620
Hypocalcemia and Pregnancy-Induced
Hypertension Produced By Maternal Fasting
Jorge A. Prada, Richardus Ross, and Kenneth E. Qark
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During pregnancy, maternal calcium needs increase as a result of increasing calcium requirements for
fetal bone development. These needs have to be completely supplied by the mother via placental transfer.
Several studies link low serum ionized calcium concentrations with the development of hypertension and
pregnancy-induced hypertension. We hypothesized that maternal hypocalcemia would develop concomitantly with the development of hypertension in sheep that were fasted in late gestation. Sixteen
instrumented ewes were used in the present study. After a 2-day baseline period, food was withdrawn from
10 animals in the experimental group (group 2) for 3 days, whereas the remaining six were allowed to eat
and drink normally (group 1). Blood pressure, uteroplacental bloodflow,and heart rate were monitored
daily. Fasted animals were given deionized water (calcium free) to drink, whereas control animals were
given tap water containing 32.9 mg/1 calcium concentration. Based on the analysis of the ionized calcium
concentration response to fasting, group 2 animals were placed in one of two groups: hypocalcemia did not
develop in group 2a, whereas in group 2b the ionized calcium concentration decreased 27% (from
1.09±0.07 to 0.80±0.06 mM,p=0.01) by the third day of fasting. Group 2b responded with a 16% elevation
in maternal blood pressure (p=0.01) and a 43% reduction in uteroplacental blood flow. Furthermore, a
positive correlation was found between maternal and fetal blood ionized calcium concentrations
(r=0.860). The intravenous infusion of calcium gluconate to the animals in group 2b resulted in a
significant (p=0.05) recovery of blood ionized calcium concentration, reduction of blood pressure, and
recovery of uteroplacental blood flow. The results of the present study are interpreted to suggest that
altered calcium concentrations during the last trimester of multiple ovine pregnancy may play an
important role in the development of pregnancy-induced hypertension. (Hypertension 1992;20:620-626)
KEY WORDS • hypocalcemia • fasting • sheep • hypertension, pregnancy-induced
N
umerous explanations have been proposed to
account for the development of pregnancyinduced hypertension, including incompatibilities between the maternal and fetal blood types, high
salt (NaCl) intake, calcium deficiency, poor general
nutrition, changes in the renin-angiotensin system, and
a predisposed genetic make-up.1-7 In humans, pregnancy-induced hypertension is a hypertensive disorder peculiar to pregnancy that usually develops between the
30th week of pregnancy and the end of the first week
postpartum. If associated with albuminuria, edema, or
both, the disorder is termed preeclampsia.8 The etiology
of pregnancy-induced hypertension is still unknown: it
develops in 10-15% of pregnant women and characteristically occurs in young primigravidas9 and women with
preexisting hypertension or vascular disease.10
Several recent studies have linked hypocalcemia and
high blood pressure.11"14 Pregnancy constitutes a major
From The A.E. Seeds Perinatal Research Center and The
Perinatal Research Institute, and the Departments of Obstetrics
and Gynecology and Pediatrics, University of Cincinnati College
of Medicine, Cincinnati, Ohio.
Supported by grant HD-17742 from the National Institute of
Child Health and Human Development and grant HL-40083 from
the National Heart, Lung, and Blood Institute, National Institutes
of Health, Bethesda, Md.
Address for correspondence: Jorge A. Prada, MD, Department
of Pediatrics, University of Cincinnati College of Medicine, 231
Bethesda Avenue, ML 0541, Cincinnati, OH 45267-0541.
