Enhanced myocardial contractility but not tachycardia persists in isolated working hyperthyroid rat hearts

Summary It is generally believed that the increased contractility and tachycardia of the hyperthyroid heart are a result of thyroid hormone-induced alterations of the mechanical and electrical properties of the heart, respectively. We compared the contractility (dP/dt_max) and the spontaneous beating rate of hyperthyroid and euthyroid hearts perfused in vitro in either a non-working or a working mode. The dP/dt_max (4196±74 mm Hg s^–1) and beating rate (322±8 beats/min) of the non-working hyperthyroid hearts were significantly higher (p<0.001) than those of the euthyroid hearts (3267±115 mm Hg s^–1 and 260±6 beats/min at an external Ca²⁺ of 2.5 mM). At 2.5 mM Ca²⁺, the working hyperthyroid hearts again displayed enhanced contractility (5636±179 mm Hg s^–1 vs 4508±172 mm Hg s^–1; p<0.001) but the spontaneous beating rate (275±7 beats/min) was not significantly different from euthyroid (261±8 beats/min). When hearts were subjected to periods of alternate non-working and working perfusion, the beating rate of the hyperthyroid hearts was significantly higher than euthyroid during non-working (p<0.02) but not during working perfusion. Increasing the afterload on the non-working preparations in a stepwise fashion from 75 cm H₂O to 120 cm H₂O caused significant changes in left ventricular pressure and dP/dt_max in both heart types but the tachycardia in the hyperthyroid hearts persisted (at 120 cm H₂O; hyperthyroid, 294±9 beats/min; euthyroid, 224±10 beats/min; p<0.001). Alteration of the preload (10 to 25 cm H₂O) and afterload (75 to 105 cm H₂O) on working hyperthyroid and euthyroid hearts caused changes in both left ventricular pressure and dP/dt_max but the beating rates of both heart types were never significantly different. We conclude from our results that (i) the increased contractility of the hyperthyroid rat heart is due to thyroid hormone-induced alteration of the mechanical properties of the heart; (ii) the tachycardia of hyperthyroidism is not due to thyroid hormone-induced changes in the electrical properties of the heart, but probably involves some as yet unidentified chronotropic agent.     

Evaluation of parameters for the assessment of regional myocardial contractile function during asynchronous left ventricular contraction

Summary The primary purpose of this study was to evaluate parameters used for the measurement of regional myocardial contractile function in the setting of left ventricular (LV) asynchrony. Secondarily, we tested whether the peak negative value of left ventricular dP/dt (-dP/dt) can be used to estimate global LV end-systole during asynchrony. In seven anesthetized (Isoflurane) swine the left anterior descending coronary artery was cannulated and perfused at constant blood flow rates. To produce LV asynchrony, dobutamine (D) was infused into the perfusion system. This was repcated later during coronary hypoperfusion (HYPO) sufficient to produce regional contractile dysfunction. The amount of LV wall thickening during systole (% WT, sonomicrometry) was calculated using either - dP/dt or the closure of the aortic valve (AO, electromagnctic flow probc) for estimating the timing of global LV end-systole. % WT was compared to other paramcters which are not dependent upon the timing of global LV end-systole, including the amplitude of the first harmonic of the Fourier transform (AMP) and regional myocardial work (WI) estimated from the left ventricular pressure-wall thickness relationship. A close correlation between global LV end-systole defined by the AO or - dP/dt existed during control. D or HYPO. During HYPO+D no such relationship was found (r=.22, NS), and % WT calculated using - dP/dt as an estimate of end-systole was underestimated when comparcd to % WT calculated by use of the AO to estimate end-systole (2.9±6.8% vs 6.3±6.6%, p<.05). % WT, AMP, and WI showed similar results during control, D and HYPO. However, D during HYPO increased the AMP from .59±.23 mm to .76±.32 mm and WI from 67±20 mm Hg*mm to 95±24 mm Hg*mm (p<.05), respectively. This increase in regional myocardial function, however, was not detected by % WT (10.5±6.4% vs 6.3±6.6%). Thus, during left ventricular asynchrony, the measurement of LV-dP/dt to estimate the timing of global LV end-systole is inappropriate and can lcad to inaccuracies in the measurement of regional contractile function. Parameters such as AMP or WI are advantageous since global LV end-systole docs not need to be accurately defined.Supported in part by the American Heart Association California Affiliate grant-in-aid #86-S105, and by the German Research Foundation (He 1320/3-2). Dr. Guth is the recipient of a Research Fellowship from the Alexander von Humboldt-Stiftung, Jean-Paul-Straße 12, D-5300 Bonn 2     

