Protective role of the angiotensin AT2 receptor in a renal wrap hypertension model

Nitric oxide: a physiological mediator of the type 2 (AT2) angiotensin receptor

Virtually all of the biological actions of angiotensin II (ANG II) have been thought to be mediated by the type 1 (AT1) angiotensin receptor and the function of the type 2 (AT2) receptor is unknown. We now describe a novel physiological action of ANG II to release nitric oxide (NO) mediated by the AT2 receptor in both the kidney and gastrointestinal tract. We present an integrated model for a counter-regulatory protective action of the AT2 receptor mediated by nitric oxide. In the kidney, ANG II at the AT2 receptor stimulates a vasodilator cascade of bradykinin (BK), NO and cyclic GMP which is tonically activated only during conditions of increased ANG II, such as sodium depletion. In the absence of the AT2 receptor, pressor and antinatriuretic hypersensitivity to ANG II is associated with BK and NO deficiency. In angiotensin-dependent hypertension, the hypotensive effect at AT1 receptor blockade is due at least in part to AT2 receptor stimulation and consequent increased activity of the vasodilator cascade. In the gastrointestinal tract, physiological quantities of ANG II stimulate the AT2 receptor releasing NO and cGMP leading to increased sodium and water absorption. In conclusion, NO is an important physiological mediator of ANG II at the AT2 receptor.

Neurohormonal modulation in cardiovascular disease

The renin-angiotensin system (RAS) is one of the oldest known hormone systems. Its effector hormone, angiotensin (Ang) II, acts through 2 receptor subtypes, AT(1) and AT(2). Most physiologic effects of Ang II, including vasoconstriction, renal salt and water retention, aldosterone and vasopressin release, and sympathetic facilitation, are mediated by AT(1). Recent data, however, suggest that Ang II also contributes to cell proliferation, left ventricular hypertrophy, vascular media hypertrophy, neointima formation in atherosclerosis, and nephrosclerosis by stimulation of AT(1) receptors. AT(2) receptors are associated with antiproliferation, cell differentiation and development, tissue regeneration, and apoptosis. They also antagonize AT(1) receptor-mediated effects, which suggests that the ratio of angiotensin receptors expressed on a particular cell can determine the net effect of Ang II. Selective AT(1) receptor antagonists ("sartans") have been used to treat several million hypertensive patients worldwide. These agents offer a powerful therapeutic alternative to angiotensin-converting enzyme (ACE) inhibitors, which reduce the generation of Ang II. Conversely, AT(1) receptor antagonists block the RAS by acting on cellular angiotensin receptors and do not interfere with the breakdown of kinins. These medications inhibit the RAS more completely than do the ACE inhibitors because their action is independent of Ang II-generating pathways. At the same time, early, preliminary data suggest that AT(1) receptor antagonists offer target-organ protection similar to that provided by the ACE inhibitors. Because AT(2) receptors are left unopposed and Ang II levels are increased with AT(1) receptor antagonist treatment, it is important to understand the function of AT(2) to fully appreciate the mechanisms of action of AT(1) receptor antagonists, especially their potential for target-organ protection.

The angiotensin type 2 receptor: variations on an enigmatic theme

Since its discovery and molecular characterization, the angiotensin AT2.receptor has been enigmatic with respect to signalling pathways and function. Evidence now emerges that angiotensin II exerts actions through the AT2 receptor which are directly opposed to those mediated by the AT1 receptor. This can be exemplified e.g. by mutually antagonizing effects on cell growth. Upregulated by the endogenous agonist itself, as well as by several growth- and differentiating factors in development and tissue injury, the AT2 receptor appears to act as a modulator of complex biological programmes involved in embryonic development, cell differentiation, tissue protection and regeneration, as well as in programmed cell death. Research on the AT2 receptor has thus unveiled hitherto unknown functions of the renin-angiotensin system extending far beyond the classical role of this old hormonal system in cardiovascular control.

