The enigma of continual plasma volume expansion in pregnancy: critical role of the renin-angiotensin-aldosterone system

Abstract

Pregnancy is characterized by avid renal sodium retention and plasma volume expansion in the presence of decreased blood pressure. Decreased maternal blood pressure is a consequence of reduced systemic vascular tone, which results from an increased production of vasodilators [nitric oxide (NO), prostaglandins, and relaxin] and decreased vascular responsiveness to the potent vasoconstrictor (angiotensin II). The kidneys participate in this vasodilatory response, resulting in marked increases in renal plasma flow and glomerular filtration rate (GFR) during pregnancy. In women, sodium retention drives plasma volume expansion (∼40%) and is necessary for perfusion of the growing uterus and fetus. For there to be avid sodium retention in the presence of the potent natriuretic influences of increased NO and elevated GFR, there must be modifications of the tubules to prevent salt wasting. The purpose of this review is to summarize these adaptations.

when a woman with normal renal function becomes pregnant, her renal handling of sodium undergoes profound changes. A healthy pregnancy involves renal and systemic hemodynamic adaptations, which allow continual renal sodium retention and marked, cumulative plasma volume expansion (PVE), accompanied by declines in blood pressure (BP) due to reductions in the total peripheral vascular resistance. These are changes that are “physiological” in pregnancy, but that would not normally coexist in the nonpregnant individual. When these adaptations do not occur, the pregnancy is compromised.

This review considers the renal mechanisms involved in these remarkable gestational alterations.

Time Course and Magnitude of the PVE

There have been many clinical studies documenting the large rise that occurs in plasma volume during a normal pregnancy. Populations in the UK, US, Australia, and Chile all demonstrate a cumulative increase in plasma volume, reaching a maximum of <30% above the nonpregnant value during the third trimester (15, 18, 30, 34, 74). Despite this PVE, BP does not increase and, in fact, remains below nonpregnant values throughout most of the pregnancy (Fig. 1; solid lines) (74). In an impressive longitudinal study conducted in the US, which included preconception data, PVE (∼10%) was detectable as early as 6 wk after conception, with further increases through 36 wk of gestation (18). Both total peripheral vascular resistance and renal vascular resistance were reduced at 6 wk gestation, resulting in a reduction in BP and increased renal plasma flow and glomerular filtration rate, respectively (18). Normal pregnancy in the rat (which lasts only 22 days) exhibits qualitatively similar systemic and renal hemodynamic changes with an astounding PVE of ∼70–80% above the nonpregnant (virgin) value, as well as a fall in BP of ∼10% in late pregnancy (Fig. 2) (9). Many of the studies described in this review were conducted on rodents.

Fig. 1.

Curve fit and observed mean values for mean blood pressure (A) and plasma volume (B) values throughout pregnancy in controls and pregnancies complicated by fetal growth restriction (FGR) or preeclampsia (PE). [From Salas et al. (74) with permission.]

Fig. 2.

Cartoon showing time course of change in plasma volume (PV; ○) and blood pressure (BP; ■) during normal rat pregnancy, expressed as %change compared with the virgin (nonpregnant) value. Figure is compiled from data in Refs. 9, 52, 53, 76, 77 based on studies in normal pregnant Munich Wistar and Sprague-Dawley rats.

Physiological Importance of the PVE

The gestational PVE is a critical part of the maternal remodeling, which accommodates the developing fetus during pregnancy. The fetus has a high metabolic rate and grows rapidly, requiring a high rate of delivery of substrates, including glucose, amino acids, electrolytes, and oxygen. Thus a high rate of placental blood flow is important for maintenance of substrate supply and fetal fluid balance (22). Suboptimal gestational PVE in otherwise normal women is associated with “small for gestational age” and growth-restricted babies (30, 74). This relationship has also been reported in women with essential hypertension (15). In fact, as far as fetal development is concerned, maternal hypovolemia resembles maternal malnutrition, or impaired feto-placental perfusion due to defective placentation. Since “small for gestational age” and growth-restricted babies are at increased risk for future cardiovascular, renal, and metabolic diseases (5, 47), it is important to understand what leads to an optimal maternal PVE.