Received October 1,1991; accepted inrevisedform July 9,1992.
challenge for calcium homeostasis because calcium is
actively transported from mother to fetus across the
placenta. The fetus requires a considerable amount of
calcium for skeletal development during the last trimester of pregnancy. The fetal calcium requirements increase sharply at approximately the 30th week in the
human fetus. By term, the fetus has accumulated as
much as 28 g calcium, 80% of this during the third
trimester.15-17 Thatcher and Keith18 have reported that
fasting pregnant sheep for 3 days resulted in a significant elevation in blood pressure, as well as proteinuria,
ketonuria, decreased glomerular filtration rate, decreased cardiac output, and decreased uteroplacental
blood flow (UBF). We hypothesized that hypocalcemia
in fasted pregnant sheep would be associated with
maternal hypertension. The present study was therefore
designed to investigate the effects of 3 days of fasting on
maternal blood ionized calcium concentration [Ca ]i,
mean arterial blood pressure (MBP), and UBF in
pregnant sheep and to evaluate the effects of these
changes on parameters of fetal well-being such as blood
pressure, heart rate, [Ca2*],, and oxygenation.
Methods
Animal Preparation
Sixteen pregnant ewes of mixed breed were obtained
from Morris & Co., Reistertown, Md. Animals (110-115
days of gestation and weighing between 50 and 60 kg)
were sedated with diazepam (10 mg i.v.) (Valium,
Prada et al
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Hoffmann-La Roche Inc., Nutley, N.J.) and then anesthetized with sodium pentobarbital (15 mg/kg) (Steris
Laboratories, Phoenix, Ariz.) before receiving a hyperbaric spinal anesthetic (15 mg) (1% tetracaine hydrochloride, Winthrop Pharmaceuticals, New York). Supplemental doses of sodium pentobarbital were
administered as needed for maintenance of anesthesia.
Animals were secured in the supine position and draped
asepticalry. The maternal femoral artery and vein were
isolated and cannulated with polyvinyl catheters that
were advanced to the level of the distal aorta and
inferior vena cava, respectively. Electromagnetic flow
probes (Dienco, Los Angeles, Calif.) of appropriate size
were placed on the left and right middle uterine arteries
through a 15-cm sterile, lower abdominal incision to
subsequently monitor blood flow to the uterus. After
hysterotomy, polyvinyl catheters were inserted into the
fetal femoral artery and vein. Catheters and flow probe
cables were exteriorized through the midline incision,
passed subcutaneously to the ewe's left flank, placed in
a cloth pouch, and secured to the ewe's side.
Antibiotics (6 cc i.m.) (Combiotics, G.C. Hanford
Manufacturing Co., Syracuse, N.Y.) were administered
on the day of surgery and 3 days after surgery. All
animals were housed in individual portable stainless
steel cages and provided with standard laboratory diets
(Rumilab, Purina Ralston, St. Louis, Mo.) and water ad
libitum. Animals were allowed an adequate recovery
period before fasting. Animals were not exposed to any
experiments preceding the fasting study.
Maternal catheters were flushed daily with heparin
sodium (1,000 USP units/ml) (Elkins-Sinn Inc., Cherry
Hill, N.J.) and fetal catheters with heparin (500 USP
units/ml) to maintain patency. All surgical and experimental procedures were performed in accordance with
the Institutional Animal Care and Use Committee
guidelines of the University of Cincinnati.
Maternal and Fetal Measurements
Maternal and fetal systemic arterial blood pressures
were measured via the appropriate catheter using a
model MP-15 blood pressure transducer (Micron Instruments, Los Angeles, Calif.) anchored at the level of
the sternum of the mother and the fetal heart. Fetal
blood pressures were corrected for amniotic fluid pressure. Heart rates were determined with a cardiotachometer (SensorMedics Instruments, Fullerton, Calif.) triggered by the arterial pressure pulse. UBF was measured
by an electromagnetic flowmeter (model RF-1000, Dienco, Los Angeles, Calif.). Electromagnetic flow probes
(Dienco) were calibrated with saline and were linear
over the flow rates measured. Blood pressure, heart
rate, and UBF were recorded continuously for 60
minutes twice dairy on a pen-writing dynograph physiological recorder (model R612, SensorMedics).
