CHBPR - Excellence Award for Hypertension Research - THE INTRARENAL RENIN-ANGIOTENSIN AND DOPAMINERGIC SYSTEMS: CONTROL OF RENAL SODIUM EXCRETION AND BLOOD PRESSURE

The renin-angiotensin system (RAS) is a coordinated hormonal cascade critical to the control of renal sodium (Na⁺) excretion and blood pressure (BP) (1). Angiotensin II (Ang II), the principal RAS effector peptide, binds to two distinct receptors, the Ang type-1 receptor (AT₁R) and the Ang type-2 receptor (AT₂R) with high affinity (1,2). The vast majority of actions of Ang II are transmitted via AT₁Rs, including cellular dedifferentiation and proliferation; vasoconstriction, reduction of vascular compliance, cardiac contractility, increased renal tubule sodium (Na⁺) reabsorption; aldosterone, vasopressin and endothelin secretion; salt appetite; thirst and activation of the sympathetic nervous system (1,2). In contrast, AT₂Rs generally oppose the actions of Ang II via AT₁Rs under most circumstances (1–4).

Another major regulatory system in cardiovascular and renal physiology is the peripheral dopaminergic system. Dopamine is mainly synthesized in renal proximal tubule cells (RPTCs) via the decarboxylation of L-dihydroxyphenylalanine (L-DOPA) that has been filtered at the glomerulus and transported into the RPTC across the apical brush border (5). Synthesized DA exits the cell mainly across the apical plasma membrane into the lumen where it can bind to and activate specific DA D₁-like receptors (D_1-likeR: D₁ and D₅) (5,6). D₁-like receptor activation induces vasodilation and inhibits renal Na⁺ reabsorption, actions which also oppose those of Ang II via AT₁Rs.

The purpose of this brief review is to summarize some of the key findings leading to our present concepts of AT₂Rs and D₁-like receptors that oppose the actions of Ang II via AT₁Rs within the kidney.

Evidence for an independent functional intrarenal RAS

Although renin was identified in the brain and the adrenal cortex in the late 1960s and early 1970s, the intrarenal RAS was the first independent functional tissue RAS to be described (7–9). The initial observations were from in vivo studies which demonstrated that intrarenal inhibition of the RAS with angiotensin converting enzyme (ACE) inhibitors or Ang receptor blockers, at infusion rates that did not alter systemic BP during the experimental period, increased renal plasma flow, glomerular filtration rate and Na⁺ and water excretion (7) (Figure 1). These results were later confirmed by more rigorous approaches showing that small intrarenal doses of Ang receptor blocker, while not altering pressor responses to systemically administered Ang II, induced marked increases in renal hemodynamic and tubular function (8,9) (Figure 2). Later, it was demonstrated that the mRNAs and proteins for all of the system components (renin, angiotensinogen (Agt), ACE and AT₁Rs) are localized in a site-specific manner within the kidney and that intrarenal formation of Ang II occurs independently of renal uptake of the peptide (1).

Figure 1.

Evidence for a functional intrarenal renin-angiotensin system in uninephrectomized conscious dogs. Panel A: Renal blood flow (RBF); Panel B: glomerular filtration rate (GFR); Panel C: Urinary Na⁺ excretion (U_NaV) in response to intrarenal arterial administration of ACE inhibitor SQ-20881 (2 μg/kg/min) or Ang receptor blocker P-113 (saralasin; 2 μg/kg/min). Control vehicle infusion, white bars; experimental agent infusion, black bars. Sham data include vehicle infusion only. Data are expressed as mean ± 1 SE. Adapted from Kimbrough HM et al. Circ Res. 1977;40:174–178.

Figure 2.

Validation of an independent functional intrarenal renin-angiotensin system in uninephrectomized conscious dogs. Estimated renal plasma flow (ERPF; Panel A), glomerular filtration rate (GFR; Panel B), urine flow rate (U_VOL; Panel C) and urinary Na⁺ excretion (U_NaV; Panel D) in response to low-dose intrarenal arterial infusion of Ang receptor blocker saralasin (0.07 μg/kg/min). Numbers on abscissa represent 20 min clearance periods. Adapted from Levens NR et al. Endocrinology. 1983:112:43–49 with permission.

