Angiotensin I, angiotensin II and their biologically active peptides

The study of the molecular mechanisms by which angiotensin II effects the regulation of body fluid volumes and blood pressure has experienced a remarkable impetus in recent years thanks to the efforts of both physiologists and clinicians. Cloning of angiotensin II receptor subtypes and the synthesis of orally active and selective angiotensin II antagonists to AT1 and AT2 receptors has accelerated the uncovering of additional regulatory actions, which only indirectly impinge upon the blood pressure control mechanisms. From the early concept of a circulating renin–angiotensin system (RAS), knowledge has evolved to identification of the chemical machinery for the formation of angiotensin peptides in the interstitial microenvironment of diverse tissues, and the suggestion that angiotensin II synthesis may occur within the cell itself. Clinically, type 1 angiotensin II antagonists have proven beneficial in retarding or even reversing the cardiovascular sequela of hypertensive disease [1–3], and even demonstrated that blockade of angiotensin II actions may interfere with the mechanisms by which atherosclerosis progresses or develops [4,5]. Overwhelmingly, the study of the regulatory actions of the RAS in the preservation of homeostatic processes remains focused on angiotensin II and its effects on blood pressure, cardiac and vascular mechanisms. Hypertension research has ignored the fact that the component of the RAS exists in annelids, crustaceans, molluscs, leeches and insects, invertebrates whose survival as biological entities has little to do with blood pressure regulation or cardiovascular remodelling [6]. Interestingly, in these biological entities angiotensin II, angiotensin-(2-8) and angiotensin-(3-8) participate in osmoregulation, immune mechanisms and neurotransmission, findings that should raise questions as to the primordial functions of angiotensin peptides. In this issue of the journal, Cesari et al. [7] review the current biochemical and physiological evidence for the actions of angiotensin peptides other than angiotensin II in the cardiovascular system underlying a need for further research into this area. At this stage of our knowledge, only a few laboratories, including ourselves, have had the intellectual tenacity to pursue this line of investigation, and seldom have been called to lecture at major national or international meetings. I dare say that it is not because their science is somehow flawed or their results are not reproducible. Possibly, however, it appears that there is little scientific need to assume that the ubiquity of actions mediated by angiotensin II do not suffice to explain the contribution of the RAS on blood pressure and vascular function. Cesari et al. document the physiological effects of three angiotensin peptides, angiotensin-(2-8), angiotensin-(3-8) and angiotensin-(1-7), within the angiotensin peptide family, a term that to my knowledge was first advanced in 1991 [8]. Wisely, these authors had the prudence to place the term ‘angiotensin fragments’ between quotes, to stress the idea that these are not mere ‘fragments’ of the angiotensin II molecule but hormones which are biologically active in their own right, regardless of whether they are cleaved from angiotensinogen (Aogen), angiotensin I or even angiotensin II. To understand the significance of this difference, the established concept of a linear and sequential processing of Aogen into the biologically active angiotensin II ‘fragment’ needs to be challenged. In truth, the formation of biologically active angiotensin peptides engages the actions of several enzyme systems: endoproteases (renin, tonin and cathepsins), endopeptidases (neutral endopeptidase 24.11., prolyl endopeptidase 24.26 and thimet oligopeptidase 24.15), carboxypeptidases [angiotensin-converting enzyme (ACE), ACE2 and the post proline carboxypeptidase] and aminopeptidases. At several steps in the enzymatic cascade, biologically active C-terminus [from angiotensin-(1-9) to angiotensin-(3-8)] and N-terminus [angiotensin-(1-7), angiotensin-(3-7)] peptides are produced to exert endocrine, paracrine and autocrine regulatory functions on neurotransmission, circulatory, immune and intracellular signalling events. Synthesis of specific molecular forms of angiotensin peptides are determined by several factors, including substrate availability, phenotypic expression and localization of the enzymes in the tissue microenvironment or the cell membrane, and ligand receptor interactions. In this commentary, I wish to address some of the findings reviewed by Cesari and colleagues in this issue of the journal. In focusing the discussion on angiotensin-(1-7), it is not my intention to belittle the importance of other members of the angiotensin peptide family [angiotensin-(2-8) and angiotensin-(3-8)]. As the investigation of the cardiovascular functions of angiotensin-(1-7) and its role in hypertension has now attracted the attention of many investigators, the reader may find some justification on this approach. The physiological actions of angiotensin-(1-7) on vascular tone, cardiac contractility, fluid homeostasis and renal function have been confirmed by several independent laboratories. The concept that angiotensin-(1-7) has a counterbalancing effect to angiotensin II in the regulation of body fluid volume and blood pressure, as first proposed by us [8], has sustained the passage of time and recently confirmed in isolated rabbit afferent arterioles [9]. Through a series of experiments, we showed that the vasodilator effects of angiotensin-(1-7) are best demonstrable in situations in which the RAS is overactive (i.e. salt depletion, renin-dependent hypertension or chronic blockade of ACE). In other words, any situation in which the balance of the pressor control mechanism is tilted towards systemic vasoconstriction. This explains why angiotensin-(1-7) may be a weak vasodilator in salt-replete spontaneously hypertensive rats (SHR) [10] and, conversely, a potent vasodilator in chronically salt-depleted SHR [11]. Studies showing the degree of endogenous activation of the RAS influences the magnitude of vasodilator responses to angiotensin-(1-7) led to the proposal that enzymatic cascade negative feedback loops exist within the RAS that act to control the activity or production of angiotensin II. This possibility gained support with the demonstration that the primary route for the degradation of angiotensin-(1-7) is via ACE [12,13]. This was a critical observation as any factor affecting the enzymatic activity of ACE has the inherent capacity to alter simultaneously both the rate of angiotensin II formation from angiotensin I and the degradation of angiotensin-(1-7) into angiotensin-(1-5). Moreover, Deddish et al. [14] reported that angiotensin-(1-7) degradation into angiotensin-(1-5) is mediated by the N-domain of somatic ACE, whereas angiotensin-(1-7) may also act as a weak endogenous inhibitor of the C-domain of ACE. These findings explain our previous observations that ACE inhibitors [13,15] and the vasopeptidase inhibitor (omapatrilat) [16] increase plasma and urinary concentrations of angiotensin-(1-7). Our results further showed that ACE-mediated increases in angiotensin-(1-7) contribute to the antihypertensive effect of these drugs [13,17] and may mediate the inhibition of neointima formation produced by vascular endothelial injury [18]. As indicated by Cesari and colleagues, current limitations regarding an identification of the molecular nature of the receptor subtype mediating the actions of angiotensin-(1-7) and the unconfirmed observation that angiotensin-(1-7) levels in biological fluids are at concentrations that are miniscule compared to angiotensin II [16] have in the past handicapped additional efforts. Nevertheless, the current availability of a selective angiotensin-(1-7) competitive receptor antagonist and the publication of several studies describing binding characteristics of angiotensin-(1-7) to a non AT1/AT2 receptor in vascular and renal tissues and derived cell lines should accelerate both acceptance of these concepts and stimulate further research into the molecular signalling processes mediated by the heptapeptide. In this context, the recent demonstration that insulin-regulated aminopeptidase is a receptor for angiotensin-(3-8) suggest new strategies for seeking the molecular mechanisms by which angiotensin-(1-7) contributes to homeostasis [19].