Effects of phosphoramidon, BQ 788, and BQ 123 on coronary and cardiac dysfunctions of the failing hamster heart

Effects of chronic endothelin-1 stimulation on cardiac myocyte contractile function

Endothelin-1 (ET-1) has acute positive inotropic effects, but consequences of chronically increased ET-1 on contractile function of cardiac myocytes are largely unknown. In the present study, effects of long-term treatment with ET-1 (10 nM) for 5 days on both force development [force of contraction (FOC)] and kinetics of contraction were determined in heart tissue reconstituted from rat cardiac cells. Isometric force was measured in response to cumulative concentrations of Ca2+and isoprenaline. ET-1 augmented basal FOC by 64 ± 11% ( P < 0.05), which was associated with a significantly blunted contractile response to Ca2+and isoprenaline. Moreover, ET-1 significantly prolonged relaxation (62 ± 3 vs. 53 ± 2 ms). Selective ETA(BQ-123) and ETBreceptor blockade (BQ-788) demonstrated that effects of ET-1 on contractile function were mediated through the ETAreceptor subtype. Effects of ET-1 were prevented by cotreatment with either Ro31-8425, a PKC inhibitor, or dimethylamiloride, an inhibitor of the Na+/H+exchanger. In contrast to long-term ET-1 treatment, no changes in contractile parameters were observed after ET-1 treatment for 3 h before force measurement. These data suggest that chronic ET-1 stimulation has dual effects on contractility: improvement of basal force but impairment of twitch kinetics and inotropic responsiveness to β-adrenoceptor stimulation. The signaling pathways involved include ETAreceptors, PKC, and the Na+/H+exchanger. The present in vitro findings raise the possibility that ET-1 may exert both adaptive and maladaptive effects in the failing myocardium in which local accumulation of ET-1 is present.

Role of nitric oxide synthase, cytochrome P-450, and cyclooxygenase in the inotropic and lusitropic cardiac response to increased coronary perfusion

Although studies have reported that increase in coronary perfusion (CP) results in positive inotropic effects, the underlying mechanisms of these actions and possible alterations in myocardial diastolic function are not well defined. Hypothesis was that nitric oxide (NO) and derivatives of cytochrome (CYT) P-450 or cyclooxygenase (COX) might contribute to interplay between coronary and myocardial compartments in these conditions. Using isovolumically contracting, isolated perfused hamster heart model, coronary flow (CF) was increased mechanically, stepwise in the physiologic range (+2 to +10 ml/min), before and after inhibition of NO synthase by NG-nitro-l-arginine methyl ester (l-NAME) (30 microM), CYT P-450 by SKF525A (1 microM), or COX by indomethacin (10 microM). CP pressure, left ventricular systolic pressure (VSP) and ventricular diastolic pressure (VDP), and heart rate (HR) were monitored continuously during the experiments. Mechanical increases in CF resulted in gradual change in CP pressure (+20% to +100%), left VSP (+5% to +40%) and VDP (+2% to +25%), whereas HR was not affected. In presence of l-NAME, the positive inotropic response and negative lusitropic effect of CF changes were similar. Exposure to SKF525A did not modify cardiac response to mechanical increases in CF. In presence of COX inhibitor indomethacin, left VSP rose to a level similar to that observed in control conditions, whereas VDP deteriorated further. These results suggest that mediators originating from NO synthase, CYT P-450, or COX do not contribute to positive inotropic response elicited by increased CP. However, COX derivatives seem to attenuate impairment of myocardial relaxation observed in these conditions. Such findings may have implications in development of therapeutics for patients with myocardial diastolic dysfunction.

Role of endothelin in cardiovascular disease

Endothelins are a family of peptides, which comprises endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3), each containing 21 amino-acids. ET-1 is a peptide secreted mostly by vascular endothelial cells, the predominant isoform expressed in vasculature and the most potent vasoconstrictor currently known. ET-1 also has inotropic, chemotactic and mitogenic properties. In addition, it influences salt and water homeostasis through its effects on the renin-angiotensin-aldosterone system (RAAS), vasopressin and atrial natriuretic peptide and stimulates the sympathetic nervous system. The overall action of endothelin is to increase blood pressure and vascular tone. Therefore, endothelin antagonists may play an important role in the treatment of cardiac, vascular and renal diseases associated with regional or systemic vasoconstriction and cell proliferation, such as essential hypertension, pulmonary hypertension, chronic heart failure and chronic renal failure. Long-term anti-endothelin therapy may improve symptoms and favourably alter the progression of heart failure. Endothelin appears to participate in induction and progression of sclerotic renal changes, leading to progression to end-stage renal disease. Anti-endothelin therapy might offer additional benefits in the prevention of progression of chronic renal failure in addition to the known benefits of RAAS inhibition. Clinical trials have demonstrated potentially important benefits of endothelin antagonists for patients with essential hypertension, pulmonary hypertension and heart failure. Further studies are necessary to determine the role of anti-endothelin therapy in the treatment of cardiovascular diseases and determine the different roles of selective receptor antagonism vs. mixed ET(A/B)-receptor antagonism in human diseases.