Additional evidence for a separate renal tissue RAS came from studies showing that intrarenal Ang II levels were elevated in the nanomolar range in renal interstitial fluid compared to picomolar concentrations in plasma, that intrarenal Ang II concentrations were markedly increased compared to plasma levels in response to Na⁺ restriction and that this response was blocked with intrarenal renin inhibition (10). Cellular studies demonstrated that Agt, Ang I and Ang II could be co-localized with renin in proximal tubule and juxtaglomerular cells, that Ang peptides were released from these cells and that the release was regulated (11). Further functional studies demonstrated that combined intrarenal RAS blockade with low doses of ACE inhibitor, AT₁R blocker and renin inhibitor, while confined to the kidney, augmented major increases in renal function that were blocked with concurrent intrarenal Ang II administration (12). Altogether, these studies provided strong support for the existence of an independent functional intrarenal RAS.

Definitive molecular evidence for an independent intrarenal RAS and its importance in the control of BP was obtained using a transgenic mouse model over-expressing Agt either in the kidney or the systemic circulation (13). Expression of Agt selectively within the kidney induced chronic hypertension independently of the endocrine RAS (13). Within the kidney, there is now substantial evidence for a separate intratubular RAS in which Ang II formation is auto-amplified by Ang II-induced up-regulation of Agt, creating a positive feedback loop that may play a role in renal tissue damage and hypertension (14). Current studies are also providing evidence for intracellular RASs that are independently functioning in a specific subcellular compartment. Such subcellular RASs have recently been described both within nuclei and mitochondria (15, 16).

AT₂R Expression and Cell Signaling Pathways

The AT₂R is a 7-transmembrane G-protein-coupled receptor, encoded on the X -chromosome, with only 34% amino acid sequence homology with the AT₁R (2). AT₂Rs are expressed ubiquitously at very high levels in the fetus, but decline precipitously in the neonatal period in most, but not all tissues. Although the expression of AT₂Rs is substantially lower than that of AT₁Rs in the adult, AT₂R mRNA and protein can be easily detected in the adult kidney, adrenal cortex, heart and vasculature and predominates over AT₁Rs in the uterus, ovary, adrenal medulla and in discrete areas of the brain (17–20). Within the kidney, AT₂Rs are expressed predominantly in RPTCs and glomeruli (18, 19).

The cell signaling mechanisms of AT₂Rs differ substantially from those of AT₁Rs. AT₂R activation initiated by binding of Ang II to the receptor in the plasma membrane triggers G protein coupling of Giα2 and Giα3 via the third intracellular loop of the receptor. G protein coupling initiates the activation of phosphotyrosine phosphatases, which dephosphorylate and inactivate mitogen-activated protein (MAP) kinases including extracellular-regulated kinase (ERK)-1 and ERK-2. Phosphotyrosine phosphatase activation can also occur through a non-G protein-coupled mechanism. MAP kinase inhibition via AT₂Rs opposes MAP kinase activation as a result of AT₁R activation. The opposing action of AT₁Rs and AT₂Rs on MAP kinases is considered a fundamental signaling mechanism for receptor-receptor interactions (1–4).

AT₂Rs can also activate the phospholipase A₂ pathway leading to arachidonic acid release and long-term AT₂R activation can also increase the biosynthesis of ceremides, which can stimulate stress kinases and caspases to induce apoptosis (4).

Vascular AT₂R Actions and Mechanisms

Overwhelming evidence currently exists that AT₂Rs mediate vasodilation and oppose the AT₁R-mediated vasoconstrictor actions of Ang II (21–35). AT₂R-mediated vasodilation has been demonstrated in small resistance arteries of the mesenteric, uterine, adrenal, coronary and peripheral circulations in many animal models and in humans. AT₂R-induced vasodilation has also been demonstrated in large capacitance vessels such as the aorta and in the fetus (29–31). AT₂R-stimulated vasodilation is mediated by a signaling cascade composed of bradykinin (BK), nitric oxide (NO) and 3′,5′-cyclic guanosine monophosphate (cGMP) (Figures 3 and 4)(21;36–38). AT₂Rs increase NO and cGMP production either by increasing BK production with activation of BK B2 receptors or by direct activation of NO production independently of BK (39–41).

Figure 3.

Ang II releases renal bradykinin (BK) by AT₂R activation. Renal interstitial fluid BK levels in response to intravenous infusion of Ang II, Losartan, an AT₁R antagonist; PD, PD-123319, an AT₂R antagonist, and combinations in Sprague-Dawley rats. Control vehicle infusions, white bars; experimental agent infusions, black bars. Data are expressed as mean ± 1 SE. * P<0.0001 from control; + P<0.05, ++P<0.0001 from Ang II alone. Data from Siragy HM et al. Am J Physiol Reg Int Comp Physiol. 1996;271:R1090–R1095 and Siragy HM and Carey RM, Hypertension 1999;33:1237–1242.