Natriuretic and Anti-Natriuretic Changes that Occur in Normal Pregnancy

The PVE results from slow, cumulative renal sodium retention, despite conflicting signals to the kidney in pregnancy. Changes that promote increases in sodium excretion include large (30–50%) increases in glomerular filtration rate (increasing filtered load) and increased renal plasma flow and declines in plasma protein concentration (reducing tubular sodium reabsorption) (9, 10). Increases in circulating atrial natriuretic peptide (ANP) occur in normal pregnant women (35, 88, 91), and no change or increase has been reported in the pregnant rat (15, 17, 60). There is considerable evidence in the rat that the gestational renal vasodilation is the result of increased renal nitric oxide (NO) (11, 16, 21, 65, 85), and NO is also potently natriuretic (28, 49, 108). In addition, our laboratory reported decreased renal cortical Na-K-ATPase abundance and activity during pregnancy in the rat (48), although an earlier study reported an increase (46). The elevated progesterone levels of pregnancy are also widely believed to promote natriuresis by direct antagonism of the mineralocorticoid receptor (MR), and this will be discussed below. Opposing these many natriuretic changes in pregnancy are the marked increase in circulating angiotensin II (ANG II) and aldosterone levels seen in women and rats (7, 10, 13, 27). There is also a large increase in circulating (sodium retaining) deoxycorticosterone in late pregnancy, which is not regulated by ANG II but arises from 21-hydroxylation of circulating progesterone (23, 105). In addition, the increased ureteral pressure and decreased systemic BP will both promote renal sodium retention (10).

Thus the gestational PVE presents a physiological puzzle. In normal, nonpregnant individuals, a transient volume expansion activates vasodilatory, natriuretic systems and suppresses vasoconstrictor, sodium-retaining systems until the volume imbalance is rapidly corrected. A normally functioning kidney should be incapable of continual sodium retention and PVE. When the kidney malfunctions and fails to maintain sodium balance with subsequent PVE, salt-sensitive hypertension develops with peripheral vasoconstriction, largely maintaining the high BP over the long term (32). Pregnancy is a physiological state, during which a normal kidney is somehow persuaded to continually retain sodium, while persistent marked PVE is occurring. In addition, BP decreases rather than increases, reflecting peripheral vasodilation. Perhaps the most remarkable pregnancy adaptation of volume perception/regulation is that both the renin-angiotensin-aldosterone system (RAAS) and the antagonist NO/ANP systems are activated. How can these two counterregulatory systems be activated at the same time and yet permit continual PVE and peripheral vasodilation?

Adaptive Changes in RAAS Responses During Pregnancy

As shown in Fig. 3, pregnancy is an unusual hemodynamic state, where the renal tubular actions of the major anti-natriuretic, vasoconstrictor arm of volume regulation (RAAS) predominate (to allow PVE), whereas the vascular actions of the natriuretic, vasodilatory systems must predominate to allow peripheral vasodilation (to accommodate the expanded plasma volume). The vasoconstrictor responsiveness to ANG II decreases early in pregnancy (7, 13, 26, 69) and most animal studies suggest that this blunted responsiveness extends to the renal circulation in rabbits and rats (14, 66). There is also general agreement that activation of the RAAS is the primary effector of the continual renal sodium retention and PVE of pregnancy (6, 15, 92, 103). Despite marked, baseline activation of the RAAS, when volume status is “challenged” in pregnant women or rats, the RAAS responds appropriately. For example, the elevated baseline renin, ANG II and aldosterone are increased further by sodium restriction or assumption of the upright posture and are decreased by volume expansion (7, 15, 33, 98). In compromised (preeclamptic) pregnancy, there are declines in RAAS and plasma volume that precede increases in BP (15).

Fig. 3.

Scheme showing proposed adaptations in the renin-angiotensin-aldosterone system (RAAS) and nitric oxide (NO)/atrial natriuretic peptide (ANP) systems, to permit both plasma volume expansion (PVE) and reductions in total peripheral vascular resistance (TPVR) during normal pregnancy.

