Modulation of angiotensin II pressor responsiveness by circulating levels of angiotensin II in pregnant sheep

Angiotensin receptor subtypes in the uterine artery during ovine pregnancy

This study was undertaken to determine if changes in receptor density or affinity could account for the reduced vascular sensitivity to angiotensin II seen during pregnancy. Angiotensin receptor subtypes in the uterine arteries of non-pregnant, pregnant and postpartum ewes were investigated using saturation and competition receptor binding techniques with the specific receptor antagonists, losartan (DuP-753) and PD-123319 (S)1-[[4-(dimethylamino)-3-methylphenyl]-methyl]-5-(diphenylacetyl )-4,5,6,7-tetrahydro-1H-imidazo(4,5-c)pyridine-6-carboxylic acid, ditrifluoroacetate, monohydrate). Receptor density and affinity of total angiotensin receptors, as well as the angiotensin AT1 and AT2 receptor subtypes in uterine arteries were compared with those in the mesenteric artery and aorta. The uterine artery contains both AT1 and AT2 receptor subtypes, whereas the mesenteric artery and aorta contain primarily the AT1 receptor subtype. In uterine arteries from pregnant sheep, angiotensin receptor density was increased because AT2 receptors were increased. AT1 receptor density was not altered. This change was not seen in the aorta. In the uterine artery, receptor affinity for [Sar1,Ile8]angiotensin II decreased in mid-gestation (IC50 7.7 +/- 1.2 x 10(-9) M) compared with non-pregnant ewes (IC50 3.0 +/- 0.6 x 10(-9) M, P = 0.006), and there was decreased affinity of angiotensin AT1 receptors for losartan during pregnancy (IC50 2.8 +/- 1.0 x 10(-4) M) compared with non-pregnant ewes (IC50 2.2 +/- 1.3 x 10(-6) M, P = 0.025). Our results show changes in the density and affinity of the angiotensin receptor subtypes in the uterine artery which could explain its reduced responsiveness to circulating angiotensin II during pregnancy.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

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.

Effect of endothelin-1 on the uterine vasculature of the pregnant and estrogen-treated nonpregnant sheep

OBJECTIVE

This study was designed to evaluate the uterine vascular responses to endothelin-1 in pregnant and estrogen-treated nonpregnant sheep.

STUDY DESIGN

Seven pregnant and five nonpregnant oophorectomized ewes received local uterine artery infusions of endothelin-1, norepinephrine, and phenylephrine. Arterial blood pressure, heart rate, and uterine blood flow were recorded.

RESULTS

Endothelin-1 (0.01 to 3.0 micrograms/min), norepinephrine (0.1 to 3 micrograms/min), and phenylephrine (0.1 to 10 micrograms) produced significant dose-related decreases in uterine blood flow and increases in uterine vascular resistance. On a nanomoles infused per minute basis, endothelin-1 was much more potent than norepinephrine and phenylephrine as a uterine artery vasoconstrictor in both pregnant and nonpregnant sheep. The uterine vascular responses to norepinephrine and phenylephrine were similar in pregnant and nonpregnant ewes, whereas response to endothelin-1 was blunted in pregnancy.

CONCLUSION

Endothelin-1 is an extremely potent uterine vasoconstrictor in both pregnant and nonpregnant ewes, but the uterine vascular responsiveness to endothelin-1 is decreased in pregnancy.

The blood pressure response to infusions of angiotensin II during normal pregnancy: relation to plasma angiotensin II concentration, serum progesterone level, and mean platelet volume

OBJECTIVE

This study is a survey of the determinants of refractoriness to the pressor effects of angiotensin II during normal pregnancy.

STUDY DESIGN

In 25 normal pregnant women, the effective angiotensin II pressor dose was determined 88 times from the twenty-fifth to the thirty-second week of gestation. Immediately before the angiotensin II infusion, blood samples were collected and measured for plasma angiotensin II concentration, serum progesterone level, platelet count, and mean platelet volume.

RESULTS

The effective pressor dose had a significantly positive correlation with plasma angiotensin II concentration and serum progesterone level and a negative correlation with mean platelet volume.

CONCLUSION

The pregnancy-associated refractoriness to angiotensin II is physiologically determined, at least in part, by the elevated circulating levels of endogenously produced angiotensin II and by the progesterone produced by the placenta, whereas platelet activation attenuates the relative refractoriness during normal pregnancy.

