Aliskiren-binding increases the half life of renin and prorenin in rat aortic vascular smooth muscle cells.

OBJECTIVE: Renin inhibition with aliskiren has been reported to cause a greater rise in renin than other types of renin-angiotensin system blockade, thereby potentially leading to angiotensin generation or stimulation of the human (pro)renin receptor (h(P)RR). Here we studied whether this rise in renin is attributable to an aliskiren-induced change in the prorenin conformation, allowing its detection in renin assays, or a change in renin/prorenin clearance. We also investigated whether aliskiren affects (pro)renin binding to its receptors, using rat aortic vascular smooth muscle cells (VSMCs) overexpressing the h(P)RR. METHODS AND RESULTS: A 48-hour incubation with aliskiren at 4 degrees C converted the prorenin conformation from "closed" to "open," thus allowing its recognition in active site-directed renin assays. VSMCs accumulated (pro)renin through binding to mannose 6-phosphate receptors (M6PRs) and h(P)RRs. Aliskiren did not affect binding at 4 degrees C. At 37 degrees C, aliskiren increased (pro)renin accumulation up to 40-fold, and M6PR blockade prevented this. Aliskiren increased the intracellular half life of prorenin 2 to 3 times. CONCLUSIONS: Aliskiren allows the detection of prorenin as renin, and decreases renin/prorenin clearance. Both phenomena may contribute to the "renin" surge during aliskiren treatment, but because they depend on aliskiren binding, they will not result in angiotensin generation. Aliskiren does not affect (pro)renin binding to its receptors.

Different contributions of the angiotensin-converting enzyme C-domain and N-domain in subjects with the angiotensin-converting enzyme II and DD genotype.

BACKGROUND: Angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphism-related differences in ACE concentration do not result in differences in angiotensin levels. METHODS AND RESULTS: To investigate whether this relates to differences in the contribution of the ACE C-domain and N-domain, we quantified, using the C-domain-selective inhibitors quinaprilat and RXPA380, and the N-domain-selective inhibitor RXP407, the contribution of both domains to the metabolism of angiotensin I, bradykinin, the C-domain-selective substrate Mca-BK(1-8), and the N-domain-selective substrate Mca-Ala in serum of IIs, DDs, and 'hyperACE' subjects (i.e., subjects with increased ACE due to enhanced shedding). During incubation with angiotensin I, the highest angiotensin II levels were observed in sera with the highest ACE activity. This confirms that ACE is rate-limiting with regard to angiotensin II generation. C-domain-selective concentrations of quinaprilat fully blocked angiotensin I-II conversion in DDs, whereas additional N-domain blockade was required to fully block conversion in IIs. Both domains contributed to bradykinin hydrolysis in all subjects, and the inhibition profile of RXP407 when using Mca-Ala was identical in IIs and DDs. In contrast, the RXPA380 concentrations required to block C-domain activity when using Mca-BK (1-8) were three-fold higher in IIs than DDs. CONCLUSION: The contributions of the C-domain and N-domain differ between DDs and IIs, and RXPA380 is the first inhibitor capable of distinguishing D-allele ACE from I-allele ACE. The lack of angiotensin II accumulation in DDs in vivo is not because of the often quoted concept that ACE is a nonrate-limiting enzyme. It may relate to the fact that in IIs both the N-domain and C- domain generate angiotensin II, whereas in DDs only the C-domain converts angiotensin I.

Prorenin is the endogenous agonist of the (pro)renin receptor. Binding kinetics of renin and prorenin in rat vascular smooth muscle cells overexpressing the human (pro)renin receptor.

OBJECTIVE: Mannose 6-phosphate receptors (M6PR) bind both renin and prorenin, and such binding contributes to renin/prorenin clearance but not to angiotensin generation. Here, we evaluated the kinetics of renin/prorenin binding to the recently discovered human (pro)renin receptor (h(P)RR), and the idea that such binding underlies tissue angiotensin generation. METHODS AND RESULTS: Vascular smooth muscle cells from control rats and transgenic rats with smooth muscle h(P)RR overexpression were incubated at 4 or 37 degrees C with human renin or prorenin. Incubation at 37 degrees C greatly increased binding, suggesting that (pro)renin-binding receptors cycle between the intracellular compartment and the cell surface. Blockade of the M6PR reduced binding by approximately 50%. During M6PR blockade, h(P)RR cells bound twice as much prorenin as control cells, while renin binding was unaltered. Incubation of h(P)RR (but not control) cells with prorenin + angiotensinogen yielded more angiotensin than expected on the basis of the activity of soluble prorenin, whereas angiotensin generation during incubation of both cell types with renin + angiotensinogen was entirely due to soluble renin. The renin + angiotensinogen-induced vasoconstriction of isolated iliac arteries from control and transgenic rats was also due to soluble renin only. The recently proposed (P)RR antagonist 'handle region peptide', which resembles part of the prosegment, blocked neither prorenin binding nor angiotensin generation. CONCLUSIONS: H(P)RRs preferentially bind prorenin, and such binding results in angiotensin generation, most likely because binding results in prorenin activation.

Transendothelial transport of renin-angiotensin system components.

