BLOOD PRESSURE AND THE RENAL ACTIONS OF AT₂ RECEPTORS

The renin-angiotensin system (RAS) is the most thoroughly studied and important hormonal system regulating blood pressure (BP) and renal function. The RAS is composed of a single substrate, angiotensinogen, and multiple enzymes [prorenin, renin, angiotensin converting enzyme (ACE), ACE-2, cathepsin B, chymase, and aminopeptidases A and N], peptides [angiotensins (Ang) I, II, III, (1–7), (1–12) and alamandine] and receptors [AT₁, AT₂, Mas and MrgD] (1,2). Despite the complexity of the system, however, the vast majority of the actions of the RAS, including vasoconstriction, are propagated by Ang II via AT₁ receptors (AT₁Rs), and AT₁R blockers comprise an effective first line pharmacologic class in the treatment of hypertension.

Worldwide, hypertension is the most important risk factor for cardiovascular disease, stroke and death and affects approximately one-third of the adult population. Surprisingly, therefore, the fundamental mechanisms by which hypertension develops and is sustained are not understood. A major proposed mechanism for the initiation of hypertension is a fundamental defect in the capacity of the kidneys to excrete sodium (Na⁺). Over time, a compensatory increase in renal perfusion pressure permits appropriate Na⁺ excretion (pressure-natriuresis), but also induces hypertension.

Ang II, the primary peptide mediating the effects of the RAS, acts at two major receptors, AT₁R and AT₂R (1). The functional role of AT₂Rs was clarified by studies in the mid-1990s and early 2000s demonstrating that receptor activation induces a “vasodilator cascade” by activating kininogen, increasing, bradykinin (BK) and facilitating BK action at its B₂ receptors, resulting in nitric oxide (NO) production and, consequently, cyclic GMP (cGMP) formation (3). The vasodilator cascade could function either in the presence or absence of BK. NO and cGMP were established as common mediators of the majority of AT₂R actions. By demonstrating that genetic deletion of AT₂Rs resulted in pressor and antinatriuretic hypersensitivity to Ang II, we generated the hypothesis that AT₂Rs might be endogenous natriuretic receptors which could be important in the pathophysiology of hypertension (4).

In addition to vasoconstriction, Ang II acts via AT₁Rs to induce Na⁺ retention. Renal AT₁Rs are both necessary and sufficient for inducing and sustaining hypertension during Ang II infusion, and increased Na⁺ reabsorption in the renal proximal tubule (RPT) is a major determinant of this response (5,6). In contrast, the role of AT₂Rs in the control of Na⁺ excretion and BP has been less clearly defined. AT₂Rs are expressed in the adult kidney primarily in the RPT (7).

Our recent studies have provided clear evidence for a major role of RPT AT₂Rs in inhibition of Na⁺ reabsorption (8–11). These studies indicate that in normal Sprague-Dawley rats, Ang II must be metabolized to des-aspartyl¹-Ang II (Ang III) in order to induce an AT₂R-mediated natriuretic response (11). We established that intrarenal Ang III (1) inhibits Na⁺ reabsorption predominantly at the RPT, (2) induces natriuresis by a cGMP-dependent mechanism, (3) induces natriuresis in the absence of systemic AT₁R blockade when Ang III metabolism by aminopeptidase N is blocked and (4) is the preferred endogenous AT2R agonist inducing natriuresis (8–11). For decades, Ang II has been considered the major agonist for both AT₁Rs and AT₂Rs. Our studies demonstrated for the first time that Ang III is the preferred endogenous agonist for AT₂R-induced natriuresis, likely due to preferential agonist binding to AT₂Rs. Receptor crystallization in the presence of different endogenous peptide agonists will be required to determine the structural characteristics of receptorbinding pocket-peptide interaction that facilitates Ang III over Ang II.

Importantly, we have also shown that the natriuretic response to Ang III is accompanied by translocation of AT₂Rs from intracellular sites along the microtubules to the apical plasma membranes of RPT cells and that the natriuresis is accompanied by internalization and inactivation of major RPT Na⁺ transporter molecules Na⁺-H⁺ exchanger-3 (NHE-3) and Na⁺/K⁺ATPase (NKA) (11). Consistent with these observations, we also demonstrated that acute activation of RPT AT₂Rs with exogenous AT₂R non-peptide agonist Compound-21 (C-21) induces natriuresis in normotensive rats via the BK-NO-cGMP-dependent signaling pathway accompanied by recruitment of AT₂Rs to RPT apical plasma membranes and internalization and inactivation of NHE-3 and NKA (12). C-21 is able to reduce BP in this model in the absence of concurrent AT₁R blockade. Most recently, we have confirmed and extended these studies in a model of Ang II-dependent hypertension (the Ang II infusion model) (13). Here we have shown that chronic AT₂R activation with C-21 prevented the initial (24h) Ang II-induced Na⁺ retention, induced sustained negative Na⁺ balance and lowered BP to near-control levels during a 7-day infusion period (13). As with the aforementioned acute studies (12), chronic administration of C-21 was equally effective administered intrarenally or systemically, translocated AT₂Rs to the apical plasma membranes and internalized/inactivated NHE-3 and NKA in RPT cells (13). C-21-induced natriuresis derived selectively from RPT inhibition of Na⁺ reabsorption because it was additive to that of diurectics acting in the distal tubule (chlorothiazide) or cortical collecting duct (amiloride). In addition to preventing Na⁺ retention and hypertension, C-21 was equally effective in normalizing BP once the Ang II-induced hypertension had been established (13). These results strongly suggest that AT₂R agonists are effective natriuretic and diuretic agents that improve the pressure-natriuresis relationship and, therefore, are potential therapeutic agents in Na⁺ retaining states and hypertension.

Returning to the role of endogenous Ang III and AT₂Rs in the pathogenesis of hypertension, we have been unable to show a natriuretic responseeither to exogenous or endogenous (intrarenal AT₁R blockade) Ang III in the hypertensive or pre-hypertensive spontaneously hypertensive rat (SHR) whereas their Wistar-Kyoto (WKY) and SD controls have robust natriuretic responses (14,15). These results strongly suggest a defect in AT₂R-mediated natriuresis in SHR that predates the hypertension (Figure 1). Studies are underway to establish the mechanisms that lead to defective AT₂R-mediated natriuresis in hypertension. These mechanisms most likely involve a pre-receptor defect, in which there may be accelerated intrarenal Ang III metabolism by aminopeptidase N or, alternatively, an AT₂R/post-receptor signaling defect in hypertension.

Figure 1.

Schematic representation of ligand-receptor interactions mediating natriuresis in the RPT in normal rodents (left) and absence of natriuresis in pre-hypertensive and hypertensive rodents (right). CAND: candesartan, AT₁R antagonist; Ang III: angiotensin III, preferred AT₂R agonist; Ang II: angiotensin II; Na⁺: sodium; ⇨: AT₂R-dependent inhibition of Na⁺ reabsorption; :reduced inhibition of Na⁺ reabsorption; response reduced.