Angiotensin II production and distribution in the kidney: I. A kinetic model

The intrarenal renin-angiotensin system in hypertension: insights from mathematical modelling

The renin-angiotensin system (RAS) plays a pivotal role in the maintenance of volume homeostasis and blood pressure. In addition to the well-studied systemic RAS, local RAS have been documented in various tissues, including the kidney. Given the role of the intrarenal RAS in the pathogenesis of hypertension, a role established via various pharmacologic and genetic studies, substantial efforts have been made to unravel the processes that govern intrarenal RAS activity. In particular, several mechanisms have been proposed to explain the rise in intrarenal angiotensin II (Ang II) that accompanies Ang II infusion, including increased angiotensin type 1 receptor (AT1R)-mediated uptake of Ang II and enhanced intrarenal Ang II production. However, experimentally isolating their contribution to the intrarenal accumulation of Ang II in Ang II-induced hypertension is challenging, given that they are fundamentally connected. Computational modelling is advantageous because the feedback underlying each mechanism can be removed and the effect on intrarenal Ang II can be studied. In this work, the mechanisms governing the intrarenal accumulation of Ang II during Ang II infusion experiments are delineated and the role of the intrarenal RAS in Ang II-induced hypertension is studied. To accomplish this, a compartmental ODE model of the systemic and intrarenal RAS is developed and Ang II infusion experiments are simulated. Simulations indicate that AT1R-mediated uptake of Ang II is the primary mechanism by which Ang II accumulates in the kidney during Ang II infusion. Enhanced local Ang II production is unnecessary. The results demonstrate the role of the intrarenal RAS in the pathogenesis of Ang II-induced hypertension and consequently, clinical hypertension associated with an overactive RAS.

Concentrations of Transition Metal Ions in Rat Lungs after Tobacco Smoke Exposure and Treatment with His-Leu Dipeptide

Tobacco smoking is deleterious to the lungs because it exposes them to many toxic substances. These include transition metal ions, such as cadmium. However, there is a lack of information about the influence of endogenous metal-binding peptides, such as His-Leu (HL), on the lung distribution of transition metals in smokers. To address this, we administered HL subcutaneously to rats exposed to tobacco smoke for six weeks, then we measured the concentrations of transition metal ions in the lungs. We found that exposure to tobacco smoke elevates the concentrations of Cd(II) and Cu(II). Administration of the HL peptide, whose elevation is a consequence of angiotensin receptor blocker anti-hypertension therapy, increases the concentration of Fe in the lungs of rats exposed to smoke. These findings suggest that smoking is a risk factor for patients receiving angiotensin receptor blockers to treat hypertension.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

Modulation of the rat angiotensin type 1a receptor by an upstream short open reading frame

The rat angiotensin type 1a receptor (AT_1aR) is a peptide hormone G protein-coupled receptor (GPCR) that plays a key role in electrolyte homeostasis and blood pressure control. There is a highly conserved short open reading frame (sORF) in exon 2 (E2) that is downstream from exon 1 (E1) and upstream of the AT_1aR coding region located in exon 3 (E3). To determine the role of this E2 sORF in AT_1aR signaling, human embryonic kidney-293 (HEK293) cells were transfected with plasmids containing AT_1aR cDNA with either an intact or disrupted E2 sORF. The intact sORF attenuated the efficacy of angiotensin (Ang) II (p < 0.001) and sarcosine¹,Ile⁴,Ile⁸-Ang II (SII), (p < 0.01) to activate AT_1aR signaling through extracellular signal-related kinases 1/2 (ERK1/2). A time-course showed agonist-induced AT_1aR-mediated ERK1/2 activation was slower in the presence of the intact compared to the disrupted sORF [Ang II: p < 0.01 and SII: p < 0.05]. Ang II-induced ERK1/2 activation was completely inhibited by the protein kinase C (PKC) inhibitor Ro 31-8220 regardless of whether the sORF was intact or disrupted. Flow cytometric analyses suggested the intact sORF improved cell survival; the percentage of live cells increased (p < 0.05) while the percentage of early apoptotic cells decreased (p < 0.01) in cells transfected with the AT_1aR plasmid containing the intact sORF. These findings have implications for the regulation of AT₁Rs in physiological and pathological conditions and warrant investigation of sORFs in the 5' leader sequence (5'LS) of other GPCRs.

