Endogenous atrial natriuretic factor after endopeptidase 24.11 inhibition by Thiorphan

Differential regulation of renal angiotensin-converting enzyme (ACE) and ACE2 during ACE inhibition and dietary sodium restriction in healthy rats

Angiotensin-converting enzyme (ACE) 2 is thought to counterbalance ACE by breakdown of angiotensin (Ang) II and formation of Ang(1-7). Both enzymes are highly expressed in the kidney, but reports on their regulation differ. To enhance our understanding of the regulation of renal ACE and ACE2, we investigated renal ACE and ACE2 expression during conditions of physiological (low-sodium diet) and pharmacological changes (ACE inhibition) in activity of the renin-angiotensin-aldosterone system (RAAS). Healthy rats were treated with vehicle or lisinopril with either a control or a low-sodium diet, and renal ACE2, ACE and plasma angiotensins were studied. During vehicle treatment, low sodium reduced renal ACE mRNA and activity without affecting ACE2 mRNA or activity and plasma Ang(1-7) and Ang II balance. Lisinopril significantly reduced renal ACE activity without affecting renal ACE2 activity. During ACE inhibition, low sodium reduced both ACE and ACE2 mRNA without affecting ACE2 activity or further reducing ACE activity. Measurements of renal neprilysin activity revealed no significant differences between any of the treatment groups. Plasma Ang(1-7) and Ang II balance is positively shifted towards the beneficial vasopeptide Ang(1-7) by the ACE inhibitor lisinopril, especially during a low sodium intake. In conclusion, modulation of the RAAS, by low sodium intake or ACE inhibition, does not affect renal ACE2 despite major variations in renal ACE. Thus, ACE and ACE2 are differentially regulated by low sodium and ACE inhibition. Therefore, we propose that the beneficial effects of ACE inhibitors are predominantly mediated by modulation of ACE and not ACE2. Whether this also applies to renal disease conditions should be investigated in future studies.

First report on the efficacy of l-alanine-based in situ-forming implants for the long-term parenteral delivery of drugs

The recent advent of biotechnologies has led to the development of labile macromolecular therapeutic agents that require complex formulations for their efficient administration. This work reports a novel concept for the systemic, sustained delivery of such agents. The proposed approach is based on the spontaneous self-assembly of low-molecular weight amphiphilic amino acid derivatives in a hydrophobic pharmaceutical vehicle. The injectable, in situ-forming organogels were obtained by mixing N-stearoyl l-alanine (m)ethyl esters with a vegetable oil and a biocompatible hydrophilic solvent. The gels' in vivo-delivering properties were evaluated in rats with leuprolide, a luteinizing hormone-releasing hormone agonist used in prostate cancer, endometriosis and precocious puberty treatment. Following subcutaneous injection, the gels degraded and gradually released leuprolide for 14 to 25 days. Drug release was accompanied by sustained castration lasting up to 50 days, as assessed by testosterone levels. This study demonstrates that in situ-forming implants based on l-alanine derivatives represent a novel injectable platform for the controlled delivery of hydrophilic compounds, which is simpler than currently available implant and microsphere technologies.

Inhibition of neutral endopeptidase potentiates the effects of atrial natriuretic peptide on acute cyclosporin-induced nephrotoxicity

BACKGROUND

The use of cyclosporin A (CyA) is limited by its significant nephrotoxicity. Atrial natriuretic peptide (ANP) has been shown to ameliorate the reduction in glomerular filtration rate (GFR) induced by CyA, but its effect is transient. One explanation may be the rapid breakdown of this hormone by neutral endopeptidase (NEP) which is highly active in the kidney. In the present study, we examined the effect of the NEP inhibitor thiorphan on the acute fall in GFR induced by CyA.

METHODS

After a first set of experiments to investigate the renal hemodynamic effects of CyA (20 mg.kg(-1), i.v. bolus), we studied four additional conditions where acute CyA treatment was followed by the administration of: (2) ANP alone (10 microg.kg(-1) i.v. as bolus and a maintenance infusion of 1 microg. kg(-1).min(-1)); (3) thiorphan alone (5 mg.kg(-1) i.v. as bolus and a maintenance infusion of 0.5 mg.kg(-1). min(-1)); (4) ANP plus thiorphan (as in 2 and 3), and (5) an infusion of 0.9% saline, increased from 1.2 to 3 ml.h(-1). The GFR was measured as the clearance of (3)H-methoxyinulin (ml.min(-1).100 g(-1) body weight).

