The renin-angiotensin system plays a critical role in sodium and fluid homeostasis. Genetic or acquired alterations in the expression of components of this system are strongly implicated in The renin-angiotensin system (RAS) is one of the primary physiological regulators of sodium and fluid balance (1). The RAS regulates body-fluid homeostasis through several distinct mechanisms-including effects on hemodynamics and vascular tone, direct stimulation of sodium reabsorption by the kidney, and stimulation of aldosterone production by the adrenal glands (2). The propensity of the RAS to cause elevated blood pressure was first recognized in acquired disorders such as renovascular hypertension (3), and alterations in the activity of this system have been strongly implicated in the pathogenesis of essential hypertension. For example, variations in genes encoding renin (4, 5), angiotensinogen (6, 7), angiotensinconverting enzyme (8,9), and angiotensin receptors (10)(11)(12) have been associated with hypertension in human populations and in animal models of sodium-sensitive hypertension.The major biologically active product of the RAS is the multifunctional peptide angiotensin 11 (1). The physiological effects of angiotensin II are elicited through binding to specific cell-surface receptors (13). Angiotensin II receptors belong to the large family of rhodopsin-like G protein-associated receptors and have been divided into two pharmacologically distinct types designated type 1 (AT1) and type 2 (AT2) (13-17). AT1The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.receptors are thought to mediate the known functions of angiotensin 11 (13-15); they are defined pharmacologically by their high-affinity binding to the nonpeptide antagonist losartan (Dup 573). AT2 receptors exhibit high-affinity binding to the antagonists PD 123177 and CGP 42112, but the physiological function of and signaling mechanisms used by AT2 receptors are not known (13,16,17).Among the AT1 receptors, two subtypes ATlA and ATlB have been identified in human, rat, and mouse (18-21). These receptors are products of separate genes, share substantial sequence homology, and have wide tissue distributions. The ATlA receptor seems to predominate in most tissues except the adrenal gland and the anterior pituitary (18-26), and expression of ATlA and ATlB receptors may be differentially regulated in the heart and the adrenals (22,25,26). This differential tissue distribution and regulation of AT1 receptor subtypes may serve to modulate the biological effects of angiotensin II. However, due to the lack of discriminatory pharmacological antagonists, the individual functions of the two AT1 receptor subtypes (A and B) have not been defined.Associations between alterations in AT1 receptor-encoding genes and hypertension have been identified in previous studies. For example, Deng and associates (10, 11) fou...

Variants of the human angiotensinogen gene have been linked in some studies to increased circulating angiotensinogen levels and essential hypertension. To test for direct causality between genotypes at the angiotensinogen locus and blood pressures, we have studied mice carrying zero, one, two, three, or four functional copies of the murine wild-type angiotensinogen gene (Agt) at its normal chromosomal location. Plasma angiotensinogen levels increase progressively, although not linearly, from zero in the zero-copy animals to 145% of normal in the four-copy animals. Mice of all genotypes are normal at birth, but most zero-copy animals die before weaning. The kidneys of the zero-copy animals show pathological changes as adults, but the kidneys are normal in the other genotypes. One adult zero-copy male tested was fertile. The blood pressures of the one-copy through four-copy animals show significant and almost linear increases of approximately 8 mmHg per gene copy despite their normal compensatory mechanisms being intact. These results establish a direct causal relationship between Agt genotypes and blood pressures.

