Spatial-temporal lipidomics profile of acute myocardial injury

Background

Lipidome disturbance has long been recognized to occur after myocardial infarction (MI). Accumulation of excessive fatty acids induces production of reactive oxygen species and consequently deteriorates cardiac injury in MI. However, the spatial and temporal lipid profile in the heart following ischemic injury remains unknown.

Purpose

We aim to uncover the temporal-spatial lipidome profile of the heart following ischemia reperfusion (I/R) injury and identify circulating lipids released from injured myocardium that are potentially useful for diagnosis of ischemic heart disease.

Methods

C57/BL6 mice were subjected to 30 min myocardial ischemia followed by removal of the ligature to establish reperfusion injury. Porcine I/R injury was induced by 105 min myocardial ischemia followed by reperfusion. Human plasma was obtained from 143 post-MI patients. Myocardial lipid profiles were generated by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MALDI-MSI) in different regions (infarct, remote and peri-infarct) at different time points. Moreover, the lipids in the heart and plasma were analysed by LC-MS/MS.

Results

We observed a drastic alteration in the lipidome with distinct spatial-temporal features in the injured heart by both MALDI-MSI and LC-MS/MS. In the infarct heart tissue, as revealed by LC-MS/MS, we observed an elevation of glycerolipids that peaked at 3 hours after I/R, and a sustained elevation of phospholipids and sphingolipids up to 3 days. Similar alternations in lipid profile was observed but much weaker in the remote and peri-infarct heart tissue compared to the infarct tissue. Among those lipids, PC 32:0 detected by MALDI-MSI highly overlapped CD68 staining at a single-cell level, showing a strong correlation of PC 32:0 with macrophage infiltration in mouse hearts (R2=0.93, p<0.0001). A similar increase of PC 32:0 in the infarct area was also observed in porcine hearts following I/R injury. Surprisingly, plasma levels of PC 32:0 in the mice decreased after I/R injury. In humans, plasma levels of PC 32:0 in post-MI patients were lower than that in healthy individuals (p=0.03). Further analysis demonstrated that plasma levels of PC 32:0 determined within 72 hours after percutaneous coronary intervention were negatively correlated with the 6-month post-MI cardiac ejection fraction in patients (R2=0.08, p<0.001).

Conclusions

A temporal-spatial lipidome profile was established in heart injury by synergizing LC-MS/MS and mass spectrometry imaging. PC 32:0 levels are positively correlated with myocardial macrophage infiltration but negatively correlated with cardiac function in cardiac I/R injury. Our findings indicate that PC 32:0 is a potential biomarker for cardiac injury and the inflammatory status in the injured heart.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Singapore Ministry of Health's National Medical Research Council

P4639Plasma tissue factor coagulation activity in post-acute myocardial infarction patients

Background

Coagulation is involved in fibroproliferative responses following acute myocardial infarction (AMI). Left ventricular (LV) remodeling following AMI is closely associated with progression to heart failure.

Purpose

We aimed to evaluate the association of plasma tissue factor (TF) coagulation activity with LV remodeling prior to heart failure in post-AMI patients.

Methods

This study was conducted in 228 subjects from the Post-AMI Left Ventricular Remodeling Biomarker Analysis (PAMILA) study and 57 healthy subjects. The post-AMI patients were divided into two age- and sex-matched groups: patients with adverse LV remodeling defined as an increase in LV end systolic volume by ≥15% over 6 months and patients with reverse LV remodeling defined as an decrease in LV end systolic volume by ≥15% over 6 months. TF coagulation activity was determined using human coagulation factor Xa generation based TF chromogenic activity assay and converted into concentrations of active TF (pM). Sodium-citrate anticoagulated plasma was collected at baseline (within 3 days after revascularization), 30 days and 6 months post-AMI. Results are presented as mean±S.E.M. One-way or two-way repeated measures ANOVA or a multiple multi-level longitudinal data analysis with structural equation model was used to assess differences in coagulation activity. P<0.05 was considered statistically significant.

Results

Plasma from healthy subjects and post-AMI patients at baseline had similar concentrations of active TF (TFa): 29.0±1.4 versus 29.1±0.7 pM. Patients treated with warfarin (15 out of 228 patients) showed lower plasma levels of TFa (mean difference −15.2 pM, [95% CI: −18.7, −11.7], p<0.001). Compared to baseline, plasma levels of TFa in the patients was significantly lower at 30 days post-AMI (mean difference −6.9 pM, [95% CI: −4.8, −8.9], p<0.001) and 6 months post-AMI (mean difference −2.8 pM, [95% CI: −0.8, −4.8], p=0.003). Intriguingly, plasma levels of TFa tended to recover from 30 days to 6 months post-AMI (mean difference 4.1 pM, [95% CI: 2.8, 5.4], p<0.001) toward the baseline level and the level in healthy subjects. Similar trends of temporal changes of plasma TFa levels were observed in patients with adverse LV remodeling and those with reverse LV remodeling although TFa levels were slightly higher in patients with reverse LV remodeling (F(2,448)=3.112, p=0.045 for interaction). After adjusting for age, gender, ethnicity, medications, lipid profile and risk factors, the temporal changes of plasma TFa levels in patients remain significant, however, the difference between patients with adverse versus reverse LV remodeling was not significant.

Conclusion

Plasma TF coagulation activity decreased post-AMI but did not differ in patients with adverse versus reverse LV remodeling.

Acknowledgement/Funding

National University Health System Singapore (NUHS O-CRG 2016 Oct-23) to JW Wang

P2582Signature of plasma extracellular vesicles associated proteins in acute myocardial infarction patients

Background

Prediction of left ventricular (LV) remodeling post-acute myocardial infarction (AMI) remains challenging. Several circulating biomarkers have been associated with post-AMI LV remodeling, however, there is no biomarker available to distinguish adverse versus reverse LV remodeling.

Purpose

In this study, we aimed to assess the association of extracellular vesicles (EVs) associated proteins with LV remodeling post-AMI.

Methods

Plasma EVs were isolated via precipitation with dextran sulphate as we previously reported. The protein levels of EV associated von Willebrand factor (VWF), SerpinC1 (antithrombin-III), plasminogen and SerpinF2 (alpha 2-antiplasmin) were determined in the citrate-anticoagulated plasma from 57 healthy subjects and 200 patients recruited in the Post-AMI Left Ventricular Remodeling Biomarker Analysis (PAMILA) study. Patients were categorized into two groups: adverse LV remodeling (n=100) characterized by an increase or reverse LV remodeling (n=100) characterized by a decrease, in LV end systolic volume by ≥15% over 6 months. Patients' plasma was collected at baseline (within 3 days after percutaneous coronary intervention), 1 month and 6 months post-AMI. Log transformation of EV protein levels was performed for assessment in a multiple multi-level longitudinal data analysis with structural equation model (with level of significance fixed at 0.05).

Results

Compared to healthy subjects, baseline protein levels of EV associated VWF and SerpinF2 were significantly higher in post-AMI patients, whereas no difference was observed in SerpinC1 and plasminogen. Among the patients, those on statins (196 out of 200 patients) showed lower protein levels of EV associated VWF (p<0.001) and plasminogen (p=0.003), whereas patients treated with P2Y12 platelet inhibitors (195 out of 200 patients) showed higher protein levels of EV associated VWF (p=0.003) and plasminogen (p=0.035). Multiple multi-level longitudinal data analysis with structural equation model showed that protein levels of EV associated VWF (p<0.001) and SerpinC1 (p=0.021) were lower in patients with adverse LV remodeling than that in patients with reverse LV remodeling during the 6-month follow-up post-AMI. In contrast, protein levels of EV associated plasminogen (p=0.002) and SerpinF2 (p=0.002) were higher in patients with adverse LV remodeling. The differences in the four EV associated proteins between patients with adverse versus reverse LV remodeling remain significant after adjusting for age, gender, ethnicity, medications, lipid profile and risk factors (diabetes, hypertension, dyslipidemia and smoking).

Conclusions

Lower levels of EV associated coagulation proteins (VWF and SerpinC1) and higher levels of EV associated fibrinolytic proteins (plasminogen and SerpinF2) were presented in patients with adverse LV remodeling compared to those with reverse LV remodeling post-AMI.

Acknowledgement/Funding

National University Health System Singapore (NUHS O-CRG 2016 Oct-23) to JW Wang

The enrichment of an alkaliphilic biofilm consortia capable of the anaerobic degradation of isosaccharinic acid from cellulosic materials incubated within an anthropogenic, hyperalkaline environment

Anthropogenic hyperalkaline sites provide an environment that is analogous to proposed cementitious geological disposal facilities (GDF) for radioactive waste. Under anoxic, alkaline conditions cellulosic wastes will hydrolyze to a range of cellulose degradation products (CDP) dominated by isosaccharinic acids (ISA). In order to investigate the potential for microbial activity in a cementitious GDF, cellulose samples were incubated in the alkaline (∼pH 12), anaerobic zone of a lime kiln waste site. Following retrieval, these samples had undergone partial alkaline hydrolysis and were colonized by a Clostridia-dominated biofilm community, where hydrogenotrophic, alkaliphilic methanogens were also present. When these samples were used to establish an alkaline CDP fed microcosm, the community shifted away from Clostridia, methanogens became undetectable and a flocculate community dominated by Alishewanella sp. established. These flocs were composed of bacteria embedded in polysaccharides and proteins stabilized by extracellular DNA. This community was able to degrade all forms of ISA with >60% of the carbon flow being channelled into extracellular polymeric substance (EPS) production. This study demonstrated that alkaliphilic microbial communities can degrade the CDP associated with some radioactive waste disposal concepts at pH 11. These communities divert significant amounts of degradable carbon to EPS formation, suggesting that EPS has a central role in the protection of these communities from hyperalkaline conditions.

