-l; CSHRs 28 t 6 ml min. 100 g-l), or maternal placental blood flow (ESHRs, 122 t_ 31 ml min-’ 100 g-l; CSHRs 78 t 21 ml min-’ 100 g-‘) among the groups. Exercise altered the relationship between maternal placental and uterine blood flow and Pa in the SHR; SHRs with lower Pa maintained higher placental and uterine blood flow after training. Before gestation ESHRs ran on average more kilometers per week than EWKYs (43 t 3 vs. 34 t 4), but during gestation ESHRs averaged fewer kilometers per week than EWKYs (16 t 4 vs. 22 & 4). Succinate dehydrogenase activity was higher in the white vastus lateralis (1.02 t 0.2 pmol cytochrome c reduced . min-l g wet wt-‘) and vastus intermedius (3.1 t 0.5 ,umol cytochrome c reduced. min-’ g wet wl?) muscles of ESHRs. No difference was observed in fetal number, fetal weight, or number of fetal resorptions among the groups. In conclusion, voluntary exercise during pregnancy is not harmful to SHRs and may cause enhanced perfusion of reproductive tissues. l
l
l
l
l
l
l
l
muscle metabolism; microcirculation; reproductive gastrointestinal circulation; hypertension
circulation;
HYPERTENSION is an important clinical problem among pregnant women and has been shown to be associated with decreased uteroplacental perfusion, which can lead to intrauterine growth retardation, increased incidence of fetal distress, and perinatal death (3). There are several hypertensive disorders in pregnancy, some of which are 1) preeclampsia and eclampsia, 2) chronic or essential hypertension, 3) chronic hypertension with superimposed preeclampsia, 4) late or transient hypertension, and 5) pregnancy-induced hypertension (22). At the pres-
no laboratory species spontaneously develops hypertension during gestation (1). The spontaneously hypertensive rat (SHR) has been used extensively as an animal model for chronic or essential hypertension because its hemodynamic and endocrine characteristics parallel those observed in human hypertension (27). Regular exercise has been shown to ameliorate hypertension in the nonpregnant SHR (26, 27), indicating that this animal model may be appropriate for studies on the effects of exercise on pregnant chronic hypertensive subjects. However, no studies have be en located that address the potential beneficial effects of regular exercise on pregnant hypertensive rats. The possible beneficial role of regular exercise in treatment of hypertensive pregnant patients has not been experimentally determined, probably for several reasons.-First, hypertension during pregnancy is considered to be life threatening to the developing fetus and, in extreme situations, the mother. Obstetricians typically prescribe bed rest an d sometimes antihypertensive medication for hypertensive pregnant patients in an attempt to bring blood pressure to within normal range. Only a few exercise studies have been conducted that used pregnant hypertensive worn .en as subjects (17,20). One epidemiological study t hat assess ed the relationsh ip between maternal physical activity and the development of pregnancy-induced hypertension found that physical activity during the first half of pregnancy is likely to reduce the risk of pregnancy-induced hypertension (16). Second, an animal model that spontaneously develops pregnancyinduced hypertension has not been described. While the SHR has been a good animal model for studying chronic hypertension in nonpregnant subjects, during pregnancy there is a tendency for their blood pressure to decline from gestational day 18 until term (15), which is approximately gestational day 23. Experiments using pregnant SHRs to study the effect of regular exercise on chronic hypertensives should be conducted on or before gestational day 17; otherwise animals may no longer be hypertensive. Results from previous exercise studies using SHRs are unclear as to what mode (swimming, running) and intensity (percent maximal 0, uptake) are necessary to elicit an attenuation of resting blood pressure. Studies that have used swimming or forced treadmill running at a
0161-7567/92 $2.00 Copyright 0 1992 the American Physiological
Society
713
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (163.015.154.053) on November 27, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
EFFECT OF VOLUNTARY EXERCISE DURING PREGNANCY
714
wheels (Lafayette n i 75 2 wY
Instruments,
Lafayette,
IN). Activity
wheels were very similar in design to those previously described (13). Daily distance run by each rat was determined by multiplying
the wheel circumference by the
number of wheel revolutions listed on the mechanical counter that was attached to the activity wheel.