Despite considerable evidence that the activated RAAS is essential for volume regulation in pregnancy (discussed below), there is also a widely held belief that the high progesterone levels of pregnancy competitively inhibit the actions of aldosterone on the MR. This concept arose several decades ago, when the potassium retention of pregnancy was not considered consistent with an activated RAAS. In vitro studies showed that progesterone competes with aldosterone for occupancy of the MR (2), although later work indicated that, in vivo, progesterone is a much less potent competitor (59). We now know that potassium homeostasis is very complex (106), and that progesterone has direct effects to increase collecting duct potassium reabsorption via the hydrogen-potassium-ATPase. In fact, adrenal progesterone plays an important physiological role in potassium retention, in both sexes, during potassium deficiency (24). Furthermore, recent work shows that the pregnant hydrogen-potassium-ATPase 2 knockout mouse develops both renal and colonic potassium wasting (75). Thus we suggest that the commonly held notion that high progesterone in pregnancy blunts mineralocorticoid responsiveness is incorrect. Indeed, elegant early clinical studies clearly demonstrate that MR continues to regulate sodium excretion in pregnancy, and that progesterone administration to nonpregnant individuals does not alter sodium excretion (23). Below we discuss some of the evidence that supports an essential role for aldosterone in the renal sodium retention of pregnancy.

Adaptations of Renal Sodium Transporters/Channels

Renal sodium excretion determines volume homeostasis, and reabsorption of sodium along the renal tubule is regulated by the individual apical tubular transporters and channels, such that a change in the activity of any transporter or channel can lead to altered volume status (41). The mRNA expression of several renal sodium transport proteins has been shown to be increased at midpregnancy (1); however, protein abundance is either decreased or unchanged for proximal tubule transporters (38) and Na-K-ATPase (48). Since examination of protein abundance of distal transporters was lacking, we performed a renal tubule protein profile of the major apical sodium transporters/channels to determine the time course adaptations in virgin and mid- and late-pregnant rats. The only sodium transport protein increased in mid- and late pregnancy (in a homogenate of whole kidney) was the α-subunit of the epithelial sodium channel (α-ENaC) (103) (Fig. 4). ENaC is a heteromultimeric channel made up of α-, β-, and γ-subunits, with the α-subunit being rate-limiting for channel formation (56). ENaC is located in the aldosterone-sensitive distal nephron, and increases in ENaC abundance, trafficking, and activity are modulated by aldosterone via the MR (54, 55, 56, 64).

Fig. 4.

Cartoon describing transporter changes in the aldosterone-sensitive distal nephron that occur in normal pregnancy in the rat. Figure is compiled from data in Refs.99, 100, 102, 103.

In late pregnancy, where the PVE and renal sodium retention are at the highest level (3), we observed an increase in the “in vivo,” benzamil-inhibitable ENaC activity, which, in turn, led to inhibition of the PVE (103). The increased ENaC activity was also prevented by MR blockade with eplerenone, demonstrating that the increased ENaC activity of pregnancy was the result of aldosterone signaling (103). The importance of the enhanced ENaC-mediated sodium reabsorption of pregnancy in the maintenance of volume expansion was further highlighted by chronic inhibition studies using pharmacological and genetic intervention. In these studies, treatment with either chronic benzamil or small interfering RNA targeting renal α-ENaC, abolished the renal sodium retention of pregnancy, and inhibited the ability of the dams to maintain BP. These pregnancies resulted in growth-restricted pups (99). Importantly, these results mirrored those of pregnancies in adrenalectomized rats and aldosterone synthase knockout mice (6, 92), supporting the premise that an intact aldosterone signaling cascade is critical for sodium reabsorption, volume expansion, BP regulation, and fetal development.

Since aldosterone has been shown to be critical for sodium reabsorption in pregnancy (6, 92, 99), we investigated the role of the aldosterone-responsive sodium-chloride cotransporter (NCC). Our laboratory had previously reported that renal NCC protein abundance from whole kidney homogenates was unchanged at mid- and late pregnancy (103); however, in cortical homogenates, the total NCC abundance was unchanged in midpregnant and decreased in late-pregnant rats (100). In addition to NCC total protein abundance, phosphorylation and apical localization were unchanged in midpregnancy and were decreased in late pregnancy (100) (Fig. 4). Since apical localization and phosphorylation are required for NCC activation, the findings suggest decreased NCC activity in late pregnancy. These changes in NCC were surprising, considering the hormonal milieu and the physiological demand for sodium, and may be related to the increased potassium intake of pregnancy, which provides a potent stimulus to decrease NCC activity, capable of overriding elevations in aldosterone (106). This unexpected finding regarding NCC highlights the complex and sometimes counterintuitive adaptations that occur in pregnancy, to accommodate the many changing requirements.

