Nifedipine does not adversely affect uteroplacental blood flow in the hypertensive term-pregnant rat

Nifedipine does not adversely affect uteroplacental blood flow in the hypertensive term-pregnant rat

Nifedipine does not adversely affect uteroplacental blood flow in the hypertensive term-pregnant rat R. A. Ahokas, PhD, B. M. Sibai, MD, W. C. Mabie, ...

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Nifedipine does not adversely affect uteroplacental blood flow in the hypertensive term-pregnant rat R. A. Ahokas, PhD, B. M. Sibai, MD, W. C. Mabie, MD, and G. D. Anderson, MD Memphis, Tennessee The short-term effect of the calcium channel blocker, nifedipine, on maternal hemodynamics and organ perfusion was investigated in 12 hypertensive term-pregnant, spontaneously hypertensive rats by means of the radioactive-labeled microsphere technique. The normal fall in blood pressure during pregnancy was prevented by reducing litter size to two conceptuses on day 7 of gestation. Nifedipine (200 µg/kg) effectively lowered mean arterial pressure 25% by decreasing total peripheral resistance 38%. Cardiac output was increased 15%. Blood flows to the splanchnic region and the reproductive organs were increased after nifedipine administration. The increase in blood flow to the reproductive organs was the result of increased ovarian and uterine wall perfusion caused by large reductions in vascular resistances. Placental blood flow was not significantly altered, but resistance was decreased. Thus, the use of nifedipine to lower maternal blood pressure in pregnancy complicated by extreme hypertension does not necessarily decrease uteroplacental perfusion. (AM J OBSTET GYNECOL 1988;159:1440-5.)

Key words: Hypertension in pregnancy, blood pressure, placental perfusion, calcium channel blockers, spontaneously hypertensive rat

Antihypertensive drugs have been used reluctantly in pregnancy because of concern about the effects on the fetus and disagreement about whether they provide any real benefit to the mother. 1 However, very high blood pressure directly damages small arteries and arterioles, and experimental evidence suggests that this damage occurs within 10 minutes after a short-term rise to ;;.150 mm Hg mean blood pressure. 2 Patients with preeclampsia may be very suddenly exposed to blood pressures of this magnitude. Although arteries and arterioles exposed long-term to hypertension withstand short-term elevations in pressure better than vessels acclimatized to normal pressure, severe chronic essential hypertension during pregnancy is also associated with a risk of convulsions, fatal cerebral hemorrhage, left ventricular failure, renal impairment, and disseminated intravascular coagulation.' Therefore extreme hypertension is a direct threat to maternal wellbeing, and lowering the blood pressure is an urgent necessity. Ideally, this should be accomplished without adversely affecting uteroplacental perfusion and placing the fetus at undue risk. Nifedipine, a dihydropyridine calcium channel

From the Division of Maternal/ Fetal Medicine, Department of Obstetrics and Gynecology, University of Tennessee, Memphis. Presented in part at the Thirty-fifth Annual Meeting of the Society For Gvnecologic Investigation, Baltimore, Maryland, March 17-

20, 1988. Reprint requests: Robert A. Ahokas, PhD, Departments of Obstetrics and Gynecology and Phvsiologyl Biophysics, L'niversity of Tenne.1see, Memphis, Memj;his, TN 38163.

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blocker, is a particularly effective antihypertensive agent with extremely low toxicity and teratogenicity. 3 It causes vasodilation by interfering with the excitationcontraction coupling of vascular smooth muscle by blocking the entrance of ionized calcium through slow channels in the plasma membrane into the cell. It induces a prompt, consistent, and predictable pressure reduction in patients with severe primary hypertension by diminishing peripheral vascular resistance with a rise in cardiac output and pulse rate.u Of additional benefit in pregnancy is the fact that nifedipine effectively suppresses uterine smooth muscle activity, arresting preterm labor in animals 7 and humans. 6 · 9 It would seem then that nifedipine is well suited for the treatment of severe hypertension during pregnancy. However, little information is available, regarding the effects of nifedipine on the uteroplacental circulation and the fetus. In pregnant ewes 10 nifedipine crossed the placenta to the fetal circulation. When given as a large enough dose to reduce maternal blood pressure, a significant reduction in uteroplacental blood flow with a trend toward fetal hypoxemia was observed. In contrast, nifedipine did not induce a change in uterine perfusion in the pregnant goat. 11 In pregnant rhesus monkeys it was reported to have a marked tocolytic effect associated with significant fetal acidosis and hypoxemia. 12 In all of these studies the experimental models used were normotensive. It would be more useful to evaluate the effects of nifedipine on uteroplacental perfusion in a hypertensive pregnant animal model. Unfortunately, no laboratory species spontaneously develops hyper-

