Left ventricular wall stress distribution in chronic pressure and volume overload: effect of normal and depressed contractility on regional stress-velocity relations

Summary Regional stress-velocity relations were determined in a first group of patients (n=15) with normal (five controls, five patients with aortic stenosis, and five patients with aortic insufficiency) and a second group of patients (n=10) with depressed contractility (five patients with aortic stenosis and five with aortic insufficiency). LV circumferential wall stress was calculated from high-fidelity pressure and frame-by-frame angiocardiographic data using the Wong thick-wall model. Regional wall stress and shortening velocity were calculated from the endo-to the epicardium, and from the equator to the apex at 35 points.Regional LV wall stress was in all patients lower at the epi- than the endocardium, and lower at the apex than the equator. Regional stress-velocity relations were downward shifted from the endo- to the epicardium and from the equator to the apex (family of curves) in both groups. At corresponding LV regions stress-velocity relations showed significantly smaller slopes and intercepts (downward depression) in group 2 than in group 1.Thus, wall stress distribution is inhomogeneous in the normal, as well as in the pressure and volume overloaded left ventricle. Regional differences in stress-velocity relations within groups (family of curves) are probably related to changes in preload rather than to changes in regional contractility. Downward depression of the regional stress-velocity relations in group 2 is caused by depressed myocardial contractility.Supported by the Swiss National Science Foundation     

Cell death in ischemic,reperfused porcine hearts: A histochemical and functional study

Summary The temporal development of infarcts was histochemically and functionally determined in porcine hearts. In one series of experiments (22 pigs), the distal third of the left anterior descending coronary artery (LAD) was transiently occluded for periods between 20 and 90 min and was reperfused for another 24h. At the end of the experiments, the infarcted myocardium of four tissue slices was determined with a tetrazolium stain and related to the risk region which was delineated by a fluorescent dye. Infarcts started to develop in the ischemic septum and the subendocardial layer of the free anterior wall between 20 and 35 min of ischemia. Thereafter, infarctions progressed rapidly from the inner towards the outer layer at risk. The jeopardized anterior left ventricular wall became almost completely infarcted within 60 min of ischemia. In a second series of experiments (10 pigs) recovery of systolic shortening was studied with implanted ultrasonic crystals over 3 weeks of reperfusion. At the end of the experiments, systolic shortening was about 75% of baseline level when ischemia had lasted between 20 and 35 min. Almost no recovery was observed when the occlusion time lasted 45 to 60 min. This study suggests that the assessment of myocardial infarction with a tetrazolium stain after 24 h of reperfusion corresponds very well with functional recovery after 3 weeks of reperfusion. Furthermore, determination of regional myocardial function of the ischemic, reperfused segment in the chronic stage may be considered an additional tool to evaluate therapeutic effects on infarct size in this model.The study was supported by a grant of Deutsche Forschungsgemeinschaft (DFG) Sonderforschungsbereich 330 Organprotektion Göttingen.This paper contains parts of the Habilitationsschrift of Dr. H. H. Klein.     

Atrial and ventricular myosins from human hearts II. Isoenzyme distribution after myocardial infarction

Summary Human atrial and ventricular myosins were prepared from autopsy specimens from subjects with coronary heart disease. Cardiac myosin light chain isotypes were resolved using twodimensional gel electrophoresis, whereas myosin isozymes were detected by pyrophosphate gel electrophoresis.Myocardial infarction and associated work overload cause a transition in the light chain complements of the myosins. Thus ventricular myosin light chains were found in pressure overloaded atria and atrial light chains have also been identified in the infarct ventricle of the human heart.Two molecular isoenzymes of the human atrial myosin, the relative proportions of which are changed after infarction, were separated under non-dissociating conditions by gel electrophoresis. A decrease in HA-3 and a corresponding increase in HA-1 were observed. Ventricular hypertrophy in patients with coronary insufficiency induces a second ventricle isomyosin, called HV-1, with the same electrophoretic mobility as HA-1. The relative part of this myosin type amounts to 20%. Comparative peptide mapping studies were carried out on myosin subfragment-1 preparations from normal and infarct ventricles. In the primary structures, the chymotrypsic digestions produced slight differences.These data demonstrate the heterogeneity of human atrial and ventricular myosins in patients with coronary heart disease.     

