Conversion of big endothelin-1 and characterization of its contractile effects on isolated human placental arteries

OBJECTIVES

To study the conversion of human big endothelin-1 (bigET-1) to endothelin-1 (ET-1) and to characterize contractile ET-1 receptors in human placental arteries.

METHODS

BigET-1 was incubated with artery membranes and the formation of ET-1 was investigated. ET-1 and bigET-1-induced contractile responses were studied in the absence or presence of the metalloprotease inhibitor phosphoramidon, the ET(A)-receptor antagonist BQ 123, or the ETB-receptor antagonists IRL 1038 and RES 701-1.

RESULTS

The artery membranes hydrolysed bigET-1 to ET-1 through a partly phosphoramidon-sensitive pathway. The contractile responses to ET-1 and bigET-1 were similar, with pEC50% values of 8.1 +/- 0.2 and 7.8 +/- 0.1, respectively (NS; n = 17). Phosphoramidon decreased pEC50% for bigET-1-evoked contractions (p < 0.05; n = 8), without affecting the response to ET-1. A Schild plot of BQ 123 effects on ET-1 and bigET-1-induced contractions resulted in identical pA2 values and a slope of 0.56 +/- 0.2 and 0.47 +/- 0.01, respectively. IRL 1038 and RES 701-1 did not affect the contractile responses.

CONCLUSION

BigET-1-evoked contractions in isolated human placental arteries depend on a rapid and metalloprotease-dependent hydrolytic conversion to ET-1, which in turn causes a, mainly ETA-receptor-mediated, contraction.

Release of atrial natriuretic peptide during pulmonary artery clamping in man

The vasodilating hormone atrial natriuretic peptide (ANP) is secreted from the heart in response to atrial wall stretch, but knowledge of the time course of ANP secretion after acute releasing stimuli is limited. The time from stimulus to release of immunoreactive atrial natriuretic peptide (irANP) was investigated by unilaterally clamping the pulmonary artery in 12 patients undergoing pulmonary surgery. The second messenger to ANP-induced vascular relaxation, plasma cyclic guanosine monophosphate (p-cGMP), was measured as an indirect marker of the vascular effects of ANP. Immediately after applied clamping, p-irANP (baseline level 15.4 +/- 2.9 pmol l-1) started to increase, reaching a significant, although moderate, increase (11 +/- 4%, P < 0.05) after 2 min. This elevation of p-irANP remained during the entire clamping period (+13 +/- 6%, P < 0.05 vs. baseline). Within 1 min of declamping, p-irANP returned to the baseline level. No conclusive alterations in p-cGMP were observed. The prompt ANP response to the applied stimulus, and the return to baseline after declamping, may indicate the presence of a short time-acting releasing mechanism of ANP.

Characterization of angiotensin II formation in human isolated bladder by selective inhibitors of ACE and human chymase: a functional and biochemical study

1. Functional recordings of smooth muscle tension and biochemical experiments on membrane fractions were performed to characterize angiotensin II (AII) formation in human isolated bladder smooth muscle. 2. A novel human chymase inhibitor CH 5450 (Z-Ile-Glu-Pro-Phe-CO2Me) and a recently developed human chymase substrate Pro11-,D-Ala12)-angiotensin I, claimed to be resistant to angiotensin converting enzyme (ACE) and carboxypeptidase, were used. 3. Angiotensin I (AI) (0.3 microM) induced a contractile response amounting to 58 +/- 5% (n=12) of the initial K+ (124 mM)-induced contractions. This response was reduced to 36 +/- 3% (n=8) by the ACE-inhibitor enalaprilat (10 microM), while pretreatment with soybean trypsin inhibitor (STI 200 microg ml(-1)) or CH 5450 (10 microM) had no effect. However, the combination of enalaprilat and STI reduced the AI-induced contractions to 19 +/- 5% (n=6), and the combination of enalaprilat and CH 5450 caused an almost complete inhibition of the AI-induced contractions to 1+/-1% (n=6). 4. The substrate (Pro11-,D-Ala12)-AI (3 microM) produced contractions which amounted to 57 +/- 4% (n=13) of the initial K+ (124 mM) contractions. These contractions were not affected by enalaprilat (10 microM). On the other hand, STI (200 microg ml(-1)) and CH 5450 (10 microM) added separately, depressed the (Pro11-,D-Ala12)-AI-induced contractions to 34 +/- 5% (n=6) and 24 +/- 4% (n=6), respectively. The combination of enalaprilat and STI or enalaprilat and CH 5450 did not produce any further inhibition. 5. Experiments with detrusor membrane fractions incubated with AI (50 microM) were performed. In the presence of enalaprilat (100 microM), carboxypeptidase inhibitor CPI (10 microg ml(-1)) and aprotinin (15 microM), CH 5450 (10 nM-1 microM) caused a concentration-dependent inhibition of AII formation. 6. The results confirm that AII is a potent contractile agent in the human isolated detrusor muscle. They also indicate that the serine protease responsible for AII formation in the human bladder in vitro is human chymase or an enzyme similar to human chymase.