Nifedipine and placental perfusion

Volume 159 Number 6

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tension during pregnancy, and pregnancy lowers blood pressure in all models of experimental hypertension. However, in the pregnant spontaneously hypertensive rat blood pressure is inversely related to litter size at term" and spontaneously hypertensive rats with litters of one to two fetuses provide a model of severe essential hypertension during pregnancy. This study was undertaken to investigate the short-term effects of nifedipine on maternal hemodynamics and the uteroplacental circulation in the hypertensive term-pregnant spontaneously hypertensive rat.

Material and methods This research conforms with the "Guiding Principles in the Care and Use of Laboratory Animals" approved by the Council of the American Physiological Society and with federal laws and regulations. The protocol used was approved by the University of Tennessee, Memphis, Animal Care and Use Committee. Virgin female spontaneously hypertensive rats ( l 0 to 12 weeks ofage) were purchased from Harlan/SpragueDawley, Inc. (Indianapolis, Ind.). They were housed in a temperature-controlled room (23 ± l ° C) with lights on from 5:00 AM to 7:00 PM and were fed Purina laboratory rodent chow (Ralston Purina, St. Louis, Mo.) ad libitum with tap water to drink. For breeding, the

females were housed l : l with mature male spontaneously hypertensive rats, and vaginal smears were checked daily in the morning for the presence of spermatozoa. The day vaginal smears were sperm positive was designated as day 0 of pregnancy. Systolic blood pressure was measured by tail-cuff plethysmography and was monitored weekly throughout gestation. On day 7 of gestation, a laparotomy was performed under light methoxyflurane anesthesia. Litter size was reduced to two (one in each uterine horn) by aspirating embroyos through small incisions in the antimesometrial uterine wall. After closing the uterine and abdominal incisions, the rats were injected intramuscularly with penicillin G (30,000 IU) and returned to their cage for the remainder of pregnancy. On day 21 of gestation, the rats were again lightly anesthetized with methoxyflurane, and polyethylene catheters were inserted into the left ventricle of the heart, via the right carotid artery, and into the abdominal aorta, via the left femoral artery. The catheters were led subcutaneously to the back of the neck and brought exterior through a small incision. The incisions were sutured closed and the rats placed in a plexiglass restrainer to recover from anesthesia (a minimum of 2 hours). During recovery blood pressure was recorded continuously with a Statham P23Gb pressure trans-

1442 Ahokas et al.

December 1988

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ducer and Gould 22005 physiologic recorder (Cleveland, Ohio). After recovery baseline arterial blood pressure and heart rate were recorded, and cardiac output and organ blood flows were measured with the reference sample radioactive microsphere technique." While withdrawing an arterial reference sample with a syringe-pump (Harvard Apparatus, South Natick, Mass.) approximately l X 106 microspheres (15 ± 3 µm in diameter labeled with either gadolinium 153 or tin 113 and suspended in 0.9% physiologic saline solution with 0.01 % Tween 80 added to prevent aggregation (New England Nuclear, Boston, Mass.) were flushed into the left ventricle of the heart. Blood pressure was recorded again to verify that microsphere infusion and blood reference withdrawal per se did not significantly alter hemodynamics. Nifedipine (200 µg/kg) was then administered intraventricularly, as a

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bolus, while continuously monitoring blood pressure. Nifedipine was prepared immediately before infusion by dissolving 50 mg in 10 ml of absolute ethyl alcohol. This solution was diluted l : 25 with 0.9% saline solution for injection. No vehicle (4% ethanol) effects on maternal hemodynamics were observed in a preliminary experiment. After 30 minutes cardiac output and organ blood flows were measured again as previously described by means of the alternately labeled microspheres. The rats were then killed with an overdose of anesthesia, and the organs of interest were removed by dissection, weighed, and placed in separate -y-counting

