Effects of the ACE inhibitor, perindoprilat, and of angiotensin II on the transient inward current of guinea pig ventricular myocytes

Hypothetically, certain ischemic and reperfusion arrhythmias may result from the activity of the calcium-dependent transient inward current. The effects of the angiotensin converting enzyme inhibitor, perindoprilat, on the transient inward current of guinea pig ventricular myocytes were studied. The transient inward current was evoked by superfusing the cell with a modified Tyrode's solution (5.4 mM CaCl2 and 0.54 mM KCl). Repetitive voltage clamp steps from a holding potential of -55 to +20 mV (1,000 ms, 0.1 Hz) were applied while dialyzing the cell internally. When administered simultaneously with the change over to the low K+ high Ca2+ solution, perindoprilat (1 microM) decreased the transient inward current from -9.55 +/- 0.31 to -3.24 +/- 0.24 microA/cm2 (p less than 0.05). A similar decrease was observed when perindoprilat was administered after first inducing the transient inward current. Perindoprilat also protected from the effects of norepinephrine (0.01 and 0.1 microM), which increased the amplitude of the transient inward current from -9.76 +/- 0.17 and -9.99 +/- 0.32 microA/cm2 at the end of the 15-min control period to -11.13 +/- 0.67 and -12.67 +/- 0.49 microA/cm2, respectively (p less than 0.05). The effects of perindoprilat were independent of angiotensin II, which in this preparation decreased the transient inward current. Based on our results, we conclude that perindoprilat decreases the transient inward current and prevents the action of norepinephrine on the transient inward current. The direct effect of the angiotensin converting enzyme inhibitor observed on the transient inward current might explain why angiotensin converting enzyme inhibitors reduce calcium-dependent ouabain-induced or reperfusion arrhythmias.

Postischemic stunning--the case for calcium as the ultimate culprit

Two phases of the stunning phenomenon are proposed. The first causative phase occurs almost immediately with reperfusion and is thought to be associated with cytosolic calcium overload and an apparently normal or nearly normal mechanical function. Agents enhancing calcium influx, if introduced at this stage, may worsen subsequent stunning, whereas those inhibiting calcium influx may lessen the extent of subsequent stunning. The second phase, true stunning, is associated with established hypocontractility and responds favorably to agents enhancing calcium influx, whereas calcium antagonists further impair mechanical function when given at this stage. These patterns, derived from data obtained on isolated rat-heart studies, cannot directly be extrapolated to the large animal heart, such as that of the dog, where the presence of added circulating leukocytes may confound the issue and explain the apparently contradictory benefit of the late administration of calcium antagonists. The harmful effects of free radicals are not discounted but could be explained, at least in part, by multiple membrane damage, with a consequent rise of cytosolic calcium during the reperfusion period.

Effect of pH shifts induced by oxygenating crystalloid cardioplegic solutions

Oxygenation of a bicarbonate-containing crystalloid cardioplegic solution alters the partial pressure of both oxygen (O2) and carbon dioxide (CO2). Therefore, oxygenating St. Thomas' Hospital II plus glucose (11 mmol/L) cardioplegic solution with 95% O2 + 5% CO2 induces a pH shift to 7.0 (10 degrees C) as opposed to pH 9.3 with 100% O2. In an isolated working rat heart model, we show that pH 7.0 (10 degrees C) improves mechanical postischemic recovery in the absence or presence of O2. However, in the absence of O2, pH 7.0 appears to inhibit glycolysis and diminish the stability of cellular membranes. The provision of O2 independently improved mechanical recovery and at pH 7.0, improved the preservation of the sarcolemma. Increasing the O2 content by including a perfluorocarbon (FC-43) in the oxygenated St. Thomas' plus glucose cardioplegia is not additionally beneficial. St. Thomas' Hospital plus glucose cardioplegic solution should be oxygenated, but with 95% O2 + 5% CO2 and not 100% O2.

Captopril, nifedipine and their combination for therapy of hypertensive urgencies

Twenty patients with acute severe hypertension were randomised to therapy with either nifedipine capsules (10 mg) or captopril tablets (25 mg) given sublingually and the blood pressure recorded for 240 minutes. Oral monotherapy with either agent followed for 3 weeks, then the agents were combined for a further 2 weeks and in the final 6 weeks of the trial a beta-blocker and diuretic were added, if needed. Thirteen patients completed the trial. The major results were: (i) nifedipine decreased blood pressure more rapidly than captopril 60 minutes after first ingestion but at 240 minutes equal degrees of fall in blood pressure had been obtained; (ii) neither agent given as sustained monotherapy was able to reduce blood pressure adequately, although nifedipine was better than captopril; and (iii) combination therapy with both agents was conspicuously successful in achieving reduction in blood pressure. It is suggested that combination nifedipine-captopril therapy be subject to a formal trial for early therapy in acute severe hypertension.