Thyroxine induces transition of red towards white muscle in cultured heart cells

Summary Thyroid hormones (TH) have previously been shown to alter the force and velocity of cardiac muscle contractions. To investigate the mechanism responsible for these alterations, excess amounts of thyroxine (T₄, 1M) were applied on rat heart cells grown in cell culture. We found the following biochemical alterations: a) 40% decrease in the myoglobin content within 2 days; b) 25% increase in the rate of Ca-uptake into sacroplasmic reticulum (SR) in myocytes following chemical skinning; and c) a two-fold increase in Na–K-ATPase activity measured by⁸⁶Rb-uptake. These changes support our hypothesis that TH induce the transition of slow-twitch (red) muscles towards the fast-twitch (white) muscle type. This may explain the changes in contractile activity known to occur under TH influence.     

Alterations in the myocardial capillary vasculature accompany tachycardia-induced cardiomyopathy

Summary Changes in capillary structure and distribution within the left ventricle (LV) occur with pressure and volume overload hypertrophy. These changes may cause an impairment in myocardial blood flow (MBF) and oxygen delivery resulting in myocyte injury and LV dysfunction. However, it is unknown whether changes in the capillary vasculature accompany the development of LV dysfunction in dilated cardiomyopathies. Accordingly, this study examined the relation between LV function, MBF and capillary architecture after the development of dilated cardiomyopathy in pigs produced by chronic supraventricular tachycardia (SVT). LV function was examined by echo-catheterization, and capillary morphometrics were computed using lectin histochemistry in two groups of pigs: sham controls (n=8); and after 3 weeks of pacing-induced SVT (n=8). Chronic SVT resulted in significantly incresed LV end-diastolic dimension and pressure with a 50% reduction in LV fractional shortening compared to CON (p<0.05). Although no significant change in capillary density (2180±164 vs 2402±147/mm², p=0.25) occurred in the SVT group compared to CON, a significant reduction in the volume fraction of capillaries (12.2±0.5 vs 9.9±0.7%, p<0.05) and increased capillary diffusion distance (8.4±0.5 vs 7.5±0.3 m, p<0.05) was observed. Frequency distribution analysis revealed a higher percentage of smaller diameter capillaries with chronic SVT vs CON (p<0.05). Ultrastructural examination revealed an increased capillary-myocyte distance with chronic SVT (0.95±0.08 vs 1.95±0.12 m, p<0.05). These changes were accompanied by ultrastructural evidence of significant subendocardial injury (p<0.05). MBF was measured using microspheres in five additional conscious pigs in each group. MBF was significantly reduced and coronary vascular resistance increased in the SVT group compared to CON (p<0.05). Chronic SVT caused significant remodeling of the capillary vasculature; these changes were associated with reduced MBF, myocyte injury, and LV dysfunction.     

Effects of ouabain and low-Na+ perfusion on rest-decay and post-rest recovery of cellular Ca content in ventricular muscle of guinea-pig heart

Summary Ca²⁺ shifts in isolated, perfused ventricular muscle of guinea-pig hearts were investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with isotopecontaining solutions. The content of⁴⁵Ca in stimulated preparations (rate 60/min) was 1.30 ±0.12 mmol/kg of wet weight (w.w.). 6 min rest resulted in the drop of this content to 0.37 ±0.05 mmol/kg w.w. despite continued perfusion with⁴⁵Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to fraction Ca₂ (15) and is labelled accordingly. Ouabain (1 M) increased the⁴⁵Ca content to 1.53±0.15 mmol/kg w.w. in the stimulated and to 1.12±0.23 mmol/kg w.w. in the rested muscle. The respective values after low (50 mM) sodium perfusion were 1.70±0.11 and 1.07±0.13 mmol/kg w.w. The differences between the stimulated and rested preparations (Ca₂ fraction) were 0.41 and 0.63 mmol/kg, respectively. In the control experiments the force of the first post-rest beat dropped to 20±5% of the force of steady-state beats. During oubain and low-sodium perfusion, the force of the first contraction increased markedly and its peak was larger than that of the few subsequent beats. It is concluded that Na–Ca exchange is the important factor in the rate-dependent control of Ca₂ fraction content and of contractile force.This study was supported by the grant CPBR, 11.6     