Hydrolysis of big endothelin-1 by a serine protease in the membrane fraction of human lung

The hydrolysis of human big endothelin 1-38 (bigET-1) was investigated in the membrane fractions from three human lung specimens. The hydrolysis products were identified by HPLC or by amino acid analysis, peptide sequencing and mass spectrometry, and the contractile effects of synthetic bigET-1, synthetic ET-1 and the major metabolite were tested on isolated rabbit pulmonary arteries. The dominating hydrolysis product was identified as bigET1-31, formed by a chymostatin-sensitive enzyme. Soybean trypsin inhibitor also suppressed bigET1-31 formation, while two other serine protease inhibitors, 3,4-dichloroisocoumarin and aprotinin, had no (or a limited) inhibitory effect. Through a partly phosphoramidon-sensitive enzymatic activity, endothelin-1 (ET-1) was formed independently of bigET1-31. On isolated pulmonary arteries, bigET1-31 had a contractile effect similar to that of synthetic bigET-1, with pEC50% values of 7.3 +/- 0.1 (n = 6) and 7.1 +/- 0.1 (n = 8), respectively. The pEC50% value of ET-1 was 9.2 +/- 0.3 (n = 6). These results indicate that human pulmonary membranes, besides hydrolysing bigET-1 to ET-1, also express serine protease activity that is responsible for the formation of the biologically active product, bigET1-31.

Conversion of angiotensin I to angiotensin II by chymase activity in human pulmonary membranes

An aprotinin-insensitive, angiotensin II (Ang II)-forming chymase has recently been identified in human heart tissue. We studied the hydrolysis of Ang I in human lung membranes. The hydrolysis products Ang II, Ang III, Ang-(1-9), Ang-(2-9), Ang-(1-7) and Ang-(8-10) appeared in membrane preparations from four patients. Two metabolic pathways for the formation of Ang II were identified; one depending on ACE activity (1.4 nmol Ang II/min/mg membrane protein) and the other on serine protease activity (2.1 nmol/min/mg). The serine protease activity was inhibitable to only 30 +/- 8% (mean +/- SEM) by aprotinin, suggesting chymase activity to play a role in the Ang I-conversion of human lung.

Contractile effect of big endothelin-1 and its conversion to endothelin-1 in rabbit cerebral arteries

The effect of big endothelin-1 (big ET-1) and its conversion to endothelin-1 (ET-1) in rabbit cerebral arteries were examined. Big ET-1 and ET-1 induced concentration-dependent contractions in the basilar artery; ET-1 was approximately 8 times more potent than big ET-1. The metalloprotease inhibitor phosphoramidon (30 mumol/l) almost abolished the contractile response to big ET-1, whereas the ET-1-induced contraction was unaffected. Removal of the endothelium did not attenuate the big ET-1-induced contraction. ET-1 was approximately 14 times more potent than endothelin-3 (ET-3) to elicit contraction. The contractions induced by big ET-1, ET-1 and ET-3 were all inhibited by ET(A) receptor antagonist BQ 123 (3 mumol/l). The ET(B) receptor antagonist IRL 1038 (3 mumol/l) had no effect on the contractile responses to big ET-1 and ET-1, but produced a small inhibition of the ET-3-induced contraction. Formation of ET-1 was demonstrated in membrane fractions of cerebral arteries incubated with big ET-1 as measured by high pressure liquid chromatography followed by radioimmunoassay. These results suggest that externally applied big ET-1 is converted to ET-1 by a phosphoramidon-sensitive "endothelin converting enzyme" present in the vascular smooth muscle cells. The ET-1 formed subsequently mediates the big ET-1-induced contraction by activation of mainly ET(A) receptors, although a small contribution of ET(B) receptors cannot be excluded.