Nifedipine and placental perfusion

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vials (60 mm high by 25 mm diameter). The placentas were detached from the uterine wall and placed in one vial, and the entire uterus (myometrium/ endometrium), including implantation sites and surgical sites, was placed in a separate vial. The uterine incisions were completely healed with no signs of infection. The skin was removed and weighed, and the remaining carcass was weighed and cut into small pieces. The skin and carcass sections were also placed in separate 'Y-counting vials. The vials containing the arterial blood reference samples and tissues were counted by differential spectroscopy in a multichannel 'Y-well counter (Minaxi Gamma Counter A5530, United Technologies/Packard, Downers Grove, Ill.). The counts per minute were transferred directly to a computer for background and channel overlap correction and for calculation of cardiac output and organ blood flows. 15 Total cardiac output was determined from the sum of the counts per minute in the whole rat. Total peripheral resistance and organ vascular resistances were calculated by dividing mean arterial blood pressure by cardiac output and organ blood flow, respectively. The data were analyzed statistically with the Student t test for paired samples. The 95% confidence level was accepted as statistical significance. The data are presented as means ± standard errors of the mean.

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Results

The short-term effects of nifedipine were examined in 12 hypertensive, term-pregnant, spontaneously hypertensive rats. Systolic blood pressure on day 0 of gestation was 191 ± 6 mm Hg and 18 ± 6 mm Hg on day 21 of gestation, not significantly lower than on day 0. Nine of the rats had two fetuses at term; two rats spontaneously aborted one and carried one fetus to term. The effects of nifedipine on maternal hemodynamics are summarized in Fig. I. Mean baseline blood pressure was 157 ± 6 mm Hg, and nifedipine induced a prompt (i.e., within I to 2 minutes) reduction in blood pressure. At 30 minutes after administration, mean blood pressure was 118 ± 5 mm Hg, 25% lower than baseline. On the other hand, heart rate was not significantly different than baseline 30 minutes after nifedipine injection. Cardiac index was significantly increased 15% after nifedipine; thus the fall in blood pressure was the result of systemic vasodilation. Total peripheral vascular resistance 30 minutes after nifedipine administration was decreased 38%. Nifedipine did not significantly increase arteriovenous shunting of blood, since the fractional distribution of cardiac output to the lungs was unchanged (l.8% ± 0.4% vs 1.9% ± 0.5% for nifedipine and baseline, respectively). Fig. 2 summarizes the effect of nifedipine on regional blood flows and vascular resistances. Nifedipine in-

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duced significant increases in blood flow to the splanchnic region and reproductive organs. Renal, cardiac, and brain blood flows were unaffected by nifedipine, but blood flow to the skin was reduced 32%. Significant reductions in the vascular resistances occurred in all body regions with the exception of the skin, which did not change significantly. Thus the reduction in skin blood flow was simply caused by the fall in perfusion pressure. The increase in splanchnic blood flow was the result of increases in perfusion of the small intestine and hepatic artery; the vascular resistances were significantly reduced (Fig. 3). Perfusion of the large intestine and spleen was significantly decreased by nifedipine. Vascular resistances of the large intestine and pancreas

1444 Ahokas et al.

were not significantly affected, but that of the spleen was significantly increased 30 minutes after nifedipine. The increase in total blood flow to the reproductive organs was the result of increased ovarian and uterine wall perfusion only because of large decreases in the vascular resistances (Fig. 4). Placental blood flow remained constant as a result of a moderate but significant reduction in preplacental vascular resistance.

Comment Experimental models of hypertension during pregnancy are lacking. No laboratory species spontaneously develops hypertension during pregnancy (i.e., preeclampsia/ eclampsia), and pregnancy has a blood pressure-lowering effect in laboratory animals in which hypertension has been experimentally induced. Even genetically hypertensive animals such as the spontaneously hypertensive rat become normotensive by the day before parturition. 13 However, reduction of litter size to two conceptuses shortly after implantation completely prevented the blood pressure-lowering effect of pregnancy in the spontaneously hypertensive rat. Thus the spontaneously hypertensive rat with small litters is a unique and useful experimental model of extreme essential hypertension during pregnancy. It is probably more appropriate to use this model as opposed to a normotensive pregnant animal for investigating the hemodynamic and organ perfusion effects of antihypertensive drugs. Changes in perfusion pressure induced by antihypertensive drugs may cause different vascular responses in hypertensive than in normotensive animals. The calcium channel blockers have been used effectively for the treatment of coronary artery disease, systemic hypertension, and supraventricular arrhythmias,3·5 but there is little experience using them for the treatment of hypertension during pregnancy. The present results have shown that nifedipine, a dihydropyridine calcium channel blocker, significantly alters maternal hemodynamics in the hypertensive termpregnant, spontaneously hypertensive rat. It lowered blood pressure by decreasing total peripheral vascular resistance, which was accompanied by a significant increase in cardiac output. Heart rate was not significantly affected, so the increase in cardiac output must have been caused by increased stroke volume. The reduction in total peripheral resistance was from a generalized decrease in the vascular resistances of all regions of the body with the exception of the skin, which remained unchanged. The present results of the hemodynamic effects of nifedipine in the hypertensive pregnant rat are in general agreement with those in normotensive pregnant sheep, 10 goats, 11 and rhesus monkeys. 12 Nifedipine induced a significant increase in splanchnic blood flow in the hypertensive, term-pregnant rat