Integrated microscopy techniques for comprehensive pathology evaluation of an implantable left atrial pressure sensor

The safety and efficacy of an implantable left atrial pressure (LAP) monitoring system is being evaluated in a clinical trial setting. Because the number of available specimens from the clinical trial for histopathology analysis is limited, it is beneficial to maximize the usage of each available specimen by relying on integrated microscopy techniques. The aim of this study is to demonstrate how a comprehensive pathology analysis of a single specimen may be reliably achieved using integrated microscopy techniques. Integrated microscopy techniques consisting of high-resolution gross digital photography followed by micro-computed tomography (micro-CT) scanning, low-vacuum scanning electron microscopy (LVSEM), and microground histology with special stains were applied to the same specimen. Integrated microscopy techniques were applied to eight human specimens. Micro-CT evaluation was beneficial for pinpointing the location and position of the device within the tissue, and for identifying any areas of interest or structural flaws that required additional examination. Usage of LVSEM was reliable in analyzing surface topography and cell type without destroying the integrity of the specimen. Following LVSEM, the specimen remained suitable for embedding in plastic and sectioning for light microscopy, using the positional data gathered from the micro-CT to intersect areas of interest in the slide. Finally, hematoxylin and eosin (H&E) and methylene blue staining was deployed on the slides with high-resolution results. The integration of multiple techniques on a single specimen maximized the usage of the limited number of available specimens from the clinical trial setting. Additionally, this integrated microscopic evaluation approach was found to have the added benefit of providing greater assurance of the derived conclusions because it was possible to cross-validate the results from multiple tests on the same specimen.

Comparative pharmacokinetics and pharmacodynamics of urocortins 1, 2 and 3 in healthy sheep

BACKGROUND AND PURPOSE The urocortin (Ucn) peptides are emerging as potential therapeutic targets for heart disease. However, pharmacokinetic (PK) and pharmacodynamic (PD) data are lacking. Therefore, we investigated the PK/PD for all three Ucns. EXPERIMENTAL APPROACH Seven sheep received 1 µg·kg(-1) boluses of Ucn1, Ucn2 and Ucn3. Population PK/PD models were developed to describe the time course of the haemodynamic effects. RESULTS The population estimate for Ucn1 clearance (0.486 L·h(-1)) was lower than that for Ucn2 (21.7 L·h(-1)) and Ucn3 (220 L·h(-1)), while steady-state volumes of distribution were similar for Ucn1 and Ucn2 (∼8 L) but substantially larger for Ucn3 (23.5 L). Ucn1 disposition was adequately described by a two-compartment model, with a one-compartment model required for Ucn2 and Ucn3. The half-life for Ucn1 was 2.9 h (α phase) and 8.3 h (β phase), and 15.7 and 4.4 min for Ucn2 and Ucn3 respectively. All Ucns produced significant increases in heart rate, cardiac output and left ventricular systolic and mean arterial pressures, and decreases in left atrial pressure and peripheral resistance. Delayed-effect pharmacodynamic models best described the time course of haemodynamic responses, with effects more rapid and less prolonged for Ucn2 and Ucn3 than Ucn1. Similar and physiologically plausible estimated baseline (E(0)) effects were exhibited by all Ucns, whereas EC(50) values were generally greater for Ucn1. CONCLUSIONS AND IMPLICATIONS Relative to Ucn1, both the PK and haemodynamic responses to Ucn2 and Ucn3 occurred more rapidly. Our data provide important comparative information, useful to the rational design of future clinical studies.

Cardiac sympathetic nerve activity and ventricular fibrillation during acute myocardial infarction in a conscious sheep model

The association between cardiac sympathetic nerve activity (CSNA) and ventricular fibrillation (VF) during acute myocardial infarction (MI) has not been assessed in conscious animal models. During the first 60 min post-MI, mean blood pressure (MBP), heart rate (HR), and CSNA were recorded continuously in 20 conscious sheep. Resistant sheep ( group A, n = 10) were compared with susceptible sheep ( group B, n = 10) who developed fatal VF ( n = 7) or sustained ventricular tachycardia (VT, n = 3). The mean time to VF/VT was 28.1 ± 3.3 min. In group B, MBP, HR, and CSNA were averaged at each consecutive minute from baseline at 14 min before the onset of VF/VT and compared with time-matched values in group A. When compared with those of group A, indexes of CSNA burst size increased before the onset of VF/VT: burst area/minute (F13,208 = 2.17, P = 0.01) and burst area/100 beats (F13,208 = 1.86, P = 0.04). By contrast, burst frequency indexes were not significantly different: burst frequency (F13,208 = 1.6, P = 0.09) and burst incidence (F13,208 = 1.48, P = 0.13). In group A, CSNA burst area/min and burst area/100 beats did not change across this time period (F13,117 = 0.97, P = 0.5, F13,117 = 0.96, P = 0.7) but increased with time in group B (F13,91 = 2.3, P = 0.01; and F13,91 = 2.25, P = 0.01). Between-group comparisons demonstrated no differences in time of onset of ventricular ectopic beats: 18.5 (range 12–24) in group A versus 15.0 min (range 7–22) in group B (Mann-Whitney U-test, P = 0.09). Pre-MI baroreflex slopes were similar: R-R slopes were 11.8 ± 2 and 15.6 ± 1.1 ms/mmHg ( t18 = −1.6, P = 0.14). CSNA slopes were −1.8 ± 0.3 and −2.3 ± 0.2%/mmHg ( t18 = −1.4, P = 0.2). An early increase in CSNA burst size indexes (before 60 min post-MI), mediated by an excitatory sympathetic reflex, is important in the genesis of VF/VT.

Adrenomedullin increases cardiac sympathetic nerve activity in normal conscious sheep

The sympathetic nervous system and adrenomedullin (AM) both participate in the regulation of cardiac and circulatory function but their interaction remains uncertain. We have examined the effects of AM on cardiac sympathetic nerve activity (CSNA) and hemodynamics and contrasted these effects with pressure-matched nitro-prusside (NP) administration in normal conscious sheep. Compared with vehicle control, arterial pressure fell similarly with AM (P=0.04) and NP (P<0.001). Heart rate rose in response to both AM (P<0.001) and NP (P=0.002) but the rise with AM was significantly greater than that induced by NP (P<0.001). Cardiac output increased in response to AM compared with both control and NP (both P<0.001). CSNA burst frequency (bursts/min) were increased in response to both AM (P<0.001) and NP (P=0.005) with the rise in burst frequency being greater with AM compared with NP (P<0.001). CSNA burst area/min was also raised by both AM (P=0.03) and NP (P=0.002) with a trend for burst area being greater with AM than NP (P=0.07). CSNA burst incidence (bursts/100 beats) showed no significant differences between any treatment day. In conclusion, we have demonstrated that AM is associated with a greater increase in CSNA and heart rate for a given change in arterial pressure than seen with the classic balanced vasodilator NP.

Increased cardiac sympathetic nerve activity following acute myocardial infarction in a sheep model

The time course of cardiac sympathetic nerve activity (CSNA) following acute myocardial infarction (MI) is unknown. We therefore undertook serial direct recordings of CSNA, arterial blood pressure (MAP) and heart rate (HR) in 11 conscious sheep before and after MI, and compared them with 10 controls. Conscious CSNA recordings were taken daily from electrodes glued into the thoracic cardiac nerves. Infarction was induced under pethidine and diazepam analgesia by applying tension to a coronary suture. MI size was assessed by left ventricular planimetry (%) at postmortem, peak troponin T and brain natriuretic peptide levels (BNP). Baroreflex slopes were assessed daily using phenylephrine-nitroprusside ramps. The mean infarcted area was 14.4 +/- 2.9%, troponin T 1.88 +/- 0.39 microg l(-1) and BNP 8.4 +/- 1.3 pmol l(-1). There were no differences in haemodynamic parameters or CSNA between groups at baseline. MAP and HR remained constant following MI. CSNA burst frequency increased from baseline levels of 55.8 +/- 7.1 bursts min(-1) to levels of 77.5 +/- 8.7 bursts min(-1) at 2 h post-MI, and remained elevated for 2 days (P < 0.001). CSNA burst area also increased and was sustained for 7 days following MI (P= 0.016). Baroreflex slopes for pulse interval and CSNA did not change. CSNA increases within 1 h of the onset of MI and is sustained for at least 7 days. The duration of this response may be longer because the recording fields decrease with time. This result is consistent with a sustained cardiac excitatory sympathetic reflex.

A neural mechanism for sudden death after myocardial infarction

By monitoring efferent cardiac sympathetic nerve activity (CSNA) directly in a conscious animal we observed, for the first time, that ventricular fibrillation (VF) following myocardial infarction (MI) was preceded by a paroxysm of CSNA which was not baroreflexmodulated. This observation has potential therapeutic implications.