60
z 45
All animals were individually
6 30 -w cn i5
Rat chow (Purina) and water were provided ad libitum.
15
0
housed in a room with
the temperature maintained at 23 t 1°C (SD) with days artificially divided into 12 h of light and 12 h of darkness.
:
I 0
1
2
3
4
5 Time
6
I
I
I
7
8
9
1 1 1 I 10
11
12
I
13
14
(weeks)
FIG. 1. Average weekly distances ously hypertensive rats in cages with
(means * SE) run by spontaneactivity
wheels
(ESHRs;
closed
diamonds) and Wistar-Kyoto rats in cages with activity wheels (EWKYs; closed circles). Weelzs l-12 represent exercise period before gestation and weeks 13-14 are during first 14 days of gestation. “Significant differences between 2 groups (P -C 0.05).
high exercise intensity have failed to result in lower resting blood pressure (19,26). However, in studies that have used voluntary exercise, hypertensive rats have experienced an attenuation of resting blood pressure (23, 25), presumably because this mode of exercise is not stressful to the animals because they choose to perform it. For this reason, voluntary exercise was the mode of exercise selected for the current study. The purpose of this study was to investigate the possible beneficial influence of voluntary exercise in pregnant SHRs on 1) maintaining or enhancing perfusion of the uterus and placenta; 2) attenuating arterial blood pressure; and 3) fetal number, fetal weight, and number of fetal resorptions. MATERIALS
AND
METHODS
Animals, voluntary exercise, and impregnation. A total of 20 virgin female SHRs, 7-8 wk old and weighing -125 g, were purchased from Charles River Breeders (Wilmington, MA). Also purchased were 20 virgin female rats of the Wistar-Kyoto strain (WKY) of the same age and body weight to serve as the normotensive controls. One week after their arrival, SHR and WKY animals were randomly assigned to either sedentary (CSHR, CWKY) or voluntary exercise (ESHR, EWKY) groups. The CSHRs and CWKYs were individually housed in standard rodent cages while the ESHRs and EWKYs were placed in single-cages with spontaneous activity running
Complete data were obtained from 31 of the 40 rats in the study; therefore, the groups were smaller than was originally planned as well
as uneven in number.
Ii&en
though
we allowed what our previous experience indicated was the shortest possible time for surgical recovery, a few animals still pulled out their catheters. Also, catheter
patency is always a problem instrumented animals
in the use of chronically
At least once a wee’k each rat was removed from its cage, weighed, petted, and placed in a different empty cage for approximately 5 min. This practice was designed to familiarize the rats with being handled and placed in
new environments because hypertensive rats are hyperresponsive to external stimuli (4). Rats in the two voluntary
exercise groups were in the cages with activity
wheels for 12 wk before breeding. After completion of 12 wk of exercise, all rats were housed overnight one to one with mature male rats, and vaginal smears were taken and checked in the morning for the presence of spermatozoa. The day that vaginal smears were sperm positive was designated as gestational day 1. Of the 40 rats used in the study only two failed to conceive the first time spermatozoa were detected. During the day these two rats had access to their activity wheels and at night they were placed with males on a regular basis until spermatozoa were again detected. The ESHR and EWKY groups continued to exercise in their wheels for the first 14 days of their gestation period. Sur$cal procedures. On gestational-day I5 each rat was anesthetized by injection of pentobarbital sodium (35 mg/kg; Nembutal) into the peritoneal cavity. The animal’s neck and cervical dorsal regions were shaved and cleaned with alcohol. A catheter (Silastic, Dow Corning; 0.6 mm ID, 1.0 mm OD) filled with heparinized saline (0.114 g/ml) was surgically implanted in the ascending aorta via the right carotid artery as previously described (6,11,14) so that microspheres could be infused for blood flow measurements. The catheter was externalized on the cervical dorsum and filled with heparinized saline.