Effect of dopamine on regional myocardial function and oxygen consumption in experimental left ventricular hypertrophy

Summary We investigated the hypothesis that the capacity of left ventricular myocardium to respond to an inotropic challenge by dopamine would be diminished in left ventricular hypertrophy induced by plication of the aortic valve. Seven mongrel dogs (LVH group) aged 6–8 weeks and weighing 4–6 kg, were subjected to preliminary surgery in which the noncoronary sinus of Valsalva was plicated. Six months later these animals, as well as a control group of dogs, were subjected to acute experiments in which the effect of dopamine (7.5 and 15 g/kg/min) on regional and global myocardial function and oxygen consumption was studied. Myocardial segment length was measured with ultrasonic dimension transducers, and left ventricular and aortic blood pressures were recorded from catheter-tip transducers. Regional coronary blood flow was determined with radioactive microspheres, and regional oxygen saturation in small arteries and veins was measured using microspectrophotometry. Regional myocardial O₂ consumption was calculated from these parameters. Heart weights were significantly elevated in the LVH group, and a pressure gradient of about 25 mm Hg was observed across the aortic valve. In both groups, dopamine infusion produced a dose-dependent increase in heart rate, left ventricular pressure, and LV dP/dt_max. Prior to dopamine infusion, percent shortening per beat was greater in the LVH group (13.97 ± 1.2 %) than in the control group (9.49 ± 1.07 %). Although the maximum speed of segment shortening was elevated by dopamine in both groups, percent shortening was not elevated in the LVH group. Stimulation by the high dose of dopamine produced a threefold elevation in regional coronary blood flow in both groups. Oxygen extraction was unchanged; the proportion of small veins with low O₂ saturation was not elevated in LVH hearts, even during dopamine stimulation. Regional myocardial O₂ consumption was elevated by dopamine (15 g/kg/min) to about the same extent in both the control and LVH groups (19.1 ± 2.3 and 17.5 ± 2.3 ml O₂/min/100 g) respectively. It is concluded that, in dogs with six months of aortic stenosis, dopamine does not exhaust functional reserve and the relationship between O₂ supply and consumption is not significantly impaired.This study was supported in part by PHS research grant HL40320     

Significance of right ventricular filling for left ventricular enddiastolic pressure-volume relationship under acute hypoxia in the dog

Summary In 14 closed-chest dogs, the significance of right ventricular filling for left ventricular enddiastolic pressure-volume relationship was investigated under acute hypoxia by means of single plane cineventriculography and simultaneous intraventricular pressure recording.Both after 5 min asphyxia (respirator switched off) (n=5) and after 3 min hypoxia (ventilation with pure N₂) (n=9), there was a significant leftward shift (p<0.005) of the left ventricular enddiastolic pressure-volume curve as compared to the control curves under normoxia. To simulate the elevated filling of the right ventricle under acute hypoxia, rapid intraventricular infusion was applier under normoxic conditions to raise right ventricular enddiastolic pressure to the same values as that measured under hypoxia. The extent of the ensuing leftward shift of the left ventricular enddiastolic pressurevolume curve was on average 60% of the shift under hypoxia in both sets of experiments. Neither the slope of the relationship between volume stiffness and enddiastolic pressure, nor the relationship between tangent elastic modulus and left ventricular wall stress, was affected by hypoxia or asphyxia.Thus, the shift of the left ventricular enddiastolic pressure-volume curve in the early stage of hypoxia is predominantly due to the influence of increased right ventricular filling. Since the increased volume of the atria under acute hypoxia limits left ventricular distensibility additionally, the changes in left ventricular enddiastolic pressure-volume relationships, observed in the early stage of hypoxia are mainly, or even entircly, the result of interaction of the various heart compartments, and not a reflection of alterations in myocardial tissue elasticity.Preliminary results were presented at the symposium on Cardiac adaptation to hemodynamic overload, training and stress in Tübingen (1983)Supported by the Deutsche Forschungsgemeinschaft     

Effects of physical training on the myocardium of streptozotocin-induced diabetic rats