Figure 4.

Ang II releases renal cyclic GMP (cGMP) by AT₂R activation. Renal interstitial fluid cGMP levels in response to intravenous infusion of Ang II; Losartan, an AT₁R antagonist; PD, PD-123319, an AT₂R antagonist, and combinations in Sprague-Dawley rats. Control vehicle infusion data, white bars; experimental agent infusion, black bars. Data represent mean ± 1 SE. * P<0.001 from vehicle or time control; + P<0.001 from Ang II alone. Adapted from Siragy *HM and Carey RM. J Clin Invest. 1996;97:1978–1982 and Siragy HM and Carey RM. J Clin Invest. 1997:100:264–269 with permission.

AT₂R-mediated vasodilation is most readily demonstrated when AT₁Rs are blocked with an AT₁R antagonist (22,23,25,26). This is almost certainly because AT₁R expression predominates over that of AT₂Rs in the vasculature (42,43). AT₂R-stimulated vasodilation is also augmented when the RAS is activated, such as during Na⁺ restriction, Ang II infusion or in renal vascular hypertension (21, 22, 44). Under all three circumstances, AT₂Rs are upregulated, enhancing the vasodilator response to Ang II (18,21,44). Another condition which upregulates AT₂R expression (by 300%) and unmasks its vasodilator action is increased pressure load from aortic banding (29,30). AT₂R blockade with specific antagonist PD-123319 (PD) or BK B₂ receptor inhibition with icatibant restores the diminished Ang II contractile responses and abolishes the 9-fold increase in aortic cGMP stimulated by Ang II under these circumstances (29,30). Taken altogether, the results of these studies emphasize the likely importance of counter-regulatory AT₂R upregulation and activation in circulatory disorders associated with chronic vasoconstriction via AT₁Rs.

The vasodilator and depressor actions of AT₂Rs are both acute and chronic and are not accompanied by desensitization, rendering these receptors a potential therapeutic target in hypertension (23,25). Indeed, the BP lowering effects of AT₁R blockade may be mediated, at least in part, by AT₂R activation as a result of increased renin biosynthesis and release and increased Ang II that can act via unblocked AT₂Rs (21,22). An example of this principle was demonstrated in diabetic, hypertensive humans in whom chronic AT₁R inhibition upregulated vascular AT₂Rs and facilitated a vasodilator response to Ang II in vitro (34). In addition, in spontaneously hypertensive rats during AT₁R blockade, pharmacological activation of AT₂Rs by Compound 21 (a non-peptide AT₂R agonist with >25,000-fold selectivity for AT₂Rs over AT₁Rs) resulted in decreased BP (35). These observations indicate the potential importance of a non-peptide AT₂R agonist combined with an AT₁R blocker in the treatment of hypertension.

AT₂R-mediated vasodilation and hypotension were confirmed in AT₂R-null mice. (45). Although baseline BP was similar between AT₂R-null and wild-type (WT) mice, AT₂R-null mice demonstrated marked and sustained hypersensitivity to the pressor actions of infused Ang II over the course of 7 days, emphasizing the importance of AT₂Rs in counter-regulating Ang II actions via AT₁Rs. Ang II pressor hypersensitivity was accompanied by a highly significant reduction in baseline and Ang-II stimulated renal interstitial levels of BK, NO and cGMP in AT₂R-null mice.

Intrarenal AT₂R Actions and Mechanisms

AT₂R-null mice also had marked antinatriuresis (and inhibition of pressure- natriuresis) during the chronic Ang II infusion that was not present in WT mice (45). These results suggested the possibility that intrarenal AT₂Rs might increase renal Na⁺ excretion via BK, NO and cGMP (45).

We subsequently explored and presented definitive evidence that intrarenal AT₂R activation mediates natriuresis (46–48). These studies were enabled by the technique of renal interstitial microinfusion of pharmacological agents, which affords direct evaluation of the intrarenal mechanisms governing renal function without systemic hormonal or hemodynamic influences. Selective intrarenal AT₁R blockade in rats induced a highly significant natriruesis that was abolished by intrarenal co-administration of AT₂R-specific antagonist PD indicating that the natriuretic effect of AT₁R blockade is mediated by AT₂R activation (46).

