The renin-angiotensin-aldosterone system and cardiac ischaemia

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

Downregulation of ANG II receptor is associated with compensated pressure-overload hypertrophy in the young dog

We studied the gradual onset of pressure overload (PO) induced by a mildly constricting aortic band in 8-wk-old puppies (n = 8) that increased to 98 +/- 11 mmHg at 9 mo. Left ventricular (LV) weight/body weight was increased in PO versus sham-operated littermate controls [8.11 +/- 0.60 (SE) vs. 4.46 +/- 0.38 g/kg, P < 0.001]. LV end-diastolic diameter, diastolic pressure, and fractional shortening did not differ in PO versus control dogs. There were no inducible arrhythmias in response to an aggressive electrophysiological stimulation protocol in PO dogs. Furthermore, isolated cardiomyocyte function did not differ between control and PO dogs. LV angiotensin II (ANG II) levels were increased (68 +/- 12 vs. 20 +/- 5 pg/g, P < 0.01) as steady-state ANG II type 1 (AT(1)) receptor mRNA was decreased 40% and endothelial nitric oxide synthase mRNA levels were increased 2.5-fold in PO versus control dogs (P < 0.05). Total ANG II receptor binding sites of freshly prepared cardiac membranes demonstrated no difference in the dissociation constant, but there was a 60% decrease in maximum binding (B(max)) in PO versus control dogs (P < 0.01). LV ANG II levels correlated negatively with AT(1) receptor mRNA levels (r = -0.75, P < 0.01) and total AT(1) receptor B(max) (r = -0.77, P < 0.02). These results suggest that LV ANG II negatively regulates AT(1) receptor expression and that this is an adaptive response to chronic PO before the onset of myocardial failure in the young dog.

The effects of Z13752A, a combined ACE/NEP inhibitor, on responses to coronary artery occlusion; a primary protective role for bradykinin

The effects on the responses to coronary artery occlusion of a combined ACE/NEP inhibitor (Z13752A) were examined in anaesthetized dogs. A 1 h infusion of Z13752A (128 microgram kg(-1) min(-1) intravenously) decreased arterial blood pressure (by 11+/-3%; P<0. 05) and increased coronary blood flow (by 12+/-4%, P<0.05). There were no other significant haemodynamic changes. Z13752A inhibited both NEP and ACE enzymes both in dog plasma and in tissue (lung ACE; kidney NEP). Pressor responses to angiotensin I in vivo were inhibited and systemic vasodilator responses to bradykinin were potentiated. When the left anterior descending coronary artery was occluded for 25 min, Z13752A markedly reduced the severity of the resultant ventricular arrhythmias. No ventricular fibrillation (VF) occurred (compared to 7/16 in the controls; P<0.05), and ventricular tachycardia (VT) was reduced (VT in 2/9 dogs treated with Z13752A cp. 16/16 of controls; episodes of VT 0.2+/-0.1 c.p. 10.7+/-3.3; P<0. 05). Reperfusion of the ischaemic myocardium led to VF in all control dogs but occurred less frequently in dogs given Z13752A (survival from the combined ischaemia-reperfusion insult 67% c.p. 0% in controls; P<0.05). Z13752A reduced two other indices of ischaemia severity; epicardial ST-segment elevation and inhomogeneity of electrical activation. These protective effects of Z13752A during ischaemia and reperfusion were abolished by the administration of icatibant (0.3 mg kg(-1), i.v.) a selective antagonist of bradykinin at B(2) receptors; the ischaemic changes in dogs given both icatibant and Z13752A were similar to those in the controls. We conclude that this ACE/NEP inhibitor is effective at reducing the consequences of coronary artery occlusion in this canine model and that this protection is primarily due to potentiation of released bradykinin. British Journal of Pharmacology (2000) 129, 671 - 680

EXP3174, the AII antagonist human metabolite of losartan, but not losartan nor the angiotensin-converting enzyme inhibitor captopril, prevents the development of lethal ischemic ventricular arrhythmias in a canine model of recent myocardial infarction

OBJECTIVES

The antiarrhythmic efficacies of the competitive angiotensin II (AII) antagonist losartan, losartan's more potent noncompetitive AII antagonist human metabolite EXP3174 and the angiotensin-converting enzyme inhibitor captopril were assessed in a canine model of recent myocardial infarction.

BACKGROUND

Multiple hemodynamic and electrophysiologic effects of AII may contribute to cardiac electrical instability. In the recent Losartan Heart Failure Study, Evaluation of Losartan in the Elderly (ELITE), a 722-patient trial primarily designed to assess effects on renal function, an unexpected survival benefit was observed with losartan compared with captopril, with the lower mortality using losartan primarily confined to a reduction in sudden cardiac death.

METHODS

Intravenous losartan (1 mg/kg + 0.03 mg/kg/min), EXP3174 (0.1 mg/kg + 0.01 mg/kg/min), captopril (1 mg/kg + 0.5 mg/kg/h) or vehicle were infused in anesthetized dogs with recent (8.1 +/- 0.4 days) anterior myocardial infarction. Electrolytic injury of the left circumflex coronary artery to induce thrombotic occlusion and posterolateral ischemia was initiated 1 h after the start of treatment.

RESULTS

Losartan, EXP3174 and captopril elevated plasma renin activities and comparably and significantly reduced mean arterial pressure. No significant electrocardiographic or cardiac electrophysiologic effects were noted with any treatment. Incidences of acute posterolateral ischemia-induced lethal arrhythmias were: vehicle, 7/9 (77%); losartan, 6/8 (75%); EXP3174, 2/8 (25%; p < 0.05 vs. vehicle control); captopril, 7/10 (70%). There were no among-group differences in time to onset of acute posterolateral ischemia or underlying anterior infarct size.

CONCLUSIONS

EXP3174, but not losartan nor captopril, reduced the incidence of lethal ischemic ventricular arrhythmia in this preparation. The antiarrhythmic efficacy of EXP3174 may be due to an attenuation of deleterious effects of local cardiac AII formed during acute myocardial ischemia or, alternatively, a non-AII-related activity specific to EXP3174. These findings suggest that in humans, metabolic conversion of losartan to EXP3174 may afford antiarrhythmic protection.