St. Thomas' Hospital cardioplegic solution. Beneficial effect of glucose and multidose reinfusions of cardioplegic solution

The intention of this study was to determine whether glucose is beneficial in a cardioplegic solution when the end products of metabolism produced during the ischemic period are intermittently removed. The experimental model used was the isolated working rat heart, with a 3-hour hypothermic 10 degrees C cardioplegic arrest period. Cardioplegic solutions tested were the St. Thomas' Hospital No. 2 and a modified Krebs-Henseleit cardioplegic solution. Glucose (11 mmol/L) was beneficial when multidose cardioplegia was administered every 30 minutes. Including glucose in Krebs-Henseleit cardioplegic solution improved postischemic recovery of aortic output from 57.0% +/- 1.8% to 65.8% +/- 2.2%; p less than 0.025. The addition of glucose to St. Thomas' Hospital No. 2 cardioplegic solution improved aortic output from 74.6% +/- 1.9% to 87.4% +/- 1.9%; p less than 0.005. Furthermore, a dose-response curve showed that a glucose concentration of 20 mmol/L gave no better recovery than 0 mmol/L, and glucose in St. Thomas Hospital No. 2 cardioplegic solution was beneficial only in the range of 7 to 11 mmol/L. In addition, we showed that multidose cardioplegia was beneficial independent of glucose. Multidose St. Thomas' Hospital No. 2 cardioplegia, as opposed to single-dose cardioplegia, improved aortic output recovery from 57.4% +/- 5.2% to 74.6% +/- 1.9%; p less than 0.025, and with St. Thomas' Hospital No. 2 cardioplegic solution plus glucose (11 mmol/L) aortic output recovery improved from 65.9% +/- 2.9% to 87.4% +/- 1.9%; p less than 0.005. Hence, at least in this screening model, the St. Thomas' Hospital cardioplegic solution should contain glucose in the range of 7 mmol/L to 11 mmol/L, provided multidose cardioplegia is given. We cautiously suggest extrapolation to the human heart, on the basis of supporting clinical arguments that appear general enough to apply to both rat and human metabolisms.

Effect of oxygenation and consequent pH changes on the efficacy of St. Thomas' Hospital cardioplegic solution

The hypothesis tested is that shifts in pH, induced when a cardioplegic solution is oxygenated, can be detrimental. We added either 100% nitrogen, 95% nitrogen and 5% carbon dioxide, 100% oxygen, or 95% oxygen and 5% carbon dioxide to the cardioplegic solution (St. Thomas' Hospital No. 2 plus glucose 11 mmol/L), and determined postischemic recovery of isolated rat hearts after 3 hours of 10 degrees C cardioplegic protected ischemia. Hearts were arrested and reinfused every 30 minutes throughout the ischemic period with cardioplegic solution. When 5% carbon dioxide was added to nitrogen, the pH of the cardioplegic solution decreased from 9.1 (100% nitrogen) to 7.0 (95% nitrogen: 5% carbon dioxide), a change associated with improved postischemic functional recovery. Aortic output improved from 52.3% +/- 2.7% to 63.9% +/- 2.8%, p less than 0.05, and cardiac output from 60.8% +/- 3.6% to 75.4% +/- 3.3%, p less than 0.01. This improvement was associated with diminished efflux of lactate during ischemia but increased postischemic release of lactate dehydrogenase. When nitrogen was replaced with oxygen, the addition of 5% carbon dioxide resulted in a similar decrease of pH, which again was associated with improved postischemic functional recovery. Aortic output improved from 66.3% +/- 2.8% (100% oxygen) to 88.9% +/- 3.7% (95% oxygen: 5% carbon dioxide), p less than 0.005, and cardiac output from 75.3% +/- 4.1% to 88.9% +/- 2.4%, p less than 0.01. The efflux of lactate during ischemia and the postischemic release of lactate dehydrogenase were similar in both groups. Furthermore, provision of additional oxygen with perfluorocarbons in an electrolyte solution identical to the St. Thomas' Hospital plus glucose solution and oxygenated with 95% oxygen: 5% carbon dioxide conferred no extra protection. In conclusion, the St. Thomas' Hospital No. 2 plus glucose cardioplegic solution should be oxygenated but with 95% oxygen: 5% carbon dioxide and not 100% oxygen because of the additive effect of a relatively "acidotic" pH.

