F. Andrew Gaffney

Angiotensin II Type 2 Receptor Is Essential for Left Ventricular Hypertrophy and Cardiac Fibrosis in Chronic Angiotensin II–Induced Hypertension

Background—The roles of angiotensin II (Ang II) in the regulation of heart function under normal and pathological conditions have been well documented. Although 2 types of Ang II receptor (AT1 and AT2) are found in various proportions, most studies have focused on AT1-coupled events. In the present study, we examined the hypothesis that signaling by AT2 is important to the development of left ventricular hypertrophy and cardiac fibrosis by Ang II infusion in mice lacking the AT2 gene (Agtr 2−/Y). Methods and Results—Male Agtr 2−/Y and age-matched wild-type (WT) mice were treated long-term with Ang II, infused at a rate of 4.2 ng · kg−1 · min−1 for 3 weeks. Ang II elevated systolic blood pressure to comparable levels in Agtr 2−/Y and WT mice. WT mice developed prominent concentric cardiac hypertrophy, prominent fibrosis, and impaired diastolic relaxation after Ang II infusion. In contrast, there was no cardiac hypertrophy in Agtr 2−/Y mice. Agtr 2−/Y mice, however, did not show signs of heart failure or impairment of ventricular relaxation and only negligible fibrosis after Ang II infusion. The absence of fibrosis may be a clue to the absence of impairment in ventricular relaxation and account for the normal left ventricular systolic and diastolic performances in Agtr 2−/Y mice. Conclusions—Chronic loss of AT2 by gene targeting abolished left ventricular hypertrophy and cardiac fibrosis in mice with Ang II–induced hypertension.

Evidence for angiotensin II type 2 receptor–mediated cardiac myocyte enlargement during in vivo pressure overload

The pathophysiological roles of the angiotensin II type 2 receptor (AT(2)) in cardiac hypertrophy remain unclear. By the targeted deletion of mouse AT(2) we were able to prevent the left ventricular hypertrophy resulting from pressure overload, while cardiac contractile functions remained normal. This implies that AT(2) is a mediator of cardiac hypertrophy in response to increased blood pressure. The effects of AT(2) deletion were independent of activation of embryonic genes for cardiac hypertrophy. However, p70(S6k), one of the key factors in cardiac hypertrophy, was markedly and specifically reduced in the ventricles of Agtr2(-)/Y mice. We propose that p70(S6k) plays a major role in AT(2)-mediated ventricular hypertrophy. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

A novel angiotensin II type 2 receptor signaling pathway: possible role in cardiac hypertrophy

We describe a novel signaling mechanism mediated by the G‐protein‐coupled receptor (GPCR) angiotensin II (Ang II) type 2 receptor (AT2). Yeast two‐hybrid studies and affinity column binding assay show that the isolated AT2 C‐terminus binds to the transcription factor promyelocytic zinc finger protein (PLZF). Cellular studies employing confocal microscopy show that Ang II stimulation induces cytosolic PLZF to co‐localize with AT2 at the plasma membrane, then drives AT2 and PLZF to internalize. PLZF slowly emerges in the nucleus whereas AT2 accumulates in the perinuclear region. Nuclear PLZF binds to a consensus sequence of the phosphatidylinositol‐3 kinase p85α subunit (p85α PI3K) gene. AT2 enhances expression of p85α PI3K followed by enhanced p70S6 kinase, essential to protein synthesis. An inactive mutant of PLZF abolishes this effect. PLZF is expressed robustly in the heart in contrast to many other tissues. This cardiac selective pathway involving AT2, PLZF and p85α PI3K may explain the absence of a cardiac hypertrophic response in AT2 gene‐deleted mice.

Targeted Deletion of Angiotensin II Type 2 Receptor Caused Cardiac Rupture After Acute Myocardial Infarction

Background—Accumulating evidence has suggested that the cardiac renin-angiotensin system is activated during the remodeling process after myocardial infarction (MI). Although 2 types of angiotensin II receptors (AT1 and AT2) are upregulated in the infarcted tissue, the contribution of AT2 to the subsequent fibrogenetic phase of wound healing is less certain. This study was conducted to evaluate the role of AT2 in wound healing after MI using an in vivo intervention study in mice with MI. Methods and Results—We examined myocardial hypertrophy, cardiac fibrosis, and morphological evidence of fibrillar collagen accumulation at the infarcted and noninfarcted regions in male mice lacking the AT2 receptor (Agtr 2−/Y) and age-matched wild-type (WT) animals. Of the Agtr 2−/Y mice, 63.6% died of cardiac rupture, whereas 23.5% of the WT mice died of the same cause within 1 week. The extent of fibrosis and that of collagen gene expression in Agtr 2−/Y mice were significantly reduced compared with WT mice at 1 week after coronary ligation. Furthermore, MI resulted in a marked increase in the prostaglandin E2 (PGE2) level at 4 days after surgery in Agtr 2−/Y mice. In WT mice, the PGE2 level was also elevated after MI but to a significantly lesser extent than in Agtr 2−/Y mice. Conclusions—A chronic loss of AT2 by gene targeting prevented the collagen deposition and caused cardiac rupture. The markedly elevated PGE2 may be a mechanism that inhibits collagen synthesis in the infarcted region of Agtr 2−/Y mice.

Evidence for increased cardiac compliance during exposure to simulated microgravity

We measured hemodynamic responses during 4 days of head-down tilt (HDT) and during graded lower body negative pressure (LBNP) in invasively instrumented rhesus monkeys to test the hypotheses that exposure to simulated microgravity increases cardiac compliance and that decreased stroke volume, cardiac output, and orthostatic tolerance are associated with reduced left ventricular peak dP/d t. Six monkeys underwent two 4-day (96 h) experimental conditions separated by 9 days of ambulatory activities in a crossover counterbalance design: 1) continuous exposure to 10° HDT and 2) ∼12-14 h per day of 80° head-up tilt and 10-12 h supine (control condition). Each animal underwent measurements of central venous pressure (CVP), left ventricular and aortic pressures, stroke volume, esophageal pressure (EsP), plasma volume, α1- and β1-adrenergic responsiveness, and tolerance to LBNP. HDT induced a hypovolemic and hypoadrenergic state with reduced LBNP tolerance compared with the control condition. Decreased LBNP tolerance with HDT was associated with reduced stroke volume, cardiac output, and peak dP/d t. Compared with the control condition, a 34% reduction in CVP ( P= 0.010) and no change in left ventricular end-diastolic area during HDT was associated with increased ventricular compliance ( P = 0.0053). Increased cardiac compliance could not be explained by reduced intrathoracic pressure since EsP was unaltered by HDT. Our data provide the first direct evidence that increased cardiac compliance was associated with headward fluid shifts similar to those induced by exposure to spaceflight and that reduced orthostatic tolerance was associated with lower cardiac contractility.