Despite the findings of unchanged or reduced NCC, our laboratory subsequently found that thiazide-sensitive sodium excretion is increased in both mid- and late pregnancy (102). This dissociation between thiazide sensitivity and NCC phosphorylation/localization in pregnant rats suggests that thiazide is inhibiting another renal sodium-chloride transport mechanism during pregnancy. Indeed, despite absence of NCC in the cortical collecting duct (CCD), thiazide inhibits ∼50% of the CCD sodium transport (90). Leviel et al. (43) showed that the thiazide sensitivity of the CCD is due to electroneutral transport through the sodium-driven chloride/bicarbonate exchanger (NDCBE), which works in tandem with pendrin. Both transporters are located at the apical plasma membrane of type B intercalated cells to mediate net reabsorption of sodium and chloride. This transport mechanism may be a candidate for the increased thiazide sensitivity that we have demonstrated in pregnant rats.

Unfortunately, there are no antibodies available that can detect NDCBE in the rat kidney, but our laboratory has reported increased pendrin (chloride/bicarbonate exchanger) protein and apical localization in late-pregnant rats (102) (Fig. 4). Of note, pendrin is activated by both MR and ANG II receptor stimulation (96, 97). The exact role of pendrin in pregnancy is yet to be determined, but could serve to support sodium transport by both electroneutral (as described above) and electrogenic mechanisms. ENaC-mediated sodium reabsorption is very low in CCD from pendrin-null mice compared with wild-type mice, suggesting that functional pendrin is necessary to support ENaC activity (40, 70, 71). Given that ENaC activity is elevated in pregnancy and is required for the progression of a healthy pregnancy (99, 103), the increased pendrin abundance and apical distribution may support the high ENaC activity.

Furthermore, pendrin may also compensate for the decrease in NCC in late pregnancy. In the NCC knockout mouse, pendrin is increased in the kidneys (95). While neither the pendrin nor the NCC-single knockout mice display any salt wasting, the pendrin-NCC double knockout mice display severe salt wasting and volume depletion (86). As NCC abundance, phosphorylation and localization are all decreased in the late-pregnant rat (100), we suggest that the increased pendrin (102) could play a compensatory role. A recent double knockout study of NDCBE and NCC has also demonstrated that NDCBE can maintain sodium balance when NCC is reduced (84). Further studies are required to elucidate the roles and mechanisms of pendrin and the newly identified thiazide-sensitive sodium reabsorption (via NDCBE?) in pregnancy.

Overall, our data, as well as that of others, suggest that the renal sodium retention of pregnancy is driven by MR stimulation, and that an important transporter being regulated is the ENaC, via increased α-ENaC subunit abundance.

Adaptive Changes in Responses to NO/ANP during Pregnancy

The systemic (and renal) vasodilation that occurs during pregnancy is associated with many factors, including the blunted vascular responsiveness to ANG II, discussed above. There is also activation of vasodilatory NO in regional vascular beds, including the kidney (65, 85, 104). The gestational renal vasodilation in the rat is strongly dependent on increased local production of NO (11, 16, 21), but NO is also a potent natriuretic agent, with a major action being to inhibit ENaC activity in the collecting duct (26, 49, 108). ANP also exerts a natriuretic effect similar to NO with the primary site of action being the medullary collecting duct (107). As shown in Fig. 3, we suggest that the natriuretic actions of NO/ANP are blunted during pregnancy, while the vasodilatory actions persist. This scenario allows for vasodilation and renal sodium retention to occur when both the RAAS and NO/ANP are activated.