The effect of calcium concentration on spontaneous ventricular defibrillation and VF threshold

Summary In order to investigate the influence of the effective refractory period on spontaneous ventricular defibrillation, isolated rat hearts were perfused with Krebs-Henseleit solution containing 0.5, 2.7 and 5.1 mM calcium. After measuring the fibrillation threshold at spontaneous rate (SR), ventricular fibrillation (VF) was induced during basic ventricular pacing of 110% SR, or the highest rate permitting 11 electromechanical coupling. The VF threshold was significantly reduced from 13.6±3.5 to 7.9±5.3 and 5.1±3.4 mA at 0.5, 2.7 and 5.1 mM Ca⁺⁺ concentrations, respectively. The incidence of spontaneous recovery from VF, induced during basic pacing, was 100%, 83% and 50% at calcium concentrations of 0.5, 2.7 and 5.1 mM, respectively, (p<0.01 for the incidences at 0.5 mM versus 5.1 mM Ca⁺⁺). The incidence of spontancous defibrillation decreased when the hearts were driven rapidly, with spontaneous recovery rates of 92%, 58% and 0% (p<0.0001)) for corresponding increases in Ca⁺⁺ concentration. Induced ventricular fibrillation of fine morphology was frequently observed at 5.1 mM Ca⁺⁺. It appears that progressive impairment of spontaneous defibrillation is caused by an increase in calcium concentration, this effect being more pronounced at high ventricular rates. Variations in the effective refractory period, caused by alterations in extracellular calcium concentration and differences in intracellular Ca⁺⁺ accumulation, may account for the above results.     

Effects of acute iron loading on contractility and spontaneous beating rate of cultured rat myocardial cells

Summary Cardiac dysfunction is a well known but poorly understood complication of iron overload. We have previously shown that cultured myocardial cells are able to assimilate large amounts of iron. In the present study, the effect of iron on the rate and amplitude of beating in monolayer cultures of rat ventricular myocytes was studied. Iron had negative chronotropic and inotropic effects, both reversible upon washout. The negative chronotropic effect developed earlier and could be reversed by adrenaline. The negative inotropic effect took longer to develop and was completely reversed by caffeine. Elevated [Ca⁺⁺] also partially restored impaired contractility, while adrenaline or ouabain did not show any significant effect. These results indicate that iron toxicity in cultured heart cells impairs cellular function at both sarcolemmal and intracellular sites.     

Voltage-dependent calcium release in guinea-pig cardiac ventricular muscle as antagonized by magnesium and calcium

Summary An increase in extracellular potassium concentration from 4 to 16 mmol/l caused a decrease in membrane potential from –92 to –59 mV and selectively diminished the earlier of two contraction components of guinea-pig papillary muscles at 0.2 Hz stimulation frequency in the presence of noradrenaline. The influence on the early contraction component had a threshold of 8 mmol/l K⁺, corresponding to a membrane potential of –77 mV. However, test contractions elicited 800 ms after the 5 s stimulation interval exhibited an unimpaired early component. Since the activator calcium responsible for the early contraction component is derived, in mammalian ventricular muscle, from the junctional sarcoplasmic reticulum (20), it is assumed that the release site of the reticulum was filled with calcium shortly (800 ms) after a regular contraction, and lost its calcium at 16 mmol/l extracellular K⁺ during the 5 s stimulation interval. The potassium-induced depolarization determined the rate of calcium leakage during rest from the intracellular store. The depolarization-induced decline of the early contraction component was equally well antagonized by Mg²⁺ or Ca²⁺ without influencing the measured transmembrane potential. Both divalent cations shifted the relation between potassium concentration or membrane potential and the strength of the early contraction component to less negative membrane potentials. In order to reduce the early contraction component by 25% in the presence of 9.6 instead of 1.2 mmol/l Mg²⁺, the potassium concentration had to be increased from 9.6 to 22.0 mmol/l, with a respective decrease in resting membrane potential from –72.6 to –51.1 mV. The antagonistic effect of both divalent cations is thought to result from the neutralization of negative charges outside the sarcolemma with a respective decrease in the outside surface potential.     

