Renin-angiotensin system inhibition on noradrenergic nerve terminal function in pacing-induced heart failure

Autonomic nervous system and cardiac neuro-signaling pathway modulation in cardiovascular disorders and Alzheimer's disease

The heart is a functional syncytium controlled by a delicate and sophisticated balance ensured by the tight coordination of its several cell subpopulations. Accordingly, cardiomyocytes together with the surrounding microenvironment participate in the heart tissue homeostasis. In the right atrium, the sinoatrial nodal cells regulate the cardiac impulse propagation through cardiomyocytes, thus ensuring the maintenance of the electric network in the heart tissue. Notably, the central nervous system (CNS) modulates the cardiac rhythm through the two limbs of the autonomic nervous system (ANS): the parasympathetic and sympathetic compartments. The autonomic nervous system exerts non-voluntary effects on different peripheral organs. The main neuromodulator of the Sympathetic Nervous System (SNS) is norepinephrine, while the principal neurotransmitter of the Parasympathetic Nervous System (PNS) is acetylcholine. Through these two main neurohormones, the ANS can gradually regulate cardiac, vascular, visceral, and glandular functions by turning on one of its two branches (adrenergic and/or cholinergic), which exert opposite effects on targeted organs. Besides these neuromodulators, the cardiac nervous system is ruled by specific neuropeptides (neurotrophic factors) that help to preserve innervation homeostasis through the myocardial layers (from epicardium to endocardium). Interestingly, the dysregulation of this neuro-signaling pathway may expose the cardiac tissue to severe disorders of different etiology and nature. Specifically, a maladaptive remodeling of the cardiac nervous system may culminate in a progressive loss of neurotrophins, thus leading to severe myocardial denervation, as observed in different cardiometabolic and neurodegenerative diseases (myocardial infarction, heart failure, Alzheimer's disease). This review analyzes the current knowledge on the pathophysiological processes involved in cardiac nervous system impairment from the perspectives of both cardiac disorders and a widely diffused and devastating neurodegenerative disorder, Alzheimer's disease, proposing a relationship between neurodegeneration, loss of neurotrophic factors, and cardiac nervous system impairment. This overview is conducive to a more comprehensive understanding of the process of cardiac neuro-signaling dysfunction, while bringing to light potential therapeutic scenarios to correct or delay the adverse cardiovascular remodeling, thus improving the cardiac prognosis and quality of life in patients with heart or neurodegenerative disorders.

Impaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model

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 α-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/FR12.03±0.11 and CHF-preparations [n=7], FR2/FR11.71±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 sympathoinhibitory potency of eprosartan was similar in both groups (control pIC508.81±0.31; CHF 8.65±0.42).

Ang II (1 nM—0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax21.64±3.86 mN, pD27.63±0.02, n=7). Eprosartan (1 nM—0.1 µM) influenced the Ang IIcontractions 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 pD26.93±0.05, Emax131.0±2.7; CHF pD27.00±0.05, Emax136.7±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.

Renin-angiotensin system blockers are associated with reduced mortality and heart failure hospitalization in patients paced for complete atrioventricular block

BACKGROUND

Right ventricular apical pacing can cause dyssynchronous activation of the ventricles, increase sympathetic activation, cause abnormalities in myocardial perfusion, worsen cardiac output and endothelial function, and may be associated with adverse cardiovascular effects. The use of rennin-angiotensin system blockers (RASBs) may be beneficial in counteracting these potentially harmful effects of right ventricular pacing.

OBJECTIVE

To explore the impact of RASB use on the outcome in patients with right ventricular pacemakers implanted for complete atrioventricular (AV) block.

METHODS

Patients implanted with right ventricular pacemakers for complete AV block between 1994 and 2009 were identified from the Tayside Pacing Registry. Cox proportional hazards model was used to assess differences in all-cause mortality and congestive heart failure hospitalizations for those receiving RASB during follow-up, adjusted for confounding variables. We also performed 2 sensitivity analyses--a propensity score-matched analysis and time-dependent analyses--to minimize bias.

RESULTS

Eight hundred twenty patients (57% men; median age 73 years; range 22-103 years) received pacemakers for complete AV block between 1994 and 2008 (54% dual-chamber pacemaker and 46% ventricular demand pacemaker). Two hundred seventy-eight (34%) patients had received RASBs. Mean follow-up was 4.9 ± 4.6 years, with 540 (65%) deaths. RASB use was independently associated with significantly reduced mortality (adjusted hazard ratio 0.67; 95% confidence interval 0.47-0.94; P = .017) and reduced heart failure hospitalization (adjusted hazard ratio 0.42; 95% confidence interval 0.17-0.92; P <.001).