December 1988 Am J Obstet Gynecol

as the result of an increase in the perfusion of the small intestine and an apparent increase in hepatic blood flow. The liver is perfused by both the hepatic arterial and hepatic portal circulations, and it is not possible to · determine from these results whether the apparent increase in hepatic arterial blood flow was the result of a decrease in hepatic arterial resistance or an increase in arteriovenous shunting of blood through the gastrointestinal tract. Nifedipine also induced a significant increase in perfusion of the reproductive organs. The increase in blood flow was the result of increased blood flow to the uterine wall (endometrium/myometrium) and the ovaries because of large decreases in the vascular resistances. Placental blood flow remained unchanged, but placerital vascular resistance was also significantly reduced. These results are not in agreement with those of other investigators who used normotensive pregnant animals. Veille et al. 11 did not observe any significant change in total uterine blood flow in response to bolus injections of 20 to 40 µg/kg of nifedipine in pregnant goats. However, this dose induced only a transient fall in blood pressure, which returned to normal within 5 minutes. Other studies have indicated a decrease in total uterine blood flow associated with nifedipine administration. Constant infusion of nifedipine (10 µg/kg/min) in near term-pregnant sheep resulted in a progressive decrease in maternal mean arterial pressure and uterine blood flow.' 0 However, uterine resistance did not change significantly; thus the fall in uterine blood flow was the result of the decrease in perfusion pressure per se. Similar results have been obtained with other calcium channel blockers. Verapamil caused a transient fall in mean arterial pressure in near term-pregnant sheep when given as a bolus. 16 The fall in blood pressure was also associated with a decrease in uterine blood flow, but unlike blood pressure uterine blood flow remained significantly reduced 30 minutes after drug injection. Both short- and longterm nicardipine significantly decreased uterine blood flow in pregnant rabbits. 17 The difference between our results in hypertensive term-pregnant rats and those in normotensive pregnant animals may be because of differences in the physiologic state of the uteroplacental vasculature. The uteroplacental circulation is not maximally dilated in the hypertensive term-pregnant spontaneously hypertensive rat. In another study in which litter size was adjusted from zero to I 0 conceptuses on day 7 of gestation, we observed that mean arterial pressure at term was inversely related to conceptus number and that placental vascular resistance was increased while placental blood flow (milliliters per minute per gram) was decreased in hypertensive rats compared with normotensive rats. 13 The fact that nifedipine reducd uteroplacental resistance in the present study is

