Volume 4, Number 4, December 2003

PAPERSubfornical organ lesion attenuates chronic hypotensive effects of losartan in salt-replete rats
John P Collister, Michael D Hendel

Hypothesis/introduction: Circumventricular organs are central nervous system brain sites thought to participate in neuroendocrine regulation of neural output. We have previously demonstrated a profound chronic hypotensive response to the angiotensin II (Ang II) AT₁ antagonist, losartan (10 mg/kg/day), in normal rats. In addition, we have demonstrated that the area postrema, one of the circumventricular organs, partially mediates this response. The subfornical organ (SFO) is another circumventricular organ which has been shown to mediate actions of Ang II. The present study was designed to test the hypothesis that the SFO mediates the chronic hypotensive effects of losartan in normal rats.
Materials and methods: Rats were randomly chosen for lesion of the SFO or sham operation and instrumented with intravenous catheters and radiotelemetric blood pressure transducers. After a control period, rats were infused with losartan (10 mg/kg/day) for nine days. Mean arterial pressure and heart rate responses were measured continuously throughout the protocol and examined as 12-hour day/night averages.
Results: By day 7 of losartan treatment, night-time mean arterial pressure had dropped to 75+2 mmHg in sham rats (n=8) but only to 83+2 mmHg in SFO-lesioned rats (n=10). This trend continued throughout the treatment protocol.
Conclusions: These results suggest that the SFO partially mediates the chronic hypotensive effects of chronic losartan treatment in normal rats.

JRAAS 2003;4:207-212.

PAPERCombined renin-angiotensin system blockade and dietary sodium restriction impairs cardiomyocyte contractility
Petcharat Trongtorsak, Trefor O Morgan, Lea MD Delbridge

Introduction: Blockade of the renin-angiotensin system (RAS) by combined angiotensin-converting enzyme inhibitor and angiotensin type 1 receptor (AT₁) antagonist treatment with reduced dietary sodium intake produces suppression of cardiac growth and regression of cardiac hypertrophy. The purpose of this study was to investigate whether cardiac growth suppression by combined RAS blockade under conditions of dietary sodium restriction is associated with cardiomyocyte atrophy and contractile dysfunction and whether this intervention modifies cardiomyocyte inotropic responsiveness to angiotensin II (Ang II).
Methods: Rats were fed a high (4% w/w) or low (0.2% w/w) NaCl diet for six days. Both groups were then given a combined intraperitoneal injection of perindopril (6 mg/kg/day) and losartan (10 mg/kg/day) with maintained dietary treatment for another seven days. At the end of the treatment period, animals were anaesthetised and their hearts were removed and weighed. Left ventricular cardiomyocytes were isolated by enzymatic dissociation and cell dimensions were evaluated. A line scan camera and digital imaging technique were used to assess cardiomyocyte contraction and inotropic responses to exogenous Ang II (10 to 10^-8 M).
Results: Dietary treatment alone had no effect on body growth, whereas combined RAS blockade suppressed somatic growth in the low sodium (LS) group, compared with the high sodium (HS) group. This growth suppression in the LS group was also evident in the heart at the organ and cellular level. Studies of cardiomyocyte contraction showed that myocytes from the LS group exhibited contractile instability and depression of contractile performance. Compared with the HS group, myocytes from the LS group showed a significant reduction in maximum cell shortening (6.40+0.17 vs. 7.32+0.16% resting length, p<0.05), and maximum rate of shortening (3.85+0.14 vs. 4.29+0.11 cell length/ms, p<0.05). Myocytes of the HS group exhibited negative inotropic responses to Ang II at all concentrations tested, with a significant reduction in maximum cell shortening by 11–16% after 12 minutes peptide exposure (p<0.05 vs. non-treated control). In comparison, Ang II elicited both positive and negative responses in myocytes from the LS group, with a predominant negative inotropic effect.
Conclusions: This study provides evidence that combined RAS blockade treatment under restricted sodium intake conditions can impair cardiomyocyte contractile function in association with cardiomyocyte growth suppression. Chronic RAS blockade qualitatively alters the intrinsic inotropic status and responsiveness of ventricular cardiomyocytes, and this shift is further modulated by dietary sodium intake conditions.

JRAAS 2003;4:213-219.

PAPERImpaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model
Alexander Nap, Charly NW Belterman, Marie-Jeanne Mathy, Jippe C Balt, Martin Pfaffendorf, Pieter A van Zwieten

Congestive heart failure (CHF) is characterised by activation of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS). Both systems are known to interact and to potentiate each other’s activities. We recently demonstrated that angiotensin II (Ang II) enhances sympathetic nerve traffic via prejunctionally-located AT1-receptors. At present, little is known about the effects of Ang II at the level of the sympathetic neurones in CHF.
Accordingly, we investigated the effect of Ang II in the presence and absence of the AT1-receptor antagonist, eprosartan, on stimulation-induced nerve traffic in isolated thoracic aorta preparations obtained from rabbits suffering from experimentally-induced CHF. Control-preparations were obtained from age-matched animals. Sympathetic activity was assessed by a [3H]noradrenaline spill-over model. Additionally, Ang II constrictor responses were compared between CHF and control vessels in the presence and absence of eprosartan. Additionally, to study postjunctional facilitation, the effects of Ang II on postsynaptic a-adrenoceptor-mediated responses were studied using noradrenaline.
Stimulation-evoked SNS-neurotransmission was similar in both groups (CHF versus control). Ang II (0.1 nM–0.1 µM) caused a concentration-dependent increase of the stimulation-evoked sympathetic outflow in both groups, with a maximum at 10 nM (control [n=7], FR2/FR1 2.03+0.11 and CHF- preparations [n=7], FR2/FR1 1.71+0.07). The enhancement by Ang II was decreased in CHF- preparations compared with controls (p<0.05). Eprosartan concentration-dependently attenuated the Ang II-enhanced (10 nM) sympathetic outflow in both CHF- and control preparations. The sympatho-inhibitory potency of eprosartan was similar in both groups (control pIC50 8.81±0.31; CHF 8.65+0.42).
Ang II (1 nM–0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax 21.64+3.86 mN, pD2 7.63+0.02, n=7). Eprosartan (1 nM–0.1 µM) influenced the Ang II- contractions via a mixed form of antagonism. In CHF-preparations, Ang II caused impaired vascular contraction. The KCl-induced contraction was decreased in the CHF- compared with control preparations (13.02+0.64 mN versus 30.40+0.89 mN). The relative Ang II contraction (% of KCl) was also decreased (2.3% vs. 58.0%). Concentration-response curves to noradrenaline (%KCl) were similar (control pD2 6.93+0.05, Emax 131.0+2.7; CHF pD2 7.00+0.05, Emax 136.7+2.6) (p>0.05) and were not affected by Ang II.
We conclude that Ang II-enhanced sympathetic neurotransmission is mediated by the prejunctional AT1-receptor in both control and CHF-preparations. The decreased facilitation of SNS effects by Ang II may be explained by down-regulation or desensitisation of the neuronal AT1-receptor. Additionally, the aortic contractile capacity in heart failure rabbits appears to be decreased, probably as a result of heart failure-associated neuroendocrine and functional changes.

JRAAS 2003;4:220-227.

POPULAR
TOPICPAPERNeutral endopeptidase expression in mesangial cells
Fabiana Ebihara, Giovana Seno Di Marco, Maria Aparecida Juliano, Dulce Elena Casarini

In the kidney, neutral endopeptidase (NEP) is implicated in the metabolism of several peptides involved in blood pressure and sodium homeostasis control, such as the atrial natriuretic peptide, bradykinin and angiotensin I. Due to its physiological importance in the modulation of pressor responses, the presence of NEP in mouse mesangial cells has been investigated, since these cells control glomerular function and are able to synthesise components of the renin-angiotensin system. A NEP-like activity (NEP-like) that cleaves the fluorogenic substrates Abz-BKQ-EDDnp and Abz-DRRL-EDDnp was purified from mesangial cell lysate by ion-exchange, followed by gel filtration chromatography. The enzyme was able to hydrolyse bradykinin at the G4-F5 peptide bond and was inhibited by thiorphan. A pH study established that enzyme activity was maximal at pH 7.5 and the determined K_m was 4.86 µM using Abz-DRRL-EDDnp as substrate. NEP-like was recognised by monoclonal anti-NEP and had a molecular mass of 95 kDa. The purified enzyme was sequenced and showed similarity with human, rat, mouse and rabbit NEPs. We isolated, for the first time, NEP-like from mesangial cells. This enzyme could have an important role in the renal physiology by its action upon different peptides that are able to alter renal haemodynamics.

JRAAS 2003;4:228-233.