Comparative effects of angiotensin II on the contractility of muscularis mucosae and detrusor in the pig urinary bladder

To explore contractile actions of angiotensin II (ATII) on the muscularis mucosae (MM) of the bladder, ATII-induced contractions were compared between MM and the detrusor smooth muscle (DSM) of the pig bladder by isometric tension recordings. Effects of ATII on spontaneous Ca²⁺ transients in MM were visualized using Cal-520 fluorescence. ATII receptor type 1 (ATR1) expression in MM and DSM was also examined by immunohistochemistry. ATII (1 nM-1 μM) caused phasic contractions of MM in a concentration-dependent manner, while ATII (10 nM-10 μM) had no or marginal effects on DSM contractility. ATII (100 nM)-induced MM contractions had an amplitude of approximately 70% of carbachol (1 μM)-induced or 90% of U46619 (100 nM)-induced contractions. Candesartan (10 nM), an ATR1 blocker, prevented the contractile effects of ATII (1 nM) in MM, while ATR1 immunofluorescence was greater in MM than DSM. ATII (10-100 pM) increased the frequency but not the amplitude of spontaneous Ca²⁺ transients in MM. Both urothelium-intact and -denuded MM strips developed comparable spontaneous phasic contractions, but ATII, carbachol and U46619-induced contractions were significantly larger in urothelium-denuded than urothelium-intact MM strips. In conclusion, the MM appears to have a much greater sensitivity to ATII compared with DSM that could well sense circulating ATII, suggesting that MM may be the predominant target of contractile actions induced by ATII in the bladder while the urothelium appears to inhibit MM contractility.

Angiotensin generation in the brain: a re-evaluation

The existence of a so-called brain renin-angiotensin system (RAS) is controversial. Given the presence of the blood-brain barrier, angiotensin generation in the brain, if occurring, should depend on local synthesis of renin and angiotensinogen. Yet, although initially brain-selective expression of intracellular renin was reported, data in intracellular renin knockout animals argue against a role for this renin in angiotensin generation. Moreover, renin levels in brain tissue at most represented renin in trapped blood. Additionally, in neurogenic hypertension brain prorenin up-regulation has been claimed, which would generate angiotensin following its binding to the (pro)renin receptor. However, recent studies reported no evidence for prorenin expression in the brain, nor for its selective up-regulation in neurogenic hypertension, and the (pro)renin receptor rather displays RAS-unrelated functions. Finally, although angiotensinogen mRNA is detectable in the brain, brain angiotensinogen protein levels are low, and even these low levels might be an overestimation due to assay artefacts. Taken together, independent angiotensin generation in the brain is unlikely. Indeed, brain angiotensin levels are extremely low, with angiotensin (Ang) I levels corresponding to the small amounts of Ang I in trapped blood plasma, and Ang II levels at most representing Ang II bound to (vascular) brain Ang II type 1 receptors. This review concludes with a unifying concept proposing the blood origin of angiotensin in the brain, possibly resulting in increased levels following blood-brain barrier disruption (e.g. due to hypertension), and suggesting that interfering with either intracellular renin or the (pro)renin receptor has consequences in an RAS-independent manner.

Brain Renin-Angiotensin System: Does It Exist?