Angiotensin I is converted to angiotensin II by a serine protease in human detrusor smooth muscle

The aim of the present study was to investigate whether a pathway for conversion of angiotensin I (ANG I) to angiotensin II (ANG II) other than that via angiotensin-converting enzyme (ACE) is present in the smooth muscle of the human detrusor. Isolated detrusor strips from 11 patients were contracted by ANG I (1 microM) in the absence or presence of enalaprilat (10 microM), soybean trypsin inhibitor (STI, 200 micrograms/ml), or both. The metabolic activity in detrusor membranes from four patients was studied separately using Hip-Gly-Gly or ANG I as a substrate, with or without various protease inhibitors. The contractile response to ANG I (1 microM) was depressed by enalaprilat from 66 +/- 22 (mean +/- SD) to 39 +/- 13% of the K+ (124 mM)-induced response (P < 0.01, n = 11), and the combination of enalaprilat and STI resulted in a further reduction in contractile amplitude to 25 +/- 14% (P < 0.01 vs. K+, and P < 0.05 vs. enalaprilat alone) and a significantly slower developing contraction with a time to peak of 3.7 +/- 1.7 vs. 1.1 +/- 0.3 min for ANG I alone (P < 0.01). In detrusor membranes, a low ACE activity, inhibitable by captopril, was demonstrated by the formation of hippuric acid (0.70 nmol.min-1.mg protein-1) from the synthetic ACE substrate, Hip-Gly-Gly. However, the conversion of ANG I (166 nmol.min-1.mg protein-1) to ANG II was not affected by ACE inhibition, while serine protease inhibitors, e.g., STI and chymostatin, completely prevented ANG II formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed decrease in plasma levels of atrial natriuretic peptide during cold hemodialysis

The high plasma levels of the vasodilating hormone atrial natriuretic peptide (alpha-ANP), observed in patients with chronic renal failure, decrease substantially during hemodialysis (HD), probably owing to volume reduction. Cardiovascular stability is better maintained by the use of cold dialysate although underlying mechanisms are unknown. In order to investigate the effects of different dialysate temperatures on hemodynamic stability and plasma levels of immunoreactive ANP (p-irANP), 10 stable HD patients were dialyzed with bicarbonate dialysis fluid for 240 min with each of 3 different dialysate temperatures: 36.5 degrees C (normal HD; NHD), 38.5 degrees C (warm HD; WHD) and 34.5 degrees C (cold HD; CHD). A Cuprophan plate dialyzer was used. The ultrafiltration volume and ultrafiltration rate were identical in each patient during the treatments. p-irANP was determined by radioimmunoassay, using 2 antisera which different cross-reactivity to ANP-related peptides. During NHD a nonsignificant decrease in mean arterial blood pressure from 111 +/- 5 to 103 +/- 8 mm Hg was observed. A significant (p < 0.05) decrease in mean arterial blood pressure from 109 +/- 4 to 96 +/- 6 mm Hg occurred during WHD, while during CHD it remained stable (111 +/- 4 before, 112 +/- 5 mm Hg after). Irrespective of the dialysate temperature or the antiserum used, p-irANP decreased significantly (p < 0.05) during the treatment. The reduction in p-irANP was delayed during CHD, the decrease being significantly (p < 0.05) less pronounced after 120 min. At the end of the treatment no significant difference was observed between the regimes.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Degradation and inactivation of rat atrial natriuretic peptide 1-28 by neutral endopeptidase-24.11 in rat pulmonary membranes