BACKGROUND: Vascular (interstitial) angiotensin (ANG) II production depends on circulating renin-angiotensin system (RAS) components. Mannose 6-phosphate (man-6-P) receptors and angiotensin II type 1 (AT(1)) receptors, via binding and internalization of (pro)renin and ANG II, respectively, could contribute to the transportation of these components across the endothelium. OBJECTIVE: To investigate the mechanism(s) contributing to transendothelial RAS component transport. METHODS: Human umbilical vein endothelial cells were cultured on transwell polycarbonate filters, and incubated with RAS components in the absence or presence of man-6-P, eprosartan or PD123319, to block man-6-P, AT(1) and angiotensin II type 2 (AT(2)) receptors, respectively. RESULTS: Apically applied (pro)renin and angiotensinogen slowly entered the basolateral compartment, in a similar manner as horseradish peroxidase, a molecule of comparable size that reaches the interstitium via diffusion only. Prorenin transport was unaffected by man-6-P. Apical ANG I and ANG II rapidly reached the basolateral fluid independent of AT(1) and AT(2) receptors. Basolateral ANG II during apical ANG I application was as high as apical ANG II, whereas during apical ANG II application it was lower. During basolateral ANG I application, ANG II generation occurred basolaterally only, in an angiotensin-converting enzyme (ACE)-dependent manner. CONCLUSIONS: Circulating (pro)renin, angiotensinogen, ANG I and ANG II enter the interstitium via diffusion, and interstitial ANG II generation is mediated, at least in part, by basolaterally located endothelial ACE.

Amino-terminal pro-brain natriuretic peptide (NT-proBNP) is a biomarker of cardiac filling pressures in pre-eclampsia.

OBJECTIVE: To evaluate if amino-terminal pro-brain natriuretic peptide (NT-proBNP) plasma levels reflect intracardiac filling pressures in pre-eclamptic patients. STUDY DESIGN: In a cross-sectional study we investigated 22 untreated critically ill pre-eclamptic women between 22 and 34 weeks gestation. All patients underwent intra-arterial blood pressure and central hemodynamic measurements and NT-proBNP was determined in stored plasma. Baseline characteristics, plasma NT-proBNP concentrations and relevant laboratory variables were investigated for correlations with hemodynamic values using Spearman's rank correlation test. RESULTS: No significant correlations were demonstrated between NT-proBNP concentrations and variables associated with the severity of the pre-eclampsia. We found significant positive correlations between NT-proBNP and diastolic pulmonary pressure (r = 0.59; p = 0.005) and pulmonary capillary wedge pressure (PCWP) (r = 0.51; p = 0.015). Multiple linear regression analysis showed that the association between NT-proBNP and PCWP was not affected by creatinine level. CONCLUSION: NT-proBNP is a biomarker of left ventricular cardiac filling pressures in untreated pre-eclamptic patients.

Cardiac Renin levels are not influenced by the amount of resident mast cells.

To investigate whether mast cells release renin in the heart, we studied renin and prorenin synthesis by such cells, using the human mast cell lines human mastocytoma 1 and LAD2, as well as fresh mast cells from mastocytosis patients. We also quantified the contribution of mast cells to cardiac renin levels in control and infarcted rat hearts. Human mastocytoma 1 cells contained and released angiotensin I-generating activity, and the inhibition of this activity by the renin inhibitor aliskiren was comparable to that of recombinant human renin. Prorenin activation with trypsin increased angiotensin I-generating activity in the medium only, suggesting release but not storage of prorenin. The adenylyl cyclase activator forskolin, the cAMP analogue 8-db-cAMP, and the degranulator compound 48/80 increased renin release without affecting prorenin. Angiotensin II blocked the forskolin-induced renin release. Angiotensin I-generating activity was undetectable in LAD2 cells and fresh mast cells. Nonperfused rat hearts contained angiotensin I-generating activity, and aliskiren blocked approximately 70% of this activity. A 30-minute buffer perfusion washed away >70% of the aliskiren-inhibitable angiotensin I-generating activity. Prolonged buffer perfusion or compound 48/80 did not decrease cardiac angiotensin I-generating activity further or induce angiotensin I-generating activity release in the perfusion buffer. Results in infarcted hearts were identical, despite the increased mast cell number in such hearts. In conclusion, human mastocytoma 1 cells release renin and prorenin, and the regulation of this release resembles that of renal renin. However, this is not a uniform property of all mast cells. Mast cells appear an unlikely source of renin in the heart, both under normal and pathophysiological conditions.

Aliskiren accumulates in Renin secretory granules and binds plasma prorenin.

The vascular effects of aliskiren last longer than expected based on its half life, and this renin inhibitor has been reported to cause a greater renin rise than other renin-angiotensin system blockers. To investigate whether aliskiren accumulation in secretory granules contributes to these phenomena, renin-synthesizing mast cells were incubated with aliskiren, washed, and exposed to forskolin in medium without aliskiren (0.1 to 1000 nmol/L). (Pro)renin concentrations were measured by renin- and prorenin-specific immunoradiometric assays, and renin activity was measured by enzyme-kinetic assay. Without aliskiren, the culture medium predominantly contained prorenin, the cells exclusively stored renin, and forskolin doubled renin release. Aliskiren dose-dependently bound to (pro)renin in the medium and cell lysates and did not alter the effect of forskolin. The aliskiren concentrations required to bind prorenin were 1 to 2 orders of magnitude higher than those needed to bind renin. Blockade of cell lysate renin activity ranged from 27+/-15% to 79+/-5%, and these percentages were identical for the renin that was released by forskolin, indicating that they represented the same renin pool, ie, the renin storage granules. Comparison of renin and prorenin measurements in blood samples obtained from human volunteers treated with aliskiren, both before and after prorenin activation, revealed that