POPULAR
TOPICPAPERReduction (TELMAR) as assessed by magnetic resonance imaging in patients with mild-to-moderate hypertension – a prospective, randomised, double-blind comparison of telmisartan with metoprolol over a period of six months – rationale and study design
Matthias G Friedrich, Björn Dahlöf, Udo Sechtem, Thomas Unger, Mathias Knecht, Rainer Dietz for the TELMAR Investigators

The Telmisartan Effectiveness on Left ventricular MAss Reduction (TELMAR) trial will assess the effect of the angiotensin II (Ang II) receptor blocker, telmisartan, on left ventricular hypertrophy (LVH) compared with the b-blocker, metoprolol, at similar antihypertensive doses. The rationale is that antihypertensives reduce LVH, a cardiac adaptation to pressure overload, principally by pressure-related effects. Ang II plays a key role in pressure-independent mechanisms causing LVH, and angiotensin-converting enzyme (ACE) inhibitors induce more pronounced LVH regression than some other antihypertensives. Blocking Ang II Type 1 receptors may be more effective than ACE inhibition in reducing LVH. TELMAR is a prospective, randomised, double-blind, double-dummy, parallel-group trial. A total of 140 patients (age 18–80 years) with uncontrolled essential hypertension (mean daytime systolic blood pressure [SBP] >140 mmHg or diastolic blood pressure [DBP] >90 mmHg and/or night-time SBP >120 mmHg or DBP >70 mmHg, measured by ambulatory blood pressure monitoring [ABPM]) and left ventricular mass index related to height (LVMI) >0.8 g/cm for females, >1.1 g/cm for males (defined by magnetic resonance imaging [MRI]) will be randomised to once-daily telmisartan or metoprolol. The telmisartan dose will be 40 mg for the first two weeks, 80 mg for 5.5 months and 40 mg for the last two weeks. Metoprolol will be given at a dose of 47.5 mg for two weeks, 95 mg for 5.5 months and 47.5 mg for two weeks. Concomitant add-on medication with hydrochlorothiazide and amlodipine will be allowed. The primary endpoint is the percentage change in LVMI at treatment end versus baseline, using MRI. Secondary variables include blood pressure changes and response rates assessed by ABPM and manual cuff sphygmomanometry, and end-systolic wall stress, systolic left ventricular function (LVF) and diastolic LVF determined by MRI. A separate study was performed prior to the main trial to define the normal range of MRI data in an age-matched population.

JRAAS 2003;4:234-243.

POPULAR
TOPICPAPERHaemodynamic and pulse wave responses to intravenous infusions of angiotensin II during chronic telmisartan therapy in normal volunteers
Noreen M Vingerhoedt, Rozemarie Gilles, Jan B Howes, Matthew Griffin, Laurence Guy Howes

Introduction: This study investigated the central haemodynamic, blood pressure (BP) and pulse wave responses to progressively increasing infusion rates of intravenous angiotensin II (Ang II) in normal volunteers during chronic therapy with telmisartan or placebo.
Materials and methods: Ten normal volunteers, aged 21–33 years, completed a randomised, double-blind crossover study. Ang II was infused intravenously at increasing infusion rates (0–512 ng/minute) at the end of one week of telmisartan therapy (40–80 mg/day) and one week of placebo therapy. BP, central haemodynamics and pulse wave parameters were monitored continuously using a CardioDynamics Recorder, a Pulse Tracer Recorder and a Finipress Recorder.
Results: Baseline diastolic BP (57+12 vs. 67+13 mmHg) and pulse wave reflection index (RI) (38.4+18.6 vs. 60.6+12.5%) were significantly lower on telmisartan than on placebo therapy. Cardiac index (CI), systolic BP, systemic vascular resistance index (SVRI), RI and pulse wave stiffness index (SI) were all significantly increased in a dose-dependent manner by Ang II on placebo therapy. Telmisartan significantly (p<0.05) attenuated all of these responses to Ang II. Increases in BP during Ang II infusion were associated with increases in SVRI and CI.
Conclusions: Telmisartan effectively blocked the effects of intravenous Ang II on CI, BP, RI and SI in healthy volunteers. Changes in CI make a major contribution to increase in BP response to intravenous Ang II in normal volunteers.

JRAAS 2003;4:244-248.

POPULAR
TOPICEDITORIAL REVIEWBlocking the renin-angiotensin-aldosterone system or lowering the blood pressure: does EUROPA help?
Judy A Mackay

The EUropean trial on Reduction Of cardiac events with Perindopril in stable coronary Artery disease (EUROPA) demonstrated that the angiotensin- converting enzyme inhibitor, perindopril, compared with placebo, in patients with established coronary artery disease, reduced cardiovascular endpoints by 20% over a four-year follow-up period. Although the authors of this study claim that the risk reduction was best explained by blockade of the renin-angiotensin-aldosterone system, reference to recent meta-analyses of blood pressure (BP)-lowering trials provides compelling evidence that the benefits observed in the trial could be explained by the 5/2 mmHg BP difference between active and placebo treatments.

JRAAS 2003;4:205-206.