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.

Fetal and maternal plasma atrial natriuretic factor responses to angiotensin II infusion

Plasma atrial natriuretic factor and angiotensin II have opposing actions in the regulation of body fluid homeostasis and systemic blood pressure. Angiotensin II infusions stimulate atrial natriuretic factor release in some, but not all, studies in adult mammals. To examine the response during the perinatal period, graded intravenous angiotensin II infusions were administered to four chronically instrumented pregnant ewes and fetuses (132 +/- 1 days of gestation). Fetuses received successive 20-minute intravenous angiotensin II infusions at 25, 50, and 100 ng/kg/min. After a 90-minute recovery, maternal ewes received successive 20-minute angiotensin II infusions (5, 10, and 25 ng/kg/min). During the fetal infusions, mean (+/- SEM) fetal arterial blood pressure (47 +/- 2 to 61 +/- 2 mm Hg, p less than 0.05) and heart rate (155 +/- 16 to 196 +/- 36 beats/min, p less than 0.05) increased, although there was no change in maternal measured parameters. In response to the maternal infusion, maternal mean arterial blood pressure increased (92 +/- 7 to 110 +/- 8 mm Hg, p less than 0.05) and maternal heart rate decreased (136 +/- 4 to 125 +/- 2 beats/min, p less than 0.05) without change in fetal parameters. Fetal plasma atrial natriuretic factor levels (210 +/- 27 to 664 +/- 250 pg/ml, p less than 0.05) significantly increased during the fetal angiotensin II infusions in spite of no change in maternal plasma atrial natriuretic factor (85 +/- 21 to 124 +/- 22 pg/ml) during the maternal angiotensin II infusions. These findings indicate that similar increases in systemic blood pressure in response to angiotensin II infusions stimulate increased fetal, but not maternal, plasma atrial natriuretic factor.

Pathogenesis of sodium and water retention in high-output and low-output cardiac failure, nephrotic syndrome, cirrhosis, and pregnancy (2)

This article has analyzed the pathogenesis of sodium and water retention in several circumstances. The initiator of retention has been proposed to be either a fall in cardiac output (e.g., low-output cardiac failure and vasoconstrictor hypovolemic nephrotic syndrome) or peripheral arterial vasodilatation (e.g., high-output cardiac failure, cirrhosis, arteriovenous fistula, and pregnancy). In the only state discussed, in which the kidney is diseased and not merely responding to extrarenal reflexes--i.e., nephrotic syndrome--intrarenal mechanisms may predominate and lead to expansion of the arterial vascular tree and suppression of the renin-angiotensin-aldosterone system (i.e., hypervolemic nephrotic syndrome). Otherwise, when kidneys are healthy, either a fall in cardiac output or peripheral arterial vasodilatation may diminish arterial vascular filling and thereby initiate a series of hemodynamic and hormonal events that result in renal sodium and water retention (Fig. 7). Finally, the approach presented in this article should be considered to be a vantage point from which to evaluate states of sodium and water retention, but not to be an exclusive position.

Decreased pressor responsiveness in pregnancy: studies in experimental animals

Pregnant animals of several species are resistant to the pressor effects of vasoconstrictor substances. Although a blunted pressor response to angiotensin II (AII) has been most thoroughly documented, resistance to the pressor effects of norepinephrine (NE), and arginine vasopressin (AVP) has also been found. In this review, possible mechanisms for decreased pressor responsiveness are discussed. Although antagonism of vasoconstriction by vasodilatory prostaglandins is the most likely cause for decreased pressor responsiveness in pregnancy, other possibilities have been proposed. Reduced pressor responsiveness to AII occurs in many conditions other than pregnancy, and the mechanisms involved include down-regulation of AII receptors, increased production of vasodilatory prostaglandins (as in pregnancy), and post-receptor defects in AII action. Several recent studies have also suggested a role of the sex hormones, particularly 17-beta-estradiol and prolactin, in stimulating prostaglandin production and causing the decreased pressor response to AII in pregnancy.