Because of the presence of the blood-brain barrier, brain renin-angiotensin system activity should depend on local (pro)renin synthesis. Indeed, an intracellular form of renin has been described in the brain, but whether it displays angiotensin (Ang) I-generating activity (AGA) is unknown. Here, we quantified brain (pro)renin, before and after buffer perfusion of the brain, in wild-type mice, renin knockout mice, deoxycorticosterone acetate salt-treated mice, and Ang II-infused mice. Brain regions were homogenized and incubated with excess angiotensinogen to detect AGA, before and after prorenin activation, using a renin inhibitor to correct for nonrenin-mediated AGA. Renin-dependent AGA was readily detectable in brain regions, the highest AGA being present in brain stem (>thalamus=cerebellum=striatum=midbrain>hippocampus=cortex). Brain AGA increased marginally after prorenin activation, suggesting that brain prorenin is low. Buffer perfusion reduced AGA in all brain areas by >60%. Plasma renin (per mL) was 40× to 800× higher than brain renin (per gram). Renin was undetectable in plasma and brain of renin knockout mice. Deoxycorticosterone acetate salt and Ang II suppressed plasma renin and brain renin in parallel, without upregulating brain prorenin. Finally, Ang I was undetectable in brains of spontaneously hypertensive rats, while their brain/plasma Ang II concentration ratio decreased by 80% after Ang II type 1 receptor blockade. In conclusion, brain renin levels (per gram) correspond with the amount of renin present in 1 to 20 μL of plasma. Brain renin disappears after buffer perfusion and varies in association with plasma renin. This indicates that brain renin represents trapped plasma renin. Brain Ang II represents Ang II taken up from blood rather than locally synthesized Ang II.

Maximum renal responses to renin inhibition in healthy study participants: VTP-27999 versus aliskiren

BACKGROUND

Renin inhibition with aliskiren induced the largest increases in renal plasma flow (RPF) in salt-depleted healthy volunteers of all renin-angiotensin system (RAS) blockers. However, given its side-effects at doses higher than 300 mg, no maximum effect of renin inhibition could be established. We hypothesized that VTP-27999, a novel renin inhibitor without major side-effects at high doses, would allow us to establish this.

METHODS AND RESULTS

The effects of escalating VTP-27999 doses (75-600 mg) on RPF, glomerular filtration rate (GFR), and plasma RAS components were compared with those of 300 mg aliskiren in 22 normal volunteers on a low-sodium diet. VTP-27999 dose-dependently increased RPF and GFR; its effects on both parameters at 600 mg (increases of 18 ± 4 and 20 ± 4%, respectively) were equivalent to those at 300 mg, indicating that a maximum had been reached. The effects of 300 mg aliskiren (increases of 13 ± 5 and 8 ± 6%, respectively; P < 0.01 vs. 300 and 600 mg VTP-27999) resembled those of 150 mg VTP-27999. VTP-27999 dose-dependently increased renin, and lowered plasma renin activity and angiotensin II to detection limit levels. The effects of aliskiren on RAS components were best comparable to those of 150 mg VTP-27999.

CONCLUSION

Maximum renal renin blockade in healthy, salt-depleted volunteers, requires aliskiren doses higher than 300 mg, but can be established with 300 mg VTP-27999. To what degree such maximal effects (exceeding those of angiotensin-converting enzyme inhibitors and AT1-receptor blockers) are required in patients with renal disease, given the potential detrimental effects of excessive RAS blockade, remains to be determined.

Urinary ACE2 is associated with urinary L-FABP and albuminuria in patients with chronic kidney disease

AIM

Angiotensin-converting enzyme 2 (ACE2) is expressed in the kidney and may be a renoprotective enzyme since it converts angiotensin (Ang) II to Ang-(1-7). In addition, ACE2 has been detected in urine from patients with chronic kidney disease (CKD). The aim of this study was to determine the urinary ACE2 levels in patients with various stages of CKD and to identify the factors associated with the presence of ACE2.

METHODS

We assessed 152 patients with CKD stage G1-G4. The patients were classified according to the presence or absence of diabetes mellitus (DM) (DM group, n = 72; non-DM group, n = 80) and according to the estimated glomerular filtration rate (CKD stage G1/2 group, n = 40; CKD stage G3 group, n = 74; and CKD stage G4 group, n = 38). Parameters were urinary ACE2, urinary albumin/ creatinine ratio (UACR), urinary liver-type fatty acid binding protein (L-FABP), estimated glomerular filtration rate, and other factors determined to be associated with elevated urinary ACE2.

