The conscious instrumented rabbit: a model for the study of mechanisms of blood pressure regulation during pregnancy

Real-time fetal monitoring using photoacoustic measurement of placental oxygen saturation in a rabbit hypoxia model

INTRODUCTION

Ensuring adequate fetal oxygenation is an essential aim of fetal monitoring. The purpose of this study was to establish a basic technique for real-time measurement of blood oxygen saturation of the placenta by photoacoustic (PA) technique as a new fetal monitoring method.

METHODS

The hypoxia model established in our previous study was applied to 7 pregnant rabbits. Three phases were induced: normal phase, hypoxia phase, and recovery phase. Three methods were simultaneously used for real-time fetal monitoring: fetal heat rate (FHR) monitoring, oxygen saturation (SO2) measurement by near-infrared spectroscopy (SNO2), and placenta SO2 measured by PA technique (SplO2). The maternal hypoxia was assessed by skin SO2 measured by PA technique (SsO2), and arterial blood SO2 by blood gas analysis (SaO2).

RESULTS

The average of SplO2 in normal phase was 52.6 ± 13.9 %. The averages of SNO2, SSO2, and SplO2 in the seven rabbits changed in parallel from the normal phase to hypoxia phase. In the recovery phase, the SplO2 rose in parallel with recovery of SaO2. There was lag in increase of the FHR compared to the change in the other values. In the detailed analysis of PA signals from the labyrinth and decidua, a unique change in oxygen saturation was seen in one case.

DISCUSSION

Results of this study showed that sensitivity of our novel PA technique in detecting tissue hypoxia was similar to near-infrared spectroscopy (NIRS). As an advantage, unlike NIRS, monitoring with PA technique was unaffected by ischemia and surface changes in oxygen saturation because of its higher spatial resolution. We conclude that PA technique provides more accurate information about fetal blood placenta than NIRS. Ultrasound imaging, combined with oxygen saturation monitoring by PA technique, would improve fetal monitoring and fetal diagnosis in the future.

A1M Ameliorates Preeclampsia-Like Symptoms in Placenta and Kidney Induced by Cell-Free Fetal Hemoglobin in Rabbit

Preeclampsia is one of the most serious pregnancy-related diseases and clinically manifests as hypertension and proteinuria after 20 gestational weeks. The worldwide prevalence is 3-8% of pregnancies, making it the most common cause of maternal and fetal morbidity and mortality. Preeclampsia lacks an effective therapy, and the only “cure” is delivery. We have previously shown that increased synthesis and accumulation of cell-free fetal hemoglobin (HbF) in the placenta is important in the pathophysiology of preeclampsia. Extracellular hemoglobin (Hb) and its metabolites induce oxidative stress, which may lead to acute renal failure and vascular dysfunction seen in preeclampsia. The human endogenous protein, α₁-microglobulin (A1M), removes cell-free heme-groups and induces natural tissue repair mechanisms. Exogenously administered A1M has been shown to alleviate the effects of Hb-induced oxidative stress in rat kidneys. Here we attempted to establish an animal model mimicking the human symptoms at stage two of preeclampsia by administering species-specific cell-free HbF starting mid-gestation until term, and evaluated the therapeutic effect of A1M on the induced symptoms. Female pregnant rabbits received HbF infusions i.v. with or without A1M every second day from gestational day 20. The HbF-infused animals developed proteinuria and a significantly increased glomerular sieving coefficient in kidney that was ameliorated by co-administration of A1M. Transmission electron microscopy analysis of kidney and placenta showed both intracellular and extracellular tissue damages after HbF-treatment, while A1M co-administration resulted in a significant reduction of the structural and cellular changes. Neither of the HbF-treated animals displayed any changes in blood pressure during pregnancy. In conclusion, infusion of cell-free HbF in the pregnant rabbits induced tissue damage and organ failure similar to those seen in preeclampsia, and was restored by co-administration of A1M. This study provides preclinical evidence supporting further examination of A1M as a potential new therapy for preeclampsia.