CONCLUSIONS

This study suggests that RASBs may confer outcome benefits in patients with right ventricular pacemakers implanted for complete AV block.

Abnormalities of angiotensin regulation in postural tachycardia syndrome

BACKGROUND

Postural tachycardia syndrome (POTS) is a disorder characterized by excessive orthostatic tachycardia and significant functional disability. We previously reported that POTS patients have low blood volume and inappropriately low plasma renin activity (PRA) and aldosterone. In this study, we sought to more fully characterize the renin-angiotensin-aldosterone system (RAAS) to gain a better understanding of the pathophysiology of POTS.

OBJECTIVE

The purpose of this study was to prospectively assess the plasma levels of angiotensin (Ang) peptides and their relationship to other RAAS components in patients with POTS compared with healthy controls.

METHODS

Heart rate, PRA, Ang I, Ang II, Ang (1-7), and aldosterone were measured in POTS patients (n = 38) and healthy controls (n = 13) while they were consuming a sodium-controlled diet.

RESULTS

POTS patients had larger orthostatic increases in heart rate than did controls (52 ± 3 [mean ± SEM] bpm vs 27 ± 6 bpm, P = .001). Plasma Ang II was significantly higher in POTS patients (43 ± 3 pg/mL vs 28 ± 3 pg/mL, P = .006), whereas plasma Ang I and angiotensin 1-7 [Ang-(1-7)] were similar between groups. Despite the twofold increase of Ang II, POTS patients trended to lower PRA levels than did controls (0.9 ± 0.1 ng/mL/h vs 1.6 ± 0.5 ng/mL/h, P = .268) and lower aldosterone levels (4.6 ± 0.8 pg/mL vs 10.0 ± 3.0 pg/mL, P = .111). Estimated angiotensin-converting enzyme-2 (ACE2) activity was significantly lower in POTS patients than in controls (0.25 ± 0.02 vs 0.33 ± 0.03, P = .038).

CONCLUSION

Some patients with POTS have inappropriately high plasma Ang II levels, with low estimated ACE2 activity. We propose that these abnormalities in Ang regulation may play a key role in the pathophysiology of POTS in some patients.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Coronary vasoconstrictor influence of angiotensin II is reduced in remodeled myocardium after myocardial infarction

The renin-angiotensin system plays an important role in cardiovascular homeostasis by contributing to the regulation of blood volume, blood pressure, and vascular tone. Because AT(1) receptors have been described in the coronary microcirculation, we investigated whether ANG II contributes to the regulation of coronary vascular tone and whether its contribution is altered during exercise. Since the renin-angiotensin system is activated after myocardial infarction, resulting in an increase in circulating ANG II, we also investigated whether the contribution of ANG II to the regulation of vasomotor tone is altered after infarction. Twenty-six chronically instrumented swine were studied at rest and while running on a treadmill at 1-4 km/h. In 13 swine, myocardial infarction was induced by ligation of the left circumflex coronary artery. Blockade of AT(1) receptors (irbesartan, 1 mg/kg iv) had no effect on myocardial O(2) consumption but resulted in an increase in coronary venous O(2) tension and saturation both at rest and during exercise, reflecting coronary vasodilation. Despite increased plasma levels of ANG II after infarction and maintained coronary arteriolar AT(1) receptor levels, the vasodilation evoked by irbesartan was significantly reduced both at rest and during exercise. In conclusion, despite elevated plasma levels, the vasoconstrictor influence of ANG II on the coronary circulation in vivo is reduced after myocardial infarction. This reduction in ANG II-induced coronary vasoconstriction may serve to maintain perfusion of the remodeled myocardium.