However, we were surprised to find that intrarenal Ang II infusion did not alter Na⁺ excretion even at high infusion rates. This finding provoked a question as to whether a downstream metabolite of Ang II might be required for renal AT₂R activation. Indeed, intrarenal infusion of des-aspartyl¹ Ang II (Ang III) into systemically AT₁R blocked rats induced a significant natriuretic response, which was abolished with intrarenal co-infusion of PD (46) (Figure 5). Intrarenal Ang III infusion in the absence of systemic AT₁R blockade did not change Na⁺ excretion, similar to AT₂R-mediated vascular responses (46). In follow up of this observation, we hypothesized that Ang II needs to be converted to Ang III to interact with AT₂Rs within the kidney. Ang II is converted to the heptapeptide Ang III by aminopeptidase A (APA), and Ang III is converted to the hexapeptide Ang IV by aminopeptidase N (APN). In the presence of systemic AT₁R blockade, intrarenal infusion of Ang III induced a natriuretic response that was markedly augmented by intrarenal co-administration of APN inhibitor 2-amino-methylsulfonyl-butane-thiol, methane-thiol (PC-18) (47). The PC-18-augmented natriuresis was abolished by intrarenal AT₂R inhibition with PD, indicating an AT₂R-mediated effect (47). The necessity for conversion of Ang II to Ang III for AT₂R-mediated natriuresis was confirmed by demonstrating that intrarenal administration of Ang II is only effective in inducing natriuresis when APN is blocked and that this response is abolished by intrarenal co-administration of APA inhibitor 3-amino-4-thio-butyl-sulfonic acid (EC-33) (48). Taken altogether, these studies demonstrate that Ang III is the preferred agonist for AT₂R-mediated natriuresis. In systematic receptor binding studies, Ang III has been found to have about 30-fold selectivity over Ang II for AT₂Rs (49). Confirming these results, Ang III has been shown to be the preferred AT₂R agonist in other tissues such as the coronary microcirculation and adrenal cortex (50, 51).

Figure 5.

Ang III is the preferred endogenous AT₂R agonist mediating natriuresis. Urinary Na⁺ excretion (U_NaV) in anesthetized Sprague-Dawley rats in response to direct renal interstitial infusion of vehicle (white bars), Ang II (black bars), Ang III (gray bars) or Ang III +PD (PD-123319, an AT₂R antagonist) (striped bars). Data are expressed as mean ± 1 SE. * P<0.05, ** P<0.01, *** P<0.001 from time control. Adapted from Padia SH et al. Hypertension. 2006.

Recent in vivo studies have demonstrated that Ang III induces natriuresis via AT₂R activation in the renal proximal tubule by a NO/cGMP signaling mechanism (52). In vitro studies also have recently shown that AT₂Rs reduce AT₁R function in the proximal tubule by the common NO/cGMP pathway and also reduce AT₁R mRNA via the ubiquitous transcription factor Sp1 (53). Ligand-activated AT₂Rs also heterodimerize with AT₁Rs reducing their expression via protein-protein action in the plasma membrane (53). Thus, AT₂Rs may oppose AT₁Rs by several pathways in the kidney.

The recent in vivo investigations cited above have confirmed that Ang III is the preferred endogenous ligand for the activation of renal AT₂Rs (52). Unexpectedly, these studies were unable to elicit a natriuretic effect of Ang (1-7), which has counter-regulatory effects offsetting AT₁R actions in other tissues (55). No natriuretic response to Ang (1-7) was observed at equimolar doses as Ang III even in the presence of AT₁R blockade, ACE inhibition to reduce Ang (1-7) metabolism or APA blockade to augment Ang (1-7) formation from Ang II via ACE-2 (52). However, these studies did demonstrate that intrarenal administration of APN inhibitor PC-18 induces natriuresis even in the absence of systemic AT₁R blockade. Furthermore, renal interstitial Ang peptide levels during Ang III administration with and without PC-18 demonstrated a marked augmentation of renal interstitial and tissue Ang III concentrations and AngIII/Ang II ratios during PC-18 administration, consistent with the role of Ang III in the augmented natriuretic effect (52). These studies also demonstrated that systemic administration of the highly selective non-peptide AT₂R agonist Compound 21 induces natriruesis that is abolished with intrarenal AT₂R antagonist PD in both male and female rats even in the absence of AT₁R blockade, suggesting the potential for this compound as a natriuretic/diuretic agent in the treatment of disorders associated with extracellular fluid volume expansion and hypertension.