Enhanced vascular effects of the Ca(2+) channel agonist Bay K 8644 in pregnant rabbits

Hemodynamic studies were performed to determine if blunting of vascular pressor responsiveness to vasoconstrictors during pregnancy may be due to impaired L-type voltage-dependent calcium channels (L-VDCC). Bay K 8644 (BAY), an L-VDCC agonist, was infused in pregnant and nonpregnant anesthetized rabbits (10, 20, 40, and 60 microg/kg) and pregnant and nonpregnant conscious, chronically instrumented (conscious) rabbits (10, 25, and 50 microg/kg). BAY infusions resulted in greater elevation of mean arterial pressure in both anesthetized pregnant (n = 6) vs. nonpregnant (n = 6) (P < 0.05) and conscious pregnant (n = 10) vs. nonpregnant (n = 10) rabbits (P < 0.05). Fractional increase over baseline of total peripheral resistance index was greater in pregnant (36 +/- 5 to 78 +/- 14%) vs. nonpregnant rabbits (14 +/- 4 to 52 +/- 6%) (P < 0.02). Cardiac output index did not differ. There was a single high-affinity L-VDCC antagonist aortic binding site with similar number and affinity in pregnant (n = 7) and nonpregnant (n = 7) rabbits. In conclusion, stimulation of L-VDCC induces greater pressor responses in pregnant rabbits with heightened peripheral vasoconstriction. This does not appear to be due to a change in L-VDCC receptor parameters.

The rabbit as a model for reproductive and developmental toxicity studies

The rabbit has many advantages as a nonrodent and second model for assessing the effects of toxic agents on semen quality, fertility, developmental toxicity, and teratology. The male and female reproductive systems of the rabbit are described, and data on growth, sexual development and reproduction are compared with mice, rats, and humans. Techniques for semen collection and evaluation in the male, and artificial insemination, superovulation, embryo culture, and embryo transfer in the female are included as useful procedures in toxicity testing. Examples of the use of rabbits and experimental replication for toxicity testing are given. Special features of the visceral yolk sac and development of the chorioallantoic placenta of the rabbit are compared with rodents. The rabbit extraembryonic membranes more closely resemble the human than do the rodents, in some respects. The use of the rabbit in developmental toxicity and teratology studies is discussed.

Hemodynamic effect of 17 beta-estradiol in absence of NO in ovariectomized rats: role of angiotensin II

Previous reports correlate plasma levels of estrogen with increased nitric oxide (NO) production. To investigate whether the hemodynamic effects of estrogens are mediated by NO, we compared the hemodynamic changes induced by 17 beta-estradiol (100 micrograms/kg) in the absence and presence of the NO synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). All protocols were performed in ovariectomized, conscious rats. Estradiol alone resulted in no significant changes in cardiac index (CI) or mean arterial pressure (MAP). However, in the presence of L-NAME, estradiol induced a significant increase in total peripheral resistance (TPR) of 37.3 +/- 11.7% and a decrease in CI of 27 +/- 4.9%, without changes in MAP. Previous blockade of angiotensin II AT1 receptors with losartan prevented any change in CI and TPR induced by 17 beta-estradiol in the presence of L-NAME. These observations suggest that NO is necessary to offset a vasoconstrictor action of angiotensin II, which is stimulated by estradiol administration.

Myometrial angiotensin II receptor subtypes change during ovine pregnancy

Although regulation of angiotensin II receptor (AT) binding in vascular and uterine smooth muscle is similar in nonpregnant animals, studies suggest it may differ during pregnancy. We, therefore, examined binding characteristics of myometrial AT receptors in nulliparous (n = 7), pregnant (n = 24, 110-139 d of gestation), and postpartum (n = 21, 5 to > or = 130 d) sheep and compared this to vascular receptor binding. We also determined if changes in myometrial binding reflect alterations in receptor subtype. By using plasma membrane preparations from myometrium and medial layer of abdominal aorta, we determined receptor density and affinity employing radioligand binding; myometrial AT receptor subtypes were assessed by inhibiting [125I]-ANG II binding with subtype-specific antagonists. Compared to nulliparous ewes, myometrial AT receptor density fell approximately 90% during pregnancy (1,486 +/- 167 vs. 130 +/- 16 fmol/mg protein) and returned to nulliparous values > or = 4 wk postpartum; vascular binding was unchanged. Nulliparous myometrium expressed predominantly AT2 receptors (AT1/AT2 congruent to 15%/85%), whereas AT1 receptors predominated during pregnancy (AT1/AT2 congruent to 80%/20%). By 5 d postpartum AT1/AT2 congruent to 40%/60%, and > 4 wk postpartum AT2 receptors again predominated (AT1/AT2 congruent to 15%/85%). In studies of ANG II-induced force generation, myometrium from pregnant ewes (n = 10) demonstrated dose-dependent increases in force (P < 0.001), which were inhibited with an AT1 receptor antagonist. Postpartum myometrial responses were less at doses > or = 10(-9) M (P < 0.05) and unaffected by AT2 receptor antagonists. Vascular and myometrial AT receptor binding are differentially regulated during ovine pregnancy, the latter primarily reflecting decreases in AT2 receptor expression. This is the first description of reversible changes in AT receptor subtype in adult mammals.