Maternal and fetal arterial blood samples were collected anaerobicalry into heparinized syringes. Arterial
blood gas values (pH, Pao^, and PacOj) were immediately determined with a blood gas analyzer (model
BMS3 MK2 Micro System, Radiometer, Copenhagen,
Denmark), operating at 39°C. Oxygen content was
determined using model LEX-O2 CON TL oxygen
content analyzer (Cavitron, Waltham, Mass.). Maternal
and fetal blood glucose concentrations were determined
with a glucose analyzer (model 27, Yellow Springs
Calcium and Pregnancy-Induced Hypertension
621
Instrument Co., Yellow Springs, Ohio). Samples for
blood [Ca 2+ ]| were collected anaerobicalry in nonheparinized syringes and immediately determined by use of
an ionized calcium analyzer (model ICA1, Radiometer).
Blood [Ca2+]i was adjusted to pH 7.4.
Experimental Protocol
Ten ewes in late gestation (121 ±2 days of gestation)
with either singleton or multiple fetuses (two singletons,
seven twins, and one triplet) were used to study the
effects of fasting. The blood pressure and heart rate of
the mother and fetus were measured daily in the
morning and afternoon for 60 minutes for a period of 5
days. Blood samples were taken daily in the morning to
determine maternal and fetal blood [Ca2+]i and glucose
concentrations, arterial PaO2, Pacch, pH, hematocrit,
and O2 content. Tap water and food were provided ad
libitum between 9 AM and 5 PM for the first 2 days. Food
was then withdrawn, and deionized (calcium free) water
was provided for the next 3 days. At the end of the 3-day
fast, an intravenous administration of calcium gluconate
(Invenex, Melrose Park, 111.) at the rate of 0.465 meq/
min was slowly infused for 20 minutes into four of the
five animals that became hypocalcemic. The effects of
the calcium infusion on blood [Ca2+]i, MBP, and UBF
were determined at 60 minutes after the end of the
infusion.
Identical measurements were made over a period of 5
days in a control group of six instrumented pregnant
ewes provided with standard laboratory diets and tap
water. Additionally four animals received an intravenous infusion of calcium gluconate.
Calculations, Statistical Analysis
In each animal, the mean value (n=4) for each
sample parameter for the 2 days before fasting (morning
and afternoon) served as the baseline value for that
animal. Similarly, daily morning and afternoon values
were calculated as the mean±SEM for each of the 3
experimental days. For statistical analyses, fasted animals (group 2) were subdivided according to the direction and degree of the blood [Ca2+], response by the
third day of fasting. For example, an animal was considered hypocalcemic and was assigned to group 2b if
the ionized calcium concentration by the third day of
fasting had decreased below two standard deviations
from its prefasting baseline value. All fasted animals
which did not meet this criterion were considered to be
normocalcemic and were assigned to group 2a. All
results are expressed as mean±SEM. Cardiovascular
and biochemical responses of the two fasted groups
were compared with each other and with the control
group (group 1) by analysis of variance. The statistical
significance of differences between means was tested by
Newman-Keuls test, and a value of p=0.05 was considered significant. Cardiovascular and biochemical responses to calcium gluconate infusions (before and
after) were analyzed using Student's t test, and a value
of p^0.05 was considered significant.
Results
Maternal Changes
Group 1 (n=six twins). As shown in Table 1, control
animals demonstrated no significant changes in blood
622
Hypertension
TABLE 1.
Vol 20, No 5 November 1992
Maternal and Fetal Cardiovascnlar and Blood Gas Values for Group 1 (Control)
Baseline
Parameters
MatemaJ
MBP (mm Hg)
Heart rate (bpm)
UBF (ml/min)
Fetal
Ca2+, (mM)
MBP (mm Hg)
Heart rate (bpm)
PH
PaOj
Pacch
O2 content (vol %)
Day 1
Day 2
Day 3
U93±127
80±2
106±4
1,256+137
83±2
104±5
1,160+90
82±2
103±5
1,069+102
1.43 ±0.02
51±3
162±7
7.385±0.0O2
21.8±1.0
36.6±1_5
6.3 ±0.4
1.45 ±0.04
48±4
161±6
7.383±0.008
21.1±0.8
36.5±1.8
5.8±0.3
1.40+0.07
50±2
150±6
7390+0.015
20.7+0.9
35.2+2.4
5.6±0.6
1.44±0.03
48±4
152±6
7.381 ±0.009
21.4±1.0
33.8±2.6
6.3 ±0.4
80±3
101 ±6
MBP, mean blood pressure; bpm, beats per minute; UBF, uteroplacental blood flow, and Ca2"1!,
ionized calcium corrected to pH=7.4. Values are mean±SEM; n=6.