Blood pressure response to angiotensin II in pregnant and lactating goats maintained on high or low sodium intake

Goats were kept on either high (274 mmol day-1) or low (68 mmol day-1) sodium intake during pregnancy, lactation, and anoestrus. High salt (HS) animals had about three times greater renal Na excretion than low salt (LS) animals, but the differences in plasma volume, protein concentration, and haematocrit were generally not significant. Angiotensin II (AII) was infused intravenously for 28-min periods in doses of 0.30, 0.75, or 1.00 nmol min-1. The increase in mean arterial blood pressure (MAP) during AII infusions was greater in HS than in LS goats, but became attenuated in all animals during the course of the pregnancy. In LS goats the MAP rise was smaller in late pregnancy than during lactation and anoestrus. In HS goats the rise in MAP was exaggerated in early pregnancy and was similar to lactation and anoestrus values in late pregnancy. During lactation and anoestrus the goats showed similar MAP increase in response to AII infusions. The pulse pressure was greater during AII infusions performed in early pregnancy, lactation and anoestrus than in late pregnancy. Cardiac arrhythmias were noted during AII infusions, most frequently during lactation and in HS goats in early pregnancy. The results show that a moderately elevated sodium intake causes a significantly higher MAP increase in response to AII infusions during pregnancy. This abolishes the reduction of the MAP increase to AII infusions during late pregnancy as compared to anoestrus; a phenomenon which is observed in goats on a low sodium intake.

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.

In vitro prostacyclin production by ovine uterine and systemic arteries. Effects of angiotensin II

Normal pregnancy is associated with reduced systemic pressor responses to infused angiotensin II (ANG II); furthermore, the uterine vascular bed is even less responsive to vasoconstriction by ANG II than the systemic vasculature overall. The mechanism(s) for this refractoriness remains unknown. To determine if vessel production of prostacyclin may be responsible, uterine and omental artery segments were obtained from four groups of sheep, nonpregnant (NP), pregnant (P; 131 +/- 4 d), early postpartum (2.2 +/- 0.4 d), and late postpartum (16 +/- 2 d), and incubated in Krebs-Henseleit alone or with ANG II in the absence or presence of Saralasin. Prostacyclin was measured as 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha). Synthesis of 6-keto-PGF1 alpha was de novo, since aspirin inhibited its formation. P and early uterine arteries produced more 6-keto-PGF1 alpha than NP and late vessels (P less than 0.05): 386 +/- 60 (X +/- SE) and 175 +/- 23 vs. 32 +/- 5 and 18 +/- 4 pg/mg X h, respectively. A similar relationship was observed for omental arteries: 101 +/- 14 and 74 +/- 14 vs. 36 +/- 10 and 22 +/- 4 pg/mg X h, respectively. Furthermore, synthesis by arteries from P and early animals was greater in uterine than omental vessels (P less than 0.05); this was not observed in NP or late vessels. ANG II increased 6-keto-PGF1 alpha production 107 +/- 20% and 92 +/- 16% in P and early uterine arteries only; the threshold dose was between 5 X 10(-11) and 5 X 10(-9) M ANG II. This ANG II-induced increase in 6-keto-PGF1 alpha by uterine arteries was inhibited by Saralasin, which by itself had no effect. During pregnancy, the reduced systemic pressor response to ANG II and the even greater refractoriness of the uterine vascular bed may be reflective of vessel production of the potent vasodilator, prostacyclin. Furthermore, in the uterine vasculature, this antagonism may be potentiated by specific ANG II receptor-mediated increases in prostacyclin.

Metabolic clearance of angiotensin II in pregnant and nonpregnant sheep

Reduced vascular responsiveness to infused angiotensin II (ANG II) has been observed during pregnancy. It has been proposed that infusions produce lower circulating concentrations of ANG II in pregnancy, due to an increase in the metabolic clearance rate of ANG II (MCRangii). We have evaluated the MCRangii and the arterial plasma concentrations of ANG II during constant infusions of 1.15 micrograms ANG II/min into chronically instrumented pregnant (n = 6) and nonpregnant (n = 9) sheep. Although the pressor responses were significantly less in the pregnant than in the nonpregnant sheep (17.5 +/- 0.5 vs. 34.9 +/- 3.2 mmHg, P less than 0.001), the values for MCRangii were not different: 56.2 +/- 6.3 ml X min-1 X kg-1 in nonpregnant and 55.9 +/- 4.3 ml X min-1 X kg-1 in pregnant sheep. The steady-state plasma ANG II concentrations during the infusions were slightly less in pregnant than in nonpregnant sheep (388 +/- 36 vs. 454 +/- 36 pg/ml); however, this difference would be responsible for only a 2-mmHg reduction in the pressor response. We conclude that the reduced pressor response to infused ANG II in pregnancy is not due to an increase in MCRangii nor to lower plasma ANG II concentrations.