Adrenomedullin modulates the neurohumoral response to acute volume loading in normal conscious sheep

The physiological role of adrenomedullin (ADM) in volume and pressure homeostasis remains unclear. Accordingly, we assessed possible modulatory actions of ADM infusions on the neurohumoral response to acute volume loading with dextran in normal conscious sheep. Dextran (15 ml/kg), given with concurrent ADM (5.5 pmol/kg per min--raising plasma ADM from below detection to approximately 10 pmol/l) or vehicle control infusions, induced matched significant (P<0.001 by ANOVA) falls in hematocrit (27-30%) during both ADM and control and similar increases in right atrial pressure (approximately 10 mmHg). Compared with control, both systemic (P=0.033) and pulmonary (P=0.005) arterial pressure and peripheral resistance (P=0.004) were reduced during ADM but were raised post-infusion. The dextran-induced increase in cardiac output was augmented by ADM (P=0.048). Dextran-induced increases in plasma atrial natriuretic peptide (ANP; P=0.008), brain natriuretic peptide (BNP; P=NS) and cyclic guanosine monophosphate (cGMP; P=0.003) were augmented post-ADM infusions. The dextran-induced fall in plasma renin activity (PRA) was attenuated by ADM (P=0.039) whereas plasma aldosterone levels were unaltered. ADM augmented the increase in urinary volume during the second 2-h clearance period post-dextran. Our data indicate that ADM modifies the hemodynamic and hormonal response to an acute volume challenge, enhances natriuretic peptide secretion and reduces systemic vascular resistance. These results provide further evidence that ADM plays a physiological role in volume and pressure homeostasis.

Comparative actions of adrenomedullin and nitroprusside: interactions with ANG II and norepinephrine

The role of adrenomedullin (ADM) in volume and pressure homeostasis remains undefined. Accordingly, we compared the biological responses to infusions of ADM and nitroprusside (NP; matched for reduction of arterial pressure) and assessed their effects on responses to ANG II and norepinephrine in eight conscious sheep. During matched falls in arterial pressure (8-10 mmHg, both P < 0.001) ADM and NP induced similar increases in heart rate. ADM increased cardiac output (P < 0.001), and the fall in calculated peripheral resistance was greater with ADM than NP (P = 0.013). ADM infusions raised plasma ADM levels (P < 0.001), plasma renin activity (P = 0.001), and ANG II (P < 0.001) but tended to blunt any concurrent rise in aldosterone compared with NP (P = 0.056). ADM maintained both urine flow (P < 0.001) and sodium excretion (P = 0.01) compared with falls observed with NP. ADM attenuated the vasopressor actions of exogenous ANG II (P = 0.006) but not norepinephrine. In addition, ADM antagonized the ANG II-induced rise in plasma aldosterone (P < 0.001). In conclusion, ADM induces a different spectrum of hemodynamic, renal, and endocrine actions to NP. These results clarify mechanisms by which ADM might contribute to volume and pressure homeostasis.

Adrenomedullin augments the neurohumoral response to haemorrhage in non-pregnant but not in pregnant sheep

Adrenomedullin (ADM) is a novel peptide with actions which include reduction of arterial pressure and interaction with a number of hormone systems. In order to assess possible interactions with the renin-angiotensin system (RAS) and the hypothalamo-pituitary-adrenal (HPA) axis, we have examined neurohumoral responses to hypotensive haemorrhage (15 ml/kg over 15 min) with or without co-infusions of ADM (5.5 pmol/kg per min) in six non-pregnant and eight pregnant conscious sheep. Haemorrhage induced a greater decrease in arterial pressure, but a blunted increase in heart rate in pregnant sheep. There was no significant effect of ADM on haemodynamic responses to haemorrhage in either group. In non-pregnant sheep, haemorrhage-induced activation of both RAS and HPA was significantly augmented by ADM, as indicated by greater increases in plasma renin activity (P<0.01), angiotensin II (P<0.05) and arginine vasopressin (P<0.01). In contrast, ADM did not augment these responses to haemorrhage in pregnant sheep. Rather, plasma concentrations of aldosterone (P=0.039) and adrenocorticotrophic hormone (P=0.012) were decreased by ADM. In conclusion, ADM-induced augmentation of the RAS and HPA responses to hypotensive haemorrhage is abolished in the pregnant state.

Delayed metabolism of human brain natriuretic peptide reflects resistance to neutral endopeptidase

Metabolism of natriuretic peptides is regulated by two degradative pathways: uptake by the clearance receptor (natriuretic peptide receptor C--NPR-C) and hydrolysis by neutral endopeptidase (NEP). Affinity studies favour a dominant role of NPR-C in hormone degradation in several species but do not account for the efficacy of NEP inhibitors in vivo, nor the uniquely prolonged half life (t((1/2))) of human brain natriuretic peptide (hBNP). Postulating that (1) delayed metabolism of hBNP reflects resistance to NEP and (2) interactions between NPR-C and NEP increase enzyme activity, we have used purified ovine and human NEP, plus ovine lung plasma membranes to study the relative importance of receptor and enzyme pathways. We have also related the findings to hormone metabolism in vivo. Binding affinities of atrial natriuretic peptide (ANP), hBNP and ovine BNP (oBNP) to oNPR-C were similar (K(d)=8-16 pM). In contrast, unlike ANP and oBNP, hBNP was not significantly degraded by purified oNEP or plasma membranes. Despite similar (and high) affinity of oNPR-C for oBNP and hBNP, the t((1/2)) of hBNP (12.7 min) was more than fourfold that of oBNP (2.6 min). Although we found no evidence for receptor-enzyme interaction, our results show that the delayed metabolism of hBNP reflects resistance to NEP. These findings have important implications for future treatment strategies in human disease.

Adrenomedullin attenuates pressor response to angiotensin II in conscious sheep

Biologic actions attributed to adrenomedullin include reduction of arterial pressure and suppression of aldosterone secretion. To assess possible in vivo antiangiotensin II actions of adrenomedullin, we examined hemodynamic and adrenal responses to stepped angiotensin II infusions with or without co-infusions of adrenomedullin (33 ng/kg/min) in conscious sheep under controlled conditions of a low sodium intake. Plasma adrenomedullin levels rose during peptide infusions (p < 0.001) to plateau at approximately 15-18 pM. The dose-dependent pressor response (15-20 mm Hg) of angiotensin II was both delayed and markedly attenuated (p = 0.017) by adrenomedullin, which also stimulated heart rate (p < 0.001) and cardiac output (p < 0.001). Adrenomedullin prevented the angiotensin II-induced increase in peripheral resistance (p < 0.001). Plasma aldosterone responses to angiotensin II were variable and were not significantly altered by concomitant adrenomedullin infusion. In conclusion, low-dose infusion of adrenomedullin administered to conscious sheep on a low-salt diet clearly antagonized the vasopressor actions of administered angiotensin II while stimulating cardiac output and heart rate. The data suggest a possible role for adrenomedullin in cardiovascular homeostasis in part through antagonism of the vasopressor action of angiotensin II.

Myocardial infarction with and without reperfusion in sheep: early cardiac and neurohumoral changes

There are few stable and reproducible large-animal models of chronic heart failure produced by ischaemic damage to the myocardium. Here we characterize a novel method of inducing myocardial damage in closed-chest sheep by catheter delivery of thrombogenic coils, and compare this with a newly described open-artery model of cardiac injury in sheep. Sham controls were compared with animals subjected to (a) 90 min of coronary artery occlusion/reperfusion by PTCA (percutaneous transluminal coronary angioplasty) balloon, and (b) permanent coronary artery occlusion induced by catheter delivery of thrombogenic coils (seven sheep/group). Both balloon occlusion/reperfusion and permanent coil occlusion resulted in well-defined anteroapical infarcts, as documented by ECG changes, significant rises in creatine kinase (both groups P<0.001) and troponin-T (both groups P<0.05), and post-mortem examination. Washout of enzymes was much more rapid in the reperfused group (P<0. 01). Infarction resulted in significant reductions in left ventricular (LV) ejection fraction (both groups P<0.01) and regional wall abnormalities. Ejection fraction 7 days post-coil (21.3+/-4.2%) was significantly lower (P<0.01) than that 7 days post-balloon (38. 8+/-4.5%). Coil-induced infarction was associated with acutely reduced arterial pressure (P<0.05), and increases in heart rate (P<0. 05), atrial pressures (P<0.05), plasma brain natriuretic peptide levels (P<0.05) and adrenaline levels (P<0.05). Rises seen in plasma endothelin levels in sham controls were blunted in the coil group (P<0.001). Haemodynamic changes were less marked in the balloon group. In conclusion, restriction of coronary artery occlusion to 90 min results in infarction, but less LV dysfunction with reduced early remodelling, compared with permanent occlusion. Acute changes in biochemical markers, haemodynamics, neurohormones and LV function confirm that these are excellent models of open- and closed-artery myocardial infarction leading to asymptomatic LV dysfunction.