Recent studies also have suggested that AT₂Rs in the thick ascending limb of Henle (TAL) may contribute to the natriuretic response (54,55). Ang II increases NO production in TALs via activation of AT₂Rs and NO inhibits the Na⁺/K⁺/2Cl^- co-transporter and reduces Na⁺ reabsorption in this nephron segment (55). Whether this response requires Ang II conversion to Ang III awaits further study.

The Intrarenal Dopaminergic System

The renal dopaminergic system is a major hormonal system controlling renal Na⁺ excretion and BP (5). D_1-likeR activation inhibits renal Na⁺ reabsorption through an adenylyl cyclase-cyclic AMP (cAMP) mechanism. In both humans and experimental animals highly selective D_1-likeR agonist fenoldopam elicits a substantial natriuretic response that is based almost exclusively on inhibition of renal proximal tubule Na⁺ reabsorption (5, 56–60). Thus, the renal dopaminergic system is an important counter-regulatory system offsetting the antinatriuretic actions of AT₁Rs. Indeed, fenoldopam was demonstrated to be close to ideal as an antihypertensive agent in that it normalized BP without reflex tachycardia and induced natriuresis in patients with primary hypertension (61) (Figure 6). In spite of its low bioavailability, these and other favorable observations led to FDA approval for emergency treatment of hypertension in intensive care settings.

Figure 6.

Oral fenoldopam lowers BP in hypertensive humans. BP responses to oral fenoldopam (SKF-82526-J) in patients with primary hypertension. Data are expressed as mean ± 1 SE. Adapted from Carey RM et al. J Clin Invest. 1984;74:2198–2207 with permission.

The physiological importance of the renal dopaminergic system in the control of Na⁺ excretion was demonstrated initially during the 1980s. Studies employing intrarenal arterial administration of highly selective D_1-likeR antagonist SCH-23390 revealed that, similar to the intrarenal RAS, DA synthesized within the kidney acts in a local cell-to-cell (paracrine) manner exclusively at the renal proximal tubule to control Na⁺ excretion (62,63) (Figure 7). These studies demonstrated that approximately 60% of basal Na⁺ excretion in the Na⁺-replete state is controlled by intrarenal dopaminergic mechanisms acting at the proximal tubule. These observations were later confirmed using renal interstitial infusion of D₁R antisense oligodeoxynucleotides to inhibit receptor expression directly (64). The importance of the renal dopaminergic system in the control of Na⁺ excretion was further underscored by the demonstration that the natriuretic and diuretic effects of D_1-likeRs are dependent on the state of Na⁺ balance. In Na⁺ - deplete states, D_1-likeR - mediated natriuresis does not occur, whereas in normal or high Na⁺ states D_1-likeRs induce a robust natriuretic response (60). Additional evidence for the physiological importance of renal dopaminergic control of Na⁺ excretion included the observation that renal DA production is increased during Na⁺ surfeit but reduced during Na⁺ depletion (65). Elegant studies in mice with selective proximal tubule knockout of aromatic amino acid decarboxylase, the enzyme generating DA from L-DOPA, inducing intrarenal DA depletion have recently confirmed the importance of intrarenal DA in the control of Na⁺ excretion and BP (66). Taken altogether, these studies strongly support the physiologic importance of renal DA and D_1-likeRs as counter-regulatory systems limiting at least in part the Na⁺-retaining actions of intrarenal Ang II via AT₁Rs.

Figure 7.

Evidence that intrarenal dopamine controls renal Na⁺ excretion by a paracrine mechanism acting at the level of the renal tubule. Urinary Na⁺ excretion (U_NaV) in uninephrectomized conscious dogs infused intrarenally with D_1-LIKE receptor antagonist SCH-23390. Data are expressed as mean ± 1 SE. * P<0.001, ** P<0.0001 from pre-control; † P<0.05, †† P<0.01 from time control. Adapted from Siragy HM et al. Am J Physiol Renal Physiol. 1988;257:F469–F477 with permission.

In the mid-1990s, with antibodies directed towards the extracellular domain of D₁Rs, receptor protein was localized in the renal proximal tubule and in several other cells and tissues (67–69). Similar to the AT₂R, D₁R mRNA is expressed only in low copy and was difficult to demonstrate using standard molecular techniques. However, D₁R protein cellular distribution was later confirmed using more sensitive in situ amplification of D₁R mRNA (70).

