Nicholas A. Flores

Capillary filtration is reduced in lungs adapted to chronic heart failure: morphological and haemodynamic correlates

OBJECTIVE To determine pulmonary capillary filtration in experimental chronic heart failure and to investigate some morphological and haemodynamic mechanisms that could account for reduced filtration in lungs adapted to chronic heart failure. METHODS We studied pulmonary capillary filtration, vascular resistances and morphology in lungs from guinea-pigs adapted to chronic heart failure. Heart failure was induced by banding of the ascending aorta (n=66) or sham control operation (n=78) in guinea-pigs which were studied at 150+/-8 days post-operation. RESULTS Reduced cardiac output, increased systemic vascular resistance and LV end diastolic pressure and increased LV and RV weight:body weight ratio (all P<0.05) indicated chronic heart failure at 5 months following aortic banding in guinea-pigs. Lung weight was increased (61%, P<0.05) in heart failure compared with controls, but lung water content was reduced (5.5%, P<0.05), a reversal of the pattern seen acutely. Studies in isolated perfused lungs demonstrated a reduced capillary filtration coefficient (0. 018+/-0.003 vs. 0.003+/-0.002 ml min(-1)mmHg(-1)g(-1), P<0.001), increased arterial (61%) and venous resistance (50%) in heart failure lungs, P<0.05. Wall thickness:lumen ratio was increased in small (<250 microm) pulmonary arterioles (0.15+/-0.02 vs. 0.08+/-0. 01) and venules (0.06+/-0.005 vs. 0.04+/-0.002) in heart failure, P<0.01. Alveolar septal volume fractions (35.2+/-5.1 vs. 23.1+/-2.7) and septal:air-space volume ratios (60.5+/-13.6 vs. 31.9+/-5.3) were also increased in heart failure, P<0.05. CONCLUSIONS Pulmonary adaptation to chronic heart failure is associated with vascular and alveolar remodelling that contributes to increased vascular resistance and reduced capillary filtration. These changes are likely to be important in mediating resistance to pulmonary oedema in chronic heart failure.

Nitric oxide and prostacyclin modulate the alterations in cardiac action potential duration mediated by platelets during ischaemia.

OBJECTIVE To investigate the effects of alterations of nitric oxide (NO) and prostacyclin (PGI2) availability on platelet-mediated electrophysiological effects during myocardial ischaemia. METHODS Transmembrane action potentials and electrograms were recorded from isolated, Langendorff-perfused guinea-pig hearts during normal perfusion, global myocardial ischaemia and reperfusion during infusion of washed human platelets. Experiments were performed in the presence of 100 microM NG-nitro-L-arginine methyl ester (L-NAME), 30 microM L-arginine, 10 microM haemoglobin, 100 microM sodium nitroprusside and 2.3 nM iloprost, or using hearts obtained from DL-lysine monoacetylsalicylate (Aspisol, 50 mg.kg-1 i.p.)-treated animals. RESULTS Perfusion with L-NAME and haemoglobin increased perfusion pressure by 33% (P = 0.0017) and 23% (P = 0.0026) while sodium nitroprusside and iloprost reduced it (17%, P = 0.0004, and 24%, P = 0.0006). In the absence of platelets, these compounds had no effect on arrhythmogenesis, but in the presence of platelets L-NAME reduced the onset time of ventricular tachycardia during ischaemia from 19.4 (s.e.m. 2.0) min to 12.9 (2.1) min, P = 0.04 and accentuated the ischaemia-induced reduction of action potential duration at 95% repolarization (APD95): 95(6) vs. 115(5) ms, P < 0.05 at 25 min. Sodium nitroprusside in the presence of platelets attenuated the ischaemia-induced reduction in APD95, while iloprost in the presence of platelets was antiarrhythmic (ventricular fibrillation 25 vs. 75%, P = 0.04) and attenuated the reduction in APD95 during ischaemia 115(4) vs. 94(4) ms, P < 0.05 at 20 min. Infusion of platelets into hearts obtained from DL-lysine-monoacetylsalicylate-treated guinea-pigs accentuated the ischaemia-induced reduction in APD95 (94(4) vs. 119(7) ms, P < 0.05 at 20 min) and this was reversed by sodium nitroprusside (117(7) ms, P < 0.05 at 20 min). L-NAME and haemoglobin had no effect on the aggregatory responses of the platelets to 5 microM ADP and 4 micrograms.ml-1 collagen, while sodium nitroprusside and iloprost ablated the responses to ADP and reduced the responses to collagen (maximum height of the aggregatory response reduced by 75 and 84%, respectively, both P = 0.03.) CONCLUSIONS Inhibition of NO and PGI2 synthesis exacerbates the reduction in cardiac action potential duration associated with platelet activation during ischaemia, while provision of exogenous NO and PGI2 attenuates the reduction in cardiac action potential duration. Provision of exogenous NO and PGI2 (as iloprost) was associated with inhibition of platelet reactivity.