Neurohormones in an ovine model of compensated postinfarction left ventricular dysfunction

Clinical heart failure, often the result of myocardial infarction, may be preceded by a period of compensated left ventricular impairment. There is substantial need for an experimental model that reflects this human condition. In sheep, coronary artery ligation produced consistent left ventricular anteroapical myocardial infarctions resulting in chronic (5 wk), stable hemodynamic changes compared with sham controls, including reductions in ejection fraction (51 +/- 2 vs. 30 +/- 5%, P < 0.001), cardiac output (6.3 +/- 0.2 vs. 5.1 +/- 0.2 l/min, P < 0.01), and arterial pressure (93 +/- 2 vs. 79 +/- 3 mmHg, P < 0.001), and increases in cardiac preload (left atrial pressure, 3.3 +/- 0.1 vs. 8.3 +/- 1.3 mmHg, P < 0.001). These changes were associated with acute and sustained increases in plasma concentrations of atrial natriuretic peptide (ANP; 5 wk, 11 +/- 2 vs. 27 +/- 5 pmol/l, P < 0.001), brain natriuretic peptide (BNP; 3 +/- 0.2 vs. 11 +/- 2 pmol/l, P < 0.001), and amino-terminal pro-brain natriuretic peptide (NT-BNP; 17 +/- 3 vs. 42 +/- 12 pmol/l, P < 0.001). Significant correlations were observed between plasma levels of the natriuretic peptides (ANP, day 7 to week 5 samples; BNP and NT-BNP, day 1 to week 5 samples) and changes in left ventricular volumes and ejection fraction. In contrast, renin activity, aldosterone, catecholamines, and endothelin were not chronically elevated postinfarction and were not related to indexes of ventricular function. Coronary artery ligation in sheep produces the pathological, hemodynamic, and neurohormonal characteristics of compensated left ventricular impairment secondary to myocardial infarction. Plasma concentrations of the cardiac natriuretic peptides are sensitive markers of left ventricular dysfunction. This is a reproducible model that reflects the clinical condition and should prove suitable for investigating the pathophysiology of, and experimental therapies in, early left ventricular dysfunction.

Bioactivity of natriuretic peptide coinfusions; no evidence for unique effects of BNP in conscious sheep

Few studies have addressed the possibility that brain natriuretic peptide (BNP) possesses a profile of bioactivity that is distinct from that of atrial natriuretic peptide (ANP). Accordingly, we assessed the biologic actions of BNP in the setting of maximal or near-maximal ANP-induced biologic activity. Background ANP infusions (7.5 pmol/kg/min) administered on all study days, increased plasma ANP (approximately 120 pM) and cyclic guanosine monophosphate (GMP) levels (approximately 40 nM), and induced significant decreases in arterial pressure and cardiac output associated with increased heart rate, hematocrit, diuresis, and natriuresis. Increasing the dose twofold after 1 h (experiment 1, n = 5) showed no enhancement of these actions despite a further twofold increase in plasma ANP and cyclic GMP (both p values <0.001). Addition of low-dose BNP (2 pmol/kg/min) after 1 h background infusion (experiment 2, n = 8), increased plasma BNP levels (30 pM, p < 0.001) but caused no significant effects on the hemodynamic, renal, or hormonal indices measured. In conclusion, in the setting of maximal hemodynamic, renal, and endocrine responses to high-dose background infusions of ANP, coinfusion of BNP exhibits no enhancement of, or additional, biologic activity. This study provides no evidence for unique short-term biologic actions of ANP and BNP.

Chronic infusions of brain natriuretic peptide in conscious sheep: bioactivity at low physiological levels

1. The circulating cardiac hormones atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) have similar bioactivity, as judged by comparative short-term studies. However, no study has reported the effects of longer-term administration of BNP. Accordingly, we have compared the haemodynamic, hormonal and renal actions of chronic (4-day) administration of BNP and ANP (0.5 pmol. min-1.kg-1) in a vehicle-controlled study in normal conscious sheep. 2.BNP infusions raised plasma BNP levels within the physiological range (4 pmol/l increment, P<0.001) and increased cyclic GMP levels (P=0.01). BNP infusions induced significant falls in right atrial pressure (P=0.048), stroke volume (P=0.014) and cardiac output (P=0. 003) associated with a rise in haematocrit (P=0.001). There were no significant renal effects or changes in renin-aldosterone. By comparison, equimolar infusion of ANP induced a smaller increment in plasma ANP levels (2 pmol/l, P=0.03) with qualitatively similar but statistically non-significant changes in plasma cyclic GMP and haemodynamic indices.3.In conclusion, chronic low-dose infusions of BNP elevate plasma cyclic GMP levels and induce significant haemodynamic actions. This study provides evidence that subtle variations in circulating BNP levels, well within the physiological range, may be important in long-term cardiovascular control.

Arginine vasopressin V1-receptor antagonism in an ovine model of acute myocardial infarction

Arginine vasopressin (AVP) has an uncertain role in the pathogenesis of increased myocardial and other regional vascular resistance after cardiac injury. The extreme increase in plasma AVP levels observed in some individuals after myocardial infarction potentially exacerbates already compromised myocardial perfusion. We assessed whether blockade of AVP, by administration of the specific V1-receptor antagonist OPC-21268, can reduce the severity of myocardial injury after embolization in our ovine model of acute myocardial infarction. Embolizations resulted in a pattern of changes in ECG and cardiac enzymes consistent with moderate to severe acute myocardial infarction, resulting in left ventricular dysfunction [left ventricular ejection fraction (LVEF) reduced from 52-53% to 38%]. However, no statistically significant differences were observed between groups in hemodynamic or neurohumoral indices of myocardial damage. By contrast, the hypothalamus-pituitary-adrenal (HPA) response [including plasma AVP (p < 0.01), adrenocorticotropic hormone (ACTH; p < 0.01), and cortisol (p < 0.01)) to embolizations was significantly increased in the sheep infused with OPC-21268. In conclusion, whereas plasma HPA responses differed between the two groups, the similar responses in cardiac enzymes, LVEF, and hemodynamic and neurohumoral factors in both groups does not support a role of V1 receptor-mediated exacerbation of myocardial injury in this model of myocardial infarction. Assessment of different receptor antagonists or examination of other models of cardiac injury may further clarify the role of the markedly increased AVP levels that can occur in myocardial infarction.

Amino-terminal proBNP in ovine plasma: evidence for enhanced secretion in response to cardiac overload

We have recently identified a novel amino-terminal fragment of pro-brain natriuretic peptide (NT-proBNP) in the circulation of humans, the concentration of which increases progressively as the left ventricle fails. To clarify the origins of NT-proBNP in experimental animals, we have developed an RIA for NT-proBNP based on residues 52-71 of ovine proBNP-(1-103) and used it to study cardiac processing, secretion, and metabolism of BNP in sheep with cardiac overload induced by coronary artery ligation (CAL) or rapid left ventricular pacing (rLVP). The concentration of NT-proBNP in left atrial plasma extracts drawn from normal control sheep was threefold that of mature BNP. Size-exclusion and reverse-phase HPLC analyses of plasma extracts coupled to RIA revealed a single peak of immunoreactive (ir) NT-proBNP [ approximately 8,000 relative molecular weight (Mr)], quite distinct from a single peak of ir-mature BNP ( approximately 3,000 Mr). In contrast, ovine cardiac tissue contained only a single immunoreactive peak of high-molecular-weight BNP ( approximately 11,000 Mr), consistent in size with proBNP-(1-103). Sampling from the cardiac coronary sinus in normal control sheep (n = 5) and sheep with CAL (n = 5) revealed that the molar ratio of NT-proBNP to mature BNP was similar. There was a significant gradient of both mature and NT-proBNP across the heart in normal sheep, whereas after CAL the gradient was significant for mature BNP only. In both forms of cardiac overload (CAL and rLVP), left atrial plasma levels of NT-proBNP were significantly increased above normal levels, in contrast with mature BNP levels, which were raised only in the rLVP group of animals. Blockade of natriuretic peptide metabolism in sheep with heart failure (induced by rLVP) raised mature BNP levels threefold but did not affect levels of NT-proBNP. In conclusion, these studies show that NT-proBNP is formed from proBNP stores during secretion and, compared with mature BNP, accumulates in plasma because metabolism of NT-proBNP appears to differ from that of mature BNP. Although its function, if any, remains unclear, plasma NT-proBNP may prove to be a sensitive marker of cardiac overload and/or decompensation.

Hemodynamic, hormonal, and renal effects of intracerebroventricular adrenomedullin in conscious sheep

Adrenomedullin, the recently described vasodilator that exhibits potent hypotensive actions when administered systemically, is also found in the central nervous system, suggesting a role for adrenomedullin as a neurohormone. However, only a limited number of studies have examined the central effects of adrenomedullin. Therefore, we have examined the integrative hemodynamic, renal, and hormonal effects of intracerebroventricular (I.C.V.) adrenomedullin in conscious sheep. Eight surgically prepared sheep received I.C.V. infusions of adrenomedullin at two doses (2 ng/kg x min followed immediately by 20 ng/kg x min each for 90 min) in a vehicle-controlled study. Water deprivation for 48 h before control infusion resulted in sheep drinking 2617 +/- 583 ml in the 90-min period following reintroduction of water. On the adrenomedullin day, drinking was halved to 1392 +/- 361 ml (P < 0.05). Adrenomedullin had no significant effect on urinary volume and sodium excretion. Plasma adrenomedullin levels remained unchanged during control infusions but were elevated by the end of I.C.V. adrenomedullin infusions (P < 0.001). Plasma ANP levels were also increased approximately 50% (P < 0.05). Plasma levels of both ACTH and cortisol were also increased 3- to 4-fold in response to I.C.V. adrenomedullin (P < 0.05). There was no significant difference in arterial pressure, heart rate, or cardiac output between study days. In conclusion, adrenomedullin within the central nervous system may have at least two roles: modulation of the hypothalamo-pituitary-adrenal axis and protection against fluid overload.