Summary Effects of endurance swimming training on myocardial contractility and left ventricular myosin isoenzymes were examined in diabetic rats. A diabetic condition was induced in 15-weck-old male Wistar rats, by intravenous injection of streptozotocin (50 mg/kg). Swimming training was carried out for five to six weeks (90 min/day, 6 days/week). In order to estimate myocardial contractility, the isometric developed tension of the isolated left ventricular papillary muscle was measured. Myosin isoenzymes were obtained by pyrophosphate gel electrophoresis. Fasting blood glucose of the trained group was significantly lower than that of the sedentary group (sedentary vs. trained=409.6±25.9 vs. 266.3±20.5 mg/dl, p<0.001). There was no significant difference in isometric developed tension (T) between the two groups, and the dT/dt_max of the trained group showed a tendency to increase (sedentary vs. trained, T: 2.8±0.8 vs. 2.9±0.8 g/mm², dT/dt_max: 23.1±3.6 vs. 26.2±3.5 g/mm² · 2, p<0.1). Myocardial mechanical responses to isoproterenol and dibutyryl cAMP were increased in the trained group. Left ventricular myosin isoenzyme pattern was shifted towards VM-1 by endurance swimming (sedentary vs. trained, VM-1: 5.6±4.5 vs. 19.6±8.8%, p<0.001, VM-3: 75.1±10.0 vs. 54.9±14.7%, p<0.001). These results indicate that endurance swimming can improve disordered glucose metabolism and also influence myocardial contractility, myocardial catecholamine responsiveness, and energetics in myocardial contraction.     

Coronary thrombolysis with and without nifedipine in pigs

Summary to investigate whether addition of Ca²⁺ antagonists adds to the beneficial effects of thrombolysis we studied recovery of regional myocardial performance in pigs, in which occlusive thrombi were induced by electrical stimulation, with and without addition of nifedipine to the thrombolytic agent. To this end, four different groups of animals with thrombotic corobnary occlusion were studied. Groups 1 and 2 received either saline or intracoronary nifcdipine (0.1 g·kg^–1·min^–1) 15 min after coronary artery occlusion. Groups 3 and 4 were treated with the thrombolytic agent plasmin which was infused directly into the left anterior descending coronary artery (LADCA) at a rate of 2 U·min^–1. The animals in group 4 also intracoronary nifedipine. 4 h after thrombus formation the animals were sacrificed. No important differences in systemic hemodynamics were observed between the four groups of animals. Reperfusion occurred only in the animals which received plasmin, with or without nifedipine. After intracoronary plasmin regional blood flow increased from 7±2 to 40±7 ml·min^–1·100 g^–1 in the LADCA-perfused subepicardial and from 9±2 to 30±6 ml·min^–1·100 g^–1 in the LADCA-pefrfused subendocardial layers. The combination of plasmin and nifedipine increased flow to the LADCA-perfused subepicardial layers from 8±2 to 74±21 ml·min^–1.100 g^–1 and that to the subendocardial layers from 8±2 to 57±16 ml·min^–1·100 g^–1 (in both cases: p<0.05 vs. plasmin alone). However, addition of nifedipine did not enhance recovery of regional myocardial function or high-energy phosphate metabolism. Because reperfusion was accomischemic myocardium which does not result in functional recovery could be deleterious.This study was supported by a grant from the Dutch Heart Foundation.     

Lack of significant effects of superoxide dismutase and catalase on development of reperfusion arrhythmias

