*3.5. Angiotensin-III*/*-IV*

Arterial concentration of Ang-III (hexapeptide 2–8) was first documented in 1980 in sheep, and accounted for 42% of that of Ang-II [58]. Ang-III is generated from Ang-II by the removal of the first amino terminus aa by Aminopeptidase A (ENPEP) [8] (Figure 1 and Table 1). In addition, it can be generated from Ang-I by a two-steps pathway involving ENPEP and ACE, respectively [102]. Studies have shown that Ang-III exerts similar, but less potent, actions as compared to Ang-II [2,103], by acting on AT1R and AT2R, with higher a ffinity to the former [104]. Indeed, Ang-III was shown to

increase blood pressure, vasopressin and aldosterone release, in addition to inducing inflammatory genes expression [2,103].

Ang-III in turn can be converted into Ang-IV (pentapeptide 3–8) by the action of the aminopeptidase N (ANPEP) [105], and possibly aminopeptidase B (RNPEP) [106] (Figure 1 and Table 1). Ang-IV acts through its Angiotensin type 4 receptor (AT4R), which is the insulin-regulated membrane aminopeptidase (IRAP). The latter is a type II integral membrane spanning protein belonging to the M1 family of aminopeptidases that is expressed in several tissues, including the brain, adrenal gland, kidney, lung, liver, and heart [107] (Figure 1 and Table 1).

Recent studies have shown that certain local Ang-II-mediated e ffects could be attributed to Ang-III. For example, Padia et al. showed that the conversion of Ang-II to Ang-III is critical for AT2R-mediated natriuresis in Sprague–Dawley rats [104]. Similarly, in Wistar rats, the Ang-II-mediated enhancement in baroreceptor heart reflex was abrogated in the presence of ENPEP inhibitor, indicating that Ang-III is the active angiotensin peptide involved in central blood pressure regulation [108]. On the other hand, Handa et al. showed that intrarenal injection of Ang I, Ang-II, or Ang-III induce dose-dependent vasoconstriction in Sprague–Dawley rats. However, Ang-IV or Ang-(3–10) injection produced a dose-dependent rapid vasoconstriction, lasting for seconds, followed by a transient vasodilatation, lasting for minutes [109]. This indicates that RAS induces peptide-specific e ffects at the tissue level.

The major e ffects of AT4R activation are thought to be in the brain where it can enhance learning and memory [107]. However, the mechanism by which Ang-IV exerts its e ffects through IRAP is still not clear [110]. One suggestion is that Ang-IV inhibits the catalytic activity of IRAP, thereby extending the half-life of its neuropeptide substrates. Another suggestion is that it may modulate glucose uptake by modulating GLUT4 tra fficking. Others sugges<sup>t</sup> that it may act directly on cellular mechanisms by inducing cellular signaling after its binding [110]. Ang-IV in the brain was also shown to be implicated in regulating blood pressure by acting on the AT1R [111], which was shown to mediate several Ang-IV effects. Indeed, Ang-IV mediates pressure and renal vasoconstrictor e ffects in mice via AT1a receptor whereas AT4R is not involved [112]. Finally, Ang-IV-mediated non-prostaglandin renal vasodilatory activity was found to be linked to renal vascular AT1R [109].

The Ang-III/Ang-IV axes have added a new level of complexity to the system and identified novel mechanisms by which Ang-II may exert its e ffects. This needs to be further studied to elucidate possible flows in the interpretation of the e ffects of Ang-II agonists and to identify possible mechanisms that would improve Ang-II antagonists' mode of action.