Combined neutral endopeptidase and angiotensin-converting enzyme inhibition in heart failure: role of natriuretic peptides and angiotensin II

We examined for the first time the specific roles of angiotensin II and the natriuretic peptides during inhibition of angiotensin-converting enzyme (captopril, 25 mg bolus + 6 mg/3 h infusion) and endopeptidase 24.11 (SCH32615, 5 mg/kg bolus + 3 mg/kg/3 h infusion), both separately and in combination, in eight sheep with pacing-induced heart failure. Plasma atrial and brain natriuretic peptide levels were similarly increased by SCH32615 and to a lesser extent during combined inhibition but decreased with captopril. Captopril and combined inhibition induced identical increases in plasma renin activity and reductions in angiotensin II, whereas neither was changed by SCH32615 alone. Mean arterial pressure and peripheral resistance decreased during SCH32615 and further still during captopril and combined treatment. Left atrial pressure was reduced to a similar extent by SCH32615 and captopril alone and reduced further by combined inhibition. Cardiac output increased during all treatments. Urine volume and sodium excretion were significantly increased during SCH32615 and combined inhibition. Creatinine clearance increased during SCH32615, decreased during captopril, and was maintained during combined treatment. In conclusion, compared with captopril alone, cotreatment with an endopeptidase 24.11 inhibitor further improved filling pressures and induced a diuresis and natriuresis with preservation of renal glomerular filtration.

Ovine brain natriuretic peptide in cardiac tissues and plasma: effects of cardiac hypertrophy and heart failure on tissue concentration and molecular forms

Whereas numerous studies have examined the cardiac tissue content and secretion of atrial natriuretic peptide (ANP), the response of brain natriuretic peptide (BNP) in states of experimental cardiac overload is less well documented. Our recent partial cloning of the ovine BNP gene has enabled us to study changes in cardiac tissue concentration, together with tissue and circulating molecular forms of ANP and BNP, in response to cardiac overload induced by rapid ventricular pacing (n = 7) and aortic coarctation (n = 6). In normal sheep, although highest levels of BNP were found in atrial tissue (15-fold those of the ventricle), the BNP/ANP concentration ratio in the ventricles was 10- to 20-fold higher than the ratio calculated for atrial tissue. Compared with normal sheep, significant depletion of both ANP and BNP concentrations within the left ventricle occurred after rapid ventricular pacing. Size exclusion and reverse phase HPLC analysis of atrial and ventricular tissue extracts from normal and overloaded sheep showed a single peak of high molecular weight BNP consistent with the proBNP hormone. In contrast, immunoreactive BNP extracted from plasma drawn from the coronary sinus was all low molecular weight material. Further analysis of plasma BNP using ion exchange HPLC disclosed at least 3 distinct immunoreactive peaks consistent with ovine BNP forms 26-29 amino acid residues in length. These findings show that BNP is stored as the prohormone in sheep cardiac tissues and that complete processing to mature forms occurs at the time of secretion. The capacity to process the prohormone at secretion is not impaired by chronic heart failure.

Clearance receptors and endopeptidase: equal role in natriuretic peptide metabolism in heart failure

The effects of separate and combined endopeptidase inhibition (by SCH-32615) and natriuretic peptide receptor C blockade [by C-ANP-(4-23)] on the clearance and bioactivity of atrial (ANP) and brain (BNP) natriuretic peptides was investigated in eight sheep with heart failure. SCH-32615 and C-ANP-(4-23) administered separately induced significant and proportionate dose-dependent rises in plasma ANP, BNP, and guanosine 3',5'-cyclic monophosphate (cGMP) levels. Associated with these changes were reductions in arterial pressure, left atrial pressure, and peripheral resistance and increases in cardiac output, urine volume, sodium excretion, and creatinine clearance. SCH-32615 induced greater diuresis and natriuresis than C-ANP-(4-23). Combined administration of SCH-32615 and C-ANP-(4-23) induced greater than additive rises in plasma ANP, BNP, and cGMP concentrations, with enhanced hemodynamic effects, diuresis, and natriuresis and reduced plasma aldosterone levels. In conclusion, we find that the enzymatic and receptor clearance pathways contribute equally to the metabolism of endogenous ANP and BNP in sheep with heart failure. Combined inhibition of both degradative pathways was associated with enhanced hormonal, hemodynamic, and renal effects and may have greater potential therapeutic value than either agent separately.

Comparison of the effects of ouabain and brain natriuretic peptide in saline-loaded sheep

1. It has been claimed that ouabain is an endogenous hormone that may be pivotal in the pathogenesis of some forms of hypertension and may exaggerate natriuresis in situations characterized by volume overload. We compared the haemodynamic, renal and endocrine effects of ouabain (at approximately 187 ng/kg per min for 2 h) with those of brain natriuretic peptide (BNP; at 5 pmol/kg per min for 2 h) in nine saline-loaded sheep in a balanced, randomized, single-blind, placebo-controlled crossover study. 2. Brain natriuretic peptide infusion reduced mean arterial pressure whereas ouabain infusion caused no change. Haematocrit rose steadily during BNP infusion but fell during ouabain infusion. Neither ouabain nor BNP affected urine volume, sodium, potassium or creatinine excretion. Mean heart rate declined during the ouabain and placebo infusions, but was not altered during BNP infusion. Endogenous ouabain concentrations were not detectable at baseline or during BNP or placebo infusions, but rose to concentrations of 11 +/- 1.3 nmol/L during the ouabain infusion. 3. These results suggest that ouabain is not an endogenous hormone present at physiologically relevant concentrations. Furthermore, ouabain does not cause natriuresis during saline-loading in sheep and is therefore unlikely to be responsible for the exaggerated natriuresis seen in some forms of hypertension.

Beneficial hemodynamic and renal effects of adrenomedullin in an ovine model of heart failure

BACKGROUND

Adrenomedullin is a recently discovered endogenous peptide with hypotensive and natriuretic actions in normal animals. Circulating and ventricular adrenomedullin are elevated in congestive heart failure, suggesting a possible role in the pathophysiology of this disease. No studies have previously examined the effects of adrenomedullin in heart failure.

METHODS AND RESULTS

Eight sheep with pacing-induced heart failure received human adrenomedullin(1-52) at 10 and 100 ng x kg(-1) x min(-1) I.V. for 90 minutes each. Compared with vehicle control data, adrenomedullin increased plasma cAMP (high dose, P<.05) in association with dose-dependent falls in calculated peripheral resistance (13 mm Hg x L(-1) x min(-1), P<.001), mean arterial pressure (9 mm Hg, P<.001), and left atrial pressure (5 mm Hg, P<.001) and increases in cardiac output (0.5 L/min, P<.001). Adrenomedullin increased urine sodium (threefold, P<.05), creatinine (P<.05) and cAMP excretion (P<.01), creatinine clearance (P<.05), and renal production of cAMP (P<.05), whereas urine output was maintained during infusion and raised after infusion (P<.05). Adrenomedullin reduced plasma aldosterone levels (P<.05), whereas plasma atrial and brain natriuretic peptide concentrations were unchanged during infusion and rose after infusion (P<.01 and P<.05, respectively). Plasma catecholamine, cortisol, renin, calcium, and glucose concentrations were not significantly altered.

CONCLUSIONS

Adrenomedullin reduced ventricular preload and afterload and improved cardiac output in sheep with congestive heart failure. Despite the clear fall in arterial pressure, adrenomedullin increased creatinine clearance and sodium excretion and maintained urine output. These results imply an important pathophysiological role for adrenomedullin in the regulation of pressure and volume in heart failure and raise the possibility of a new therapeutic approach to this disease.

An ovine model of acute myocardial infarction and chronic left ventricular dysfunction

In order to develop and validate an ovine model of myocardial infarction with subsequent impairement of left ventricular function, 15 instrumented sheep underwent selective microembolization of the left coronary arteries with 0.5 mL 90 microns polystyrene beads. Hemodynamics and plasma hormones were measured preembolization (baseline) and then at hours 2, 4, 6, and 12 and days 1, 2, 3, 5 and 7 postembolization. Of the 15 sheep studied, 2 (13%) died on the day of embolization from arrhythmias. In the remaining sheep, left ventricular systolic pressure and stroke work (both P < 0.001) were reduced promptly and remained below basal levels. Mean arterial pressure (P < 0.001) increased initially, then decreased to below basal levels by hour 6. Heart rate (P < 0.001) and left atrial pressure (P < 0.05) were increased while cardiac output was decreased (P < 0.05). Left ventricular ejection fraction at day 7 was reduced (38.8 +/- 3.5 vs 46.0 +/- 3.9% preembolization; P < 0.05). The cardiac enzymes creatine kinase (P < 0.001) and troponin-T (P < 0.001) were increased following microembolization and returned to basal levels by days 2 and 5 respectively. Plasma atrial and brain natriuretic peptides (both P < 0.001) and plasma renin activity (P < 0.005) were all increased following embolization. This ovine model mimics the hemodynamic and neurohumoral features of acute myocardial infarction, resulting in left ventricular dysfunction, and should prove suitable for the study of interventions in a number of these conditions.