Electrophysiological and arrhythmogenic effects of platelet activating factor during normal perfusion, myocardial ischaemia and reperfusion in the guinea‐pig

1 Platelet activating factor (PAF) is often used to study the effects of platelet activation. While direct myocardial electrophysiological effects of PAF have been described in superfused myocardial tissue, little is known about its actions on the whole heart. 2 The cellular electrophysiological and arrhythmogenic effects of PAF (10−11 m, 10−10 m and 10−9 m) were studied during normal perfusion, global myocardial ischaemia and reperfusion in Langendorff‐perfused guinea‐pig hearts at 32°C. 3 PAF (10−9 m) increased the incidence of ventricular fibrillation during ischaemia and reduced action potential duration (APD) during normal perfusion and early myocardial ischaemia (10−9 m and 10−10 m). PAF also reduced refractory period (RP) during normal perfusion (10−9 m) and early ischaemia (10−9 m and 10−10 m). PAF prevented recovery of APD (10−9 m) and RP (10−9 m and 10−10 m) during reperfusion. PAF at a concentration of 10−11 m had no electrophysiological effects. 4 PAF (10−9 m) increased the QRS width of the electrocardiogram during late ischaemia while 10−10 m PAF raised pacing threshold during late ischaemia. 5 Perfusion pressure was increased, and developed tension decreased by 10−9 m PAF. 6 These results demonstrate that PAF has direct myocardial electrophysiological effects in the whole heart which occur during normal perfusion and are capable of augmenting the effects of myocardial ischaemia, but are independent of the presence of platelets.

Investigation of mechanisms that mediate reactive hyperaemia in guinea-pig hearts: role of KATP channels, adenosine, nitric oxide and prostaglandins

Reactive hyperaemia is a transient vasodilatation following a brief ischaemic period. ATP‐dependent K+ (KATP) channels may be important in mediating this response, however it is unclear whether mitochondrial KATP channels contribute to this in the heart. We examined the involvement of KATP channels and the relative role of mitochondrial channels as mediators of coronary reactive hyperaemia and compared them to mechanisms involving NO, prostaglandins and adenosine in the guinea‐pig isolated heart. Reactive hyperaemic vasodilatation (peak vasodilator response and flow debt repayment) were assessed after global zero‐flow ischaemia (5 – 120 s) in the presence of nitro‐L‐arginine methyl ester (L‐NAME, 10−5 M, n=9), 8‐phenyltheophylline (8‐PT, 10−6 M, n=12) and indomethacin (10−5 M, n=12). Glibenclamide (10−6 M, n=12) a non‐selective KATP channel inhibitor and 5‐hydroxy‐decanoic acid (5‐HD, 10−4 M, n=10) a selective mitochondrial KATP channel inhibitor were also used. The specificity of the effects of glibenclamide and 5‐HD (n=6 each) were confirmed using pinacidil (38 nmol – 10 μmol) and diazoxide (42 nmol – 2 μmol). Glibenclamide was most effective in blocking the hyperaemic response (by 87%, P<0.001) although 5‐HD and 8‐PT also had a marked effect (40% inhibition, P<0.001 and 32%, P<0.001, respectively). L‐NAME and indomethacin had little effect. Perfusion with L‐NAME and glibenclamide significantly reduced baseline coronary flow (22%, P<0.01 and 33%, P<0.01) while 8‐PT, indomethacin and 5‐HD had no effect. KATP channels are the major mediators of the coronary reactive hyperaemic response in the guinea‐pig. Although mitochondrial KATP channels contribute, they appear less important than sarcolemmal channels.