Renal autoregulation in midterm and late-pregnant rats

OBJECTIVES

The normal kidney autoregulates (that is, maintains) renal blood flow over a wide range of arterial blood pressures. During normal pregnancy blood pressure falls and the kidney vasodilates. The purpose of this study was to investigate whether renal autoregulatory ability was influenced during pregnancy in the normal rat.

STUDY DESIGN

Experiments were conducted on three groups of anesthetized rats, studied in the virgin state, at midterm, and late in pregnancy (gestation lasts 22 days in the rat). Renal blood flow was measured by electromagnetic flow probe at normal renal perfusion pressures, during elevation in renal perfusion pressure with bilateral carotid occlusion, and at subnormal renal perfusion pressures after application of an aortic snare.

RESULTS

Blood pressure was similar in midterm and virgin rats but lower in late pregnancy in the control state. Control renal blood flow was similar between virgin and late-pregnant rats but significantly elevated at midterm. During elevation in blood pressure with bilateral carotid occlusion, autoregulation was maintained in all three groups. During graded aortic occlusion, renal autoregulatory ability was lost. The autoregulatory threshold was between 90 and 100 mm Hg for midterm pregnant and virgin rats and was slightly lower at about 88 mm Hg for late-pregnant rats.

CONCLUSIONS

Midterm pregnant rats are able to autoregulate renal blood flow as well as virgins are in spite of underlying gestational renal vasodilation. The slight shift in renal autoregulatory threshold seen in late-pregnant rats may prevent the kidney from hypoperfusion during late-gestation hypotension.

Increased peripheral resistance during reduced uterine perfusion pressure hypertension in pregnant rabbits

The role of an increase in total peripheral resistance (TPR) and the contribution of angiotensin II (ANG II) to the hypertension induced by reduced uterine perfusion pressure (RUPP) was explored in pregnant rabbits. On the 22nd day of gestation, a catheter and a microthermocouple were placed in the aorta to measure mean arterial pressure (MAP) and cardiac output (CO), respectively. Three days later, RUPP was induced by a clip on the aorta proximal to the ovarian and distal to the renal arteries. Mean arterial pressure distal to the clip (uterine perfusion pressure) was reduced to 56 +/- 8% (mean +/- SD) of the initial level. Twenty-four hours later, MAP rose from 65 +/- 3 to 84 +/- 11 mm Hg; CO index decreased from 207 +/- 18 to 169 +/- 27 ml/min/kg; and TPR index increased from 0.32 +/- 0.03 to 0.51 +/- 0.08 mm Hg kg/ml/min, respectively (n = 7, all p less than 0.01). Sham-operated pregnant rabbits (n = 7) and non-P rabbits (n = 5) with a comparable distal aortic pressure reduction experienced no change in MAP or CO. Infusion of a receptor antagonist of angiotensin II (Sar1,Ile8-Ang II, 1 microgram/kg/min for 20 min) decreased MAP in sham-operated pregnant rabbits from 64 +/- 6 to 54 +/- 6 mm Hg (p less than 0.01) but did not change MAP in RUPP hypertensive rabbits (86 +/- 9 mm Hg before and 87 +/- 8 at the end of infusion, n = 6). These data indicate that RUPP in pregnant rabbits leads to a high resistance form of hypertension in which the formation of Ang II is not increased.

Angiotensin II vascular smooth-muscle receptors are not down-regulated in near-term pregnant sheep