Hemodynamic, hormonal, and renal effects of adrenomedullin in conscious sheep

Adrenomedullin is a recently discovered peptide that has been shown to reduce arterial pressure and induce natriuresis. However, few studies have examined the biological actions of adrenomedullin in conscious animals in an integrative manner. Accordingly, we have examined the hemodynamic, renal, and endocrine actions of adrenomedullin infused intravenously at 10 and 100 ng.kg-1.min-1 (each 90 min) in a vehicle-controlled study in eight normal conscious sheep. Adrenomedullin reduced right atrial pressure (P < 0.05) and diastolic (15 mmHg, P < 0.01) and mean arterial pressure (10 mmHg, P < 0.05) and increased cardiac output (3 l/min, P < 0.001). Total peripheral resistance was reduced 40% (P < 0.001). Urinary sodium was reduced to 35% of control during the 90-min clearance period immediately postinfusion (P < 0.05). Adrenomedullin increased plasma adenosine 3',5'-cyclic monophosphate levels (P < 0.001). Plasma renin activity was elevated during adrenomedullin (P < 0.001) coincident with the peak hypotensive effect, whereas plasma aldosterone was not affected and plasma norepinephrine levels fell (P < 0.05). In conclusion, adrenomedullin had clear blood pressure-lowering effects with increased cardiac output and stimulation of renin but suppressed sympathetic activation in conscious sheep. The physiological implications of these findings require further study.

Hypothalamo-pituitary-adrenal axis response to coronary artery embolization: an ovine model of acute myocardial infarction

Although previous studies have described the hypothalamo-pituitary-adrenal (HPA) response to the stress of acute myocardial infarction, it is not possible to study the hormone changes immediately after infarction in humans. Accordingly, we have examined the HPA response to microembolization of coronary arteries in 13 sheep compared with 5 sham control sheep. Plasma vasopressin (AVP; P < 0.001), ACTH (P = 0.005) and cortisol (P = 0.005) were all increased 2 h (first sample time) after embolization. Plasma ACTH and cortisol levels returned to baseline levels by 6 h but plasma AVP levels did not return to baseline levels until more than 12 h after embolization. Plasma corticotrophin-releasing hormone (CRH) showed no significant change in response to embolization. In a subset of six animals which were sampled more frequently, the peak responses for plasma AVP, ACTH and cortisol occurred at 40 min after embolization. The maximum responses in any individual sheep observed at this time point were 744 pmol/l for AVP, 144 pmol/l for ACTH and 492 nmol/l for cortisol. CRH levels tended to increase across the first hour but these changes were not statistically significant. In conclusion, the stress hormone responses to microembolization of the coronary arteries have been defined in an ovine model of myocardial infarction. This model is suitable for studying the effects of novel treatments to reduce the stress of myocardial infarction.

Comparative bioactivity of atrial and brain natriuretic peptides in an ovine model of heart failure

1. Whereas many studies have detailed the effects of exogenous atrial natriuretic peptide (ANP) infusions in heart failure, and a limited number have examined the effects of brain natriuretic peptide (BNP), none have directly compared the bioactivity of similar doses of ANP and BNP under standard conditions of impaired cardiac function. We compared the hormonal, haemodynamic and renal effects of 3 h infusions of ANP, BNP and a vehicle control in eight sheep with pacing-induced heart failure (225 beats/min for 8-12 days). 2. Infusion of ANP and BNP increased plasma ANP (P < 0.001) (276 +/- 27 versus control 142 +/- 26 pmol/l) and BNP (P < 0.001) (257 +/- 34 versus control 45 +/- 5 pmol/l) respectively, in association with increased cyclic 3',5'-guanosine monophosphate [control, 40 +/- 6; ANP, 53 +/- 6 (P < 0.05); BNP, 57 +/- 7 nmol/l (P < 0.001)]. Metabolic clearance rate and half-life were similar for both peptides. Infusion of ANP and BNP similarly reduced mean arterial pressure [control, 73.0 +/- 1.6; ANP, 67.6 +/- 1.2 (P < 0.01); BNP, 65.7 +/- 1.7 mmHg (P < 0.001)], left atrial pressure (both P < 0.05) (control, 22.0 +/- 0.7; ANP, 19.9 +/- 1.0; BNP, 19.8 +/- 0.9 mmHg) and peripheral resistance [control, 50.3 +/- 4.1 mmHg l-1 min-1; ANP, 46.0 +/- 2.8 (P < 0.05); BNP, 43.8 +/- 4.5 (P < 0.01)], and increased urine volume (2-3-fold, both P < 0.05), sodium excretion (> 10-fold, both P < 0.01) and haematocrit levels relative to control (both P < 0.05). Infused BNP tended to raise plasma ANP levels (by 28 pmol/l), while ANP increased plasma BNP (by 18 pmol/l). Plasma aldosterone was reduced by approximately 40% by both peptides (both P < 0.05). 3. In conclusion, ANP and BNP are both powerfully, natriuretic, similarly suppress aldosterone and appear equipotent in reducing preload and afterload in this model of pacing-induced heart failure.

Natriuretic peptides in sheep with pressure overload left ventricular hypertrophy

To examine tissue and plasma atrial (ANP) and brain natriuretic peptide (BNP) responses to left ventricular hypertrophy (LVH) 7 sheep underwent suprarenal aortic banding (20 mmHg initial pressure differential). Median survival time was 15 days. Proximal mean aortic pressure (MAP) increased from 65.1 +/- 5.0 mmHg (baseline) to 111.6 +/- 7.5 mmHg (day 7, p < 0.0001). Distal systolic aortic pressure fell from 85.5 +/- 8.7 mmHg (baseline) to 55.6 +/- 6.4 mmHg (day 7, p = 0.0002). Maximal plasma ANP (26.9 +/- 3.6 vs 10.1 +/- 1.2 pmol/L, p = 0.005) and BNP (15.3 +/- 3.6 vs. 3.5 +/- 1.0 pmol/L, p = 0.006) were recorded at 15 +/- 4.0 days. Coarctation induced rapid increases in PRA and plasma aldosterone and a fall in urinary sodium. Post-mortem examination of hearts confirmed LVH. Compared with controls, tissue ANP concentration was reduced in left atrium (p = 0.04) and LV (p = 0.04). BNP concentration was reduced in left atrium (p = 0.02) but tended to be higher in LV. In conclusion, suprarenal aortic coarctation leads to progressive hypertension resulting in LVH, progressive increases in plasma ANP and BNP and, in most cases, death from heart failure.

Neutral endopeptidase inhibition: augmented atrial and brain natriuretic peptide, haemodynamic and natriuretic responses in ovine heart failure

1. Atrial and brain natriuretic peptide are both circulating hormones subject to degradation by neutral endopeptidase 24.11. Whereas endogenous levels of atrial natriuretic peptide are increased by neutral endopeptidase inhibition in most pathophysiological states, the effect on brain natriuretic peptide and the influence of cardiac status is less clear. To further evaluate the role of neutral endopeptidase 24.11, we directly compared the responses of atrial and brain natriuretic peptide, together with the effects on other vasoactive hormones, haemodynamics and renal indices, to a neutral endopeptidase inhibitor, SCH32615, and a vehicle control in eight conscious sheep before and during pacing-induced heart failure. 2. In normal animals, SCH32615 significantly increased concentrations of plasma atrial natriuretic peptide (22 +/- 5 pmol/l compared with 14 +/- 2 pmol/l in control, 1.6-fold increase) and brain natriuretic peptide (6.5 +/- 1.2 pmol/l compared with 4.1 +/- 0.7 pmol/l in control, 1.6-fold increase), whereas in heart failure, plasma levels of atrial natriuretic peptide (306 +/- 38 pmol/l compared with 187 +/- 25 pmol/l in control, 1.6-fold increase) and brain natriuretic peptide (93 +/- 11 pmol/l compared with 55 +/- 9 pmol/l in control, 1.7-fold increase) were elevated to a significantly greater absolute, but proportionately similar, extent. In both normal and heart-failed animals, SCH32615 induced reductions in mean arterial pressure and left atrial pressure and increases in haematocrit, plasma cGMP and endogenous creatinine clearance. However, only in heart failure did neutral endopeptidase inhibition induce a significant and marked natriuresis (> 10-fold increase) and diuresis (4-fold increase), together with suppression of renin activity and haemodynamic effects including decreased peripheral resistance and raised cardiac output. 3. In conclusion, neutral endopeptidase inhibition increases plasma concentrations of atrial and brain natriuretic peptide to a proportionately similar extent in both normal and heart-failed sheep. The striking natriuresis and diuresis and additional haemodynamic effects demonstrated in sheep with heart failure, where natriuretic peptide levels are elevated compared with normal sheep, supports the concept that neutral endopeptidase inhibition augments endogenous atrial and brain natriuretic peptide.

Clearance receptors and endopeptidase 24.11: equal role in natriuretic peptide metabolism in conscious sheep

Although many studies have examined the effects of administering natriuretic peptide receptor C (NPR-C) ligands and endopeptidase 24.11 (EP 24.11) inhibitors on clearance and bioactivity of atrial natriuretic peptide (ANP), none have systematically compared their effects on the endogenous levels of both ANP and brain natriuretic peptide (BNP) under physiological conditions. Accordingly, we examined the hemodynamic, hormonal, and renal actions of an EP 24.11 inhibitor, SCH-32615, and an NPR-C ligand, C-ANP-(4-23), both alone and in combination in eight normal conscious sheep. NPR-C blockade and EP 24.11 inhibition induce similar rises in plasma ANP, BNP, and guanosine 3',5'-cyclic monophosphate (cGMP). Synergistic increments in plasma ANP, BNP, and cGMP observed during combined administration are likely to be due to the reduced clearance of C-ANP-(4-23) in the setting of EP 24.11 inhibition, leading to increased inhibition of the receptor pathway. Combined administration was also associated with enhanced hemodynamic actions and diuretic and natriuretic effects. Our findings show that both enzymatic and receptor clearance pathways contribute equally to natriuretic peptide clearance and induce potentially important hemodynamic and renal effects in normal conscious sheep.