Effects of the PAF antagonists BN50726 and BN50739 on arrhythmogenesis and extent of necrosis during myocardial ischaemia/reperfusion in rabbits

1 The effects of two novel platelet activating factor (PAF) antagonists BN50726 and BN50739 on arrhythmias, haemodynamics and extent of necrosis during myocardial ischaemia and reperfusion were investigated in anaesthetized rabbits subjected to coronary artery ligation. 2 BN50739 reduced heart rate prior to coronary artery occlusion (P < 0.005) but had no other significant haemodynamic effects at this time. BN50739 and BN50726 did not significantly alter heart rate or blood pressure during 30 min of ischaemia or 30 min of reperfusion, compared to control hearts. 3 BN50739 and BN50726 had no effect on the incidence of arrhythmias during ischaemia. BN50726 significantly reduced the incidence of reperfusion ventricular fibrillation compared to controls (0% v 40%, P < 0.05), and improved survival (80% v 39%, P < 0.05). Similar trends were observed with BN50739. 4 BN50726 reduced the extent of necrosis compared to control hearts (18 ± 2% v 30 ± 3%, P <0.01). A similar trend was observed with BN50739. 5 These results demonstrate that PAF antagonism with BN50726 attenuates reperfusion‐induced arrhythmias and preserves myocardium in the early phase of ischaemia, independently of haemodynamic effects.

Platelet activation during myocardial ischaemia: a contributory arrhythmogenic mechanism.

Experimental and clinical observations of the involvement of platelets in the pathophysiology of myocardial ischaemia indicate the importance of interactions between these formed elements and the heart. The aim of this review is to outline evidence linking platelet activation, myocardial ischaemia and infarction, and to present evidence for a link between platelet activation, arrhythmogenesis and sudden death. A brief review of platelet physiology and pharmacology is provided, with a review of the cardiac electrophysiological effects of ischaemia and the electrophysiological effects of platelet-derived substances. The concept that platelet activation during myocardial ischaemia is a contributory arrhythmogenic mechanism is discussed.

Electrophysiological and antiarrhythmic effects of UK 52,046–27 during ischaemia and reperfusion in the guinea-pig heart

1 We studied the antiarrhythmic and electrophysiological effects of UK 52,046–27 (10−8 m and 5 × 10−8 m), a highly selective α1‐adrenoceptor antagonist, during global ischaemia (flow reduced to 10% of control for 30 min) and reperfusion, in isolated, buffer‐perfused hearts of guinea‐pigs. 2 The compound had few electrophysiological effects during normal perfusion, although action potential amplitude and were reduced with 10−8 m (by 9% and 22%) and refractory period was increased with 5 × 10−8 m (by 13%) compared to control hearts. 3 Perfusion with 5 × 10−8 m UK 52,046–27 reduced the incidence of ventricular tachycardia during ischaemia from 67% to 25%, and during reperfusion reduced the incidence of ventricular tachycardia (from 83% to 17%) and ventricular fibrillation (from 67% to 8%). 4 The compound prolonged significantly action potential duration and refractory period during ischaemia and reperfusion. was reduced to a greater extent during reperfusion in the treated hearts, while greater increases in QRS width and stimulation threshold occurred during ischaemia in the treated group. 5 These results confirm that blockade of the α1‐adrenoceptor subpopulation during myocardial ischaemia and reperfusion decreases the incidence of arrhythmias and alters cellular electrophysiology during ischaemia and reperfusion.