Normal human and ovine pregnancies are associated with elevated plasma angiotensin II levels and refractoriness to the vasoconstrictor effects of infused angiotensin II, which is greater in the ovine uteroplacental vascular bed than in the systemic vasculature. It remains unclear whether this refractoriness reflects alterations in angiotensin II vascular smooth-muscle receptor density or affinity. We examined the angiotensin II vascular smooth-muscle receptor in nonpregnant (n = 12) and near-term pregnant (130 +/- 3 days [mean +/- SD], n = 10) sheep, comparing binding characteristics on plasma membranes prepared from the medial layer of aorta, mesenteric artery, and uterine artery. Plasma angiotensin II levels were increased threefold to fourfold (p less than 0.001) in pregnant ewes. A single class of high-affinity angiotensin II vascular smooth-muscle receptor was identified in each type of artery. Receptor density was similar in nonpregnant and pregnant mesenteric artery (92 +/- 21 vs 103 +/- 40 fmol/mg protein, respectively), aorta (186 +/- 29 vs 220 +/- 46 fmol/mg protein), and uterine artery (59 +/- 20 vs 77 +/- 20 fmol/mg protein) tissue. Receptor affinity also was unchanged during pregnancy. Because changes in the density and affinity of the angiotensin II vascular smooth-muscle receptor were not observed in near-term pregnant ewes, the attenuated vasoconstrictor responses seen during pregnancy do not reflect receptor down-regulation or decreased affinity.

Effect of protein intake on regional vascular resistance and reactivity to angiotensin II in the rat

Male Sprague-Dawley rats (200-250 g) were fed low protein (6%) diets (LP rats), high protein (50%) diets (HP rats), or regular rat chow (approximately 16% protein) (control rats) and studied under anesthesia after 2 weeks. Dietary protein intake did not affect mean arterial pressure, but renal blood flow was increased in the HP rats and decreased in the LP rats compared with the control rats. Mesenteric blood flow was not significantly different in the three diet groups. Captopril (10 mg.kg-1 i.v.) had no effect on renal vascular resistance in the HP rat but did reduce the elevated renal vascular resistance seen in the LP rat. Meclofenamate (5 mg.kg-1 i.v.) did not significantly affect renal hemodynamics in either HP or LP rats. Finally, the HP rat exhibited resistance to the systemic pressor, renal, and mesenteric vasoconstrictor effects of angiotensin II. Captopril restored the systemic pressor and the mesenteric vasoconstrictor response but not the renal vasoconstrictor response to angiotensin II. Meclofenamate, on the other hand, restored both the systemic pressor response and the renal vasoconstrictor response. Thus, in the LP rat, the vascular response to angiotensin II remains intact, and renal vasoconstriction appears to be mediated by angiotensin II. In contrast, in the HP rat, the renovascular response to angiotensin II is blunted apparently because of enhanced renal prostaglandin production. However, neither increased renal prostaglandin synthesis nor blunting of the renovascular response to angiotensin II appears to account for the chronic vasodilation seen in the HP rat.

Prostaglandin E2 and gestational hypotension in rabbits

Arterial levels of 13,14-dihydro-15-keto-PGE2 (PGE2-M), a stable metabolite of prostaglandin E2 (PGE2) were compared between unanesthetized pregnant (n = 12) and nonpregnant (n = 8) rabbits with the aim of elucidating the role PGE2 in the development of physiological hypotension associated with pregnancy. On the 20th and 22nd days of the 30 day gestation period the mean arterial concentrations of PGE2-M were about 10-times higher (p less than 0.05) and largely variable as compared to that of nonpregnant rabbits. Mean arterial pressure was not lower on either the 20th (69 +/- 4 mmHg, mean +/- SD) or the 22nd (70 +/- 3 mmHg) days of gestation (dg) than in nonpregnant rabbits (69 +/- 4 and 73 +/- 6 mmHg, respectively). On the 23rd dg hypotension was invariably present (61 +/- 5 mmHg vs 72 +/- 4 in nonpregnants, p less than 0.001), but arterial levels of PGE2-M (31.0 +/- 31.6 ng/ml) did not overcome those measured on earlier, normotensive days of gestation. Hypotension was also evident in a subgroup of pregnant rabbits (n = 4) with low PGE2-M concentrations in the nonpregnant range (3.2 +/- 1.5 ng/ml vs 1.9 +/- 1.2 in nonpregnant rabbits, ns). Since the arterial level of PGE2-M proved to correlate (p less than 0.001) with both the uteroplacental venous and renal venous PGE2 concentrations, we suggest that a key role of uteroplacental and renal PGE2 played in the development of gestational hypotension is not probable in rabbits.

NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase: immunochemical characterization of the lung enzyme from pregnant rabbits

A polyclonal antibody was produced in guinea pig against the lung NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase (PGDH) purified from pregnant rabbits. Western blot analysis demonstrated that the protein identified by this antibody in the 105,000g supernatant fraction of lung tissue from pregnant rabbits had a molecular mass of 30 kDa and comigrated with the purified PGDH. The specific activity of the lung PGDH in pregnant rabbits (25- to 28-day gestations) was 36.7 nmol NADH formed/min/mg protein compared to 0.3 nmol NADH formed/min/mg protein in nonpregnant rabbits. Although the PGDH activity in the lung cytosol of nonpregnant rabbits was inhibited by the anti-lung PGDH antibody, the 30-kDa protein was not detected by Western blot analysis. An examination of this 30-kDa protein during the gestational period indicated that the protein was present after 10 days and the amount of the protein increased from Day 10 to Day 28. This increase in the immunochemically reactive protein correlated with the marked increase in PGDH specific activity between 10 and 28 days. An immunochemically reactive protein also was observed in the ovary of 25- to 28-day pregnant rabbits and the specific activity of the ovary PGDH was 19.3 nmol NADH formed/min/mg protein. Only trace levels of the PGDH activity were detected in the ovaries of nonpregnant rabbits. A 30-kDa protein was not detected by the anti-rabbit lung PGDH in brain, kidney, bladder, uterus, liver, and heart tissue of pregnant or nonpregnant rabbits. When rabbit or human placental cytosol was examined with the anti-rabbit lung PGDH only faint 30-kDa bands were observed by Western blot analysis. A monoclonal antibody prepared against human placental PGDH did not recognize the 30-kDa band in the pregnant rabbit lung. Localization studies indicated a marked increase in immunochemical staining in pulmonary epithelial cells of pregnant rabbits as compared to nonpregnant rabbits. Lung epithelial cells but not endothelial cells were identified as containing the PGDH.

Eicosanoid production in rabbit vascular tissues and placentas

In pregnant rabbits, systemic levels of prostaglandin E2 (PGE2) are elevated. To evaluate the source of the rise in circulating levels of PGE2, eicosanoid production was determined in rabbit placental cotyledons and the following vascular tissues: mesenteric arteries, main renal arteries, and intrarenal preglomerular vessels (n = 6 or 7 rabbits). Comparisons were made with vascular tissues from nonpregnant rabbits (n = 6 or 7). Freshly isolated tissues were incubated for 15 min at 37 degrees C. Medium aliquots were analyzed for eicosanoids by radioimmunoassay. Maximal net production and release (pg/microgram protein) was determined by subtraction of the quantity of eicosanoids present in medium before incubation from that present at the end of incubation. Net production and release of 6-ketoprostaglandin F1 alpha (PGF1 alpha), a stable metabolite of prostacyclin, and of PGE2 was similar in vascular tissues from pregnant and nonpregnant rabbits. Thromboxane B2 (TxB2) generation in preglomerular vessels from pregnant rabbits was lower (P less than 0.05) than that of nonpregnant rabbits. This could contribute to renal vasodilation in pregnancy. The pattern of eicosanoid production in vascular tissues was 6-keto-PGF1 alpha greater than PGE2 greater than TxB2, and in cotyledons it was PGE2 greater than 6-keto-PGF1 alpha greater than TxB2. In cotyledons, PGE2 and TxB2 production was 56-219 and 2-12 times that in maternal vascular tissues, respectively. These data suggest that physiologically significant quantities of eicosanoids may be released by rabbit placental tissue.

The influence of indomethacin on blood pressure during the infusion of vasopressors

The effect of indomethacin and its vehicle on blood pressure was studied in conscious rabbits during the infusion of three vasopressors. The cyclooxygenase inhibitor raised mean arterial pressure 12 (vehicle: 3) mm Hg during norepinephrine infusion, 5 (vehicle: 0) mm Hg during angiotensin II infusion, and 5 (vehicle: -8) mm Hg during arginine vasopressin infusion. When saline was given in place of vasopressors, indomethacin failed to alter blood pressure. Since indomethacin elevated pressure in the presence, but not the absence, of all three vasopressors, the possibility that elevation of blood pressure per se stimulates synthesis of vasodilator prostaglandins was considered. A pressor action of indomethacin was observed in ganglion-blocked animals, in which absolute blood pressure remained below normotensive levels during angiotensin II infusion. Thus, indomethacin raised arterial pressure during the infusion of norepinephrine, angiotensin II, and vasopressin, and this action was not influenced by manipulation of blood pressure. These results suggest that each vasopressor promotes prostaglandin synthesis independently to a degree sufficient to restrain its pressor action.