Altered coronary and cardiac adrenergic response in the failing hamster heart: role of cyclooxygenase derivatives

Although the influence of the adrenergic system has been studied in the presence of heart failure, controversies still exist. Since cyclooxygenase derivatives appear to modulate coronary and cardiac adaptation in the failing heart, we hypothesized that cyclooxygenase derivatives may participate in the altered adrenergic responses in this situation. Isolated hearts from cardiomyopathic (UM-X7.1 subline) and normal hamsters, aged > 240 days, were utilized. Coronary and cardiac response to alpha1-, beta1-, and beta2-adrenergic stimulations was observed before and after pretreatment with indomethacin, a cyclooxygenase inhibitor. Reduction of coronary flow elicited by alpha1-adrenergic stimulation was unchanged in the presence of heart failure, while beta1- and beta2-induced vasodilatations were reduced. Inotropic response to alpha1 and beta1 stimulations were also reduced in failing hearts, while beta2-adrenergic action was unchanged. Pretreatment with indomethacin exacerbated coronary flow reduction observed with alpha1 stimulation in failing hearts only. Beta2-induced coronary vasodilatation and inotropic response to alpha1 and beta2 stimulations were impaired similarly in the presence of indomethacin in normal and failing hearts. The results suggest a complex interaction between adrenergic and cyclooxygenase activation.

Rationale and perspective of endothelin-1 antagonism in acute heart failure

A role of the potent and long-acting vasoconstrictor peptide endothelin (ET)- I in the pathophysiology of chronic human heart failure has been postulated, based upon indirect evidence such as elevated plasma ET-1 levels and their relationship to the degree of haemodynamic impairment. Acute heart failure shares many features of chronic heart failure, albeit in an exaggerated fashion. As both the mixed ETA/ETB-receptor antagonist bosentan and the selective ETA receptor antagonist BQ 123 acutely improved the haemodynamics of chronic heart failure patients, there seems to be good reason to believe that ET-1 receptor antagonism may also be of benefit in the setting of acute heart failure. However, appropriate trials will have to be performed to document the clinical benefit of such an approach. Finally, the question remains open as to whether mixed ET-1 receptor antagonists like bosentan will prove better, worse or equal to antagonists that block the ETA, receptor only.

Chronic administration of losartan plus hydrochlorothiazide improves vascular status in young cardiomyopathic hamsters

The combination of an angiotensin II receptor antagonist and a thiazide has been used extensively in the treatment of patients with overt heart failure. The effect of this combination on the vascular wall early in the disease, however, has not been investigated. To evaluate this effect, the vascular status of 3-month-old cardiomyopathic hamsters was assessed after daily administration of a combination of losartan (25 mg/kg, p.o.) and hydrochlorothiazide (6.5 mg/kg, p.o.) over an 8-week period. Age-matched golden hamsters were used as healthy controls. The contractile response of aortic rings to endothelin-1 was significantly higher in cardiomyopathic hamsters than in control animals. Concentration-response curves for the endothelin-1-induced contraction were displaced to the right after hydrochlorothiazide+losartan treatment (toward the curves for healthy controls); however, E(max) from treated hamsters was significantly reduced when compared to E(max) from untreated cardiomyopathic animals (1.016+/-0.073 vs. 1.346+/-0.153 g, P<0.05, n=6). No significant differences in the EC50 values from these curves were observed between hydrochlorothiazide+losartan treated and untreated cardiomyopathic animals (2.90+/-0.95 vs. 1.10+/-0.85 nM, P>0.05). The acetylcholine-induced relaxation observed in cardiomyopathic animals was not improved after treatment with hydrochlorothiazide+losartan or hydrochlorothiazide alone, but the combination of these drugs increased significantly the basal production of nitric oxide (NO). Angiotensin-converting enzyme activity increased in plasma (from 29.9+/-1.23 to 41.16+/-1.82 nmol x mg(-1) x min(-1), n=8, P<0.05) but decreased in the aorta (from 0.33+/-0.02 to 0.25+/-0.017 nmol x mg(-1) x min(-1), n=6, P<0.05) after treatment with hydrochlorothiazide+losartan. In addition, the combination of these drugs reduced the heart-to-body mass ratio (3.96+/-0.07 for treated vs. 5.01+/-0.20 mg/g for untreated animals, n=7, P<0.05), and the thickness of the aortic media (0.076+/-0.003 for treated vs. 0.149+/-0.009 mm for untreated animals, n=8, P<0.05). Although hydrochlorothiazide alone lowered systolic blood pressure to the same level achieved with both drugs in combination (from 166+/-10 for untreated cardiomyopathic animals to 84+/-1 mm Hg for hydrochlorothiazide+losartan, and 80+/-5 mm Hg for hydrochlorothiazide alone, P<0.05), no significant reduction in heart-to-body mass ratio was observed in animals treated with the diuretic alone (P>0.05). In conclusion, in this model of heart failure, chronic hydrochlorothiazide+losartan administration normalizes the vascular responses to endothelin-1, improves basal vascular tone, and prevents the development of cardiac and vascular hypertrophy.