Calcium antagonists post-infarction: the significance of experimental studies on potentially lethal early ischemic ventricular arrhythmias

The DAVIT-II trial has shown that the verapamil type of calcium antagonist can beneficially be used in post-infarct patients. A recent re-analysis suggests that verapamil may also prevent post-infarct sudden death. There are good theoretical reasons to suppose that calcium antagonists should help prevent ventricular fibrillation. If calcium antagonist agents could be found which have negligible negative inotropic effects, such drugs might be ideal for further testing in the post-infarct phase.

Prevention and treatment of ischemic injury with nucleosides

Using Langendorff rat hearts, we tested whether 1. adenosine as a cardioplegic agent, and 2. inosine administered during reperfusion could prevent and treat ischemic injury, respectively. For cardioplegic arrest (37 degrees C), buffer supplemented with 20 mM K+ (K), K + 1 mM adenosine (KA), or none (Control, C), was infused for 3 min at 3 ml/min. Arrest time was 260 +/- 16 s (C), 22 +/- 4 s (K) and 10 +/- 2 s (KA, p less than 0.02 vs K). During 20 min total ischemia, resting tension increased only in C, and remained elevated after 20 min reperfusion. In treated hearts resting tension rose somewhat and returned to baseline. Developed tension: heart rate (g/min) after reperfusion was superior with KA:C (3,180 +/- 830), K (4,380 +/- 390), and KA (6,250 +/- 740, p less than 0.05 vs. K.). Our electrophysiological studies suggest that adenosine increases K(+)-permeability and thereby arrests the sinus node. It did not affect high-energy phosphates. We also tested whether inosine could regenerate nucleotides. We perfused hearts with buffer containing glucose +/- pyruvate. After 15 min no-flow, hearts were reperfused for 45 min with 20 microM inosine and 0.5 mM ribose. Adenine nucleotide levels tended to recover better in the purine-treated groups. Inosine decrease the ATP/ADT ratio by 15% (p less than 0.05) and increased the IMP level 2 times (p less than 0.01) whom pyruvate was absent. It increased the effluent adenosine concentration 6 times (p less than 0.005). Inosine administration +/- pyruvate did not affect function recovery, heart rate or coronary flow. Thus adenosine as adjunct to K(+)-cardioplegia shortened arrest time, and was also beneficial for post-ischemic recovery. Inosine given during reperfusion failed to improve heart function. Both treatments hardly affected cardiac adenine nucleotide levels.

Role of calcium and other ions in reperfusion injury

Calcium-related mechanisms may play a role in several aspects of reperfusion injury. One proposal is that internal cytosolic calcium concentration is elevated early in the reperfusion period and that excess oscillations of calcium can very significantly contribute to reperfusion ventricular arrhythmias. Alternate hypotheses, such as those involving free radicals and the local tissue renin-angiotensin system, can be married to the existing hypothesis. Furthermore, the hypothesis allows for a role of the sodium-calcium exchange system and the proton-sodium exchanger. The hypothesis also provides an explanation for "stunning," as it is proposed that early excessive cytosolic calcium damages the organelles regulating the contractile cycle, which subsequently develops into imperfect functioning of the contractile apparatus. Calcium antagonist drugs given during the ischemic period may lessen reperfusion injury by decreasing the severity of ischemic damage. When given at the time of reperfusion, results are complex and to some extent conflicting. The biggest challenge is to understand how relatively low doses of calcium antagonists given after the onset of reperfusion help to decrease delayed reperfusion "stunning." A logical but untested proposal is that calcium antagonists help to prevent delayed contraction-band necrosis, one of the causes of delayed no-reflow.

Adenosine cardioplegia: reducing reperfusion injury of the ischaemic myocardium?