Our laboratory and others have reported that the natriuretic responsiveness to ANP in the rat is lost during normal pregnancy (42, 52, 68). As shown in Fig. 5, both conscious (52) and anesthetized pregnant rats (42) exhibit a blunted natriuretic response to infused ANP, and this also occurs in the pregnant goat (67). Acute isotonic saline expansion produces a rapid natriuretic response, which is partly dependent on endogenous ANP release, and this is also blunted in the pregnant rat (Fig. 5) (63). In addition, the pressure natriuresis is blunted in the pregnant rat (37, 53) (Fig. 5), and, while not dependent on ANP, renal NO production plays an important role in this natriuretic response (50). To directly assess the natriuretic effect of NO in pregnancy, we infused the NO donor NONOate into one renal artery and again observed a blunted natriuretic response in pregnancy (Fig. 5) (79).

Fig. 5.

Change in sodium excretion (UNaV) expressed as %change from control (baseline) to experimental condition in virgin (V; open bars) and pregnant (P; solid bars) rats. Data were obtained from intravenous infusion of atrial natriuretic peptide (IV ANP) in the conscious rat (52) and anesthetized rat (42), endogenous (End) ANP released by an acute infusion of isotonic NaCl (63), pressure natriuresis (Press Nat) (53), and intrarenal (IR) infusion of the nitric oxide donor NONOate (79). *P < 0.05 in the natriuretic response between virgin and pregnant rats.

Despite the blunted natriuretic response to ANP in pregnancy, we found that circulating levels of ANP were similar in virgin and pregnant rats during both infusion of exogenous ANP and stimulation of endogenous ANP (52, 63). Furthermore, there were no differences in either ANP affinity or maximum binding capacity to ANP receptors in the inner medullary collecting duct of virgin vs. pregnant rats (63). This suggested a postreceptor effect of pregnancy, perhaps involving the second-messenger cGMP, which is the common signal for both ANP- and NO-mediated natriuresis. Indeed, the natriuretic actions of dopamine are mediated by cAMP and are intact during pregnancy (76), supporting the hypothesis that the gestational loss of natriuretic responsiveness is limited to the cGMP-dependent natriuretic agents, NO and ANP.

Using in vitro techniques, we determined that there is an increased rate of cGMP breakdown in the inner medullary collecting duct cells (IMCD) of the pregnant rat kidney, a major site of natriuretic action of ANP and NO (63). The cyclic nucleotides are degraded within the cell by a family of enzymes, the phosphodiesterases, several of which specifically degrade cGMP, including phosphodiesterase 5 (PDE5) (12). Earlier work by Humphreys and colleagues (61, 62, 93, 94) had implicated increased renal medullary PDE5 in the natriuretic resistance to ANP seen in the pathological states of nephrotic syndrome and liver disease. Increased renal PDE5 activity was also observed in a canine model of congestive heart failure (89), raising the possibility that, in these pathological sodium-retaining conditions, increased medullary PDE5 activity was blunting tubular ANP signaling. For this reason, we focused on the PDE5 isoform in our studies on pregnancy.

In a cross-sectional time course study in the rat (J. M. Sasser, T. Tsarova, M. H. Humphreys, and C. Baylis, unpublished observations), we observed that inner medullary PDE5 protein abundance was significantly elevated at day 6 of pregnancy vs. virgins (177 ± 26 vs. 100 ± 14%, P < 0.05), increasing further to a maximum at day 16 of 218 ± 33% (P < 0.01 vs. virgin), and had returned to nonpregnant values by 2–4 days postpartum (126 ± 22 vs. virgin, P value nonsignificant). All our in vivo studies investigating natriuretic responses in pregnancy have been conducted at gestational day 16, the time of maximum increase in inner medullary PDE5. Interestingly, this increase in PDE5 was confined to the inner medulla and was not observed in the outer medulla or cortex (J. M. Sasser, T. Tsarova, M. H. Humphreys, and C. Baylis, unpublished).