Effects of in vivo manganese administration on calcium exchange and contractile force of rat ventricular myocardium

Summary The purpose of this study was to investigate the effect of prolonged (14 days) intragastric administration of Mn²⁺ (0.25 mmol/kg daily) on Ca²⁺ exchange and contractility of rat ventricular myocardium. Left-ventricular pressure and its first derivative (dP/dt) were recorded by means of a balloon catheter inserted via the left atrium into the left ventricle of the rat heart perfused by Langendorff method. Ca²⁺ exchange in the stimulated and rested ventricular myocardium was investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with a solution containing⁴⁵Ca²⁺. The cellular⁴⁵Ca²⁺ content was calculated by subtraction of⁴⁵Ca²⁺ dissolved in the free water of extracellular space from the total tissue⁴⁵Ca²⁺ content. The cellular⁴⁵Ca²⁺ content in the stimulated (60/min) ventricles of control rats (without Mn²⁺) was 0.83 ± 0.09 mmol/kg wet weight (w.w.). Ten minutes of rest resulted in a gain of 0.06 mmol⁴⁵Ca/kg w.w. (not statistically significant). Fourteen days' exposure to Mn²⁺ resulted in an increase of the mean⁴⁵Ca content to 1.61 ± 0.09 mmol/kg w.w. in the stimulated preparations and to 1.35 ± 0.06 mmol/kg w.w. in the rested ones (p < 0.001). Thus, the control rest preparations did not change their Ca²⁺ content, while in the rats treated with Mn²⁺ the rest resulted in an increase at exchangeable Ca by 52 %. The maximal ventricular developed pressure (P_max) after 14 days of Mn²⁺ administration was increased by 35 % and dP/dt_max was 228 % of the value in the control group. The mean time from the maximal value of the first derivative to P_max (t-dP/dt_max) was reduced by one-half and mean time to peak developed pressure (TPT) was shortened to one-third of these in the control group. These results, although far from conclusive, do suggest that the long-lasting exposition to Mn²⁺ resulted in increased myocardial contractility caused, most probably, by the inhibition of Ca²⁺ efflux from the cell. Perhaps this is a compensation for the inhibitory effect of Mn²⁺ on the slow calcium channels.     

Mitochondrial respiration in myocardial biopsy samples as a criterion of postischemic recovery of the cardiac contractility

Summary Isolated perfused guinea pig hearts were arrested by a high K⁺ cardioplegic solution containing (PG group) or lacking (control group) 10 mM phosphocreatine +15 mM glutamate. Total normothermic ischemia lasted 45 min followed by 30 min reperfusion. Mitochondrial respiration in the absence and presence of different concentrations of ADP and creatine was studied in biopsy samples (6–8 mg) after saponin treatment. The samples were taken before and after ischemia, as well as after the reperfusion period. A slightly better relative recovery of developed pressure (RRDP) in PG group was associated with higher mitochondrial acceptor control ratio after reperfusion (5.74±0.32 vs. 4.54±0.21 in PG and control groups, resp., p<0.01). When the results obtained in both groups were treated together, tight correlations between the pre- or postischemic mitochondrial state and RRDP were revealed. Higher values of RRDP were found for the hearts with lower preischemic values of (low ADP+creatine)-stimulation of mitochondrial respiration (r=–0.57, p<0.01). Relative changes in this mitochondrial parameter during ischemic period were in a good correlation with the RRDP (r=0.82, p<0.001). The data suggest that the study of the mitochondrial function in myocardial biopsy samples before ischemia and reperfusion could provide a useful information for the prognosis of cardiac function recovery.     