Midkine is a potent regulator of the catecholamine biosynthesis pathway in mouse aorta

To discover regulatory pathways dependent on midkine (Mk the gene, MK the protein) signaling, we compared the transcriptional profiles of aortae obtained from Mk -/- and wild type (WT, +/+) mice; the comparison demonstrated an extraordinary high level expression of tyrosine hydroxylase (12-fold), the rate-limiting enzyme in catecholamine biosynthesis, DOPA decarboxylase (73-fold), and dopamine beta-hydroxylase (75-fold) in aortae of Mk -/- mice compared with aortae of WT (+/+) mice. Phenylethanolamine-N-methyltransferase, the enzyme catalyzing the conversion of norepinephrine into epinephrine, was not detected in either Mk -/- and WT (+/+) mouse aorta. The protein levels of tyrosine hydroxylase, DOPA decarboxylase and dopamine beta-hydroxylase confirmed the analysis of the transcriptional profiles. Surprisingly, MK failed to regulate the enzymes of the catecholamine biosynthesis pathway in 10 other tissues studied. Furthermore, the expression levels of the enzymes of catecholamine biosynthesis in aortae of Mk -/- mice were effectively the same as those in aortae of Pleiotrophin (Ptn the gene, PTN the protein) genetically deficient (Ptn -/-) mice when compared with WT (+/+) mice. The remarkable increases in levels of expression of tyrosine hydroxylase, DOPA decarboxylase and dopamine beta-hydroxylase suggest that MK together with PTN are very important regulators of the catecholamine pathway in mouse aorta and may critically regulate catecholamine biosynthesis and function in inflammatory and the other pathological conditions in which Mk or Ptn are upregulated. The data also establish that norepinephrine is effectively the only catecholamine synthesized in mouse aorta.

Cardiac Overexpression of the Norepinephrine Transporter Uptake-1 Results in Marked Improvement of Heart Failure

A hyperadrenergic state is one of the key features of human and experimental heart failure. Decreased densities and activities of the presynaptic neuronal norepinephrine (NE) transporter uptake-1 occur both in patients and animal models. It is currently unclear to what extent the reduction of uptake-1 contributes to the deterioration of heart failure. Therefore, we investigated the effects of myocardial overexpression of uptake-1 in both nonfailing rabbit hearts and in an animal model of heart failure. Heart failure was induced in rabbits by rapid ventricular pacing. Adenoviral gene transfer was used to overexpress uptake-1 in the myocardium. Uptake-1 overexpression led to increased NE uptake capacity into the myocardium. In contrast, systemic plasma NE levels in uptake-1-overexpressing failing rabbits (uptake-1-CHF) did not differ from controls. Downregulation of SERCA-2 and beta-adrenergic receptors in the failing myocardium was significantly reversed after uptake-1 overexpression. Uptake-1 overexpression significantly improved left ventricular (LV) diameters (LV end-diastolic diameter: in GCP-overexpressing failing rabbits (GFP-CHF), 17.4+/-0.4 mm; in uptake-1-CHF rabbits, 15.6+/-0.6 mm) and systolic contractility (fractional shortening: GFP-CHF, 20.7+/-0.6%; uptake-1-CHF, 27.3+/-0.7%), as assessed by echocardiography at the end of the heart failure protocol. Intraventricular tip catheter measurements revealed enhanced contractile reserve (dP/dt max with isoproterenol 1.0 microg/kg: GFP-CHF, 6964+/-230 mm Hg/sec; uptake-1-CHF, 7660+/-315 mm Hg/sec) and LV relaxation (dP/dt min with isoproterenol 1.0 microg/kg: GFP-CHF: -3960+/-260 mm Hg/sec; uptake-1-CHF, -4910+/-490 mm Hg/sec). End-diastolic filling pressures (GFP-CHF, 8.5+/-1.2 mm Hg; uptake-1-CHF, 5.6+/-0.7 mm Hg) tended to be lower in uptake-1 overexpressing animals. In summary, local overexpression of uptake-1 in the myocardium results in marked structural and functional improvement of heart failure, thus underlining the importance of uptake-1 as a key protein in heart failure.

Importance of antioxidant and antiapoptotic effects of β-receptor blockers in heart failure therapy

The present study was carried out to determine whether beneficial effects of carvedilol in congestive heart failure (CHF) are mediated via its β-adrenergic blocking, antioxidant, and/or α-adrenergic blocking action. Rabbits with heart failure induced by rapid cardiac pacing were randomized to receive subcutaneous carvedilol, metoprolol, propranolol plus doxazosin, or placebo pellets for 8 wk and compared with sham-operated rabbits without pacing. We found rapid cardiac pacing produced clinical heart failure, left ventricular dilation, and decline of left ventricular fractional shortening. This was associated with an increase in left ventricular end-diastolic pressure, decrease in left ventricular first derivative of left ventricular pressure, and myocyte hypertrophy. Tissue oxidative stress measured by GSH/GSSG was increased in the heart with increased oxidation product of mitochondrial DNA, 8-oxo-7,8-dihydro-2′-deoxyguanosine, increase of Bax, decrease of Bcl-2, and increase of apoptotic myocytes as measured by anti-single-stranded DNA monoclonal antibody. Administration of carvedilol and metoprolol, which had no effect in sham animals, attenuated cardiac ventricular remodeling, cardiac hypertrophy, oxidative stress, and myocyte apoptosis in CHF. In contrast, propranolol plus doxazosin, which has less antioxidant effects, produced smaller effects on left ventricular function and myocyte apoptosis. In all animals, GSH/GSSG correlated significantly with changes of left ventricular end-diastolic dimension ( r = −0.678, P < 0.0001), fractional shortening ( r = 0.706, P < 0.0001), and apoptotic myocytes ( r = −0.473, P = 0.0001). Thus our findings suggest antioxidant and antiapoptotic actions of carvedilol and metoprolol are important determinants of clinical beneficial effects of β-receptors in the treatment of CHF.