Angiotensin II receptor blockers: review of the binding characteristics

Several angiotensin II receptor blockers (ARBs), including candesartan cilexetil, irbesartan, losartan, telmisartan, and valsartan, are currently approved by the US Food and Drug Administration (FDA) for the treatment of patients with hypertension. These agents share a common mechanism of action-antagonism of the angiotensin type 1 (AT1) receptor-and as a result, they block a number of angiotensin II effects that are relevant to the pathophysiology of cardiovascular disease, including vasoconstriction, renal sodium reabsorption, aldosterone and vasopressin secretion, sympathetic activation, and vascular and cardiac hyperplasia and hypertrophy. Unlike the angiotensin converting enzyme (ACE) inhibitors, these new drugs block the effects of angiotensin II regardless of whether it is produced systemically in the circulation or locally via ACE- or non-ACE-dependent pathways in tissues. ARBs also block the angiotensin II-induced feedback regulation of renin release, resulting in an increase in angiotensin II levels. With the AT1 receptor blocked, angiotensin II is available to activate the angiotensin type 2 (AT2) receptor, which mediates several potentially beneficial effects in the cardiovascular system, including vasodilation, antiproliferation, and apoptosis. Thus, ARBs provide a highly selective approach for regulating the effects of angiotensin II.

The AT2 receptor: fact, fancy and fantasy

The angiotensin AT2 receptor subtype was recently cloned and pharmacologically characterized but its function still remains elusive and controversial. It is a member of the G-protein coupled receptor superfamily with a minimal sequence homology with the AT1 receptor, responsible for the known effect of angiotensin II. The AT2 receptor displays a totally different signaling mechanisms from the AT1 receptor and involves various phosphatases. It is expressed at low density in adult tissues but up-regulated in pathological circumstances. Clearly, the AT2 receptor has antiproliferative properties and therefore opposes the growth promoting effect linked to the AT1 receptor stimulation. It is also reported that the AT2 receptor regulates ionic fluxes, affects differentiation and nerve regeneration, has anti-angiogenic and anti-fibrotic properties and stimulates apoptosis. However, the results, although suggestive, are sometimes equivocal. Obviously, the AT2 receptor plays a role in the pathogenesis and remodeling of cardiovascular and renal diseases. A more extensive knowledge of the AT2 receptor could therefore contribute to the understanding of the clincial beneficial effects of the AT1 receptor antagonists.

Novel actions of angiotensin II via its renal type-2 (AT(2)) receptor

The vast majority of the biologic effects of angiotensin II have been considered to be mediated by the subtype-1 (AT(1)) receptor. The AT(2) receptor is expressed to a low degree in most adult cells and tissues, and its function has not been understood. Recent studies, however, have identified novel actions of angiotensin II mediated by the AT(2) receptor in the kidney. These AT(2) receptor actions have importance in the control of blood pressure and hypertension. The AT(2) receptor mediates a renal vasodilator cascade, including generation of bradykinin, nitric oxide, and cyclic GMP. This action of angiotensin II occurs when the renin-angiotensin system is activated, as in sodium depletion. The AT(2) receptor also appears to mediate prostaglandin (PG) F(2)(a) formation, probably by stimulating conversion of PGE2 to PGF(2)(a). The AT(2) receptor plays a counter-regulatory vasodilator role opposing the vasoconstrictor actions of angiotensin II. The AT(1) and AT(2) receptors engage in inter-receptor "cross-talk." In the absence of the AT(2) receptor, sustained angiotensin II pressor and antinatriuretic hypersensitivity occurs, mediated by a deficiency of bradykinin, nitric oxide, and cyclic GMP. The AT(2) receptor may play an important role in stimulating pressure natriuresis, but definitive studies are required to resolve this issue. The AT(2) receptor mediates several renal actions of angiotensin II, appears to be important in the physiologic regulation of blood pressure, and may be involved in the pathophysiology of hypertension.