Binding of bepridil to isolated rat heart mitochondria

Summary We investigated mass action of isolated rat heart mitochondria with the calcium antagonist bepridil. At pH 7.20 bepridil in basic form b associates rapidly with the mitochondrial membrane but the amount fixed is higher in non-energized mitochondria than in mitochondria energized by succinate or ATP Mg²⁺. This effect is related to the dissociation state of the drug since conditions favoring the acidic form bH⁺ suppress this difference. Tritiated bepridil bound to mitochondrial membrane is only partially displaced by high concentrations of unlabeled drug (5 10^–5 M). No membrane energization effect is noted on this displacement. Binding values of bepridil to mitochondrial membrane (K_D 1.7 10^–5 M; B_max 23.8 nmol·mg^–1 protein) show only low affinity receptor sites. Bepridil binding to the lipid part of the inner membrane surface is postulated. This interaction is used to explain some of the in vitro effects of this calcium antagonist on membrane bound enzyme activities.     

The effect of calcium ions and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis in chick embryo hearts

Summary The effect of increases in extracellular calcium [Ca]₀ and the calcium ionophore A23187 on choline uptake and phosphatidylcholine biosynthesis was assessed in isolated cardiac myocytes. The cells were obtained from 7-day old chick embryos and were maintained in culture. Choline uptake was examined using [methyl 3H] choline. A23187 was found to increase choline uptake through the saturable choline uptake process. Pulse chase experiments using [methyl 3H] choline showed that after a 2 h incubation with choline, about 85% of the label was recovered in phosphocholine with most of the rest in phospholipid and a small amount in CDP-choline and glycerol phosphocholine. Increases in [Ca]₀ up to 10 mM did not affect the amount of label in phosphocholine or phospholipid, the rate of disappearance of label from phosphocholine, or the rate of appearance of labelled choline in phospholipid. In contrast, A23187, at concentrations up to 10^–4 M, was associated with a significant (p<0.05) increase in choline in the phosphocholine and phospholipid pool compared to control cells. The time course of the disappearance of choline from the phosphocholine pool and appearance in phospholipid pool was not significantly different between control cells and those treated with A23187. A23187 increased choline uptake via the specific uptake process. The effect on choline uptake may be attributed to the action of A23187 to facilitate the release of calcium from specific intracellular calcium storage sites rather than a nonspecific increase in [Ca]₁ that may have resulted from the increase in [Ca]₀.Supported in part by a grant from Canadian (British Columbia) Heart Foundation     

Rest- and stimulation-dependent changes in exchangeable calcium content in rabbit ventricular myocardium

Summary Ca²⁺ shifts in the isolated, perfused ventricular muscle of rabbit hearts were investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with⁴⁵Ca containing solution. The cellular⁴⁵Ca content was calculated by subtraction of⁴⁵Ca²⁺ dissolved in the free water of extracellular space from the total tissue⁴⁵Ca²⁺ content. The cellular content of⁴⁵Ca in stimulated (60 per min) preparation was 0.887±0.067 mmol/kg wet weight (w.w.). Six minutes of rest resulted in the drop of this content to 0.503±0.054 mmol/kg w.w. despite continued perfusion with⁴⁵Ca containing solution. Contractile force (CF) decreased at that time to 23% of control. The first post-rest contraction (RSC) resulted in a gain of 0.073 mmol⁴⁵Ca/kg w.w. Both the content of⁴⁵Ca and CF returned to the pre-rest values when stimulation was resumed.The difference between the⁴⁵Ca content of post-rest stimulated and rested preparations (0.384 mmol/kg w.w.) is equivalent to Ca₂ fraction (Ca₂), previously described in guinea pig ventricular myocardium (17, 12). However, the volume of rabbit Ca₂ is only about 42% of that in guinea pig. Caffeine in concentration 12.5 mM, which did not displace Ca₂ from guinea pig ventricular muscle, decreased Ca₂ in the rabbit ventricle by 44%. CCCP, a protonofore destroying the mitochondrial protone gradient essential for Ca²⁺ uptake and maintainability, displaced Ca₂ completely from rabbit ventricles. These results, although far from conclusive, do suggest that both the mitochondrial and sarcoplasmic reticulum might be the site of the rate-dependent Ca₂ fraction. The physiological meaning of differences in Ca₂ content between rabbit, guinea pig, and rat ventricular myocardium is discussed.This study was supported by the grant CPBR, 11.6     

