The main finding of this study is that endogenous ACE activity and, therefore, Ang II generation are required for the augmentation of plasma Ang II, intrarenal Ang II, and blood pressure during Ang II–induced hypertension in mice. When mice were infused with Ang II in the presence of ACE inhibition, they failed to develop increased blood pressure levels similar to those observed in mice treated with Ang II only. Thus, these results indicate that endogenous Ang II generation is required for the complete development of hypertension in this model. Furthermore, because plasma renin activity is suppressed and kidney renin activity is maintained during chronic Ang II infusions in mice,11
it is likely that most of the endogenous Ang I and Ang II are generated by the kidneys during Ang II–induced hypertension. These results are partially supported by Sadjadi et al23
in which uninephrectomized rats subjected to Ang II infusions and ACE inhibition with enalapril also failed to develop hypertension and increase intrarenal Ang II. However, it is unclear what sort of influence the presence of only 1 kidney exerted on the intrarenal RAS and its ability to respond to chronic Ang II infusions.
ACE inhibition is complex because ACE is an enzyme with several substrates, including bradykinin, a molecule with vasodilating and natriuretic activities. In the absence of ACE, bradykinin accumulates.16
The role of bradykinin accumulation in conditions where ACE activity is suppressed was explored by Xiao et al24
through the generation of a double-knockout mice lacking ACE and the bradykinin B2
receptor. The B2
receptor is considered the main pathway for bradykinin activation of endothelial NO synthase. It was found that the phenotype of the double-knockout mice was indistinguishable from mice lacking only ACE. Hence, their results confirmed that the observed changes in the ACE knockout mice are primarily because of the lack of Ang II generation and not because of bradykinin accumulation.24
On the other hand, Cervenka et al25
receptor knockout mice to chronic Ang II infusions and observed that the absence of a B2
receptor exacerbated the development of hypertension, concluding that bradykinin buffered the effect of Ang II in this model. In their study, Ang II was infused at 1800 ng/kg per minute, which is 4.5-fold greater than that used in this report. Such high doses in mice comfound the analysis, because they induce a rapidly progressive and malignant form of hypertension with losses of body and KWs, as well as extrarenal Ang II effects, including direct vasoconstriction.11,26,27
Previous experiments have convincingly demonstrated that ACE is the main pathway for Ang II generation in plasma and in the kidneys. Campbell et al16
showed that ACE knockout mice display 95% to 97% reductions in intrarenal Ang II content and that acute ACE inhibition reduces kidney Ang II content by >90% in wild-type controls. These observations emphasize the main role of ACE in intrarenal Ang II formation and do not support suggestions by others that chronic suppression of ACE activity leads to induction of alternative enzymatic pathways of angiotensin II formation, like chymase or chymase-like enzymes.28
In the present study, only a 50% reduction of intrarenal ACE activity was observed; however, based on the achieved reduction of intrarenal Ang II, it is evident that Ang II generation by ACE is a major contributor to the increases in intrarenal Ang II observed during Ang II–induced hypertension. Several studies have shown that acute or chronic systemic administration of ACEis, although effective at suppressing plasma ACE activity, fails to completely abolish intrarenal ACE activity in the kidneys of normal29,30
or Ang II–infused rats.23
These observations suggest that part of the intrarenal ACE activity is somehow inaccessible to the ACEi. At present there is no clear explanation for this finding, but it is possible that ACE localized to intracellular compartments is beyond the reach of the inhibitor. With regard to the effect of ACE inhibition on plasma ACE activity in Ang II–infused animals, Sadjadi et al23
reported that enalapril completely abolishes plasma ACE activity in controls and Ang II–infused uninephrectomized rats. In agreement with these observations, it is reported here that ACE inhibition reduced plasma ACE activity by >70% in both control and Ang II–infused mice.
Ang II–infused rats show an increased intrarenal ACE activity23
and increased ACE binding in the proximal tubule10,31
and other cell types.31
Two-kidney, 1-clip hypertensive rats display increases in ACE activity in the nonclipped kidney but not in the clipped kidney.2
In the present study, a significant difference in the activity of ACE between controls and Ang II–infused mice was not observed. It is possible that such discrepancy is because of interspecies differences, the dose of Ang II used, or the severity of the induced hypertension. Future experiments will be conducted to analyze the expression of ACE and other components of the intrarenal RAS in our mouse model. Another issue to be addressed is the effect of ACE inhibition on expression of ACE2 during chronic Ang II infusions, because it has been suggested that chronic treatment with lisinopril increases intrarenal ACE2 activity in normal rats.32
Such an augmentation in ACE2 activity could enhance Ang II degradation and contribute to the reductions in intrarenal Ang II levels. Thus, an elevation on the activity of degradative pathways for Ang II might also account for the observed results.
It was observed that ACEi inhibition failed to increase Ang I in plasma from control and Ang II–infused mice. These observations are in accord with the absence of differences in plasma Ang I displayed by systemic ACE knockout mice when compared with wild-type littermates.16
These findings have been attributed to the low levels of plasma angiotensinogen in mice that may attenuate any increase caused by chronic ACE inhibition.16
In the kidneys, where angiotensinogen is available and renin activity is much higher,11
ACE inhibition increased Ang I in controls. The lack of effect of ACE inhibition on intrarenal Ang I in Ang II–infused mice is more difficult to explain and perhaps can be attributed to changes in intrarenal angiotensinogen expression, the activation of alternative pathways for Ang I degradation, or both. Further experiments are needed to explore the various possibilities. The failure of chronic Ang II infusions to increase plasma Ang II in the presence of ACE inhibition supports the hypothesis that the kidneys are the main sites for endogenous Ang II generation during Ang II–induced hypertension. ACE inhibition may, therefore, reduce the kidney’s capability to contribute to the plasma pool of Ang II.
In summary, these results demonstrate that endogenous ACE-mediated Ang II formation plays a significant role in the increases of blood pressure, plasma Ang II, and intrarenal Ang II during Ang II–induced hypertension, because pharmacological inhibition of this enzyme with lisinopril significantly ameliorates such changes in Ang II–infused mice.
ACEis and angiotensin receptor blockers are effective in patients with essential hypertension, although the vast majority have normal or low plasma renin activity levels; they also confer renoprotection to an extent greater that can be explained by the reduction in blood pressure. These effects may be explained by modulation of the intrarenal RAS and Ang II. Therefore, studies aimed at the mechanisms that govern the intrarenal RAS and, consequently, local Ang II levels in the kidney will contribute to our understanding of hypertension and renal injury and lead to the development of better diagnostic and therapeutic tools.