Volume 159 Number 6

further confirmation that the uteroplacental vasculature is not maximally dilated in the hypertensive pregnant spontaneously hypertensive rat. If the uteroplacental vasculature in normotensive animals is already near maximally dilated, a reduction in blood pressure induced by nifedipine would be expected to cause a fall in uterine perfusion. The fact that.a decrease in uterine blood flow occurred only when calcium channel blockers induced a significant decrease in blood pressure in normotensive animals strongly supports this.'°· 12 (Holbrook RH, Katz M, Hendricks SK, et al. Maternal cardiovascular and uterine blood flow changes during acute and chronic administration of nicardipine HCI [Abstract). Presented at the thirty-third annual meeting of the Society for Gynecologic Investigation, Toronto, Canada, 1986.) We have not yet examined the effect of nifedipine on uteroplacental circulation in normotensive, term-pregnant, spontaneously hypertensive rats with large litters. Parisi et al. (Parisi VM, Salinas JK, Stockmar EJ. Placental vascular responses to nicardipine in the hypertensive ewe [Abstract]. Presented at the thirty-third annual meeting of the Society for Gynecologic Investigation, Toronto, Canada, 1986.) investigated the effects of nicardipine on blood pressure and uterine and placental vascular resistance in near term-pregnant ewes in which blood pressure was increased by the constant infusion of angioterisin I I. Angiotensin II significantly increased both uterine and placental resistance in association with the increase in blood pressure. Nicardipine decreased blood pressure and uterine vascular resistance toward control levels. However, in contrast to the effects of nifedipine in the hypertensive termpregnant rat, nicardipine did not reverse placental vasoconstrictors. Whether such a difference is the result of differences in the placental vasculature of species and their response to calcium channel blockade or the specific response of the placental vasculature to angiotensin II vasoconstriction is not known. In summary, nifedipine rapidly and effectively lowered maternal blood pressure in the hypertensive termpregnant rat by decreasing total peripheral vascular resistance with a moderate increase in cardiac output. The decrease in total peripheral resistance was the result of a generalized decrease in the resistances of most of the vascular beds in the body including the reproductive organs. Thus lowering maternal blood pressure with nifedipine does not appear in this SHR model to

Nifedipine and placental perfusion

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place the fetus at undue risk of hypoxemia and acidemia caused by a reduction in placental perfusion. However, we cannot rule out the possibility that fetal hypoxemia and acidemia might result from the direct effects of nifedipine used long-term.

REFERENCES I. Redman CWG. The use of antihypertensive drugs in hypertension during pregnancy. Clin Obstet Gynecol 1977; 4:685. 2. Goldby FS, Beilen LJ How an acute rise in arterial pressure damages arterioles. Electron microscopic changes during angiotensin infusion. Cardiovasc Res 1972;6:569. 3. Guzzi M, Olivari MT, Polese A, et al. Nifedipine, a new antihypertensive with rapid action. Clin Pharmacol Ther l 977;22:528. 4. Pederson OL, Mikkelsen E. Acute and chronic effects of nifedipine in arterial hypertension. Eur J Clin Pharmacol 1978;14:375. 5. Olivari MT, Bartorelli C, Polese A, et al. Treatment of hypertension with nifedipine, a calcium antagonistic agent. Circulation 1979;59: 1056. . 6. Houston MC. Treatment of hypertensive urgencies and emergencies with nifedipine. Am Heart J 1986; 111 :963. 7. Golichowski AM, Hathaway DR, Fineberg N, Peleg D. Tocolytic and hemodynamic effects of nifedipine in the ewe. AM J OBSTET GYNECOL 1985; 151: 1134. 8. Ulmsten U, Andersson K-E, Wingerup L Treatment of premature labor with the calcium antagonist nifedipine. Arch Gynecol 1980;229: I. 9. Read MD, Wellby DE. The use of a calcium antagonist (nifedipine) to suppress preterm labor. Br J Obstet Gynecol l 986;93:933. 10. Harake B, Gilbert RD, Ashwal S, Power GG. Nifedipine: effects on fetal and maternal hemodynamics in pregnant sheep. AMJ OBSTET GYNECOL 1987;157:1003. IL Veille JC, Bissonnette JM, Hohimer AR. The effect of a calcium channel blocker (nifedipine) on uterine blood flow in the pregnant goat. AM J 0BSTET GYNECOL 1986; 154: 1160. 12. Ducsay CA, Cook MJ, Veille JC, et al. Cardiorespiratory effects of calcium channel blocker tocolysis in pregnant rhesus monkeys. In: Jones CT, Nathanielsz PW, eds. The physiological development of the fetus and newborn. London: Academic Press, 1985:423. 13. Ahokas RA, Cannon EB, Sibai BM, Anderson GD. The relationship between blood pressure and litter size in the SHR FASEB J 1988;2:A317. 14. Ahokas RA, Reynolds SL, Anderson GD, Lipshitz J Uteroplacental blood flow in the hypertensive termpregnant spontanteously hypertensive rat. AM J OBSTET GYNECOL !987;156:1010. 15. Rudolph AM, Heymann MA. The circulation of the fetus in utero: methods studying distribution of blood flow, cardiac output and organ blood flow. Circ Res 196 7; 21:163. 16. Murad SH, Tabsh KMA, Shilyanski G, et al. Effects of verapamil on uterine blood flow and maternal cardiovascular function in the awake pregnant ewe. Anesth Analg 1985;64:7.