Inorganic phosphate inhibits contractility and ATPase activity in skinned fibers from human myocardium

Summary During hypoxic heart failure, inorganic phosphate (Pi) accumulates. We report the effects of Pi on force development and on myofibrillar ATPase-activity of human skinned atrial fibers, both at normal and at reduced levels of Mg-ATP. Pi (10 mM) depressed force production at maximal calcium activation (pCa 4.3) by about 40%. At higher pCa values (pCa 5.6), foree inhibition was even more pronounced, but at low concentrations of Mg-ATP (10 M), Pi was less effective. In contrast to contractile force, myofibrillar ATPase was only inhibited by about 10% at pCa 4.3, whereas it could be inhibited by 40–50% at submaximal calcium activation (pCa 5.6). As Pi inhibited contractile force more than ATPase activity, the ratio of ATPase-activity to foree (tension cost) was increased by inorganic phosphate. ATPase-activity and tension cost were significantly reduced by loweing Mg-ATP concentration to 10 M, whereas contractile force was less affected. Pi did not affect ATPase under these conditions at 10 mM Mg-ATP. Pi also shifted the calcium-force relationship towards higher Ca⁺⁺ concentrations, that is, it decreased calcium sensitivity. In contrast, the calcium sensitivity of myofibrillar ATPase was less affected. These findings suggest that inorganic phosphate may affect the myocardium by altering crossbridge kinetics rather than the calcium affinity of troponin-C. Because of its inhibitory effect on myofibrillar ATPase, inorganic phosphate may be partly cardioprotective in the hypoxic myocardium. However, this energy sparing effect is probably offset by the greater tension cost that decreases the efficiency of tension maintenance in the presence of inorganic phosphate.     

Open chest and open pericardium affect the distribution of myocardial blood flow in the right ventricle

Summary We have investigated the effects of open chest and open pericardium on the distribution of myocardial blood flow assessed with the radioactive microsphere technique (15 m). Five dogs with intact thorax served as controls (group I) and six dogs were studied after a right-sided thoracotomy and pericardiotomy (group II). Global myocardial blood flow (mean±S.D.) was 0.73±0.17 ml·min^–1·g^–1 in group I and 1.22±0.09 ml·min^–1·g^–1 in group II (p<0.05). Analysis of transmural blood flow distribution revealed that flow was 44% higher in the right and 60% higher in the left ventricular endocardial layers in the open-chest animals, whereas epicardial flow increased by 105% and 90%, respectively. As a result of the preferential blood flow to the epicardial layers of the right ventricle, the endo/epi ratio was reduced from 1.30±0.26 in group I to 0.86±0.11 in the open-chest group (p<0.05). Left ventricular endo/epi ratio was 1.27±0.16 and 1.06±0.11 (n.s.), respectively. External work and diastolic filling pressure of the right ventricle did not differ between the two groups and therefore cannot account for the redistribution of myocardial blood flow. It is concluded that the distribution of myocardial blood flow, particularly in the RV, is severely disturbed by thoracotomy and pericardiotomy. This is an important aspect for the planning and evaluation of studies under open-chest/open-pericardium conditions.Supported by Deutsche Forschungsgemeinschaft grant SFB 320     