We repeated the observation of a selective increase in medullary abundance/activity of PDE5 in separate animals at day 16 of pregnancy and also observed that ANP-dependent cGMP release in isolated IMCD was blunted in tissue from pregnant rats (63). Similar blunting of IMCD cGMP release was seen when using sodium nitroprusside as NO donor, in pregnant compared with virgin and postpartum tissue (79) (Fig. 6A). Of note, selective inhibition of PDE5 raised IMCD cGMP accumulation to similar values in virgin and pregnant tissues; i.e., the blunted response in pregnancy was reversed with PDE5 inhibition (79) (Fig. 6B). Restoration of cGMP accumulation in pregnancy with ANP was also restored by selective PDE5 inhibition (63).

Fig. 6.

A: sodium nitroprusside (NaNP)-dependent cGMP accumulation by isolated inner medullary collecting duct (IMCD) cells from kidneys of virgin, pregnant, and postpartum rats. *P < 0.05 vs. virgin. B: the NaNP (10-4 mol/l) stimulated cGMP accumulation in cells from both virgin and pregnant rats in the presence of the PDE5 inhibitor DMPPO (10-7 mol/l). [Reproduced from Ref. 79.]

We have strong evidence that there is a local (medullary) increase in PDE5 protein and enzyme activity in pregnancy from in vitro studies, and, to test whether this contributes to reduced natriuretic responses, we conducted additional studies, in vivo. During both intravenous ANP and intrarenal NONOate, local specific PDE5 inhibition (given directly to one kidney) restored the natriuretic responses, which remained blunted in the contralateral kidney (42, 79). Furthermore, chronic oral administration of the selective PDE5 inhibitor sildenafil, at a high dose of 90 mg·kg⁻¹·day⁻¹, to normal pregnant rats led to increased IMCD cGMP levels and suppression of the maternal PVE, with no impact on maternal BP (77). Lower doses (10–50 mg·kg⁻¹·day⁻¹) increased plasma cGMP but not renal inner medullary cGMP levels and had no effect on PVE or BP. Based on these collective findings, we conclude that increased medullary PDE5 activity is involved in the natriuretic resistance to ANP and NO and contributes to the PVE of pregnancy in the rat.

The hypothesis of local renal tubular loss of cGMP responsiveness but with maintained systemic, vascular responsiveness to cGMP is appealing, but whether this has any relevance to human pregnancy is unclear. The only study to directly address the question, by Irons et al. (35), reported that a low dose of infused ANP produced a minimal natriuresis in women in late pregnancy and a nonsignificant increase in sodium excretion when studied 4 mo postpartum. This study was different in design to the animal studies in that a prolonged, 40-min equilibration time was allowed after ANP infusion was begun, before measurement of sodium excretion. The tubular actions of ANP are very rapid, and a more robust and earlier natriuretic response may have been missed in the women when studied postpartum. This is an important area to target for further clinical study, particularly since PDE5 inhibitors have been suggested as potential antihypertensive agents for treatment of preeclamptic pregnancy (78).

One reassuring finding from our recent animal study shows that, in the pregnant Dahl salt-sensitive rat (which develops spontaneous “preeclampsia”-like symptoms), low-dose sildenafil (50 mg·kg⁻¹·day⁻¹) has beneficial effects on maternal hemodynamics and fetal development (31). Sildenafil also improved uterine perfusion and fetal outcome in a mouse model of preeclampsia induced by catechol-O-methyl transferase knockout (87). However, the animal model used is important since, in the reduced uterine perfusion pressure model, low-dose sildenafil lowers maternal BP, but does not improve the compromised fetal outcome (29). This may be because the increased medullary cGMP observed in the sildenafil-treated reduced uterine perfusion pressure rat (indicating inhibition of medullary PDE5) reduces the gestational PVE and offsets the beneficial maternal vasodilatory actions. Ultimately, these studies have to be carried out in women, and it is essential that plasma volume should be assessed when evaluating the efficacy and safety of PDE5 inhibitors in treatment of hypertensive disorders of pregnancy, particularly since preeclamptic patients usually exhibit a blunted PVE (15, 74).

What is the Signal for Increased Medullary PDE-5, α-ENaC, and PVE in Pregnancy?