Summary It has been reported that agents having the ability to scavenge oxygen-derived free radicals reduce the severity of ventricular arrhythmias that occur after brief coronary occlusion and reperfusion. Superoxide dismutase plus catalase (SOD + CAT) or placebo was administered in a blinded randomized fashion prior to coronary occlusion in rats (n = 25 each group) undergoing a 5-min left coronary occlusion followed by 15 min of reperfusion. During reperfusion, ventricular tachycardia (VT) developed in 96 % of animals in both groups. Reperfusion ventricular fibrillation (VF) developed in 60 % of the placebo group vs 56 % in the SOD + CAT group (p =1.0). Irreversible VF occurred in 40 of the placebo group vs 20 % in the SOD + CAT group (p = 0.22). Atrioventricular block occurred in 12 % of placebo and 4 % of SOD + CAT animals (p = 0.61).There were no significant differences between groups in duration of VT (85 ± 15 s (mean ± SEM) placebo vs 81 ± 14 s SOD + CAT, p = 0.81), total duration of VT plus VF (391 ± 76 s placebo vs 256 ± 64 SOD + CAT, p = 0.45) or numbers of single ventricular ectopic beats (65 ± 15 placebo vs 97 ± 18 SOD + CAT, p = 0.18). Heart rate at reperfusion was slightly higher in control than SOD + CAT animals (340 ± 33 vs 319 ± 32, p = 0.02). Risk zone size, determined by Monastral blue injection, was equal in both groups (34 ± 2 % of ventricular mass). The occurrence of reperfusion VF in this model could not be predicted by heart rate at reperfusion (331 ± 33 VF animals vs 328 ± 36 no VF, p = 0.77), or by risk zone size (34 ± 2 %, VF and no VF groups).Treatment with SOD + CAT did not reduce the overall incidence or duration of reperfusion arrhythmias in this model, though such treatment was associated with a non-significant trend toward less irreversibility of VF.Supported in part by National Institutes of Health SCOR HL-26215, Bethesda, Maryland 660     

Regional myocadial blood flow and cardiac mechanics in dog hearts with CO2 laser-induced intramyocardial revascularization

Summary Laser-induced intramyocardial revascularization (LIR) has been used to promote direct communications between blood within the ventricular cavity and that of the existing myocardial vasculature in an attempt to increase perfusion in patients with ischemic heart discase. This study was conducted to measure the effects of LIR channels on regional myocardial flood flow (microspheres), cardiac mechanics (sonomicrometers), and myocardial tissue pressures in 18 dogs. Under baseline hemodynamic conditions (mean HR=165.2±11.4 bpm, LVP=123.6±22.9/4.0±1.8 mmHg, AoP=112.8±27.1/77.0±22.5 mmHg), myocardial blood flow in laser-treated tissue (mean =1.11±.10 cc/min/gm before laser; .71±.19 cc/min/gm after laser) was reduced as compared to blood flow in control tissue (mean=1.12±.15 cc/min/gm before laser; 1.25±.22 cc/min/gm after laser). Regional myocardial systolic shortening (11.32%±3.82% before laser; 7.49%±2.86% after laser) was decreased by 33%. During simultaneous reversible ligation of the LAD and LCCA for 2 min, when intramyocardial channels represented the only tissue access for the injected microspheres, blood flow in laser-treated tissue was not increased above that of the control non-lasered tissue. However, regional blood flow was greater in laser-treated ischemic tissue (mean=.61±.12 cc/min/gm) than in untreated ischemic areas (mean=.04±.03 cc/min/gm) when left ventricular pressure (LVP) was acutely elevated (mean SLVP=207.0±16.1 mmHg). Using these measurements, a model is proposed to predict regional systolic pressure gradients between the left ventricular cavity and coronary intramyocardial vasculature required to permit restoration of blood flow to ischemic myocardium. We conclude that improved perfusion via laser-induced intramyocardial channels does not occur in otherwise normal myocardium exposed to acute coronary ligation and only small improvements in perfusion are noted when LVP is significantly elevated. Consideration of further clinical application of this approach is seriously cautioned awaiting additional experimental studies.This study was supported by U.S. Public Health Service Grant R01 HL32897-01 from the National Heart, Lung, and Blood Institute, by grants in aid from the American Heart Association, and by the Fulbright-Hays Scholarship Grant.     

Relationship of myocardial morphometry in aortic valve regurgitation to myocardial function and post-operative results

Summary In 24 patients with aortic insufficiency undergoing aortic valve replacement, a clinical and hemodynamic study was performed pre-operatively. Left ventricular biopsies were obtained perioperatively for morphometric study.No significant relations were found when morphometric data were compared to functional class, cardiothoracic radio and ECG findings.The percentage of interstitial fibrosis was not correlated with any of the measured hemodynamic parameters. Myocardial cell diameter was weakly correlated with left ventricular systolic function parameters. A decrease in the percentage of contractile material was strongly correlated with an impaired left ventricular function, assessed pre-operatively. During clinical follow-up, patients were divided into two groups: Group A (17 patients) included patients who were in class I or II of NYHA after surgery. Group B (seven patients) included patients who died or were in functional class III or IV. As compared with Group A, Group B patients had a significantly lower ejection fraction; their myocardial cell diameter was larger and the percentage of myofibrils, and the content of contractile material were significantly lower. This suggests that, in aortic regurgitation left ventricular dysfunction is correlated with contractile material loss and not with interstitial fibrosis, and that morphometric changes are good predictors of follow-up after surgery.     