RESULTS

Urinary ACE2 was significantly higher in patients with diabetes (p = 0.01) and in patients with CKD stage G4 compared with stages G1-G3 (p < 0.0001). Multivariable regression analysis revealed that urinary L-FABP and UACR were significantly associated with urinary ACE2 levels, indicating that urinary ACE2 is increased in patients with diabetes and advanced stage CKD.

CONCLUSION

ACE2 might continuously protect from both glomerular and tubulointerstitial injury during CKD progression. Taken together, urinary ACE2 might be a marker of kidney renin-angiotensin system activation in such patients.

How does the angiotensin II type 1 receptor 'trump' the type 2 receptor in blood pressure control?

BACKGROUND

A kinetic model for the binding of angiotensin II (Ang II) to AT1 receptors (AT1Rs) in arterioles did suggest a novel mechanism of association rate amplification and facilitated Ang II diffusion in vivo.

AIM OF STUDY

To examine how this mechanism, acting on AT1R, will affect the stimulation of AT2R.

METHOD

The model distinguishes between the diffusion of plasma Ang II across the endothelium layer (thickness 10(-4) - 5 × 10(-4) cm) into the vascular smooth muscle (VSM) layer (5 × 10(-4) cm), and the diffusion of tissue Ang II from perivascular interstitium (thickness of micromilieu fluid layer at abluminal VSM surface 10(-6) - 10(-5) cm, i.e. 1 to 10 times the glycocalyx). Thus, Ang II concentration [Ang II] is taken to be 0 at the abluminal and adluminal VSM cell surfaces, respectively. Tissue Ang II is defined as originating from local generation and/or from the capillary circulation. [Ang II]/AT1R and [Ang II]/AT2R occupancy curves for the two directions of diffusion are constructed from the model-based calculations.

RESULTS

Ang II, at 10(-15)-10(-13) mol/ml (~1-100 pg/ml), is much less likely to react with vascular AT2R than AT1R, though it has similar affinity for the receptor types. With plasma [Ang II] = 10(-15)-10(-13) mol/ml, AT2R occupancy is less than 10% of maximum on endothelium, and virtually 0 on VSM, whereas AT1R occupancy on VSM is virtually 0 at plasma [Ang II] < 10(-14) mol/ml, and between 0 and 30% at plasma [Ang II] = 10(-13) mol/ml. With tissue [Ang II] = 10(-15)-10(-13) mol/ml, VSM AT2R occupancy is close to 0, whereas VSM AT1R occupancy is 40-60% in the absence of endocytotic AT1R down-regulation, and up to 70-90% in its presence.

CONCLUSION

The threshold concentration of Ang II needed for response is much higher for AT2R than for AT1R. Plasma Ang II rather than tissue Ang II is the agonist of AT2R, and the reverse applies to AT1R. Thus, AT2R stimulation may come into play only at unusually high circulating levels of Ang II.

Urinary markers of intrarenal renin-angiotensin system activity in vivo

Recent interest focuses on urinary renin and angiotensinogen as markers of renal renin-angiotensin system activity. Before concluding that these components are independent markers, we need to exclude that their presence in urine, like that of albumin (a protein of comparable size), is due to (disturbed) glomerular filtration. This review critically discusses their filtration, reabsorption and local release. Given the close correlation between urinary angiotensinogen and albumin in human studies, it concludes that, in humans, urinary angiotensinogen is a filtration barrier damage marker with the same predictive power as urinary albumin. In contrast, in animals, tubular angiotensinogen release may occur, although tubulus-specific knockout studies do not support a functional role for such angiotensinogen. Urinary renin levels, relative to albumin, are >200-fold higher and unrelated to albumin. This may reflect release of renin from the urinary tract, but could also be attributed to activation of filtered, plasma-derived prorenin and/or incomplete tubular reabsorption.