Comparative bioactivity of atrial, brain, and C-type natriuretic peptides in conscious sheep

Although atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) appear to share functional homology, there is doubt concerning a possible endocrine function for C-type natriuretic peptide (CNP) and the relative potency of species-specific forms of these hormones. Accordingly, we have examined the biological effects, interactions, and pharmacokinetics of equimolar doses (0.5 followed by 2.5 pmol.kg-1.min-1, each for 2 h) of species-specific forms of ANP, BNP-26, and CNP-22 in vehicle-controlled studies in normal conscious sheep. Although pharmacokinetics (metabolic clearance rates of 5.7 +/- 1.17, 7.5 +/- 1.36, and 4.7 +/- 0.71 l/min and half-lives of 3.9 +/- 0.42, 2.5 +/- 0.21, and 2.0 +/- 0.18 min for ANP, BNP, and CNP, respectively) are similar, the biological effects and actions on endogenous natriuretic peptide levels differ. Plasma BNP was significantly increased by CNP infusion (P < 0.0001), as was CNP by BNP infusions (P = 0.0009). Compared with ANP and BNP, which were equipotent in stimulating plasma guanosine 3',5'-cyclic monophosphate (cGMP; P < 0.0001 for both) and lowering arterial pressure (P < 0.05 for both) and cardiac output, CNP infusions induced only a small increment in cGMP and had no significant hemodynamic actions. In contrast, all three peptides suppressed plasma aldosterone levels (P < 0.05 for each), yet none induced significant natriuresis. Actions of CNP to increase BNP (and ANP) may account for the observed bioactivity of CNP. The findings show that potentially important interactions occur among all three hormones that need to be considered when interpreting the effects of individual peptides, particularly CNP.

Nitric oxide inhibition in an ovine model of heart failure

1. The role of nitric oxide (NO) in congestive heart failure was investigated by studying the acute haemodynamic, hormonal and renal effects of N(G)-monomethyl-L-arginine (L-NMMA(, a nitric oxide inhibitor, given as incremental bolus doses in six sheep before (normal) and after induction of heart failure (HF) by rapid left ventricular pacing (LVoff+). 2. 6-NMMA caused significant initial dose-dependent rises in left ventricular systolic pressure, mean arterial pressure (MAP), peripheral resistance (PR) and left atrial pressure and declines in cardiac output in both normal and HF states (maximum response in 2-6 min). These responses were all but abolished when L-arginine was given concurrently with L-NMMA. The dose-response curve for the L-NMMA-induced rise in MAP was shifted to the right following LVP (P < 0.05), which is consistent with previous observations of blunted NO synthase activity in HF. A subsequent decline in MAP and PR to below prebolus levels was observed 30-60 min after L-NMMA administration in the paced state. No significant hormonal or renal effects were observed. 3. In conclusion, the present study confirms the important haemodynamic role played by endogenous NO in the normal state and demonstrates a blunted pressor response to NO inhibition in this model of heart failure.

Chronic ouabain infusion does not cause hypertension in sheep

Ouabain is claimed to be a hormone of adrenal origin, capable of raising arterial pressure in rats. We infused ouabain in conscious sheep under carefully controlled circumstances to determine its effects on blood pressure, urine electrolytes, and vasoactive hormones. Eight healthy ewes were studied while taking a constant intake of dietary sodium and potassium. Ouabain infusion at 0.25 mg daily over 22 days reduced heart rate and arterial pressure and had no effect on pressor responsiveness to incremental intravenous infusions of angiotensin II. Ouabain induced minor, but statistically significant, decrements in urine volume, urinary sodium excretion, plasma renin and angiotensin II concentrations, and a rise in plasma aldosterone and cortisol. Plasma ouabain levels averaged 1.37 +/- 0.28 nmol/l during ouabain infusion. In conclusion, high-dose chronic ouabain infusion in sheep did not elevate arterial pressure or alter pressor responsiveness to angiotensin II, was antidiuretic and antinatriuretic, and induced minor perturbations in circulating renin, angiotensin II, aldosterone, and cortisol.

Comparison of chronic neutral endopeptidase inhibition and furosemide in an ovine model of heart failure

The hemodynamic, hormonal, and metabolic effects of chronic neutral endopeptidase (NEP) 24.11 inhibition and furosemide were compared in the pacing model of ovine heart failure (HF). Intravenous SCH 39370 induced a dose-related inhibition of NEP activity during 4-days treatment, in association with a transient increase in plasma atrial natriuretic peptide (ANP) and, at the higher dose, an increased mean level of plasma cyclic guanosine monophosphate. There was a significant and persistent decrease in left atrial pressure, a brief increase in cardiac output (CO), and a tendency for arterial pressure to be reduced. Significant dose-related diuresis and natriuresis were also observed. Furosemide induced a similar hemodynamic response, associated with greater diuresis and natriuresis. Both agents significantly reduced plasma aldosterone levels. Coinfusion of captopril on day 4 of treatment resulted in similar responses in both the SCH 39370- and furosemide pretreated groups. Chronic NEP inhibition significantly alters circulatory and renal function and appears to be as effective as furosemide in reducing cardiac preload in this model of congestive HF.

Natriuretic peptide responses to acute and chronic ventricular pacing in sheep

The responses of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) were investigated in six conscious sheep paced at 120, 155, 190, and 225 beats/min for 1.5 h at each rate and at 180, 225, and 180 beats/min for 4 days at each rate. Increased pacing reduced arterial pressure, cardiac output, and urine and Na excretion and increased left atrial pressure and plasma ANP, BNP, and C-type natriuretic peptide, with delayed activation of the renin-angiotensin system (RAS). Acute pacing increased plasma ANP and BNP levels 8.6- and 3.6-fold, respectively (both P < 0.001), whereas chronic pacing increased ANP and BNP 7.8- and 9-fold, respectively (both P < 0.001). Thus the ANP-to-BNP ratio increased during acute pacing (P < 0.001) and decreased proportionately during chronic pacing (P < 0.05). Reduction in pacing improved hemodynamic status, reduced natriuretic peptides (BNP less than ANP), normalized the RAS, and induced diuresis and natriuresis. In conclusion, BNP is less responsive than ANP to acute changes in intracardiac pressure but is proportionately more responsive to chronic hemodynamic changes such as occur in congestive heart failure.

Central angiotensin II AT1-receptor antagonism in normal and heart-failed sheep

The role of the brain renin-angiotensin system (RAS) in heart failure was examined by administering intracerebroventricular (ICV) infusions of the angiotensin II (ANG II) type 1 (AT1)-receptor antagonist losartan (0.1 followed by 0.5 mg.kg-1.3 h-1) to six concious sheep before (nonpaced) and after induction of heart failure by rapid left ventricular pacing (paced). In both nonpaced and paced states, ICV losartan abolished drinking, induced a significant diuresis (P < 0.05) and anti-natriuresis (P < 0.05), and increased plasma renin activity (P < 0.05) and ANG II (P < 0.01) and aldosterone levels (0.1 > P > 0.05). Plasma arginine vasopressin was suppressed by ICV losartan only in the paced state (P < 0.05). Hemodynamics were not altered by ICV losartan in the nonpaced animals. In the paced state, however, significant reductions in left ventricular systolic, mean arterial, and left atrial pressures were observed (decrements of 13 +/- 7, 12 +/- 5, and 3.4 +/- 0.7 mmHg, respectively, all P < 0.05). In conclusion, ANG II within the brain participates in the regulation of thirst and body electrolyte and fluid homeostasis in normal and heart-failed sheep and appears to play a role in regulating resting hemodynamic status in this model of heart failure.

Endopeptidase inhibition in angiotensin-induced hypertension. Effect of SCH 39370 in sheep

To assess the efficacy of neutral endopeptidase 24.11 inhibition in the setting of elevated plasma levels of angiotensin II (Ang II), we studied the hemodynamic, renal, and hormonal effects of bolus injections of the potent and specific neutral endopeptidase inhibitor SCH 39370 or vehicle (control) in 10 sheep with Ang II-induced hypertension. Ang II infusion (5 ng/kg per minute for 6 days) sufficient to increase plasma Ang II levels 50% to 100% induced a consistent rise in mean arterial pressure (mean increment, 15 mm Hg; P < .0001) and increased plasma atrial natriuretic peptide (P = .017) and its second messenger cGMP (P = .049). Compared with time-matched control observations after vehicle alone, SCH 39370 (2.5 mg/kg) further increased plasma atrial natriuretic peptide (P = .0006), cGMP (P = .006), and plasma Ang II (P = .054). Systolic and mean arterial pressures tended to fall after SCH 39370, but these changes were not significant compared with control. No significant changes were observed in urinary volume and sodium excretion. Viewed in relation to previous studies in normotensive sheep, the current findings indicate that the vasodepressor response to neutral endopeptidase inhibition is blunted in hyperangiotensinemic sheep, in which neutral endopeptidase inhibition further augments plasma Ang II levels.