Cardiac Electrophysiological Effects of Platelet‐Derived Substances

The ability of the heart to function as an efficient pump is critically dependent on an adequate blood supply to the myocardium. When myocardial blood supply is reduced (ischaemia), contractile function of the heart is impaired and its cellular electrophysiological properties are profoundly altered. Prolonged ischaemia may result in cell necrosis which, if irreversible, leads to cell death (infarction) and predisposes the heart to ventricular arrhythmias, which in turn may be fatal. In man, myocardial ischaemia is the consequence of narrowing or occlusion of a coronary artery and is usually a manifestation of coronary artery disease which is often associated with thrombosis.

Coronary Vasomotor and Cardiac Electrophysiologic Effects of Diadenosine Polyphosphates and Nonhydrolyzable Analogs in the Guinea Pig

Platelet activation in heart disease is important owing to the effects of platelet-derived compounds on myocardial perfusion and cardiac electrophysiology. Diadenosine polyphosphates are secreted from platelets and present in the myocardium, but their electrophysiologic and vasomotor effects are incompletely understood. We used isolated guinea-pig hearts to study the effects of diadenosine triphosphate (Ap3A), tetraphosphate (Ap4A), pentaphosphate (Ap5A), and hexaphosphate (Ap6A) (10 p M–0.1 m M), comparing their actions to those of adenosine, adenosine triphosphate, and nonhydrolyzable Ap4A and Ap5A analogs. Diadenosine polyphosphates (0.1 n M–0.1 &mgr;M) transiently reduced coronary perfusion pressure, which recovered during the continued presence of the compounds. At concentrations greater than 0.1 &mgr;M effects were maximal and sustained (perfusion pressure decreased from 36.5 ± 3.4 to 18.6 ± 2.5 mm Hg, p < 0.001, with 1 &mgr;M Ap4A). The changes in action potential duration and refractory period developed slowly but were maintained (0.1 n M–1 &mgr;M). With 1 n M Ap4A, action potential duration increased from 170.6 ± 2.6 to 187.3 ± 3.8 ms, p < 0.05, and refractory period increased from 138.5 ± 1.6 to 147.9 ± 2.0 ms, p < 0.05. Ap4A and its analog reduced QRS duration (from 24.7 ± 1.1 to 13.9 ± 1.6 ms with 1 &mgr;M Ap4A, p < 0.05). P2-purinergic (adenosine triphosphate) receptor antagonism (suramin) reduced perfusion pressure but was without electrophysiologic effect. Other changes in coronary perfusion pressure and electrophysiologic variables associated with Ap4A were not seen in the presence of suramin. P1-(adenosine) antagonism (8-[p-sulfophenyl]theophylline) attenuated the electrophysiologic effects only. Diadenosine polyphosphates have potent cardiac electrophysiologic and coronary vasomotor effects via purinergic receptors, suggesting an important role during platelet activation in acute coronary syndromes.

Effects of cicletanine on haemodynamics, arrhythmias and extent of necrosis during coronary ligation in rabbits

1 The effects of cicletanine on arrhythmias, haemodynamics and extent of necrosis during myocardial ischaemia were investigated in rabbits subjected to coronary ligation. 2 Cicletanine increased cardiac output prior to coronary occlusion (P < 0.01) but had no other significant haemodynamic effects at this time and did not significantly alter heart rate, blood pressure or cardiac output during 30 min of ischaemia or 30 min of reperfusion. 3 Ventricular fibrillation and mortality were greater in control (65% and 60% respectively) than treated animals (15.4% and 15.4%, P < 0.01). 4 The extent of myocardial necrosis expressed as a percentage of the area at risk was also reduced by cicletanine from 61 ± 8% in controls to 37 ± 6% (P < 0.05). 5 These findings indicate that cicletanine attenuates arrhythmias and preserves myocardium in the early phase of ischaemia and this effect appears to be independent of an established antihypertensive action.