RESULTS

The data show: (1) the GFR fell from 1.06 +/- 0.15 to 0.59 +/- 0.09 ml.min(-1).100 g(-1) (p < 0.01) 60 min after CyA and remained depressed for at least 2 h; (2) ANP caused a marked initial rise in GFR from 0.49 +/- 0.07 to 1.23 +/- 0.18 ml.min(-1).100 g(-1) (p < 0.005 vs. CyA) which declined rapidly to the value seen after CyA injection alone, despite continuing ANP infusion; (3) thiorphan caused a modest, but significant increase in GFR within 15 min from 0.48 +/- 0.04 to 0.69 +/- 0.10 ml.min(-1).100 g(-1) (p < 0.05 vs. CyA) which was sustained during infusion and for at least 30 min after stopping infusion; (4) ANP plus thiorphan produced a marked increase in GFR from 0.58 +/- 0.09 to 1.39 +/- 0.44 ml.min(-1).100 g(-1) (p < 0.05 vs. CyA) which then decreased, but remained above the post-CyA injection value, until infusion of both drugs ended; (5) more than doubling the saline infusion rate per se had no significant effect on the GFR response to CyA. The blood pressure decreased significantly during ANP infusion, but more so when combined with thiorphan.

CONCLUSION

These data indicate that the inhibition of NEP by thiorphan is able to ameliorate partially the reduction in GFR induced by CyA and to enhance, and prolong, the vasodilator and diuretic effects of ANP.

Molecular assembly of endogenous and synthetic big atrial natriuretic peptide (ANP) and its amyloidogenic implications

The aggregation process of alpha-hANP has been investigated in vitro at physiological concentrations by gel chromatographic procedures using a radiolabeled tracer incubated in PBS and in plasma. In PBS big forms of ANP are organized as a peak eluting from both Sephacryl S-100 and S-300 HR in the void volume of the columns; in plasma, besides this major peak, a second radioactive peak is evident, eluting from Sephacryl S-100 HR around the HSA region. After gel chromatography on Sephacryl S-300 HR the major peak appears to consist of three components of different molecular size. Some information about the nature of these peak materials comes from the result of parallel incubations of partially aggregated (seed or nucleus) and aggregate depleted tracer. The comparison between the two time courses of big ANP formation indicates that: (a) ANP aggregation is a nucleation-dependent process, with a lag time longer than 8 days, at picogram peptide levels and (b) the aggregated forms of peptide are those eluting in the void volume, the other plasma peaks being probably expression of a binding, neither saturable or reversible, to some plasma components. The principle of seeded polymerization, used to detect ANP aggregates present in the plasma, indicates that: (a) the endogenous big ANP cannot act as a nucleus for polymerization and it likely consists of non-fibrillar ANP aggregates and/or bound ANP, and (b) this experimental approach can be suitable to evidence ANP binding plasma factors for further characterization studies.

Degradation and inactivation of human atrial natriuretic peptide by human pulmonary plasma membranes

Atrial natriuretic peptide (ANP) is extracted from plasma during its passage through the lungs. ANP is metabolized in rat lung membrane preparations by the enzyme neutral endopeptidase-24.11 (EC 3.4.24.11), but the hydrolysis of ANP in human lung has not been characterized. In the present study synthetic human atrial natriuretic peptide 1-28 (alpha-hANP) was separately incubated with human pulmonary plasma membranes from two non-smoking patients, and the major degradation products were separated from alpha-hANP by reverse-phase high pressure liquid chromatography. The degradation products were identified by sequence analysis and by mass-spectrometry, and biological activity was studied in vitro by exposing precontracted rabbit pulmonary arteries to alpha-hANP and the degradation products. The initial cleavage appeared, with membrane preparations from both patients, in the central ring structure between Arg14 and Ile15, followed by a cleavage of the bond Arg3-Arg4 at the N-terminal region of the peptide. The biological activity of this ring-opened product was about 1/500 of the activity of uncleaved alpha-hANP. Cleavage of the Arg3-Arg4 or Arg14-Ile15 bonds could not be inhibited by EDTA, iodoacetamide, benzamidine hydrochloride or pepstatin A. Neither did phosphoramidon (1 microM) or thiorphan (1 microM) inhibit the hydrolysis, indicating the presence in human lung of an ANP-degrading enzyme different from endopeptidase-24.11.

Atrial natriuretic peptide clearance receptors in trout: effects of receptor inhibition in vivo

Inactivation of circulating atrial natriuretic peptides (ANP) by specialized clearance (C) receptors has been characterized in mammals but has not been examined in fish. In the present study arterial blood pressure, urine flow, and urine electrolytes were measured in chronically cannulated rainbow trout, Oncorhynchus mykiss, during infusion of the specific C receptor inhibitor, SC-46542. C receptor inhibition decreased blood pressure and pulse pressure, increased heart rate and urine flow, but did not affect urinary electrolyte concentrations. These responses are consistent with those produced by exogenous ANP administration and indicate that: (1) trout possess C-type receptors capable of ANP inactivation, and (2) ANP-like molecules are continuously released and metabolized by trout in vivo. Phosphoramidon, an inhibitor of neutral endopeptidase, did not enhance the SC-46542 response, indicating that C receptors predominate in ANP inactivation in these fish.