D₁R/AT₂R Interactions

Renal interstitial administration of fenoldopam in Na⁺-loaded rats elicits a robust natriuretic response that is abolished with intrarenal co-administration of D_1-likeR antagonist SCH-23390 (71). However, we were surprised to find that fenoldopam-induced natriuresis is also completely inhibited with intrarenal co-infusion of AT₂R antagonist PD (71) (Figure 8). To explore the possible mechanism of AT₂R involvement in D_1-likeR-induced natriuresis, studies were performed to determine the intracellular trafficking of AT₂Rs in RPTCs (71,72). In vivo administration of fenoldopam was associated with translocation of AT₂Rs from intracellular sites to the apical plasma membranes of RPTCs. Fenoldopam-induced AT₂R translocation to the apical plasma membrane and natriuresis were abolished in the presence of microtubulin inhibitor nocodazole but were unaffected by actin microfilamint inhibitor cytochalasin D, suggesting that microtubules are required for the translocation process (72). Because D_1-likeRs signal via an adenylyl cyclase, cAMP and protein kinase A pathway, we explored the role of these signaling processes in D_1-likeR – induced AT₂R recruitment to the apical plasma membrane and its necessity for the natriuretic response. In vivo experiments demonstrated that intrarenal administration of direct adenylyl cyclase activator forskolin together with 3-isobutyl-1-methyl xanthine (IBMX) to inhibit its metabolism increased renal interstitial fluid levels of cAMP, stimulated AT₂R recruitment to the RPTC apical plasma membrane and induced natriruesis that was abolished with AT₂R antagonist PD (72). Direct agonist stimulation of D_1-likeRs was not necessary for AT₂R-mediated natriuresis since forskolin/IBMX-induced AT₂R translocation and natriuresis persisted in the presence of D_1-likeR blockade with SCH-23390 (72). Therefore, the mechanism by which AT₂Rs and D_1-likeRs interact during high Na⁺ conditions to induce natriuresis is D_1-likeR-cAMP signaling, which provides the necessary stimulus for AT₂R translocation and natriuresis. We also demonstrated that similar to agonist-stimulated D₁R recruitment to the plasma membrane, AT₂Rs are translocated via microtubules to the apical plasma membrane, where they are required to induce the natriuretic response (72, 73).

Figure 8.

Intrarenal interstitial fenoldopam-induced natriruesis is abolished by intrarenal D_1-LIKE receptor antagonist SCH-23390 (SCH) or AT₂R antagonist PD-123319 (PD) in anesthetized, uninephrectomized Sprague-Dawley rats. Vehicle infusion, white bars; fenoldopam (1 μg/kg/min) infusion, black bars; fenoldopam + PD infusion, gray bars; fenoldopamn + SCH infusion, striped bars. Data are expressed as mean ± 1 SE. Numbers on the abscissa refer to experimental one-hour periods. * P<0.01, ** P<0.001 from vehicle control. Adapted from Salomone LJ et al. Hypertension. 2007;49:155–161 and Padia SH et al. Hypertension. 2012;59:437–445.

Recently, AT₁R-null animals were demonstrated to have increased longevity (74). In current studies, the aging process is beginning to be linked to reduction in AT₂R and D₁R expression and/or activation in different tissues and at the mitochondrial level (16, 75,76). It is possible that D₁R and/or AT₂R pharmacological activation may provide a new target for the reversal of certain aspects of the aging process and/or for the extension of lifespan in the future.

Conclusions and Perspectives

In conclusion, the intrarenal renin-angiotensin and dopaminergic systems play a major critical role in cardiovascular and renal function, the subject of this brief review. AT₂Rs and D₁Rs cooperatively oppose the vasoconstrictor and antinatriuretic functions mediated by Ang II at AT₁Rs. Reduced AT₂R expression and/or activity may contribute to the initiation and/or acceleration of disease processes including hypertension, edema-forming states and inflammation/fibrosis leading to cardiovascular and renal damage. Conversely, pharmacological activation of AT₂Rs and/or D₁Rs may provide therapeutic advantages or even preventive strategies in the presence or absence of AT₁R blockade. Increased understanding of angiotensin and dopamine receptor functions and interactions currently provides hope for improved treatment and prevention of hypertension and other Na⁺/fluid retaining states and for the extension of healthier lives in the future.