Effects of caffeine on energy output of rabbit heart muscle

Summary The effects of caffeine (1 mmol·1^–1) on mechanical and energetic parameters in the arterially perfused interventricular rabbit septa were examined at various frequencies of stimulation. Even though 1 mmol^–1 caffeine induced a negative inotropic effect only at stimulation rates higher than 0.33 Hz. relaxation was impaired at all frequencies tested. The ratio between maximum rate of relaxation and developed tension was consistently lowered by caffeine, indicating a more marked effect on relaxation over contraction. In addition, while time-to-peak tension was unaffected by caffeine at the dose used, the last part of the relaxation (i.e., of the contractile event) was prolonged at all frequencies in the presence of the drug. Resting heat production (H _t ) was increased in the presence of caffeine (1.6±0.6 mW·g^–1). The ratios between active heat production and either developed tension (H_a/T) or tension time integral (H_a/TtI), increased at all frequencies examined (53.3±8.5 J·mN^–1·g^–1 and 68.2±9.9 J·mN^–1·s^–1·g^–1, respectively), indicating a lowered economy of the contractile process. This is consistent with the lower ATP/Ca ratio reported for the sarcoreticular Ca pump (i.e., one ATP hydrolyzed/2 Ca transported) with respect to the sarcolemmal mechanisms such as Na–Ca exchanger or the sarcolemmal Ca pump, with an ATP/Ca ratio of 1 to 1. Thus, inhibition of the SR-Ca pump by caffeine would induce a higher rate of ATP hydrolysis with the consequent increase in the H_a/T ratio. As a result of the increase in both H_a/T ratio and H_r induced by caffeine, the ratio between total heat production and developed tension (H_t/T) also increased. Therefore, the contractile process appeared to be more efficient in the presence of an active SR, since it is energetically less costly to generate a given level of isometric tension.     

Effects of the calcium antagonist gallopamil on the increase of myocardial extracellular potassium activity during LAD occlusion in dogs

Summary It has been implied that the increase of myocardial extracellular potassium activity ([K⁺]_c) in the early stage of acute myocardial ischemia is a major cause of the increased likelihood of arrhythmia after acute coronary artery occlusion. There is also experimental evidence that some calcium antagonists reduce the occurrence of ischemia-induced early ventricular arrhythmias. In order to clarify the antiarrhythmic effect of gallopamil during the early phase of acute LAD occlusion, the influence of this calcium antagonist on the time course of [K⁺]_c during acute ischemia was measured in open-chest anesthetized dogs using a K⁺-selective surface multielectrode. The regional myocardial blood flow was determined with 9 m radioactive tracer microspheres.After application of gallopamil (bolus, 25 g/kg and infusion 2.5 g/kg·min for 30 min) the maximal and mean rate of rise of [K⁺]_c as well as the plateau of [K⁺]_c reached during ischemia were significantly diminished compared with the control occlusions. 90 min after gallopamil, the rate of rise of [K⁺]_c as well as the plateau of [K⁺]_c reached were still significantly reduced, but 180 min after the gallopamil application, no significant differences between the time course of [K⁺]_c and that of the two control occlusions could be found. Gallopamil significantly elevated, myocardial blood flow in the non-ischemic area, but did not influence blood flow in the ischemic region.While collateral perfusion remains unchanged, the slowed and reduced increase of myocardial [K⁺]_c during acute coronary artery occlusion may be an important component of the antiarrhythmic effect of gallopamil during early ischemia.Preliminary results were presented at the International Symposium on Calcium entry blockers and tissue protection, Rome, 1984, at the 59th Meeting of the Deutsche Physiologische Gesellschaft, Dortmund, 1984, at the 50th meeting of the Deutsche Gesellschaft für Herz- und Kreislaufforschung, Mannheim, 1984, and at the IXth European Congress of Cardiology, Düsseldorf, 1984.     

Changes in heart rate by verapamil during carotid sinus stimulation in patients with hyperparathyroidism,pre- and postoperatively

Summary It has been investigated whether calcium- and verapamil-dependent sensitivities of carotis baroreceptors also exist in man. To answer this question, we pre- and postoperatively measured changes in heart rate during carotid sinus stimulation before and after intravenous administration of 5 mg verapamil in 23 patients with primary hyperparathyroidism. Findings during hypercalcemia were as expected: a more pronounced reduction of heart rate at comparatively low calcium levels. During normocalcemia, we found an opposite effect: a more pronounced reduction at relatively high calcium levels, which was statistically significant. This fact could be explained according to our interpretation. In previous reports, local effects on baroreceptors were examined, whereas we measured the combined effect of several calcium actions. As expected, verapamil attenuated the decrease in heart rate which, however, was not statistically significant.     