Angiotensin II receptor alterations during pregnancy in rabbits

Despite activation of the renin-angiotensin system during pregnancy, renal and peripheral vascular blood flows increase, and the systemic blood pressure and the pressor response to exogenous angiotensin II (Ang II) fall. Gestational alterations in Ang II receptors could contribute to these changes. Ang II binding parameters were determined utilizing 125I-Ang II in vascular (glomeruli and mesenteric arteries) and nonvascular (adrenal glomerulosa) tissues from 24- to 28-day pregnant rabbits. Comparisons were made utilizing tissues from nonpregnant rabbits. Binding site concentrations (N) and dissociation constants (Kd) were obtained by Scatchard analyses of binding inhibition data. In glomeruli from nonpregnant and pregnant rabbits, N was 515 +/- 84 and 300 +/- 54 fmol X mg-1 protein (P less than 0.005; n = 8), respectively. Kd did not differ (P greater than 0.05). In mesenteric artery membranes from nonpregnant (n = 3) and pregnant (n = 4) rabbits, N was 304 +/- 21 and 112 +/- 23 fmol X mg-1 (P less than 0.005), respectively. Kd did not differ. Neither N nor Kd differed in adrenal glomerulosa tissues (n = 6). Meclofenamate (M) inhibits prostaglandin synthesis, reduces plasma renin activity, and enhances the pressor response to infused Ang II in pregnant rabbits. Administration of M to pregnant rabbits increased N in glomerular and in mesenteric artery membranes from 298 +/- 16 to 381 +/- 8 fmol X mg-1 (n = 3) and from 144 +/- 13 to 218 +/- 13 fmol X mg-1 (n = 4), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against the hypothesis that prostaglandins are the vasodepressor agents of pregnancy. Serial studies in chronically instrumented, conscious rats

Renal hemodynamics increase dramatically during pregnancy, and pressor responsiveness to exogenous administration of vasoconstrictors is attenuated. We investigated whether or not vasodilatory prostaglandins mediate these phenomena. Trained, chronically instrumented, conscious pregnant rats were used. Control values of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated at midgestation (P less than 0.01 and P = 0.05 from prepregnant means, respectively), and effective renal vascular resistance was decreased (P = 0.05). Indomethacin (4.5-6.5 mg/kg body weight [BW]) failed to decrease renal hemodynamics at this stage of pregnancy; in fact, it raised GFR somewhat further (P less than 0.05). Systemic pressor responsiveness to bolus administration of norepinephrine and angiotensin II (AII) was significantly attenuated by at least gestational day 20. Neither indomethacin (7 mg/kg BW) or meclofenamate (6 mg/kg BW) affected the refractory response. The renal vasculature was also relatively unresponsive to an intravenous infusion of AII (5 ng X kg-1 X min-1) during late gestation (day 19); in particular, the fall in ERPF in response to AII (16 +/- 3%) was markedly less than that observed in the prepregnant condition (34 +/- 3%; P less than 0.05). Indomethacin (6 mg/kg BW) failed to restore this blunted response, and further attenuation was evident, despite the presence of the inhibitor (gestational day 21). We conclude that vasodilatory prostaglandins do not appear to mediate the rise in renal hemodynamics, and the attenuation of the systemic and renal pressor responsiveness observed during pregnancy, insofar as these phenomena were unaffected by acute cyclooxygenase inhibition in unstressed, conscious rats.

The effect of prostaglandin and kinin synthesis inhibition on blood pressure during infusion of angiotensin II in the conscious rabbit

The contribution of vasodilator prostaglandins and kinins to blood pressure regulation was studied during the infusion of different doses of angiotensin II in conscious rabbits. Angiotensin II was infused for 60 min. in each experiment. Indomethacin, a prostaglandin synthesis inhibitor, or Trasylol, a kallikrein inhibitor, was given at the 30 min. interval. Indomethacin caused a sustained increase in blood pressure during the infusion of pressor doses of angiotensin II. The range of the mean increase after prostaglandin synthesis inhibition was 3.4 to 6.0 and 3.0 to 9.4 mm Hg at angiotensin II infusion rates of 10 and 50 ng/kg/min respectively. In contrast, indomethacin did not alter blood pressure when the peptide was administered at subpressor levels. Trasylol did not alter blood pressure during infusion of angiotensin II. These results suggest that when blood pressure is maintained at supranormal levels by angiotensin II, the pressor action is attenuated by one or more prostaglandins; an event which is not mediated or assisted by changes in kinin metabolism.