Central C-type natriuretic peptide augments the hormone response to hemorrhage in conscious sheep

The effect of intracerebroventricular infusions of C-type natriuretic peptide (CNP-22, 5 micrograms/h for 3 h) or vehicle on the neurohumoral response of conscious sheep (n = 7) to acute moderate hemorrhage (15 ml/kg in 15 min performed 1 h after start of CNP) was studied. CNP alone (prehemorrhage) induced a transient rise (after 30 min) in heart rate (p = 0.0005), plasma aldosterone (p = 0.007), ACTH (p = 0.049), and cortisol (p = 0.049), with values returning to control levels immediately prehemorrhage. Hemorrhage caused arterial pressure to fall (15 mmHg) and heart rate to rise similarly on both study days. Compared with vehicle, posthemorrhage responses of AVP (p = 0.05) and cortisol (p = 0.004) were greater during CNP. Thus, central CNP-22 augmented the hypothalamic-pituitary-adrenal axis at baseline and in response to hemorrhage in conscious sheep.

Biological actions and pharmacokinetics of C-type natriuretic peptide in conscious sheep

C-type natriuretic peptide (CNP) is found in abundance in neural tissue and in endothelial cells of vascular tissue, where it may participate in the regulation of vascular tone. However, few studies have examined the metabolism and systemic actions of CNP in conscious animals. Accordingly, we investigated the hemodynamic, renal, and hormonal effects of intravenous CNP-22 administered at two doses (1 and 10 pmol.kg-1.min-1 as dose 1 and dose 10, respectively) in a vehicle-controlled study in normal conscious sheep (n = 8). Plasma CNP levels were raised from a mean baseline level of 2-3 pmol/l (detection limit) to plateau at 10 +/- 1.2 and 115 +/- 18 pmol/l during doses 1 and 10, respectively. Metabolic clearance rates were 3.15 +/- 0.39 and 2.48 +/- 0.36 l/min, respectively. The plasma half-life of CNP on termination of infusion was rapid (1.6 +/- 0.27 min). Dose 10 increased plasma guanosine 3',5'-cyclic monophosphate (P = 0.0002), reduced cardiac output by 18% (P = 0.01), but did not significantly affect mean arterial pressure. Similar suppression (15%) of cardiac output occurred during dose 1 (P = 0.078). Both doses were natriuretic. This study demonstrates that CNP is natriuretic in sheep and lowers cardiac output without significantly affecting arterial pressure. CNP may play an important role in the regulation of regional hemodynamics and fluid homeostasis.

Blockade of the renin-angiotensin system

HISTORICAL OVERVIEW

Milestones in understanding the renin-angiotensin system (RAS) until the development of angiotensin II antagonists are described briefly. Sites at which the RAS might be blocked are outlined.

PEPTIDE ANALOGUE ANTAGONISTS OF ANGIOTENSIN II RECEPTORS

Saralasin-like compounds were used in numerous experimental and clinical situations and clarified the role of the RAS. Some of these situations are described (e.g. hypertension and cardiac failure) particularly in regard to the control of arterial pressure and aldosterone secretion by the RAS. Saralasin and its analogues allowed visualization, for the first time, of complete angiotensin II/effector (especially blood pressure) dose-response curves.

ANGIOTENSIN CONVERTING ENZYME (ACE) INHIBITORS

The clinical usefulness of ACE inhibitors in hypertension, cardiac failure, diabetes mellitus and after acute myocardial infarction is emphasized. In particular, ACE inhibitors have actions beyond blockade of angiotensin II formation, necessitating cautious interpretation of data from their use.

RENIN INHIBITORS

Experience with renin inhibitors in documenting and confirming the role of the RAS is outlined. NON-PEPTIDE ANGIOTENSIN II RECEPTOR ANTAGONISTS: Characteristics of losartan-like compounds are discussed in brief, and the results of their limited use in clinical medicine are outlined.

CONCLUSION

Blocking agents have led to great advances in knowledge of the RAS and its physiological and pathophysiological functions. ACE inhibitors are used regularly in a number of clinical disorders and, in theory, have therapeutic potential beyond alternative antihypertensive drugs in the prevention of cardiovascular complications in essential hypertension. The place of renin inhibitors and angiotensin II receptor antagonists in clinical practice, however, remains to be determined.

Periodontal screening and recording

The perceived benefits of PSR as a simple, sensitive and efficient tool to screen patients for periodontal diseases have contributed to its incorporation into the dental practice environment. The recent introduction of PSR to the public probably will motivate patients to seek more information about periodontal health. Used as a communication and screening tool, PSR has the potential to enhance professional and patient relationships through cooperative treatment planning and increased patient treatment acceptance.

Cardiovascular actions of ANF: contributions of renal, neurohumoral, and hemodynamic factors in sheep

Whereas the short-term effects of atrial natriuretic factor (ANF) are well documented, less is known concerning the possible hypotensive action of sustained low-dose ANF infusions. Accordingly, we have examined the sequential renal, hormonal, and hemodynamic responses to 48-h low-dose ANF infusions in eight normotensive conscious sheep and evaluated the effects of these infusions on autonomic (baroreceptor), adrenocortical, and pressor responsiveness to exogenous stimulation. Plasma ANF levels tended to rise in response to ANF infusions, but the difference between ANF and control day levels was not significantly different. Systolic arterial pressure (SAP) was significantly lower after ANF infusion (P = 0.01) and was associated with a reduction in calculated total peripheral resistance (CTPR, P = 0.007). Mean arterial pressure also tended to be lower (P = 0.08) in response to ANF. No change was seen in urinary volume or sodium excretion or in plasma angiotensin II (ANG II), aldosterone, or cortisol levels. ANF significantly attenuated the pressor response to exogenous ANG II (P = 0.01) but did not affect the adrenocortical responsiveness or autonomic (baroreceptor) responsiveness to exogenous stimulation. This study demonstrates that chronic low-dose infusions of ANF that barely elevate plasma ANF levels induce significant cardiovascular effects, including lowering of SAP associated with a fall in CTPR and attenuation of pressor responsiveness to physiological increments in ANG II, thus providing further support for the importance of ANF in blood pressure homeostasis in normal sheep.

Central C-type natriuretic peptide but not atrial natriuretic factor lowers blood pressure and adrenocortical secretion in normal conscious sheep

C-type natriuretic peptide (CNP) and its specific receptor (ANPRB) are richly distributed throughout the brain, especially the anterior pituitary and hypothalamus but not in tissue of nonneural origin. These findings suggest that the actions of CNP, unlike atrial natriuretic factor (ANF), are largely confined to the brain where CNP may participate in the central regulation of hemodynamics and salt and water balance. Therefore, we have studied the hemodynamic, renal, and hormonal effects of continuous intracerebroventricular infusions of CNP (5 micrograms/h for 4 h) in a vehicle-controlled study and compared the responses to those of ANF in normal conscious sheep. Hemodynamic and hormonal responses to ANF were not different from control infusions. There was a trend for urinary sodium and potassium excretion to increase throughout the control infusion but not on the ANF day. Water intake during control infusion (358 +/- 160 ml/4 h) was almost 3-fold that ingested during ANF (127 +/- 89 ml/4 h, NS). In contrast, CNP induced a prompt fall in mean arterial pressure (mean decrement 5 mm Hg), arterial pressure remaining below time control values for the remainder of the infusion (P = 0.006). Rises in both heart rate and PRA observed on the control day tended to be attenuated by CNP. Urine electrolyte response to CNP was similar to that observed with ANF. Compared with control infusions, the responses of both plasma aldosterone (P = 0.006) and cortisol (P = 0.043) were significantly different. Following CNP-induced hypotension, plasma cortisol and aldosterone increased abruptly at 30 min after which values fell to control or lower levels until the infusion was terminated. These studies show that intracerebroventricular CNP lowers arterial pressure without increasing heart rate and also suppresses the adrenocortical response, whereas ANF given under the same conditions has no significant effects. These data support the hypothesis that CNP plays an important role in the central regulation of blood pressure and hormones.

Angiotensin II receptor antagonism in ovine heart failure: acute hemodynamic, hormonal, and renal effects

The hemodynamic, hormonal, and renal effects of angiotensin II-type 1 receptor antagonism (AT1A) have not been documented previously in heart failure (HF) or compared with angiotensin-converting-enzyme inhibition (ACEI). Accordingly, we investigated the acute (2-h) response to losartan (1 and 10 mg/kg iv) or vehicle (N saline) followed by captopril (12.5 mg) on separate days in an ovine model of HF induced by 7 days of rapid ventricular pacing. Losartan induced a significant rise in plasma renin activity (PRA) and plasma angiotensin II levels (P less than 0.01 and P less than 0.001, respectively), in association with a fall in the plasma aldosterone-to-PRA ratio (P less than 0.001) and plasma atrial natriuretic peptide (P less than 0.05). Mean arterial and left atrial pressure both fell significantly after losartan (P less than 0.001), whereas the rise in cardiac output was not sustained. The response to captopril was similar except for plasma angiotensin II, which declined (P less than 0.001). Glomerular filtration and urine sodium excretion were maintained despite a fall in renal perfusion pressure. In conclusion, the vasodilatation and renal effects of AT1A were similar to ACEI. Thus AT1A may be a useful therapeutic alternative to ACEI in HF.