Calcium repletion-induced arrhythmias after short periods of calcium-free perfusion in the isolated rat heart

Summary The occurrence of arrhythmias was studied in the calcium-paradox model in the isolated rat heart. Clear relationships were found between the duration of calcium-free perfusion and (a) the occurrence of calcium-free-induced electrophysiological changes, (b) the incidence and duration of subsequently induced calcium-repletion arrhythmias and (c) mechanical recovery at the end of the repletion period.The first signs of electrophysiological changes (i.e. decreased heart rate, T-wave amplitude and increased PQ-interval) and irreversible loss of myocardial recovery occurred during or after 60–90 s of calcium-free perfusion. The occurrence of calcium-repletion induced ventricular tachycardia parallelled this onset of irreversible cardiac injury. These results suggest that the process of calcium washout and subsequent sudden calcium overloading may play a role as a trigger in the pathogenesis of ventricular arrhythmias.     

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.     

Sensitivity to oxidants of mitochondrial and sarcoplasmic reticular calcium uptake in saponin-treated cardiac myocytes

Summary Calcium transport functions of mitochondria and sarcoplasmic reticulum (SR) were studied without prior extraction using isolated rat heart myocytes permeabilized with saponin. Calcium uptake by SR was rapid and its affinity was high in comparison to calcium uptake by mitochondria, which had a higher capacity. The sensitivity of uptake to two oxidants, H₂O₂ and HOCl (hypochlorous acid), depended on the cytosolic calcium concentration; when this was similar to the concentration in diastole (180 nM), HOCl inhibited calcium uptake by mitochondria and SR, whereas when the calcium concentration was 750 nM, mitochondrial calcium uptake showed relatively high resistance, although SR uptake was still markedly inhibited by HOCl. Calcium uptake of both mitochondria and SR was less sensitive to the action of H₂O₂ than to HOCl, and the H₂O₂ effect was less dependent on the cytosolic calcium concentration. Therefore, HOCl, when produced by activated leukocytes and supplied to the heart cells, may seriously impair the excitation-contraction coupling function of SR, whereas H₂O₂, possibly generated directly by mitochondria or generated from superoxide anions, may be tolerated relatively well by heart SR and mitochondria.Preliminary accounts of this work were presented as FASEB 1988. Dr. Kaminishi was a Canadian Heart Foundation Post-Doctoral fellow; his present address is Yamagata University, Japan.     

Inhibition of contractility during the early phase of total ischaemia in the working heart. Recovery during reperfusion

Summary Total global ischaemia of the normothermic working rat heart caused an initial positive inotropic response characterized by vigorous contractions. After ±15 s this response reached a peak whereafter the isotonic contraction amplitude started to decline. After ±3.5 min the heart ceased to beat. The low level of high energy phosphates (HEP), determined 3 min after the onset of ischaemia, indicated that these phases of contractility during ischaemia might play a significant role in depleting HEP. This was substantiated by the observation that inhibition of the contractions during ischaemia by low calcium or high potassium solutions resulted in conservation of myocardial adenosine triphosphate (ATP) and creatine phosphate (CP) stores. It also resulted in the prevention of contracture development during ischaemia and improved mechanical recovery during reperfusion. It was therefore concluded that inhibition of contractility immediatcly after the onset of total global ischaemia of the normothermic working rat heart is of prime importance in mechanical recovery during reperfusion.     

Isoproterenol induced rat endomyocardial damage in relationship to local capillary geometry

Summary Isoproterenol (2 mg/kg) injected subcutaneously into male Sprague-Dawley rats elicited morphological damage in the endomyocardium which was analyzed 16h following injection. Our aim was to study the relationship between damaged individual myocytes asnd their capillary supply. Myocardial tissue sections were differentially stained in order to distinguish arteriolar (AC) and venular (VC) capillary portions. Tissue areas surrounding individual capillaries and the position of the capillaries with respect to the damaged individual myocytes were established by using the method of capillary domains. In multicellular necrotic lesions 84 % of the capillaries located within the necrotic foci and 77 % of the capillaries in the surrounding tissue area were identified as the distal, venular portion with presumably low OZ content. The proportion of VCs related to necrotic lesions was significantly higher than in surviving endomyocardial regions. In the case of individual necrotic myocytes, we found 88 % to be supplied by VCs, while the adjacent normal myocytes were supplied by 61 % VCs. Both values were significantly higher when compared to control hearts (42 %). These results strongly support the crucial role of a lack of oxygen delivery in the pathogenesis of isoproterenol-induced necrosis.