TABLE 1. Succinate dehydrogenase activity in selected skeletal muscle and muscle parts Skeletal
Muscle/Muscle
Vastus lateralis (red) Vastus lateralis (white) Vastus intermedius
Part
CSHR
ESHR
(6)
(8)
4.4kO.4 0.5UO.l 2.9110.4
5.3*0.5 1.02&0.2*
3.lt0.5?
CWKY
(10) 4.7t0.2 0.53+0.1 2.3t0.3
EWKY (7)
F
P
4.1t0.3
2.45
0.09
0.62t0.1 1.6kO.3
5.58 3.41
0.03
0.00
Values are means + SE given in pmol cytochrome c reduced. min. g wet wt-’ for no. of rats in parentheses. CSHR and ESHR, spontaneously hypertensive rats in standard cages and cages with activity wheels, respectively; CWKY and EWKY, Wistar-Kyoto rats in standard cages and cages with activity wheels, respectively. * Significantly different from CSHR and CWKY (P < 0.05); t significantly different from EWKY (P < 0.05).
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (163.015.154.053) on November 27, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
EFFECT
+
CSHR
T
CWKY
ESHR
OF VOLUNTARY
EXERCISE
+
EWKY
2. Resting mean arterial pressure (means & SE) for spontaneously hypertensive rats in standard cages (CSHRs), ESHRs, WistarKyoto rats in standard cages (CWKYs), and EWKYs. FIG.
In the morning 2 days after implantation of the carotid catheter (gestational day 17), the animal was anesthetized with methoxyflurane gas and a second catheter of the same dimensions was placed in the caudal artery of the tail as described by Chiueh and Kopin (4) for withdrawal of the reference blood sample. After completion of the caudal cannulation (gestational day 17), the rat was placed in the cage where the experiment would be conducted. Both catheters were connected to extension lines. Mean arterial pressure and heart rate were observed and monitored continuously to determine when the animal had fully recovered from anesthesia, which usually took ~2 h. This observation is in agreement with the findings of Flaim et al. (6), who performed similar surgical procedures on rats under halothane anesthesia and found no statistically significant changes in cardiac or circulatory parameters from 1 to 6 h postsurgery. During the experiment the rat was resting quietly in the cage and had not exercised for 72 h. Heart rate and arterial pressure measurements. Heart rate and arterial pressure were recorded from the carotid catheter before and immediately after the microsphere infusion during the experiment using an Ailtech pressure transducer and a Gould Brush chart recorder (model 2400s). Only arterial pressure is reported because long small-bore Silastic catheters were used for the measurements, precluding accurate determination of systolic and diastolic pressures. Blood flow measurements. Radiolabeled (*%c, ‘13Sn, TABLE
DURING
715
PREGNANCY
and 153Gd) microspheres (New England Nuclear) of 15 pm diameter were used to measure the distribution of blood flow, as previously described (6,8,14). Labels were randomly selected, and each microsphere infusion consisted of -500,000 spheres suspended in 2.0 ml of saline containing
D. M. BLACK,
R. E. GRAHAM, AND R. B. ARMSTRONG Cornell University, Ithaca, New York 14853; Georgia 30602; and Department of New South Wales 2308, Australia
animal model for JONES,M.T.,K.I. NORTON,D.M. BLACK, R.E. GRAHAM, ent time there is not a well-established AND R. B. ARMSTRONG.Effect of regular voluntary exercise on most of the hypertensive disorders in pregnancy because resting cardiovascular responses in SHR and WKY pregnant rats. J. Appl. Physiol. 73(2): 713-720, 1992.-The purpose of this study was to assess the influence of regular voluntary exercise in pregnant normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats on 1) uteroplacental perfusion and mean arterial pressure in the resting conscious condition and 2) fetal number, fetal weight, and number of fetal resorptions. WKYs and SHRs were randomly assigned to standard cages [CWKY (n = 10); CSHR (n = 6)] or cages with activity wheels [EWKY (n = 7); ESHR (n = S)]. EWKYs and ESHRs exercised for 12 wk, and then all rats were bred and experiments were conducted on gestational day 17. Resting blood flow (microspheres), heart rate (HR), and mean arterial pressure (Pa) were measured. No significant difference was found in Pa, HR, uterine blood flow (ESHRs 52 t 8 ml min-’ al00 g-l; CSHRs 28 t 6 ml min. 100 g-l), or maternal placental blood flow (ESHRs, 122 t_ 31 ml min-’ 100 g-l; CSHRs 78 t 21 ml min-’ 100 g-‘) among the groups. Exercise altered the relationship between maternal