Atrial natriuretic peptide (ANP), a 28-residue peptide with cardiovascular and renal effects, is rapidly cleared from the circulation. Beside renal clearance, an extra-renal metabolism by the enzyme neutral endopeptidase-24.11 (NEP-24.11) has been proposed, since specific NEP-24.11-inhibitors increase endogenous plasma-ANP. NEP-24.11 is present in rat lung but its significance for ANP hydrolysis within the lung is unclear. The aim of this study was to investigate a possible degradation of rat ANP in a membrane preparation from rat lung. Hydrolysis products of ANP were separated by HPLC and further characterized by a pulmonary artery bioassay, by radioimmunoassay with different antisera, by peptide sequencing and by masspectrometry. Rat pulmonary membranes degraded ANP to one main metabolite lacking biological activity and with poor cross-reactivity to an antiserum recognising the central ring-structure of the peptide. Formation of the hydrolysis product was prevented by the NEP-24.11-inhibitor phosphoramidon (1 microM). Peptide sequencing of the metabolite revealed a cleavage between Cys7 and Phe8, which was confirmed by mass-spectrometry. The metabolite had an HPLC elution time identical to that of the product formed by purified porcine NEP-24.11. These findings suggest that ANP is metabolized and inactivated by endopeptidase-24.11 in rat lungs, the first organ exposed to ANP released from the heart.

Radio-immunoassay of atrial natriuretic peptide (ANP) and characterization of ANP immunoreactivity in human plasma and atrial tissue

A sensitive radio-immunoassay (RIA) was developed to determine the occurrence of atrial natriuretic peptide (ANP) in plasma and atrial extracts from patients undergoing open heart surgery. The immunoreactive ANP (irANP) was characterized by high-pressure liquid chromatography coupled with RIA. The plasma irANP response to releasing stimuli during the operation was determined in simultaneously sampled venous and arterial blood, in order to evaluate any differences. The antiserum recognized the intact ring-structure of alpha-humanANP (alpha-hANP) and its propeptide gamma-hANP, as well as beta-hANP, an anti-parallel dimer of alpha-hANP. Less bioactive N-or C-terminal fragments of alpha-hANP, or an N-terminal fragment of the propeptide, gamma-hANP 1-67, did not cross-react with the antiserum. Sep Pak C18-extraction of plasma resulted in an 80% recovery of synthetic alpha-hANP. The assay had a sensitivity of 1.9 pmol l-1, well below the venous plasma concentrations of irANP found in healthy volunteers (7.4 +/- 1.3 pmol l-1, mean +/- SEM, n = 19), and the local standard was identical to an international standard of alpha-hANP. In atrial extracts three major peaks of irANP were identified as alpha-, beta- and gamma-hANP, with gamma-hANP as the most abundant form. In plasma alpha-hANP dominated, but in two cases high plasma levels of beta-hANP were seen, reflecting the high atrial content in these patients. In peripheral arterial blood, irANP was on an average 56% +/- 20% (p less than 0.01, n = 18) higher than in venous blood; this was associated with more distinct arterial irANP responses to releasing stimuli during the operation.

Adsorption of atrial natriuretic peptide to different materials: a factor influencing results of in vitro experiments?

Studies on atrial natriuretic peptide (ANP) in ex vivo situations, include a risk of adsorption to surrounding materials. In order to investigate this potential source of error, known concentrations of ANP in Krebs solution were prepared in test tubes of different materials. The solutions were analyzed for ANP-concentration by radioimmunoassay (RIA), using a standard-curve of ANP in phosphate buffer supplemented with 0.1% human serum albumin (HSA) and 0.1% Triton X100. A considerable adsorption was seen to the different materials tested, also to siliconized glass and polypropylene. With 1 ml ANP-solution in concentrations from 1 x 10(-9) to 1 x 10(-5)M an adsorption varying between 10 and 31% was seen to a 15 cm2 polystyrene-surface, corresponding to a conventional test tube. With 1 ml of ANP 120 pM in Krebs solution serially dispensed into six empty polystyrene test tubes, 73% of the initial peptide amount was lost due to adsorption. The adsorption could be prevented or partly reversed by adding HSA or Triton X100 to the solutions. These findings indicate that adsorption entails a risk of disturbing the results of in vitro experiments in studies on ANP.