Reduction of Vascular Noradrenaline Sensitivity by AT 1 Antagonists Depends on Functional Sympathetic Innervation

Blockade of angiotensin II type-1 (AT 1 ) receptors has been shown to reduce the magnitude of the blood pressure response to noradrenaline in pithed rats via an unidentified mechanism. Dose-response curves were established for the noradrenaline-induced (10 −12 to 10 −7 mol/kg) increase of diastolic blood pressure in pithed rats treated with tubocurarine, propranolol, and atropine. Candesartan (1 mg/kg) increased the ED 50 of the noradrenaline response (1.3±0.1 nmol/kg) up to 20-fold. Vasopressor responsiveness to noradrenaline was attenuated specifically, whereas the vasopressin-induced increase in diastolic blood pressure was maintained. Specific involvement of AT 1 receptors was confirmed by equivalent actions of losartan. Blockade of norepinephrine transporter or α 2 -adrenoceptors using desipramine or rauwolscine reduced the losartan-induced shifts in the ED 50 values of noradrenaline by 63% and 21%, respectively. Combined blockade of norepinephrine transporter and α 2 -adrenoceptors eliminated the influence of losartan on noradrenaline sensitivity ( ED 50 5.5±1.3 versus 5.6±1.2 nmol/kg), a result also observed after sympathetic denervation by reserpine ( ED 50 7.1±0.8 versus 7.8±0.8 nmol/kg). Our experiments show that the reduction of vascular noradrenaline sensitivity by AT 1 blockade is dependent on the intact functioning of both neuronal noradrenaline uptake via norepinephrine transporter and presynaptic α 2 -mediated autoinhibition, exclusively provided by the sympathetic innervation. These newly identified mechanisms may contribute to the antihypertensive and protective actions of AT 1 blockers.

Stress kinase phosphorylation is increased in pacing-induced heart failure in rabbits

In hearts with chronic left ventricular (LV) systolic dysfunction secondary to hypertension or myocardial infarction, MAPK phosphorylation and/or activity are increased. Whether other settings of LV dysfunction not associated with ischemia-reperfusion are also characterized by increased MAPK phosphorylation or activity is unknown. After 3 wk of rapid LV pacing (400 beats/min), eight rabbits displayed clinical signs of heart failure (HF), and echocardiography revealed an increase in LV end-diastolic diameter from 15.6 +/- 0.7 (means +/- SE) to 18.8 +/- 0.7 mm and a reduced shortening fraction from 31 +/- 1to10 +/- 2% (both P < 0.05). Morphological alterations in HF included increased numbers of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cardiomyocytes, extent of fibrosis, and cross-sectional cardiomyocyte area. Total p38 MAPK did not differ between failing and normal hearts (n = 8). However, p38 MAPK phosphorylation [164,488 +/- 29,323 vs. 43,565 +/- 14,817 arbitrary units (AU), P < 0.05, densitometry] and the activities of p38 MAPK-alpha and -beta were increased in failing compared with normal hearts (149,441 +/- 38,381 and 170,430 +/- 32,952 vs. 68,815 +/- 28,984 and 81,788 +/- 22,774 AU, respectively, both P < 0.05). In failing compared with normal hearts, total and phosphorylated JNK46 and JNK54 MAPK were increased, whereas total and phosphorylated ERK MAPK remained unchanged. In pacing-induced HF, p38 and JNK MAPK phosphorylation as well as p38 MAPK activity was increased. Further studies will have to define whether or not chronic specific blockade of MAPK activity can interfere with apoptosis/fibrosis and thereby attenuate the progression of HF.