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ionized calcium concentration, MBP, UBF, or heart
rate. Blood gas status (data not shown) also demonstrated no change during the 5-day period.
Group 2a (n=5, two singletons and three twins). As
shown in Figure 1 and Table 2, there were no significant
changes in [Ca2+],, MBP, or UBF during the 3 days of
fasting when compared with the baseline parameters.
However, there was a significant (p=0.05) 20% decrease in heart rate by the third day of fasting when
compared with the baseline value. Additionally, there
was a significant (p=0.0l) decrease of 50% in plasma
glucose concentration as a result of fasting. Paco 2 , PaO2,
pH, and O2 content did not change in this group during
the experimental period (data not shown).
Group 2b (n=5, four twins and one triplet). In contrast
to group 2a animals, there was a significant 27% decrease (from 1.09±0.07 to 0.80±0.06 mM, p=0.01) in
the blood ionized calcium concentration during fasting
(Figure 1). Group 2b animals also demonstrated a
significant (/>=0.01) 16% increase in MBP (from 83±4
to 96±4 mm Hg) (Table 3). A significant (p=0.009)
negative correlation was found between maternal ionized calcium concentration and MBP (r= —0.560, Figure
2). Plasma glucose concentrations showed a significant
(/?=0.01) 41% decrease similar to those of group 2a,
10
5
0
-5
-10
-15
-20
D
D Group I
O — O Croup Iki, .
• — • Croup lib I F o s U < 1
-25-30-35
DAYS OF STUDY
FIGURE 1. Line graph shows maternal blood ionized calcium concentrations (% change) of groups 1 (control, n=6),
2a (normocalcemic, n=5), and group 2b (hypocalcemic,
n=5) during experimental days 1-3 (mean±SEM). *Significant difference from baseline (p=0.01, Newman-Keuls).
declining by the second day of fasting (Table 3). UBF
decreased significantly during fasting, falling from
l,073±132 to 608±192 ml/min (Figure 3). In one animal, the decrease in UBF was more dramatic. UBF fell
from 950 to 157 ml/min. Uterine vascular resistance
showed a significant (/?=0.01) increase. By the third day
of fasting, vascular resistance increased from
0.083±0.014 to 0.250+0.082 mm Hg • mT1 • min"1. A
high correlation (r=0.526,/>=0.009) was found between
[Ca2+], and UBF (Figure 4). Neither heart rate nor
blood gas values (data not shown) changed significantly
from baseline values.
Fetal Changes
The mean fetal blood [Ca2+]i (1.29±0.03 mM) during
the baseline period (groups 2a and 2b pooled) was
significantly higher than the mean maternal blood
[Ca2+], of 1.09±0.04 mM (p=0.05).
Fetal blood glucose concentration decreased significantly (p=0.05) in groups 2a and 2b as a result of
maternal fasting (Tables 2 and 3). As shown in Figure 5,
there was a significant (p=0.003) positive correlation
between maternal and fetal blood glucose (r=0.532).
In group 2a pregnancies, fetuses demonstrated no
significant changes in fetal blood [Ca2+], during maternal fasting when compared with the baseline period
(1.29±0.06 to 1.22±0.02 mM, Figure 6). Furthermore,
no changes in fetal MBP, heart rate, pH, Pacc^, Pac>2,
or O2 occurred during the experimental period of 3 days
as compared with baseline (Table 2).
