Regional Variation in Aortic AT1b Receptor mRNA Abundance Is Associated with Contractility but Unrelated to Atherosclerosis and Aortic Aneurysms

BACKGROUND: Angiotensin II (AngII), the main bioactive peptide of the renin angiotensin system, exerts most of its biological actions through stimulation of AngII type 1 (AT1) receptors. This receptor is expressed as 2 structurally similar subtypes in rodents, termed AT1a and AT1b. Although AT1a receptors have been studied comprehensively, roles of AT1b receptors in the aorta have not been defined. METHODOLOGY/RESULTS: We initially compared the regional distribution of AT1b receptor mRNA with AT1a receptor mRNA in the aorta. mRNA abundance of both subtypes increased from the proximal to the distal aorta, with the greatest abundance in the infra-renal region. Corresponding to the high mRNA abundance for both receptors, only aortic rings from the infra-renal aorta contracted in response to AngII stimulation. Despite the presence of both receptor transcripts, deletion of AT1b receptors, but not AT1a receptors, diminished AngII-induced contractility. To determine whether absence of AT1b receptors influenced aortic pathologies, we bred AT1b receptor deficient mice into an LDL receptor deficient background. Mice were fed a diet enriched in saturated fat and infused with AngII (1,000 ng/kg/min). Parameters that could influence development of aortic pathologies, including systolic blood pressure and plasma cholesterol concentrations, were not impacted by AT1b receptor deficiency. Absence of AT1b receptors also had no effect on size of aortic atherosclerotic lesions and aortic aneurysms in both the ascending and abdominal regions. CONCLUSIONS/SIGNIFICANCE: Regional abundance of AT1b receptor mRNA coincided with AngII-induced regional contractility, but it was not associated with AngII-induced aortic pathologies

Genetic Variants of the Renin Angiotensin System: Effects on Atherosclerosis in Experimental Models and Humans

The renin angiotensin system (RAS) has profound effects on atherosclerosis development in animal models, which is partially complimented by evidence in the human disease. Although angiotensin II was considered to be the principal effector of the RAS, a broader array of bioactive angiotensin peptides have been identified that have increased the scope of enzymes and receptors in the RAS. Genetic interruption of the synthesis of these peptides has not been extensively performed in experimental or human studies. A few studies demonstrate that interruption of a component of the angiotensin peptide synthesis pathway reduces experimental lesion formation. The evidence in human studies has not been consistent. Conversely, genetic manipulation of the RAS receptors has demonstrated that AT1a receptors are profoundly involved in experimental atherosclerosis. Few studies have reported links of genetic variants of angiotensin II receptors to human atherosclerotic diseases. Further genetic studies are needed to define the role of RAS in atherosclerosis

Regional variation in aortic AT1b receptor mRNA abundance is associated with contractility but unrelated to atherosclerosis and aortic aneurysms.

Angiotensin II (AngII), the main bioactive peptide of the renin angiotensin system, exerts most of its biological actions through stimulation of AngII type 1 (AT1) receptors. This receptor is expressed as 2 structurally similar subtypes in rodents, termed AT1a and AT1b. Although AT1a receptors have been studied comprehensively, roles of AT1b receptors in the aorta have not been defined.We initially compared the regional distribution of AT1b receptor mRNA with AT1a receptor mRNA in the aorta. mRNA abundance of both subtypes increased from the proximal to the distal aorta, with the greatest abundance in the infra-renal region. Corresponding to the high mRNA abundance for both receptors, only aortic rings from the infra-renal aorta contracted in response to AngII stimulation. Despite the presence of both receptor transcripts, deletion of AT1b receptors, but not AT1a receptors, diminished AngII-induced contractility. To determine whether absence of AT1b receptors influenced aortic pathologies, we bred AT1b receptor deficient mice into an LDL receptor deficient background. Mice were fed a diet enriched in saturated fat and infused with AngII (1,000 ng/kg/min). Parameters that could influence development of aortic pathologies, including systolic blood pressure and plasma cholesterol concentrations, were not impacted by AT1b receptor deficiency. Absence of AT1b receptors also had no effect on size of aortic atherosclerotic lesions and aortic aneurysms in both the ascending and abdominal regions.Regional abundance of AT1b receptor mRNA coincided with AngII-induced regional contractility, but it was not associated with AngII-induced aortic pathologies

AT1b receptor deficiency had no effect on AngII-induced aortic arch and descending aortic dilation.

<p>Intimal area of (<b>A</b>) aortic arch region and (<b>B</b>) descending thoracic aorta was measured using an en face method (N = 16: AT1b receptor +/+, and N = 10: AT1b receptor −/− mice). Inverted triangles represent values from individual mice, circles represent means, and error bars are SEM.</p

Region-specific differences in aortic mRNA abundance of AT1a and AT1b receptors.

<p>mRNA abundance of (<b>A</b>) AT1a receptors (N = 4) and (<b>B</b>) AT1b receptors (N = 4) in aortic regions of C57BL6/J mice. * denotes P<0.05 in supra-renal versus ascending and descending aortic regions (one way ANOVA with Holm-Sidak post hoc test). # denotes P<0.001 in the infra-renal region versus all the other aortic regions (one way ANOVA with Holm-Sidak post hoc test).</p

AngII-induced regional contractions were markedly reduced by AT1b receptor deficiency.

<p>Regional contractility of aortic rings harvested from the infra-renal aorta of (<b>A</b>) C57BL/6, (B) AT1a receptor −/−, or (C) AT1b receptor −/− mice. Aortic rings were contracted during 5-minute incubation with potassium chloride (KCl; 80 mM), 5-hydroxytryptamine (5-HT; 1 µM), or Ang II (1 µM). Contractions are represented as percent of the maximal contraction achieved during incubation with KCl (80 mM). Red arrows indicate the return to normal Krebs–Henseleit solution.</p

Characterization of AT1b receptor +/+ and −/− mice infused with AngII.

<p>Body weight, plasma cholesterol and renin concentrations were measured after termination. SBP was measured during the last week of the study. Values are represented as mean ± SEM. There were no significant differences between the two AT1b receptor genotypes.</p

AT1b receptor deficiency had no effect on AngII-induced abdominal aortic dilation in vivo.

<p>(<b>A</b>) Maximal luminal diameters of suprarenal aortas were measured in vivo by ultrasonography at baseline (Day 0) and on Day 28 during AngII infusion (N = 16: AT1b receptor +/+, and N = 10: AT1b receptor −/− mice). * denotes P<0.001 saline versus AngII within AT1b genotypes (two way repeated measures ANOVA). (<b>B</b>) Maximum width of supra-renal aortas was measured ex vivo (N = 16: AT1b receptor +/+, and N = 10: AT1b receptor −/− mice). Inverted triangles represent individual mice, circles represent means and error bars are SEM. (<b>C</b>) Examples of ultrasound images (Day 0 and Day 28) and ex vivo pictures (after termination) of suprarenal aortas, which represent aortic diameters nearest the mean of each group.</p