Effects of circulating levels of vasoconstrictors on systemic pressor responsiveness to angiotensin II and norepinephrine

Pregnancy is associated with vascular refractoriness to systemic pressor responses produced by angiotensin II. The present study was designed to determine if pregnancy-induced increases in circulating levels of angiotensin II are responsible for this refractoriness. Ten ovariectomized ewes received intravenous infusions of graded doses of angiotensin II (3, 10, 30, and 100 ng/kg/min) and norepinephrine (0.1, 0.3, 1.0, and 3.0 micrograms/kg/min) prior to (control response) and during (test response) the continuous infusion of either angiotensin II (8 ng/kg/min) or norepinephrine (0.2 microgram/kg/min). Systemic pressor responses (increases in blood pressure, millimeters of mercury) to graded doses of angiotensin II were significantly depressed during the continuous angiotensin II infusion while responses to norepinephrine were not modified. In contrast, elevation of baseline levels of a second vasoconstrictor, norepinephrine, did not alter pressor responsiveness to either angiotensin II or norepinephrine. These data suggest that vascular responses to exogenously administered angiotensin II can be selectively depressed by elevated baseline levels of angiotensin II in nonpregnant sheep.

Role of sex steroid hormones in relative refractoriness to angiotensin II during pregnancy

Normal human pregnancy is associated with significant vascular refractoriness to the pressor effects of infused angiotensin II, and in women destined to develop pregnancy-induced hypertension, this refractoriness is lost several weeks before the onset of hypertension. The plasma concentrations of sex steroid hormones gradually increase throughout pregnancy. In the present study, the effect of infusion of 17 beta-estradiol (E2), progesterone, or 5 alpha-dihydroprogesterone on the pressor response to infused angiotensin II (0.002 to 0.1 micrograms X min-1 X kg-1) was evaluated in eight unanesthetized and chronically instrumented nonpregnant ewes. Pressor response to infused angiotensin II (0.02 to 0.1 micrograms X min-1 X kg-1) was significantly suppressed by a 60-minute infusion of E2 (0.8 micrograms X kg-1), whereas infusion of progesterone (4 micrograms X min-1 X kg-1) or 5 alpha-dihydroprogesterone (0.8 micrograms X min-1 X kg-1) did not affect the pressor response. Neither the acid-base status, plasma renin activity, nor serum electrolytes were altered by the administration of E2 or progesterone. These results indicate that E2 may play an important role in the refractoriness to infused angiotensin II during pregnancy, and that this refractoriness by E2 is not mediated by changes in chemoreceptor reflex, renin-angiotensin system, or serum electrolytes, but more likely by the changes in the characteristics of the vascular wall.

Estrogen-induced refractoriness to the pressor effects of infused angiotensin II

Estrogen may be important in the hemodynamic changes that develop during pregnancy; its role in the development of vascular refractoriness to the pressor effects of infused angiotensin II is unknown. We, therefore, examined the pressor responses to six doses of angiotensin II (0.115 to 5.73 micrograms/min) in unanesthetized, nonpregnant sheep both before and after treatment with either high-dose or low-dose 17 beta-estradiol (E2) and constructed dose-response curves. Although mean arterial pressure was unchanged after the infusion of E2, cardiac output rose 28% and systemic vascular resistance fell 19% (p less than 0.001). Infused angiotensin II resulted in dose-dependent rises in mean arterial pressure before and after E2; however, the pressor response after E2 was decreased 30% to 50%. Plasma renin activity rose from 1.15 +/- 0.09 ng/ml X hr to 2.57 +/- 0.39 and 3.21 +/- 0.61 ng/ml X hr with low-dose and high-dose E2, respectively (p less than 0.05). Acutely estrogenized nonpregnant sheep develop significant alterations in both the cardiovascular and the renin-angiotensin systems in addition to decreased pressor responsiveness to infused angiotensin II. Although our findings suggest that estrogen may be important in the development of the vascular refractoriness to angiotensin II seen in pregnancy, additional studies are needed to clarify the role of each E2-induced change.