Renal responses to three types of renin-angiotensin system blockers in patients with diabetes mellitus on a high-salt diet: a need for higher doses in diabetic patients?

OBJECTIVE

Activation of the renal renin-angiotensin system in patients with diabetes mellitus appears to contribute to the risk of nephropathy. Recently, it has been recognized than an elevation of prorenin in plasma also provides a strong indication of risk of nephropathy. This study was designed to examine renin-angiotensin system control mechanisms in the patient with diabetes mellitus.

METHODS

We enrolled 43 individuals with type 2 diabetes mellitus. All individuals were on a high-salt diet to minimize the contribution of the systemic renin-angiotensin system. After an acute exposure to captopril (25 mg), they were randomized to treatment with either irbesartan (300 mg) or aliskiren (300 mg) for 2 weeks.

RESULTS

All agents acutely lowered blood pressure and plasma aldosterone, and increased renal plasma flow and glomerular filtration rate. Yet, only captopril and aliskiren acutely increased plasma renin and decreased plasma angiotensin II, whereas irbesartan acutely affected neither renin nor angiotensin II. Plasma renin and angiotensin II subsequently did increase upon chronic irbesartan treatment. When given on day 14, irbesartan and aliskiren again induced the above hemodynamic, renal and adrenal effects, yet without significantly changing plasma renin. Irbesartan at that time did not affect plasma angiotensin II, whereas aliskiren lowered it to almost zero.

CONCLUSION

The relative resistance of the renal renin response to acute (irbesartan) and chronic (irbesartan and aliskiren) renin-angiotensin system blockade supports the concept of an activated renal renin-angiotensin system in diabetes, particularly at the level of the juxtaglomerular cell, and implies that diabetic patients might require higher doses of renin-angiotensin system blockers to fully suppress the renal renin-angiotensin system.

Loss of ACE2 accelerates time-dependent glomerular and tubulointerstitial damage in streptozotocin-induced diabetic mice

As angiotensin-converting enzyme-2 (ACE2) was identified as a negative regulator of the renin-angiotensin system, there have been many reports concerning its role in several tissues, including the kidney. However, the role of ACE2 during the development of diabetic nephropathy remains undetermined, as previous reports did not necessarily support a protective role against renal injury. Thus, we performed detailed observations of kidneys in ACE2-knockout (ACE2-KO) mice at early (4 weeks) and advanced (18 weeks) stages of diabetes. ACE2-KO and wild-type C57BL/6 mice were rendered diabetic by intraperitoneal injection of streptozotocin. Diabetic ACE2-KO mice showed earlier onset and more severe progression of albuminuria than those did wild-type mice. The elevation of serum creatinine and urea nitrogen levels at 18 weeks of diabetes was more prominent in ACE2-KO mice. Periodic acid-Schiff-stained cross-section of diabetic ACE2-KO mice showed a more severe time-dependent increase in glomerular/tubulointerstitial damage than did that of wild-type mice, confirmed by the immunostaining of alpha-smooth muscle actin, collagen IV and F4-80 antigen. Glomeruli of diabetic ACE2-KO mice showed earlier and more severe decrease in the expression of nephrin, whose degradation is involved in the onset of albuminuria, and more potent increase of vascular endothelial growth factor expression. In addition, treatment with AT1 receptor blocker olmesartan significantly, but not totally, ameliorated the functional and morphological deterioration of diabetic nephropathy in ACE2-KO mice. These results suggest that ACE2 might continuously protect from both glomerular and tubulointerstitial injury during the development of diabetic nephropathy. The renal-protective effect of ACE2 might involve more than just suppressing angiotensin II-mediated AT1 receptor signaling.