In group 2b pregnancies, the fetal MBP remained
constant (Table 3) despite maternal hypertension, but
fetuses became hypocalcemic (1.30+0.04 to 1.06 ±0.07
mM,p=0.01, Figure 6) concomitantly with the development of maternal hypocalcemia. A highly significant
(p=0.0001) positive correlation was found between
maternal and fetal blood [Ca2+]i (r=0.860, Figure 7).
Although, PaOj and O2 content decreased with the
decrease in UBF, this did not reach significance. There
were no significant changes in fetal hematocrit (data not
shown). However, as mentioned earlier the mean fetal
blood glucose concentration decreased significantly
from 15.2±2.1 to 10.3±1.5 mg% (p=0.05, Table 3).
Prada et al Calcium and Pregnancy-Induced Hypertension
TABLE 2.
623
Maternal and Fetal Cardiovascular and Blood Gas Values for Group 2a (Normocalcemic)
Parameters
Baseline
Day 1
Day 2
Day 3
76±3
95±11
l,025±89
49.6±3.3*
75±3
86±10
937±78
35.5 ±0.5*
75±3
83±13'
840±53
33.7±3.8*
42±4
156±6
7.390±0.015
21.8±1.4
42±4
152±4
42±2
163 ±9
Maternal
MBP (mm Hg)
Heart rate (bpm)
UBF (ml/min)
Glucose (mg%)
Fetal
MBP (mm Hg)
Heart rate (bpm)
PH
Pach (mm Hg)
Pacch (mm Hg)
O2 content (vol %)
Glucose (mg%)
81±3
102±4
982±121
67.2±4.2
45±3
167±5
7.384±0.007
20.0±0.6
37.3±2.0
6.9±0J
20 5 ±0.7
35.9+3.0
6.8+0.5
13.6±0.8*
7.391 ±0.007
20.5±0.7
353±2.3
7.3±0.4
10.5 ±0.8*
7J98±0.014
21.1±1.2
34.8±1.7
6.8±0.7
10.0±0.7*
MBP, mean blood pressure; bpm, beats per minute; and UBF, uteroplacental blood flow. Values are
mean±SEM; n=5.
>=0.05 Newman-Keuls.
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Administration of Calcium Gluconate to Control
Group and Group 2b
Maternal changes. The intravenous infusion of calcium gluconate (0.465 meq/min for 20 minutes) to an
additional four animals in the control group resulted in
no significant changes in MBP or UBF despite a significant increase in blood [Ca2+]f (from 1.03 ±0.05 to
1.19±0.06mM,p=0.05).
Group 2b. The intravenous infusion of calcium gluconate to four of the five ewes in group 2b returned the
maternal blood [Ca2+]i toward baseline concentrations
(0.80±0.06 to 1.04±0.09 mM,p=0.01, Table 4) within
an hour. In contrast to animals in the control group,
MBP decreased progressively during the 60 minutes
after the calcium gluconate infusion (from 96±4 to
85±5 mmHg, p=0.01, Table 4). Amelioration of the
effect of fasting on the [Ca2+], was effective in restoring
UBF toward prefasting values in only three of the four
animals (p=0.001, Table 4). In contrast, the dramatic
fall in UBF that accompanied fasting in the fourth
animal was not reversed by the infusion of calcium
gluconate, and UBF continued to fall (Table 4).
TABLE 3.
Fetal changes. The mean fetal blood [Ca2+], in group
2b increased (from 1.06±0.07 to 1.23±0.01 mM,
p=0.01, Table 4) over the subsequent hour in response
to the intravenous infusion of calcium gluconate to the
ewes in group 2b. There were no corresponding effects
on fetal MBP, heart rate, Pac>2, Paco2, or O2 content.
The infusion of calcium gluconate to the ewes in group
2b did not reverse the decline in fetal pH (Table 4).