Hyperkalaemia-induced hypopolarization of the sarcolemnal membrane during standard crystalloid cardioplegic arrest potentiates calcium influx during reperfusion and is associated with depletion of high-energy phosphate reserves. Adenosine has been shown to induce fast cardiac arrest whilst preserving membrane hyperpolarization in an isolated rat heart model. In this study we compared the efficacy of adenosine, both as an arresting agent and as an ultrastructural, haemodynamic and high-energy phosphate preserving agent, in an in situ global ischemia model in the baboon with St. Thomas' Hospital solution No. 2 (ST2; n = 8) and with Krebs-Henseleit buffer (KHB; n = 7). The addition of 10 mM adenosine to the non-cardioplegic KHB (ADO; n = 8) improved haemodynamic recovery significantly in terms of cardiac index (91.6% +/- 7.2 vs 59.9% +/- 9.9) and stroke volume index (101.6% +/- 8.9 vs 55.6 +/- 10.0) and was not statistically distinguishable from the ST2 with regard to cardiac index (91.6% +/- 7.2 vs 94.8% +/- 5.8), stroke volume index (101.6% +/- 8.9 vs 114.0% +/- 8.3) or left ventricular dP/dt (73.1% +/- 9.9 vs 87.0% +/- 12.4). Adenosine triphosphate was best preserved with ADO (103.5% +/- 21.1 vs 67.9% +/- 9.3 and 48.5% +/- 8.7) although this was not statistically significant. This suggests therefore that the mechanism of cardioprotection by adenosine occurs by means other than its role as high-energy phosphate precursor.

Required beta blocker profile in the elderly

In the truly elderly, a complex balance between compensatory processes and impaired organ function allows reasonably normal physical function. It is argued that beta blockade should have certain desirable qualities to minimize any impairment of organ function, thereby upsetting the quality of life. Thus a simple pharmacokinetic pattern without hepatic metabolism is less likely to cause unexpected variation in blood levels of the beta blocking agents and to have fewer risks of interactions with other drugs including nicotine. Renal-excreted beta blockers--such as atenolol, nadolol, and celiprolol--do, however, need downward dose adjustment when the glomerular filtration rates fall. The elderly are frequently categorized as having a low renin profile, which in the view of some workers may make a vasodilatory beta blocker more desirable. Hemodynamic advantages of such agents include the prime site of attack in hypertension on the increased peripheral vascular resistance, increasingly fundamental with a prolonged duration of hypertension and therefore with the aging process. Furthermore, a normal heart rate with a sustained cardiac output may avoid symptomatic bradycardia. In the elderly, respiratory function may be impaired so that loss of elastic recoil causes elderly emphysema. A highly cardioselective beta blocker should be an advantage. Finally, minimal interference with glucose and lipid metabolism should also be desirable goals.

Compensation and overcompensation in congestive heart failure

The compensatory mechanisms that develop in response to heart failure have been well defined. In this review, it is argued that each compensatory mechanism leads to overcompensation and that there is no way to distinguish between the beneficial aspects of the former and the harmful effects of the latter. Therapeutic agents that maintain rather than decrease blood pressure might perhaps be more beneficial because of the crucial role of hypotension in initiating both compensation and overcompensation.

Adenosine as adjunct to potassium cardioplegia: effect on function, energy metabolism, and electrophysiology

Adenosine is known to induce rapid cardioplegic arrest and to improve postischemic recovery in the isolated rat heart. Long exposures to high doses of adenosine impair postischemic recovery, however. In this paper we tested the combination of low-dose adenosine (1 mmol/L) with potassium (26 mmol/L), with the aim of achieving rapid arrest (as with high-dose adenosine) but eliminating the need for postarrest washout of adenosine. Cardioplegic solutions studied were (1) Krebs-Henseleit potassium (26 mmol/L) (K); (2) K plus adenosine (1 mmol/L) (KA); (3) K plus an adenosine deaminase inhibitor [erythro-9-(2-hydroxy-3-nonyl)adenine] (0.1 mmol/L) (KE); and as control (4) Krebs-Henseleit potassium (6 mmol/L) (C). We induced cardiac arrest in Langendorff-perfused rat hearts by infusing the cardioplegic solution for 3 minutes at 3 ml/min. Total ischemia lasted 20 minutes at 37 degrees C, followed by reperfusion for 30 minutes. High potassium decreased the arrest time from 260 +/- 16 seconds (group C, mean values +/- standard error of the mean) to 22 +/- 4 seconds (group K). A further decrease to 10 +/- 2 seconds was observed with KA (p = 0.016 versus K). KE, which increased endogenous adenosine, gave intermediate effects. All hearts recovered during reperfusion; the product of developed tension and heart rate (grams per minute) was superior in KA hearts (6250 +/- 740 versus K hearts 4380 +/- 390; p = 0.050). KE gave an intermediate result (5290 +/- 900), while C showed the worst recovery (3180 +/- 830). Our electrophysiologic studies with sinus node and atrial tissue suggest that adenosine induced hyperpolarization and an increase in potassium permeability, thereby arresting the sinus node before depolarization of the membrane by potassium (26 mmol/L). We conclude that low-dose adenosine as an adjunct to potassium shortens the arrest time in this model and improves postischemic recovery.