Functional significance of ventricular dilatation

Summary On the basis of theoretical considerations and experimental data this study deals with the functional consequences of structural dilatation, particularly in view of Linzbach's concept of chronic heart failure (34–38). After a short review of the literature, a theoretical analysis of the relationship between stroke volume and ventricular inner radius is presented assuming a thick-walled sphere. Presupposing constant contractility, end-diastolic sarcomere length, end-diastolic wall thickness and end-systolic pressure, only a considerable increase of ventricular radius could be the direct cause of ventricular pumping failure — despite increasing wall stress and reduced ejection fraction. Impaired contractility, as well as insufficient hypertrophy and increased systemic pressure, would intensify the adverse consequences of ventricular enlargement to a predictable extent. Thus, hemodynamic and energetic consequences of dilatation, although mutually interacting, should in principle be distinguished. Despite considerable simplifications involved in model calculations, the relative significance of contractility, ventricular size, wall thickness, and extracardiac factors (mechanical overload; neuroendocrine reactions) can be estimated in various animal models with congestive failure. Hence, this theoretical and experimental approach permits the modification and deepening of previous concepts of structural dilatation and also has implications for interpreting the effects of therapeutical interventions.Supported by the Deutsche Forschungsgemeinschaft (Ja 172/14-1)Dedicated to Dr. Erwin Riesch, Honorary Senator of the University of Tübingen, on the occasion of his 80th birthday     

The relationship between regional blood flow and contractile function in normal,ischemic, and reperfused myocardium

Summary The prevailing paradigm of coronary physiology and pathophysiology is that a balance between blood flow (i.e., supply) and function (i.e., demand) exists under normal conditions and that an imbalance between supply and demand occurs during ischemia. However, this paradigm is derived largely from studies relating changes in total coronary inflow to global ventricular function. The present article examines the relationship between myocardial blood flow and function on a regional level and proposes that a change may be needed in the current paradigm of coronary pathophysiology. In normal myocardium, considerable heterogeneity of regional blood flow exists, indicating either similar heterogeneity of metabolic demand and function or questioning the precision of metabolic coupling between flow and function. After the onset of ischemia, a transient imbalance between the reduced blood flow and function may exist. However, myocardial function rapidly declines and during early steady-state ischemia regional myocardial blood flow and function are once again evenly matched. Such supply-demand balance may persist over prolonged periods of ischemia enabling the myocardium to remain viable through reduction of energy expenditure for contractile function, i.e., to hibernate. Whereas in hibernating ischemic myocardium, regional myocardial blood flow and function are both reduced but appropriately matched to one another, flow and function appear to be largely uncoupled in reperfused stunned myocardium. The clinical identification of viable but ischemic (hibernating) and postischemic (stunned) myocardium is of utmost importance in patients undergoing reperfusion procedures. A new paradigm of coronary and myocardial pathophysiology, encompassing a regional as well as a global view of perfusion and function, will have to include explanations for phenomena such as myocardial hibernation and myocardial stunning.     

Voltage dependence of force- and slow inward current restitution in ventricular muscle