The classically recognized functions of the renin-angiotensin system are mediated by type 1 (AT 1 ) angiotensin receptors. Whereas man possesses a single AT 1 receptor, there are two AT 1 receptor isoforms in rodents (AT 1A and AT 1B ) that are products of separate genes (Agtr1a and Agtr1b). We have generated mice lacking AT 1B (Agtr1b ؊͞؊) and both AT 1A and AT 1B receptors (Agtr1a ؊͞؊Agtr1b ؊͞؊). Agtr1b ؊͞؊ mice are healthy, without an abnormal phenotype. In contrast, Agtr1a ؊͞؊Agtr1b ؊͞؊ mice have diminished growth, vascular thickening within the kidney, and atrophy of the inner renal medulla. This phenotype is virtually identical to that seen in angiotensinogen-deficient (Agt؊͞؊) and angiotensin-converting enzyme-deficient (Ace ؊͞؊) mice that are unable to synthesize angiotensin II. Agtr1a ؊͞؊Agtr1b ؊͞؊ mice have no systemic pressor response to infusions of angiotensin II, but they respond normally to another vasoconstrictor, epinephrine. Blood pressure is reduced substantially in the Agtr1a ؊͞؊ Agtr1b ؊͞؊ mice and following administration of an angiotensin converting enzyme inhibitor, their blood pressure increases paradoxically. We suggest that this is a result of interruption of AT 2 -receptor signaling. In summary, our studies suggest that both AT 1 receptors promote somatic growth and maintenance of normal kidney structure. The absence of either of the AT 1 receptor isoforms alone can be compensated in varying degrees by the other isoform. These studies reaffirm and extend the importance of AT 1 receptors to mediate physiological functions of the renin-angiotensin system.The renin-angiotensin system (RAS) regulates blood pressure and body fluid balance and plays a role in growth and development (1-3). The biological functions of the RAS and its major effector peptide, angiotensin II, are mediated by specific receptors. Two angiotensin receptor subtypes, type 1 angiotensin receptor (AT 1 ) and AT 2 , can be distinguished pharmacologically. The classically recognized actions of the RAS are mediated by AT 1 receptors (1, 2). Whereas man possesses a single AT 1 receptor, rodents possess two AT 1 receptor isoforms, designated AT 1A and AT 1B . These receptors are the products of distinct but highly homologous genes (Agtr1a and Agtr1b) located on separate chromosomes (3, 4). Expression of the AT 1A receptor subtype predominates in nearly all tissues except the anterior pituitary gland and the adrenal cortex, where AT 1B receptors are more highly expressed (5-9). Because pharmacological AT 1 receptor antagonists block both AT 1A and AT 1B receptors, it has been difficult to separate their distinct functions (2).Experiments using gene targeting have provided insight into the roles of RAS genes in regulating blood pressure, body fluid homeostasis, and development. For example, mice that are unable to generate angiotensin II because of targeted mutations in the angiotensinogen (Agt Ϫ͞Ϫ) or angiotensinconverting enzyme (Ace Ϫ͞Ϫ) genes have virtually identical phenotypes characterized by reduced survival, l...

Most of the classic functions of the renin-angiotensin system are mediated by type 1 (AT1) angiotensin receptors, of which two subtypes, AT1A and AT1B, have been identified. However, distinct functions for these two AT1 receptors have been difficult to separate. We examined the pressor effects of angiotensin II in Agtr1A -/- mice, which lack AT1A receptors. In enalapril-pretreated Agtr1A -/- mice, angiotensin II caused significant and dose-proportional increases in mean arterial pressure. This pressor response was not blocked by pretreatment with sympatholytic agents but was completely inhibited by the AT1-receptor antagonists, losartan and candesartan, suggesting that it is directly mediated by AT1B receptors. Chronic treatment of Agtr1A -/- mice with losartan reduced systolic blood pressure from 80 +/- 5 to 72 +/- 4 mmHg (P < 0.04), suggesting a role for AT1B receptors in chronic blood pressure regulation. These studies provide the first demonstration of in vivo pressor effects mediated by AT1B receptors and demonstrate that, when AT1A receptors are absent, the AT1B receptor contributes to the regulation of resting blood pressure.

Mice lacking AT(1A) receptors for ANG II have a defect in urinary concentration manifested by an inability to increase urinary osmolality to levels seen in controls after thirsting. This defect results in extreme serum hypertonicity during water deprivation. In the basal state, plasma vasopressin levels are similar in wild-type controls and Agtr1a -/- mice. Plasma vasopressin levels increase normally in the AT(1A) receptor-deficient mice after 24 h of water deprivation, suggesting that the defect in urine concentration is intrinsic to the kidney. Using magnetic resonance microscopy, we find that the absence of AT(1A) receptors is associated with a modest reduction in the distance from the kidney surface to the tip of the papilla. However, this structural abnormality seems to play little role in the urinary concentrating defect in Agtr1a -/- mice since the impairment is largely reproduced in wild-type mice by treatment with an AT(1)-receptor antagonist. These studies demonstrate a critical role for the AT(1A) receptor in maintaining inner medullary structures in the kidney and in regulating renal water excretion.