It is difficult to imagine how two mutually antagonistic volume regulatory systems could both be activated in a physiological situation. In 1987, Schrier and Durr (82) suggested that, in pregnancy, a primary enlargement of the vascular compartment (due to peripheral vasodilation and later placental arteriovenous shunting) created an underfill signal, with secondary renal sodium and water retention, in an attempt to refill the circulation. There are parallels between the hemodynamic changes of normal pregnancy and both high output heart failure and liver cirrhosis, where an initial vasodilation leads to sodium and water retention by an apparently normal kidney (80, 81). As with pregnancy, there is activation of the RAAS (4, 36) and increases in plasma ANP in heart failure and cirrhosis (19, 51, 58), as well as regional increases in NO production in cirrhosis (51). Furthermore, refractoriness to the natriuretic actions of administered ANP has been report in humans and dogs with heart failure (19, 89), as well as in the cirrhotic rat (62), and ∼50% of patients, dogs, and rats with liver cirrhosis (44). Of note, ANP resistance also occurs in the pathological underfill state, which occurs in nephrotic syndrome (93, 94). Here, the sodium retention is secondary to fluid leakage into interstitium rather than primary vasodilation, reinforcing the notion that underfill “per se” drives ANP resistance. In animal studies in models of heart failure, cirrhosis, and nephrotic syndrome, renal ANP resistance is associated with increased PDE5 activity (62, 89, 94).

In order for pregnancy to be considered a primary “underfilled” state of PVE, the peripheral vasodilation must precede the initial renal sodium retention. The hemodynamic changes occur very early in normal pregnancy, with women exhibiting renal and systemic vasodilation, activation of the RAAS, and the beginning of the PVE by 6 wk of pregnancy (18). In fact, peripheral vasodilation had begun as early as ∼3 wk after conception (5 wk after last menstrual period) (73) in women. In the baboon, declines in peripheral resistance were detectable by 4 wk of pregnancy, whereas PVE was not detectable until ∼12 wk (72), suggesting that an underfill signal precedes PVE.

Thus, as discussed above, there is suggestive evidence to support the underfill hypothesis of PVE in pregnancy. To directly test whether chronic vasodilation in a nonpregnant animal, with normal renal function, would lead to PVE, we subjected virgin female rats to chronic pharmacological vasodilation.

Impact of Chronic Vasodilation on Normal Kidney Function

In the first series of studies, we administered two mechanistically different vasodilators over a period of 14 days to virgin female rats: nifedepine (calcium channel blocker) and sodium nitrite (NO-dependent vasodilator) (25). Both vasodilators produced mild sustained hypotension (∼7% fall vs. control rats given vehicle), and both also resulted in PVE, suggesting that the primary vasodilatory signal does not rely on a specific mechanism to produce PVE. Data for nifedipine are shown in Fig. 7. In addition to PVE and the associated decline in hematocrit, we also saw reduction in plasma sodium and plasma osmolality with both vasodilators (25). This closely resembles the responses in pregnancy where there is water retention in excess of sodium, leading to falls in plasma osmolality and complex resetting of both osmotic and nonosmotic arginine vasopressin (AVP) release (45, 83). One difference, however, was that, unlike pregnancy, we did not observe an increase in plasma renin activity after 14 days of chronic vasodilation (25), possibly because we were administering a constant dose of vasodilators, and that renal sodium retention had adequately “refilled” the circulation after 14 days of chronic vasodilation.

Fig. 7.

Mean change in blood pressure (BP; A) and change in plasma volume (PV; B) expressed as %change from control in virgin female rats after 14 days of nifedepine (NIF), angiotensin converting enzyme inhibitor (ACEI), NIF + angiotensin receptor type 1 antagonist losartan (LOS), and NIF + mineralocorticoid receptor antagonist spironolactone (SPR). *P < 0.05 in the change from control. Data were derived from Ref. 101.

