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Although it is well-known that excess renin angiotensin system (RAS) activity contributes to the pathophysiology of cardiac and vascular disease, tissue-based expression of RAS genes has given rise to the possibility that intracellularly produced angiotensin II (Ang II) may be a critical contributor to disease processes. An extended form of angiotensin I (Ang I), the dodecapeptide angiotensin-(1-12) [Ang-(1-12)], that generates Ang II directly from chymase, particularly in the human heart, reinforces the possibility that an alternative noncanonical renin independent pathway for Ang II formation may be important in explaining the mechanisms by which the hormone contributes to adverse cardiac and vascular remodeling. This review summarizes the work that has been done in evaluating the functional significance of Ang-(1-12) and how this substrate generated from angiotensinogen by a yet to be identified enzyme enhances knowledge about Ang II pathological actions.

Angiotensin-(1-12) [ANG-(1-12)] is processed into ANG II by chymase in rodent and human heart tissue. Differences in the amino acid sequence of rat and human ANG-(1-12) render the human angiotensinogen (hAGT) protein refractory to cleavage by renin. We used transgenic rats harboring the hAGT gene [TGR(hAGT)L1623] to assess the non-renin-dependent effects of increased hAGT expression on heart function and arterial pressure. Compared with Sprague-Dawley (SD) control rats (n= 11), male homozygous TGR(hAGT)L1623 (n= 9) demonstrated sustained daytime and nighttime hypertension associated with no changes in heart rate but increased heart rate lability. Increased heart weight/tibial length ratio and echocardiographic indexes of cardiac hypertrophy were associated with modest reduction of systolic function in hAGT rats. Robust human ANG-(1-12) immunofluorescence within myocytes of TGR(hAGT)L1623 rats was associated with a fourfold increase in cardiac ANG II content. Chymase enzymatic activity, using the rat or human ANG-(1-12) as a substrate, was not different in the cardiac tissue of SD and hAGT rats. Since both cardiac angiotensin-converting enzyme (ACE) and ACE2 activities were not different among the two strains, the changes in cardiac structure and function, blood pressure, and left ventricular ANG II content might be a product of an increased cardiac expression of ANG II generated through a non-renin-dependent mechanism. The data also underscore the existence in the rat of alternate enzymes capable of acting on hAGT protein. Homozygous transgenic rats expressing the hAGT gene represent a novel tool to investigate the contribution of human relevant renin-independent cardiac ANG II formation and function.

Angiotensin-(1-12) [ANG-(1-12)], a new member of the renin-angiotensin system, is recognized as a renin independent precursor for ANG II. However, the processing of ANG-(1-12) in the circulation in vivo is not fully established. We examined the effect of angiotensin converting enzyme (ACE) and chymase inhibition on angiotensin peptides formation during an intravenous infusion of ANG-(1-12) in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). WKY and SHR were assigned to a short ANG-(1-12) infusion lasting 5, 15, 30, or 60 min (n = 4-10 each group). In another experiment WKY and SHR were assigned to a continuous 15-min ANG-(1-12) infusion with pretreatment of saline, lisinopril (10 mg/kg), or chymostatin (10 mg/kg) (n = 7-13 each group). Saline or lisinopril were infused intravenously 15 min before the administration of ANG-(1-12) (2 nmol·kg(-1)·min(-1)), whereas chymostatin was given by bolus intraperitoneal injection 30 min before ANG-(1-12). Infusion of ANG-(1-12) increased arterial pressure and plasma ANG-(1-12), ANG I, ANG II, and ANG-(1-7) levels in WKY and SHR. Pretreatment with lisinopril caused increase in ANG-(1-12) and ANG I and large decreases in ANG II compared with the other two groups in both strains. Pretreatment of chymostatin had no effect on ANG-(1-12), ANG I, and ANG II levels in both strains, whereas it increased ANG-(1-7) levels in WKY. We conclude that ACE acts as the primary enzyme for the conversion of ANG-(1-12) to smaller angiotensin peptides in the circulation of WKY and SHR and that chymase may be an ANG-(1-7) degrading enzyme.