Summary This study was aimed to assess the relationship among the voltage-dependent processes underlying the excitation-contraction coupling, viz. force restitution (FR), transmembrane Ca fluxes and Ca release. The experiments (n=22) were performed on voltage-clamped dog trabeculae in which force and slow inward current were measured. Standard steady-state was achieved by clamp driving at 0.5 Hz, 300 ms, 70 mV depolarizing pulses from holding=resting potential at 30°C. Voltage and duration of single pulses and intervals in between were varied according to five protocols.The voltage dependence of Ca release was tested by varying single pulses at equal steady-state, i.e., at equal Ca availability. Contractions could be elicited in absence of I_Ca (20–30 mV step) and in the presence of disproportionately small I_Ca (above 80 mV).The voltage dependence of Ca availability for the release was tested by constant test pulses following either a variable conditioning clamp pulse or a period of rest at a variable voltage. After a low voltage pulse and, hence, depressed or absent I_Ca, the test contraction is diminished in presence of normal or even augmented I_si at any test interval (i.e., FR is depressed). Diminished Ca influx thus reduces the Ca availability of the subsequent beat. During prolonged depolarization (by 60 mV and more) a tonic response appears, but a phasic response cannot be elicited (FR is inhibited). Upon subsequent repolarization FR starts from zero and is significantly enhanced.It is concluded that, during depolarization, Ca release channels are in an open state, thus allowing free recirculation of Ca, but no build-up of a sufficient Ca gradient at the release site.     

Calcium metabolism and depressed contractility in isolated human and porcine heart muscle

Summary Contractility is often depressed in isolated heart muscle. To analyze this phenomenon, we measured the derivative of left ventricular pressure (dP/dt) in intact and in isolated, blood perfused pig hearts, and peak force (F) or stress (F/mm²) in ventricular trabeculae of man and pig. When the heart was in the steady state at a priming frequency of 2 Hz an extrasystolic interval of 0.3 s was interposed, followed by four postextrasystolic intervals of 0.8 s. In the case of isolated trabeculae the priming frequency was 0.2 Hz, the extra interval 0.4 s, and the post-extrasystolic intervals were 5 s. The exponential decay of potentiation is characterized by the constant D: a low value of D indicates a rapid decay of potentiation. DP/dt was about 1000 mm Hg/s in the intact hearts, but within 1 h after isolation dP/dt decreased to about 700 mm Hg/s, and this was associated with a decrease in D from 0.63 to 0.40. Developed stress in the isolated trabeculac was about 2 mN/mm² and D was about 0.20 under standard, in vitro conditions (a.o. 1.5 mM Ca²⁺, 0.2 Hz stimulus frequency). This stress is only 10% of the calculated stress in the intact heart. An increase of priming frequency, or of [Ca²⁺], or addition of 30 nM isoproterenol to the perfusate caused a marked increase in F and D. Properties of human and porcine trabeculae were quantitatively similar. The strong correlation between dP/dt, or F, and D suggests a causal relationship. This is consistent with the current model of e-c coupling in heart muscle, in which the activity of the Ca²⁺ pump of the sarcoplasmic reticulum determines the decay of potentiation and the amount of releasable Ca²⁺ in the reticulum determines force of contraction. Since isoproterenol stimulates the Ca²⁺ pump in the reticulum, the increase in D and F induced by this drug is consistent with the model. We conclude, that the decreased dP/dt, F, and D in isolated preparations was due to impaired sarcoplasmic reticulum function. The role of this phenomenon in the stunned heart syndrome, species differences and possible causes are discussed.     

Superoxide dismutase decreases early reperfusion release of conjugated dienes following regional canine ischemia

Summary Oxygen radical-induced myocardial lipid peroxidation may cause injury during regional ischemia and reperfusion. However, in vivo detection of lipid peroxidation is difficult. Since conjugated dienes are lipid peroxidation products of unsaturated fatty acids, we evaluated the potential value of detection of these double-bonded fatty acids as a marker of oxygen radical injury. In seven untreated and five superoxide dismutase-treated anesthetized dogs exposed to 90 min of coronary occlusion and subsequent reperfusion, coronary sinus plasma draining the ischemic and reperfused region was assayed for dienes. Lipids were extracted and diene optical density measured at 233 nm wavelength. Superoxide dismutase (5 mg/kg, total dose) was infused into the left atrium during ischemia and the first 30 min of reperfusion. Coronary sinus diene optical density increased in untreated animals at 5 and 10 min of reperfusion (reperfusion optical density (x±SEM): 5 min=1.49±0.20 absorbance units, 10 min=1.36±0.06; both p<0.05 vs preocclusion optical density=1.10±0.05 and 25 min reperfusion=1.20±0.07). No increase in diene optical density occurred in superoxide dismutase-treated dogs. Myocardial lipid peroxidation products, as conjugated dienes, increased in coronary sinus plasma during early reperfusion and this increase was prevented by superoxide dismutase infusion.     