Effects of the free radical generating system FeCl3/ADP on reperfusion arrhythmias of rat hearts and electrical activity of canine Purkinje fibres

STUDY OBJECTIVE

The aim was to evaluate the arrhythmogenic effect of a free radical generating system, FeCl3/ADP using two different approaches.

DESIGN

Ventricular arrhythmias were studied in isolated rat hearts subjected to regional ischaemia and reperfusion without or with simultaneous treatment with nicergoline (0.4 mg.litre-1). In the second part of this study the electrophysiological effects of FeCl3/ADP (0.1/1.0 microM) were investigated in normal Purkinje fibres and in Purkinje fibres from dog surviving infarction, by using conventional microelectrode method.

EXPERIMENTAL MATERIALS

Hearts were obtained from male Sprague-Dawley rats, weight 250-300 g. Purkinje fibres were dissected from hearts of mongrel dogs of either sex (10-15 kg) with or without prior myocardial infarction.

MEASUREMENTS AND RESULTS

FeCl3/ADP (0.1/1.0 microM and 1.0/1.0 microM respectively) weakly changed the incidence of reperfusion induced arrhythmias. In nicergoline pretreated hearts, in which the incidence of reperfusion arrhythmias was reduced, FeCl3/ADP (0.1/1.0 microM and 1.0/1.0 microM) did not change the incidence and the duration of reperfusion arrhythmias. In normal Purkinje fibres, FeCl3/ADP (0.1/1.0 microM) induced a decrease in action potential duration without any pronounced effect on Vmax, diastolic potential, and activation potential. In Purkinje fibres from post infarct myocardium, FeCl3/ADP decreased action potential duration, diastolic potential, and activation potential.

CONCLUSIONS

Free radical generation did not antagonise the antiarrhythmic activity of alpha adrenergic blockade. Free radical generation induced slow and minor changes in electrophysiological activity of Purkinje fibres both from normal and ischaemic hearts. Our data suggest that free radical generation may not be the only mechanism involved in the genesis of reperfusion arrhythmias.

Myocardial ischemia--metabolic pathways and implications of increased glycolysis

Evidence is reviewed that favors the hypothesis that maintenance of glycolysis plays a special role in protecting membrane function in ischemia. Therefore all procedures stimulating glycolytic flux should be beneficial in ischemia, and procedures inhibiting flux should be harmful. However, a crucial consideration is the coronary flow rate. In severe ischemia, accumulation of protons, derived not directly from glycolytic flux but from the breakdown of ATP and from proton-producing cycles, will tend to inhibit glycolysis and to minimize any benefit from increased glycolytic flux. Therefore maintenance of intracellular pH is crucial to the concept of the benefits of glycolysis. It also follows that the severity of ischemia can determine whether or not enhanced glycolysis has a beneficial effect. It is argued that a multiple approach, including enhanced glycolytic flux, control of intracellular pH, and improved coronary flow, constitutes the combination most likely to benefit ischemia.

Effects of increased heart work on release of norepinephrine and ventricular arrhythmias following reperfusion in the isolated rat heart

The influence of increasing left atrial pressures (0.5, 1.0, and 2.0 kPa) on the incidence of ventricular arrhythmias and the liberation of prelabeled norepinephrine (3H-NE) was investigated in the isolated working rat heart. Acute regional myocardial ischemia (30 min) was produced by ligature of the left main coronary artery with subsequent release of the ligature to achieve reperfusion, which consistently provoked ventricular arrhythmias. The magnitude of regional ischemia was measured by microspheres, and the efflux of 3H-labeled NE compounds was measured in the coronary effluent. Our data show that an increase in atrial pressure enhanced reperfusion arrhythmias, but the magnitude of NE release was not directly related to the occurrence of arrhythmias. It is proposed that increased heart work has an arrhythmogenic effect by enhancing the severity of regional ischemia.