Discussion
The existence of a link between calcium and hypertension has been postulated by several investigators.10-14
For example, it has been suggested that a lack of dietary
calcium is associated with elevated blood pressure in
adults. In a study by McCarron,19 hypertensive patients
were found to consume significantly less calcium. It has
further been reported20 that poor urban and rural
Guatemalan women with a diet low in calories, proteins,
and vitamins, but high in calcium have demonstrated
one of the lowest rates of pregnancy-induced hypertension in the world.
Maternal and Fetal Cardiovascular and Blood Gas Values for Group 2b (Hypocalcemk)
Parameters
Baseline
Day 1
Day 2
Day 3
85±5
93±4
82±7
97±4
96±4*
l,058±163
48.7±2.4*
806±201
40.4±4.4'
608±192*
Maternal
MBP (mm Hg)
Heart rate (bpm)
UBF (ml/min)
Glucose (mg%)
Fetal
MBP (mm Hg)
Heart rate (bpm)
PH
PaO2 (mm Hg)
Pa,a>2 (mm Hg)
O2 content (vol %)
Glucose (mg%)
83±4
107±7
1,073+132
68.4±3.7
40±2
39±3
179±6
7J95±0.009
21.4±1.4
35.7±1.3
6.0±0.7
166±3
7387±0.017
15.2±2.1
20.5±1.8
34.1 ±1.2
6.0±0.6
12.7±1.8
38±3
168±4
99±8
50.3±3.8*
41±3
163±7*
7.378±0.020
7J73±0.021
18.8±1.8
34.6±1.1
5J±0.9
17.5±2.7
32.5 ±2.1
4.7±1.1
9.0±1.8'
10.3±1.5*
MBP, mean blood pressure; bpm, beats per minute; and UBF, uteroplacental blood flow. Values are
mean±SEM; n=5.
*/>=0.05 Newman-Keuls.
624
Hypertension
Vol 20, No 5 November 1992
2000
r - - 0.580
p-0.009
< - 0.526
p - 0.009
1750
1500
Group Kb
1250
so
1000
Croup Ub
750
500
250
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
0.6
0.7
MATERNAL IONIZED CALCIUM (mM)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
MATERNAL IONIZED CALCIUM (mM)
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FIGURE 2. Scatterplot shows mean arterial blood pressure
(mm Hg) in relation to maternal ionized calcium concentration during baseline and the 3-day experimental period in
group 2b (r=-0.560, p=0.009).
FIGURE 4. Scatterplot shows uterine blood flow (ml/min) in
relation to maternal ionized calcium concentration during the
baseline and the 3-day experimental period in group 2b
(t=0.526, p=0.009).
Calcium exists in the blood in three forms, with a total
normal concentration of 2.25-2.75 mM. Under normal
conditions, 40% is present as a nondiffusible complex
with protein; 5% is present as a diffusible but undissociated complex with citrate, bicarbonate, and phosphate; and 55% is present as free [Ca2+]|. Calcium
homeostasis results from the integrated actions of parathyroid hormone (PTH), 1,25-dihydroxy [1,25-(OH)J
Vitamin D, and calcitonin.21-22 PTH, which is released in
response to a decrease in serum ionized calcium concentration, stimulates osteoclastic bone resorption to
liberate calcium from bone into the blood. As the
concentration of calcium in the blood rises, PTH activity
is decreased and the rate of bone resorption decreases.
PTH also stimulates the renal synthesis of 1,25-(OH)2
Vitamin D. 1,25 -(OH)j Vitamin D stimulates transepithelial absorption of both calcium and phosphate in the
small intestine.23
The influence of calcium on blood pressure is complex. It is proposed that when [Ca2+]i is elevated in
vascular smooth muscle cells, these cells become abnormally contracted, increasing the vascular resistance and
blood pressure. To lower the blood pressure, [Ca 2+ ]|
must be reduced. Increasing the blood [Ca 2+ ]| decreases
the [Ca2+]i by calcium-regulated mechanisms and pro-
duces relaxation of the vascular cell,24 and as a consequence the blood pressure decreases. High [Ca2+]j in
blood acts in the same manner as calcium channel
blockers, preventing calcium from entering the cell, thus
leading to relaxation.