Antihypertensive therapy upregulates renin and (pro)renin receptor in the clipped kidney of Goldblatt hypertensive rats

Recently, a (pro)renin receptor has been identified which mediates profibrotic effects independent of angiotensin II. Because antihypertensive therapy induces renal injury in the clipped kidney of two kidney-1-clip hypertensive rats, we examined the regulation of renin and the (pro)renin receptor in this model. Hypertensive Goldblatt rats were treated with increasing doses of the vasopeptidase inhibitor AVE 7688 after which the plasma renin and prorenin as well as the renal renin and (pro)renin receptor expression were measured. The vasopeptidase inhibitor dose-dependently lowered blood pressure, which was associated with a massive increase in plasma prorenin and renin as well as increased renal renin expression. The (pro)renin receptor was upregulated in the clipped kidney of the Goldblatt rat indicating a parallel upregulation of renin and its receptor in vivo. Immunohistochemistry showed a redistribution of renin upstream from the glomerulus in preglomerular vessels and renin staining in tubular cells. Expression of the (pro)renin receptor was increased in the vessels and tubules. This upregulation was associated with thickening of renin-positive vessels and tubulointerstitial damage. We propose that renin and the (pro)renin receptor may play a profibrotic role in the clipped kidney of Goldblatt rats treated for hypertension.

Loss of angiotensin-converting enzyme-2 (Ace2) accelerates diabetic kidney injury

Diabetic nephropathy is one of the most common causes of end-stage renal failure, but the factors responsible for the development of diabetic nephropathy have not been fully elucidated. We examined the effect of deletion of the angiotensin-convert-ing enzyme 2 (Ace2) gene on diabetic kidney injury. Ace2(-/-) mice were crossed with Akita mice (Ins2(WT/C96Y)), a model of type 1 diabetes mellitus, and four groups of mice were studied at 3 months of age: Ace2(+/y)Ins2(WT/WT), Ace2(-/y)Ins2(WT/WT), Ace2(+/y) Ins2(WT/C96Y), and Ace2(-/y)Ins2(WT/C96Y). Ace2(-/y) Ins2(WT/C96Y) mice exhibited a twofold increase in the urinary albumin excretion rate compared with Ace2(+/y)Ins2(WT/C96Y) mice despite similar blood glucose levels. Ace2(-/y)Ins2(WT/C96Y) mice were the only group to exhibit increased mesangial matrix scores and glomerular basement membrane thicknesses compared with Ace2(+/y)Ins2(WT/WT) mice, accompanied by increased fibronectin and alpha-smooth muscle actin immunostaining in the glomeruli of Ace2(-/y) Ins2(WT/C96Y) mice. There were no differences in blood pressure or heart function to account for the exacerbation of kidney injury. Although kidney levels of angiotensin (Ang) II were not increased in the diabetic mice, treatment with an Ang II receptor blocker reduced urinary albumin excretion rate in Ace2(-/y)Ins2(WT/C96Y) mice, suggesting that acceleration of kidney injury in these mice is Ang II-mediated. We conclude that ACE2 plays a protective role in the diabetic kidney, and ACE2 is an important determinant of diabetic nephropathy.

ACE phenotyping as a first step toward personalized medicine for ACE inhibitors. Why does ACE genotyping not predict the therapeutic efficacy of ACE inhibition?

Angiotensin (Ang)-converting enzyme (ACE) inhibitors are widely used for the treatment of cardiovascular diseases. Not all patients respond to ACE inhibitors, and it has been suggested that genetic variation might be a useful marker to predict the therapeutic efficacy of these drugs. In particular, the ACE insertion (I)/deletion (D) polymorphism has been investigated in this regard. Despite a decade of intensive research involving the genotyping of thousands of patients, we still do not know whether ACE genotyping helps in predicting the success of ACE inhibition. This review critically addresses the concept that predictive information on therapeutic efficacy of ACE inhibitors might be obtained based on ACE genotyping. It answers the following questions: Do higher ACE levels really result in higher Ang II levels? Is ACE the only converting enzyme in humans? Does ACE inhibition affect ACE expression? Why does ACE have 2 catalytically active domains? What is the relevance of ACE inhibitor-induced signaling through membrane-bound ACE? The review ends with the proposal that ACE phenotyping may prove to be a better first step toward personalized medicine for ACE inhibitors than ACE genotyping.