Can coronary systolic-diastolic flow differences be predicted by left ventricular pressure or time-varying intramyocardial elastance?

Summary In six isolated rabbit hearts perfused with a pressure source and Krebs-Henseleit as the perfusion medium, the effect of left ventricular pressure on coronary inflow in the maximally vasodilated bed was studied. This effect was determined from isovolumic beats, low afterloaded isobaric beats (afterload maintained at values below 10 mm Hg) and during cardiac arrest. For isovolumic beats end-diastolic left ventricular pressure was varied by means of an intraventricular balloon between 0–40 mm Hg and systolic left ventricular pressure varied between 90–130 mm Hg. In these ranges diastolic inflow decreased significantly 18±6% (mean±SD) with increasing pressure and systolic inflow could not be shown to depend on pressure (n=6). For isobaric beats, diastolic and systolic inflow remained at values similar to those found for the isovolumic beats (n=6). In the arrested heart inflow diminished 8±2% when the pressure in the left ventricle was increased from 0 to 40 mm Hg (n=3). We conclude that systolic coronary inflow is hardly affected by left ventricular pressure. Systolic inflow decreased by the same amount in the isovolumically and isobarically beating heart, when cardiac contractility was enhanced by epinephrine infusion. We suggest the results can be explained on the basis of the time-varying elastance concept: systolic elastance is the same for isovolumic and isobaric beats but depends on contractility. Models that relate coronary inflow impediment to left ventricular pressure should therefore be reevaluated.This investigation is supported by the Netherlands Organisation for the Advancement of Pure Research (NWO-grant: FHC-5 810-906-005).     

Effect of oxygen-derived free radical scavengers on infarct size following six hours of permanent coronary artery occlusion: salvage or delay of myocyte necrosis?

Summary Oxygen-derived free radicals (O ₂ ^– and ·OH) have been implicated in myocardial injury associated with coronary artery occlusion followed by reperfusion. While these cytotoxic oxygen species are predominantly produced upon reintroduction of molecular oxygen to previously ischemic tissue, they may also be generatedthroughout coronary occlusion in species (such, as dog and man) in which native collateral vessels permit residual blood flow into the ischemic bed. To test this theory, 20 anesthetized, open-chest dogs underwent 6 h of permanent left anterior descending coronary artery occlusion: ten dogs were treated with the potent free radical scavenging enzymes superoxide dismutase (SOD: 5 mg/kg per hour) plus catalase (5 mg/kg per hour), while the remaining ten animals received saline. Infusion of drug or saline solution was begun 15 min prior to occlusion, and maintained throughout occlusion. Infusion of SOD+catalase did not significantly affect the extent of the area at risk of infarction (19.5±1.8% vs 24.0±1.4% of the left ventricle for the treated vs control group;P=NS), did not reduce myocardial oxygen demand (heart rate and arterial pressures were comparable for both groups), and did not alter collateral blood flow to the ischemic myocardium. However, mean infarct size in dogs treated with SOD+catalase (39.6±6.6% of the area at risk; 8.4±2.1% of the left ventricle) was significantly smaller than that observed in the saline controls (73.0±6.3% of the area at risk,P<0.01; 19.8±2.2% of the left ventricle,P<0.01). Thus, infusion of SOD+catalase prior to and during 6 h of coronary occlusion significantly reduced infarct size assessed at 6 h postocclusion. To determine whether this reduction in infarct size represented long-term salvage of ischemic myocardium, an additional 14 dogs (seven treated and seven controls) underwent the same procedure as described above; in this case the artery was reperfused after the 6-hour occlusion period, infusion of the SOD+catalase or saline solution stopped at 5–10 min postreperfusion, and the hearts examined at 30–48 h postocclusion. In contrast to the results at 6 h postocclusion, the necrosis at 30–48 h postocclusion was large, confluent and transmural in all dogs (infarct size=21.2±2.5% vs 22.7±4.4% of the left ventricle for treated vs controls;P=NS). These results suggest that infusion of SOD+catalase in this model may delay, but not prevent, the development of ischemic necrosis.Presented in part at the X World Congress of Cardiology, Washington, DC, September 1986.Performed during Dr. Kloner's tenure as an Established Investigator of the American Heart Association.