Decreased facilitation by angiotensin II of noradrenergic neurotransmission in isolated mesenteric artery of rabbits with chronic heart failure

Both in human and in experimental heart failure (HF), the renin-angiotensin system and the sympathetic nervous system are activated. In a previous study a facilitatory action of angiotensin II (Ang II) was shown in the rabbit mesenteric artery, which was mediated via prejunctionally located Ang II type 1 (AT ) receptors. Very little is known about the effects of Ang II on sympathetic neurotransmission at the peripheral level in congestive heart failure (CFH). Accordingly, in the isolated mesenteric arteries obtained from rabbits with experimentally induced CHF, as well as in age-matched control rabbits, the effect of Ang II on contractions provoked by electrical field stimulation was investigated in the presence and absence of the AT receptor antagonist eprosartan. Additionally, to investigate a possible postjunctional facilitation, the effects of Ang II on alpha-adrenoceptor-mediated responses were studied using noradrenaline (NA). Lastly, the vasoconstrictor effects of Ang II were compared between HF rabbits and controls, by constructing concentration-response curves to Ang II. In control rabbits, Ang II 0.5 n caused an enhancement of stimulation-induced responses by a factor 3.2 +/- 0.5, 2.4 +/- 0.3, and 1.5 +/- 0.08, at 1, 2, and 4 Hz, respectively ( < 0.05 at all frequencies compared with vehicle). In rabbits with HF, the enhancement by Ang II (0.5 n ) amounted to a factor 2.1 +/- 0.2, 1.7 +/- 0.1, and 1.2 +/- 0.04, at 1, 2, and 4 Hz, respectively ( < 0.05 compared with vehicle at all frequencies). Accordingly, the enhancing effect of Ang II was more pronounced in the control group compared with rabbits with HF ( < 0.05 at each frequency). Eprosartan (1 nM -0.1 microM) could inhibit the facilitatory effects of Ang II in arteries from HF as well as from control rabbits. Contractile responses to exogenous NA (3 n -0.1 m ) were the same in HF rabbits and controls, and they were unaltered in the presence of Ang II 0.5 n Ang II (0.1 nM -1 microM) caused a concentration-dependent increase in contractile force, which was the same in HF rabbits and controls. From these findings it can be concluded that in rabbits with CHF as well as in control animals, Ang II facilitates the stimulation-induced vasoconstrictor responses via prejunctionally located AT receptors. The facilitating effect was decreased in vessels obtained from rabbits with CHF, whereas responses to exogenous Ang II were unchanged. These findings may be explained by downregulation or uncoupling of the prejunctional AT receptor.

Exogenous angiotensin II does not facilitate norepinephrine release in the heart

Studies on the effect of angiotensin II on norepinephrine release from sympathetic nerve terminals through stimulation of presynaptic angiotensin II type 1 receptors are equivocal. Furthermore, evidence that angiotensin II activates the cardiac sympathetic nervous system in vivo is scarce or indirect. In the intact porcine heart, we investigated whether angiotensin II increases norepinephrine concentrations in the myocardial interstitial fluid (NE(MIF)) under basal conditions and during sympathetic activation and whether it enhances exocytotic and nonexocytotic ischemia-induced norepinephrine release. In 27 anesthetized pigs, NE(MIF) was measured in the left ventricular myocardium using the microdialysis technique. Local infusion of angiotensin II into the left anterior descending coronary artery (LAD) at consecutive rates of 0.05, 0.5, and 5 ng/kg per minute did not affect NE(MIF), LAD flow, left ventricular dP/dt(max), and arterial pressure despite large increments in coronary arterial and venous angiotensin II concentrations. In the presence of neuronal reuptake inhibition and alpha-adrenergic receptor blockade, left stellate ganglion stimulation increased NE(MIF) from 2.7+/-0.3 to 7.3+/-1.2 before, and from 2.3+/-0.4 to 6.9+/-1.3 nmol/L during, infusion of 0.5 ng/kg per minute angiotensin II. Sixty minutes of 70% LAD flow reduction caused a progressive increase in NE(MIF) from 0.9+/-0.1 to 16+/-6 nmol/L, which was not enhanced by concomitant infusion of 0.5 ng/kg per minute angiotensin II. In conclusion, we did not observe any facilitation of cardiac norepinephrine release by angiotensin II under basal conditions and during either physiological (ganglion stimulation) or pathophysiological (acute ischemia) sympathetic activation. Hence, angiotensin II is not a local mediator of cardiac sympathetic activity in the in vivo porcine heart.