placental and uterine blood flow and Pa in the SHR; SHRs with lower Pa maintained higher placental and uterine blood flow after training. Before gestation ESHRs ran on average more kilometers per week than EWKYs (43 t 3 vs. 34 t 4), but during gestation ESHRs averaged fewer kilometers per week than EWKYs (16 t 4 vs. 22 & 4). Succinate dehydrogenase activity was higher in the white vastus lateralis (1.02 t 0.2 pmol cytochrome c reduced . min-l g wet wt-‘) and vastus intermedius (3.1 t 0.5 ,umol cytochrome c reduced. min-’ g wet wl?) muscles of ESHRs. No difference was observed in fetal number, fetal weight, or number of fetal resorptions among the groups. In conclusion, voluntary exercise during pregnancy is not harmful to SHRs and may cause enhanced perfusion of reproductive tissues. l
l
l
l
l
l
l
l
muscle metabolism; microcirculation; reproductive gastrointestinal circulation; hypertension
circulation;
HYPERTENSION is an important clinical problem among pregnant women and has been shown to be associated with decreased uteroplacental perfusion, which can lead to intrauterine growth retardation, increased incidence of fetal distress, and perinatal death (3). There are several hypertensive disorders in pregnancy, some of which are 1) preeclampsia and eclampsia, 2) chronic or essential hypertension, 3) chronic hypertension with superimposed preeclampsia, 4) late or transient hypertension, and 5) pregnancy-induced hypertension (22). At the pres-
no laboratory species spontaneously develops hypertension during gestation (1). The spontaneously hypertensive rat (SHR) has been used extensively as an animal model for chronic or essential hypertension because its hemodynamic and endocrine characteristics parallel those observed in human hypertension (27). Regular exercise has been shown to ameliorate hypertension in the nonpregnant SHR (26, 27), indicating that this animal model may be appropriate for studies on the effects of exercise on pregnant chronic hypertensive subjects. However, no studies have be en located that address the potential beneficial effects of regular exercise on pregnant hypertensive rats. The possible beneficial role of regular exercise in treatment of hypertensive pregnant patients has not been experimentally determined, probably for several reasons.-First, hypertension during pregnancy is considered to be life threatening to the developing fetus and, in extreme situations, the mother. Obstetricians typically prescribe bed rest an d sometimes antihypertensive medication for hypertensive pregnant patients in an attempt to bring blood pressure to within normal range. Only a few exercise studies have been conducted that used pregnant hypertensive worn .en as subjects (17,20). One epidemiological study t hat assess ed the relationsh ip between maternal physical activity and the development of pregnancy-induced hypertension found that physical activity during the first half of pregnancy is likely to reduce the risk of pregnancy-induced hypertension (16). Second, an animal model that spontaneously develops pregnancyinduced hypertension has not been described. While the SHR has been a good animal model for studying chronic hypertension in nonpregnant subjects, during pregnancy there is a tendency for their blood pressure to decline from gestational day 18 until term (15), which is approximately gestational day 23. Experiments using pregnant SHRs to study the effect of regular exercise on chronic hypertensives should be conducted on or before gestational day 17; otherwise animals may no longer be hypertensive. Results from previous exercise studies using SHRs are unclear as to what mode (swimming, running) and intensity (percent maximal 0, uptake) are necessary to elicit an attenuation of resting blood pressure. Studies that have used swimming or forced treadmill running at a
0161-7567/92 $2.00 Copyright 0 1992 the American Physiological
Society
713
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (163.015.154.053) on November 27, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
EFFECT OF VOLUNTARY EXERCISE DURING PREGNANCY
714
wheels (Lafayette n i 75 2 wY
Instruments,
Lafayette,
IN). Activity
wheels were very similar in design to those previously described (13). Daily distance run by each rat was determined by multiplying
the wheel circumference by the
number of wheel revolutions listed on the mechanical counter that was attached to the activity wheel.
