Hypertension 37(5): 1329-1335

Enhancement of Angiotensinogen Expression in Angiotensin II–Dependent Hypertension

Department of Physiology, Tulane University School of Medicine, New Orleans, La

Correspondence to Hiroyuki Kobori, MD, PhD, Department of Physiology, SL39, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, LA 70112−2699. E-mail ude.enalut@irobokh

Abstract

Chronic infusion of angiotensin (Ang) II leads to the development of hypertension and enhances intrarenal Ang II content to levels greater than can be explained from the circulating concentrations of the peptide. We previously reported that renal angiotensinogen (Ao) mRNA is enhanced in Ang II–dependent hypertension and may contribute to augmented intrarenal Ang II levels, but the Ao protein levels were not significantly increased. Because a high-salt diet (H/S) has been shown to suppress renal expression of Ao mRNA, we examined the effects of chronic Ang II infusion on kidney and liver Ao mRNA and protein levels in male Sprague-Dawley rats (n=12) maintained on an 8% salt diet. Ang II was administered via osmotic minipumps (40 ng/min) to 1 group (n=6) while the remaining rats were sham-operated. A H/S diet alone did not alter systolic blood pressure in sham animals (109±6 mm Hg at day 12); however, Ang II infusions to the H/S rats significantly increased systolic blood pressure (167±7 at day 12) and intrarenal Ang II content (459±107 fmol/g versus 270±42) despite a marked suppression of plasma renin activity (0.9±0.2 ng Ang I·mL^·h^{versus 2.8±1.3). Ang II infusions significantly increased kidney Ao mRNA compared with the H/S diet alone by 1.9±0.1-fold. Western blot analysis of kidney protein extracts showed that the Ang II–infused rats had increased kidney Ao protein levels compared with the H/S diet alone (1.9±0.1-fold). Liver Ao mRNA and protein and plasma Ao protein were also significantly increased by Ang II infusions. These data demonstrate the effects of Ang II infusion to stimulate Ao mRNA and protein. Thus, the augmented intrarenal Ang II in Ang II–dependent hypertension may result, in part, by a positive amplification mechanism to activate renal expression of Ao.}

Keywords: angiotensin II, angiotensinogen, kidney, sodium, dietary, Western blot, reverse transcriptase–polymerase chain reaction

Abstract

Angiotensin (Ang) II, an extensively characterized peptide produced by successive proteolytic cleavages of its prohormone angiotensinogen (Ao), plays a critically important role in the regulation of renal hemodynamics and electrolyte homeostasis.¹ Recent evidence on the basis of experimental animal models and transgenic mice has documented the involvement of Ao in the development of hypertension caused by activation of the renin-angiotensin system.²6 In human genetic studies, a linkage has been established between the Ao gene and hypertension.⁷ Thus, there is considerable evidence supporting the involvement of inappropriately elevated intrarenal Ang II levels in many forms of hypertension.⁸9 Chronic infusion of Ang II provides a useful experimental model of Ang II–dependent hypertension that resembles the 2-kidney, 1-clip Goldblatt hypertension model.¹⁰ The mechanisms responsible for the ability of moderate increases in circulating Ang II to cause progressive increases in arterial pressure remain incompletely understood. One possible mechanism by which Ang II produces its progressive hypertensinogenic effects may be related to the failure to downregulate renal and hepatic Ang II type 1 receptor mRNA and protein levels.¹¹ When coupled with sustained increases in the circulating Ang II, a progressive Ang II–dependent hypertension results. We suggest that stimulation of Ao synthesis by Ang II may be a key component in the maintenance of elevated circulating and, more importantly, intrarenal Ang II levels.¹²

In a previous study, we observed that Ang II infusions resulted in significant increases in Ao mRNA in the kidney and liver and liver Ao protein; however, kidney Ao protein levels were not significantly increased.¹² One possible reason for the failure to show a distinct effect on the Ao protein is that the control levels might have been already partially stimulated during normal dietary salt intake. It is known that a high-salt (H/S) diet suppresses the renal expression of Ao mRNA.¹³15 Therefore, a H/S diet was given to the animals in an attempt to minimize the endogenous Ang II influences on basal Ao production and to allow full demonstration of the effects of Ang II infusions on Ao synthesis. Thus, rats maintained on a H/S diet might provide a more optimal model to evaluate the in vivo effects of Ang II to affect Ao mRNA and protein expression in an environment of suppressed Ao production. Accordingly, this study was performed to investigate the stimulatory effect of Ang II on Ao mRNA and protein under conditions of H/S intake.

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