Role of Angiotensin II Type 2 Receptor in Blood Pressure Regulation in Obese rats
Renin angiotensin system (RAS) consists of enzymes, hormones, proteins and peptides. Angiotensin II (Ang II) is an important peptide of RAS. Ang II acts via AT1 receptor (AT1R) and AT2 receptor (AT2R). While AT1R is known to cause antinatriuresis and increase in blood pressure, the role of AT2R in renal function and long-term BP regulation is not well defined. Recently our laboratory showed that AT2R are upregulated in the kidney of obese rats and selective activation of these receptors stimulates nitric oxide/cGMP pathway, inhibits proximal tubules Na+/K+-ATPase (NKA) activity and increases urinary sodium excretion. In light of those findings, we undertook this project to investigate the role of AT2R in renal function, long-term blood pressure control and interaction with renal AT1R function in obese Zucker rats, an animal model exhibiting hyperinsulinemia, hyperglycemia and hypertension. Also, we studied the mechanism associated with hyperglycemia induced AT2R upregulation in proximal tubule cells. First, we designed experiment to determine whether AT2R has a protective role in blood pressure increase in obese rats. We treated obese Zucker rats with AT2R antagonist PD123319 (PD) for two weeks and BP was measured. Treatment with PD significantly increased the blood pressure, which was associated with increased renal renin expression in obese rats. This suggested that AT2R protect against increase in blood pressure by keeping renal renin expression low. Then, we designed experiments to determine whether chronic AT2R activation affects Na-balance and lowers BP in obese rats. We treated lean and obese Zucker rats with AT2R agonist CGP42112A (CGP) for two weeks. Two weeks treatment caused a decrease in BP by 19 mmHg and in Na-balance in obese but not in lean rats. The plasma renin activity was significantly decreased in both lean and obese CGP-treated rats. The expression of AT2R, AT1R, angiotensin converting enzyme (ACE) and renin in the kidney cortex was not affected by the CGP-treatment of obese or lean rats. However, ACE2 expression and activity was significantly increased in CGP-treated obese rats and not in lean rats. These studies suggest that long-term activation of AT2R decreases BP in obese rats. The reduction in BP by AT2R agonist treatment may have been contributed by a decrease in Na-balance and an enhanced expression and activity of ACE2 in renal cortex. In order to determine whether the reduction in BP and decrease in Na-balance might have been contributed by the ability of AT2R to antagonize renal AT1R function in CGP-treated obese rats, we again treated the obese Zucker rats with CGP for two weeks. We performed the renal function study after two weeks under anesthesia. We found that CGP-treatment of obese rats caused reduction in Ang II pressor response and blunted the candesartan-induced natriuresis/diuresis in these rats suggesting that chronic activation of AT2R antagonizes the function of AT1R. Earlier studies from our laboratory suggest that AT2R promote Na-excretion but the contribution of different nephron segments in AT2R-induced natriuresis is not known. We investigated the involvement of proximal tubule AT2R in natriuresis by blocking the two important distal tubule Na-transporters (NaCl cotransporter and ENaC). We found that selective activation of AT2R with a novel AT2R agonist C21 promoted natriuresis predominantly via proximal tubules. We also performed in vitro experiments (HK2 cells) to elucidate the potential signaling mechanism involved in the proximal tubule AT2R upregulation in diabetes/hyperglycemia. In this experiment, we exposed HK2 cells with high glucose with and without IRF-1 siRNA. High glucose increased AT2R expression in HK2 cells and is mediated via transcriptional mechanism involving the transcription factor IRF-1. Collectively, the data suggest that long-term treatment with AT2R agonist attenuates positive Na-balance, lowers renal renin expression, antagonizes the function of AT1R and decreases blood pressure in obese Zucker rats. Moreover AT2R upregulation in response to hyperglycemia may be compensatory mechanism to exert a beneficial role in kidney function. These findings highlight the therapeutic potential of AT2R for treating obesity/diabetes related hypertension.