60
z 45
All animals were individually
6 30 -w cn i5
Rat chow (Purina) and water were provided ad libitum.
15
0
housed in a room with
the temperature maintained at 23 t 1°C (SD) with days artificially divided into 12 h of light and 12 h of darkness.
:
I 0
1
2
3
4
5 Time
6
I
I
I
7
8
9
1 1 1 I 10
11
12
I
13
14
(weeks)
FIG. 1. Average weekly distances ously hypertensive rats in cages with
(means * SE) run by spontaneactivity
wheels
(ESHRs;
closed
diamonds) and Wistar-Kyoto rats in cages with activity wheels (EWKYs; closed circles). Weelzs l-12 represent exercise period before gestation and weeks 13-14 are during first 14 days of gestation. “Significant differences between 2 groups (P -C 0.05).
high exercise intensity have failed to result in lower resting blood pressure (19,26). However, in studies that have used voluntary exercise, hypertensive rats have experienced an attenuation of resting blood pressure (23, 25), presumably because this mode of exercise is not stressful to the animals because they choose to perform it. For this reason, voluntary exercise was the mode of exercise selected for the current study. The purpose of this study was to investigate the possible beneficial influence of voluntary exercise in pregnant SHRs on 1) maintaining or enhancing perfusion of the uterus and placenta; 2) attenuating arterial blood pressure; and 3) fetal number, fetal weight, and number of fetal resorptions. MATERIALS
AND
METHODS
Animals, voluntary exercise, and impregnation. A total of 20 virgin female SHRs, 7-8 wk old and weighing -125 g, were purchased from Charles River Breeders (Wilmington, MA). Also purchased were 20 virgin female rats of the Wistar-Kyoto strain (WKY) of the same age and body weight to serve as the normotensive controls. One week after their arrival, SHR and WKY animals were randomly assigned to either sedentary (CSHR, CWKY) or voluntary exercise (ESHR, EWKY) groups. The CSHRs and CWKYs were individually housed in standard rodent cages while the ESHRs and EWKYs were placed in single-cages with spontaneous activity running
Complete data were obtained from 31 of the 40 rats in the study; therefore, the groups were smaller than was originally planned as well
as uneven in number.