Given the importance of the RAAS in renal sodium retention, we conducted additional studies to investigate the role of an intact RAAS in the PVE of chronic vasodilation. Using chronic angiotensin-converting enzyme inhibition (ACEI), we obtained significant declines in BP without any PVE at 14 days (101). In fact, the hypotension was exaggerated in the last few days of ACEI, suggesting that the failure to refill caused additional falls in BP (101) (Fig. 7). Thus removal of the entire RAAS cascade completely suppressed PVE. In addition to inhibiting the RAAS, ACEI leads to kinin stimulation, causing peripheral vasodilation and a fall in BP in a nonpregnant, initially euvolemic rat. In contrast, ANG II inhibition using selective ANG II type 1 receptor blockade (AT1RB) does not lower BP in a conscious euvolemic animal, since the level of basal ANG II activation is too low to control systemic resistance (8). Therefore, we administered a combination of AT1RB + nifedipine and observed a similar exaggerated fall in BP, as seen with ACEI, and again no evidence of PVE. In both cases, the exaggerated fall in BP with RAAS inhibition could be due to failure to refill (suppression of PVE) and/or lack of compensatory ANG II activation. To further investigate, we used a combination of nifedipine and spironolactone [a MR antagonist (MRA)] and again saw vasodilation and hypotension as well as failure of PVE (101). Interestingly, the hypotension seen with nifedipine + MRA was similar to that seen with nifedipine alone, but again the PVE was suppressed, suggesting that ANG II-induced vasoconstriction is necessary to prevent exaggerated declines in BP.

In addition to suppressing PVE, ACEI prevented increases in medullary α-ENaC, as did both AT1RB and MRA in the presence of nifedipine. This suggests that stimulation of the MR with aldosterone is the most important factor in vasodilation-induced increases in medullary α-ENaC protein abundance. We also observed increases in renal medullary PDE5 protein abundance with chronic nifedipine, as well as chronic sodium nitrite administration (25). The increased medullary PDE5 was suppressed by both ACEI and AT1RB, but persisted during MRA. This suggests that the vasodilation-induced increase in medullary PDE5 is due to AT1R stimulation, not MR (101). In fact, this effect has already been reported with ANG II stimulating PDE5A expression via the AT1R in rat vascular smooth muscle (39) and the left cardiac ventricle (57).

As with pregnancy, chronic vasodilation with nifedipine increased α-ENaC protein abundance, without any change in the other ENaC subunits, and with no increase in NCC. This stimulation of α-ENaC and PDE5 with chronic vasodilation resembles the changes seen in pregnancy, discussed above. In both pregnancy and chronic vasodilation, the PVE is dependent on the RAAS and can be prevented by MRA.

Conclusion

There are vascular and tubular adaptations that occur during pregnancy that permit renal sodium retention, PVE, and reductions in BP to coexist. While many of these changes remain mysterious, we suggest that RAAS activation plays a primary driving role in the PVE, by activation of ENaC and pendrin in collecting duct. RAAS stimulation of medullary PDE5 activity is a secondary permissive factor that prevents some of the natriuretic influences activated in pregnancy from predominating. A normal kidney from a nonpregnant animal can also retain sodium and produce chronic PVE, in response to a primary vasodilatory signal, and this may also provide the initial signal in the gestational PVE.

Perspectives

The underfill hypothesis for gestational PVE is attractive when one focuses on the RAAS, but other volume sensing and regulatory systems behave differently. The increased NO/ANP discussed above is representative of an “overfilled” state, while the complex adaptations that occur in control of AVP release suggest continual resetting of both osmotic and nonosmotic (volume-dependent) AVP release to recognize “normal fill” (45). While this review addresses some of the molecular mechanisms allowing simultaneous activation of RAAS and NO/ANP to coexist, we are far from understanding how the different volume regulatory systems are reset in normal pregnancy. However, it is clear that a normal kidney can be persuaded to retain sodium and water when a volume expansion is appropriate/required, as in pregnancy. The blood volume also increases, due to aldosterone-dependent sodium retention, as a result of exercise training (20). Again, the volume expansion may be viewed as appropriate, since the increased blood volume boosts cardiac output and also aids in thermoregulation (20). In contrast, a normal kidney may retain sodium during disease states, such as heart failure and liver cirrhosis (19, 44, 51, 58, 62, 89), but here the volume expansion is inappropriate and contributes to the pathology.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

C.A.W. and C.B. prepared figures; C.A.W., J.M.S., and C.B. drafted manuscript; C.A.W., J.M.S., and C.B. edited and revised manuscript; C.A.W., J.M.S., and C.B. approved final version of manuscript.