The present study demonstrated that when pregnant
sheep carrying twin or triplet pregnancies were fasted
and given deionized water for 3 days, hypocalcemia and
elevated arterial blood pressure occurred in 50% of the
experimental animals. The hypocalcemia and elevated
blood pressure were associated with reduced UBF,
increased uterine vascular resistance, and decreased
fetal blood glucose concentration and [Ca2+]|. Also, the
present study clearly shows that fetal [Ca2+]| is significantly higher than maternal [Ca 2+ ] ( , suggesting an active
transport of calcium across the placenta. However, a fall
in maternal calcium concentration was consistent with a
fall in fetal calcium concentration, suggesting a reduced
placenta! calcium transfer by an unknown mechanism.
Nevertheless, a similar phenomenon was reported by
Mughal et al25 in intrauterine growth retarded (IUGR)perfused rat placentas. They found that the maternalfetal clearance of 45Ca across IUGR placentas was
significantly lower than control placentas.
Of the 10 sheep fasted, onlyfivebecame hypertensive,
with a significant reduction in blood calcium concentra-
• D — Q Group I
O — O Group llo
)Fa»t*d
• — • Group lib
DAYS OF STUDY
FIGURE 3. Line graph shows uterine blood flow (UBF) (%
change) in groups 1 (control, n=6), 2a (normocalcemic,
n=5), and 2b (hypocalcemic, n=5) during experimental days
1-3 (mean±SEM).
*Significant difference from day 1
(p=0.01, Newman-Keuls).
r - 0.532
p-0.003
20
30
40
50
60
70
80
90
MATERNAL BLOOD GLUCOSE (mgX)
FIGURE 5. Scatterplot shows fetal blood glucose concentration in relation to maternal blood glucose concentration
(mg%) during baseline and the 3-day fasting period in groups
2a and 2b (T=0.532,
p=0.003).
Prada et al
Calcium and Pregnancy-Induced Hypertension
1.
J
1.4
r - O.SSO
P - 0.0001
1.3
-20
^ s ^
•
Group lib
1.2
-3
i-
625
D — D Group I
O — O Group Do , - . _
faatta
• — • Group lib '
-25
DAYS OF STUDY
FIGURE 6. Line graph shows fetal blood ionized calcium
concentrations (% change) in maternal groups 1 (control,
n=6), 2a (normocalcemic, n=5), and 2b (hypocalcemic,
n=5) during experimental days 1-3 (mean±SEM). *Significant difference from baseline (p=0.01 Newman-Keuls).
Downloaded from http://hyper.ahajournals.org/ by guest on June 18, 2017
tion and UBF. The sheep in which hypertension developed showed significant reduction in blood [Ca2+],,
whereas those which remained normotensive showed no
significant changes in blood [Ca2+]i. The finding that
groups 2a and 2b both showed similar and significant
decreases in maternal plasma glucose concentrations,
but only group 2b developed hypocalcemia and hypertension, diminished the likelihood that the hypertension
in group 2b was the result of low plasma glucose
concentration. The possibility that the present experimental protocol may have altered other elements or
electrolyte concentrations that could have effects on the
parameters studied cannot be ruled out. However,
further support for the role of calcium in modulation of
the observed hypertension is provided by its reversibility after administration of calcium gluconate.