Ii&en
though
we allowed what our previous experience indicated was the shortest possible time for surgical recovery, a few animals still pulled out their catheters. Also, catheter
patency is always a problem instrumented animals
in the use of chronically
At least once a wee’k each rat was removed from its cage, weighed, petted, and placed in a different empty cage for approximately 5 min. This practice was designed to familiarize the rats with being handled and placed in
new environments because hypertensive rats are hyperresponsive to external stimuli (4). Rats in the two voluntary
exercise groups were in the cages with activity
wheels for 12 wk before breeding. After completion of 12 wk of exercise, all rats were housed overnight one to one with mature male rats, and vaginal smears were taken and checked in the morning for the presence of spermatozoa. The day that vaginal smears were sperm positive was designated as gestational day 1. Of the 40 rats used in the study only two failed to conceive the first time spermatozoa were detected. During the day these two rats had access to their activity wheels and at night they were placed with males on a regular basis until spermatozoa were again detected. The ESHR and EWKY groups continued to exercise in their wheels for the first 14 days of their gestation period. Sur$cal procedures. On gestational-day I5 each rat was anesthetized by injection of pentobarbital sodium (35 mg/kg; Nembutal) into the peritoneal cavity. The animal’s neck and cervical dorsal regions were shaved and cleaned with alcohol. A catheter (Silastic, Dow Corning; 0.6 mm ID, 1.0 mm OD) filled with heparinized saline (0.114 g/ml) was surgically implanted in the ascending aorta via the right carotid artery as previously described (6,11,14) so that microspheres could be infused for blood flow measurements. The catheter was externalized on the cervical dorsum and filled with heparinized saline.
TABLE 1. Succinate dehydrogenase activity in selected skeletal muscle and muscle parts Skeletal
Muscle/Muscle
Vastus lateralis (red) Vastus lateralis (white) Vastus intermedius
Part
CSHR
ESHR
(6)
(8)
4.4kO.4 0.5UO.l 2.9110.4
5.3*0.5 1.02&0.2*
3.lt0.5?
CWKY
(10) 4.7t0.2 0.53+0.1 2.3t0.3
EWKY (7)
F
P
4.1t0.3
2.45
0.09
0.62t0.1 1.6kO.3
5.58 3.41
0.03
0.00
Values are means + SE given in pmol cytochrome c reduced. min. g wet wt-’ for no. of rats in parentheses. CSHR and ESHR, spontaneously hypertensive rats in standard cages and cages with activity wheels, respectively; CWKY and EWKY, Wistar-Kyoto rats in standard cages and cages with activity wheels, respectively. * Significantly different from CSHR and CWKY (P < 0.05); t significantly different from EWKY (P < 0.05).
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (163.015.154.053) on November 27, 2018. Copyright © 1992 the American Physiological Society. All rights reserved.
EFFECT
+
CSHR
T
CWKY
ESHR
OF VOLUNTARY
EXERCISE
+
EWKY
2. Resting mean arterial pressure (means & SE) for spontaneously hypertensive rats in standard cages (CSHRs), ESHRs, WistarKyoto rats in standard cages (CWKYs), and EWKYs. FIG.
In the morning 2 days after implantation of the carotid catheter (gestational day 17), the animal was anesthetized with methoxyflurane gas and a second catheter of the same dimensions was placed in the caudal artery of the tail as described by Chiueh and Kopin (4) for withdrawal of the reference blood sample. After completion of the caudal cannulation (gestational day 17), the rat was placed in the cage where the experiment would be conducted. Both catheters were connected to extension lines. Mean arterial pressure and heart rate were observed and monitored continuously to determine when the animal had fully recovered from anesthesia, which usually took ~2 h. This observation is in agreement with the findings of Flaim et al. (6), who performed similar surgical procedures on rats under halothane anesthesia and found no statistically significant changes in cardiac or circulatory parameters from 1 to 6 h postsurgery. During the experiment the rat was resting quietly in the cage and had not exercised for 72 h. Heart rate and arterial pressure measurements. Heart rate and arterial pressure were recorded from the carotid catheter before and immediately after the microsphere infusion during the experiment using an Ailtech pressure transducer and a Gould Brush chart recorder (model 2400s). Only arterial pressure is reported because long small-bore Silastic catheters were used for the measurements, precluding accurate determination of systolic and diastolic pressures. Blood flow measurements. Radiolabeled (*%c, ‘13Sn, TABLE
DURING
715
PREGNANCY
and 153Gd) microspheres (New England Nuclear) of 15 pm diameter were used to measure the distribution of blood flow, as previously described (6,8,14). Labels were randomly selected, and each microsphere infusion consisted of -500,000 spheres suspended in 2.0 ml of saline containing