Previously, Morriss et al26 have reported the effect of
5 days of fasting in late gestation ewes. In those studies,
TABLE 4. Maternal and Fetal Cardiovascular and Blood Gas
Values Before and One Hour After Calcium Gluconate for
Group 2b (Hypocalcemic)
Parameter
Day 3
After calcium
Maternal
MBP (mm Hg)
Heart rate (bpm)
UBF (ml/min)
UBFt (ral/min)
Ca 2+ , (raM)
pH
96±4
99±8
680±278
0.80±0.06
7.399±0.017
85±5*
87±11
937±286* (n=3)
48
(n = l)
1.04±0.09*
7.410±0.035
41±3
163 ±7
1.06±0.07
7373±0.021
17.5±2.7
32.5±2.1
4.7±1.1
38±3
177±10
1.23±0.01t
7.234±0.136
17.4±3.7
36.4±4.8
3.9±1.5
157
Fetal
MBP (mm Hg)
Heart rate (bpm)
Ca 2+ , (mM)
PH
PaOj (mm Hg)
PacO2 (mm Hg)
O2 content (vol %)
MBP, mean blood pressure; bpm, beats per minute; UBF,
uteroplacental blood flow; and Ci 1, ionized calcium at pH=7.4.
Values are mean±SEM, n=4, except for UBF, n=3.
•p=0.05 t test, from day 3.
tUBF response to calcium gluconate of the fourth animal.
a
1.1
o
1.0
i
0.9
0.8
0.6
*
•
0.7
1.3
0.8
0.9
1.0
1.1
1.2
MATERNAL IONIZED CALCIUM (mM)
1.4
FIGURE 7. Scatterplot shows fetal ionized calcium concentration in relation to maternal ionized calcium concentration
during baseline and the 3-day experimental period in group
2b (hypocalcemic, r=0.860, p=0.0001).
in which regular tap water was provided to the animals,
none of the four pregnant animals recorded developed
hypertension. In contrast, a 9% decrease in blood
pressure was observed by the fifth day of fasting, a
change that was similar to what we observed in group 2a
animals by the third day. One major difference between
the present study and that of Morriss et al26 was the
difference in the provision of calcium in the drinking
water. We used deionized water that contained no
calcium. Other investigators have observed similar decreases in blood pressure27 and heart rate 28 in response
to fasting.
In the present study, the maternal ability to maintain
adequate blood calcium concentration seems to be the
determining factor in preventing hypertension, which
may be accomplished by the integrated actions of PTH,
1,25-(OH)2 Vitamin D, and calcitonin. Also of importance may be the maternal parity, maternal storage of
calcium, and maternal and fetal metabolic calcium
requirements.
UBF declined 43%, confirming the decrease in UBF
reported by other investigators in similar experimental
conditions.18 The effects of UBF reduction have been an
area of concern because of the risk of fetal and neonatal
morbidity and mortality. The literature abounds with
studies devoted to the investigation of many different
factors concerning UBF and IUGR. The decreased
UBF and the decreased PaOj and O2 content we observed could have serious effects on the fetus if maintained on a chronic basis. In addition to hypocalcemia,
hypertension, and reduced UBF, we found a much
higher incidence of premature labor (unpublished observation), a finding that has also been reported in
humans29 after acute starvation.
It is well known that pregnancy-induced hypertension
is common in multiple pregnancies.30 Based on our
observations from the present study, this phenomenon
might be explained as follows: the fetal content of
calcium is related to fetal weight in a linear manner,
with most of the accretion occurring late in pregnancy.
The placenta also doubles its calcium content over the
last 4 weeks of pregnancy. In ewes, placental transfer of
calcium is increased in late twin gestation, day 122 (1.76
g/day) to day 140 (2.60 g/day).31 It seems reasonable to
suggest that maternal, fetal, and placental calcium
626
Hypertension
Vol 20, No 5 November 1992
requirements could be readily met from maternal
stores; however, in a situation where calcium intake is
seriously decreased, this could cause undue disturbances to maternal and fetal calcium homeostasis.
The results of the present study identify hypocalcemia as a strong candidate as a mediator of ovine
hypertension in pregnancy during fasting. The finding
that maternal hypocalcemia and hypertension have significant detrimental effects on fetal calcium concentration warrants further detailed study of the relations
between diet, intensity of pregnancy, and fetal
well-being.
Acknowledgment
We thank the staff of the Perinatal Research Center for
their expert technical assistance.
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J A Prada, R Ross and K E Clark
Hypertension. 1992;20:620-626
doi: 10.1161/01.HYP.20.5.620
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