Left Ventricular Papillary Muscle Strips Research Articles

Protective effect of distillated Nerium oleander on heart of type 2 diabetic rats.

The current study aims to investigate the possible role of NO distillate either for therapeutic or for protective potential in diabetic cardiomyopathy. Protective and restorative effects of distillated Nerium oleander (NO) on the diabetes-induced electrophysiological and structural alterations were investigated. Type 2 diabetes was induced by combination of single dose streptozotocin injection and high fat diet for four weeks. Experimental groups were designed as follows: control, diabetic, restorative-NO treated diabetic and protective-NO treated diabetic. Intracellular action potentials (AP) and contractile activities were measured form left ventricular papillary muscle strips as well as histopathological examination of heart tissue and biochemical examinations of serum were performed. Type 2 diabetes induced AP prolongation was prevented with both ways of NO treatments. Moreover, treatments produced nearly complete restorations of diabetes-induced depressed amplitude and altered kinetics of contractile activities. In parallel to electrophysiological parameters, both histopathological and biochemical results indicates the NO induced beneficial effects on the diabetes related alterations. Distillated Nerium oleander (NO) can be a highly potential therapeutic or preventive agent on the diabetes induced excitation-contraction coupling alterations (Tab. 3, Fig. 3, Ref. 23).

Effects of β-adrenergic receptor blockers on cardiac function: a comparative study in male versus female ratsThis article is one of a selection of papers from the NATO Advanced Research Workshop on Translational Knowledge for Heart Health (published in part 2 of a 2-part Special Issue).

In heart disease, differences exist between women and men with respect to the impact of risk factors, symptoms, and therapeutic responses. The use of beta-adrenergic receptor blockers is now well established in the treatment of mild and moderate systolic heart failure. Although there are significant differences among agents, their clinical effects are predictable. To address the question of sex disparities in the heart, however, we investigated the effect of treatment with the nonselective beta-blockers timolol and propranolol on mechanical and electrical function of heart preparations from male and female rats. We examined the long-term effects of intragastric treatment with timolol (5 mg/kg per day) or propranolol (25 mg/kg per day) for 7 months on the hemodynamic and intracellular action potential parameters of the heart. Chronic administration of timolol but not propranolol produced a significant increase in the baseline activity of the left ventricular developed pressure (LVDP) in both male and female rats with no significant effect on the left ventricular end-diastolic pressure. Timolol or propranolol treatment of male rats and timolol but not propranolol treatment of female rats induced significant shortening in the repolarization phases of action potentials recorded from left ventricular papillary muscle strips of the hearts. The responses of LVDP to beta-adrenergic stimulation were similar in timolol- or propranolol-treated or untreated male rats. On the other hand, timolol treatment markedly increased, and propranolol treatment significantly decreased, the responses of increase in LVDP in female rats. Our results suggest that although treatment with beta-blockers for 7 months confirmed the role of the beta-adrenergic pathway in heart function, there are marked differences in the effects of individual beta-blockers on heart physiology. Sex differences should be taken into consideration when using beta-blockers during experimental studies and clinical therapy.

Increased Functional Importance of the Na,Ca-Exchanger in Contracting Failing Human Myocardium but Unchanged Activity in Isolated Vesicles

The present study aimed to investigate the hypothesis that the function of the Na,Ca-exchanger (NCX) is of higher importance for contractility and Ca(2+)-homeostasis in left ventricle from terminally failing than from nonfailing human hearts. The effect of decreasing extracellular [Na](e) (140 to 25 mmol/L) on force of contraction in isolated left ventricular papillary muscle strips was studied as a reflection of NCX function in multicellular preparations (terminally failing, DCM, dilated cardiomyopathy, NYHA IV, n = 13; nonfailing, NF, donor hearts, n = 10). Decreasing [Na](e) has previously been shown to increase contractility in vitro secondary to a decreased Ca(2+)-extrusion by the NCX. In addition, the NCX activity was measured as Na(+)-dependent (45)Ca(2+)-uptake into isolated myocardial vesicles as a function of time and Ca(2+)-concentration (DCM n = 8, NF n = 8). Decreasing [Na](e) enhanced the contractility of papillary muscle strips in both DCM and NF, but the contractility of DCM was increased at smaller reductions of [Na](e) than NF. The NCX activity in isolated myocardial vesicles was unchanged as a function of time (T(1/2): DCM 2.4 +/- 0.3 s versus NF 2.5 +/- 0.3 s) and as a function of Ca(2+) (DCM 0.99 +/- 0.08 versus NF 0.96 +/- 0.07 nmol/mg protein x 3 s, K(1/2): DCM 39.2 microM versus NF 38.3 microM). These results demonstrate a higher sensitivity of the failing human myocardium towards Na,Ca-exchanger mediated positive inotropic effects, suggesting a higher significance of the Na,Ca-exchanger for the extrusion of Ca(2+)-ions in intact failing versus nonfailing human myocardium. Since the activity and the Ca (2+)-affinity of the Na,Ca-exchanger in isolated vesicles was unchanged, we propose that alterations in Ca(2+)-and Na(+)-homeostasis (due to impaired function of the sarcoplasmic reticulum and the Na(+), K(+)-ATPase) or the prolonged action potential are the reason for this observation.

Bucindolol exerts agonistic activity on the propranolol-insensitive state of beta1-adrenoceptors in human myocardium.

In congestive heart failure patients, treatment with beta-adrenoceptor antagonists improves symptoms and decreases mortality. However, intrinsic sympathomimetic activity of beta-adrenoceptor antagonists might be disadvantageous in chronic heart failure. The nonselective beta1- and beta2-adrenoceptor antagonist bucindolol has failed to decrease mortality in clinical trials. A putative beta4-adrenoceptor, which mediates positive inotropic effects by activation of the adenylate cyclase has been described. Recently, this putative beta4-adrenoceptor has been identified to be a propranolol-insensitive state of the beta1-adrenoceptor. The present study aimed to characterize whether bucindolol exhibits agonistic activity on this atypical beta1-adrenoceptor state as one possible reason for clinical inefficiency. For comparison (S)-4-(3'-t-butylamino-1'-hydroxypropoxy)-benzimidozole-2 (CGP 12177), metoprolol, and nebivolol were investigated. Bucindolol did not reveal intrinsic sympathomimetic activity in electrically driven (1 Hz, 37 degrees C), forskolin-stimulated, left ventricular papillary muscle strips (donor hearts, nonfailing; n = 5) and in right auricular trabeculae (bypass operation; n = 4). Functional studies on the propranolol-insensitive state of beta1-adrenoceptors were performed in isolated muscle preparations after beta1- and beta2-adrenoceptor antagonism (propranolol, 1 microM), inhibition of beta3-mediated inotropic effects (N-nitro-L-arginine, 100 microM) and forskolin treatment (0.3 microM). Positive inotropic response to stimulation of atypical state beta1-adrenoceptors could be demonstrated in right auricular as well as left ventricular human myocardium (CGP 12177 treatment, 10 microM). Under these conditions, also bucindolol, but not metoprolol and nebivolol, significantly increased contractility (all 10 microM). In conclusion, bucindolol but not metoprolol or nebivolol mediate positive inotropic effects in human myocardium due to activation of atypical state beta1-adrenoceptors. Thus, the agonistic activity of bucindolol may influence outcome in heart failure patients.

Myocardial cross-bridge kinetics in transition to failure in Dahl salt-sensitive rats.

The role of altered cross-bridge kinetics during the transition from cardiac hypertrophy to failure is poorly defined. We examined this in Dahl salt-sensitive (DS) rats, which develop hypertrophy and failure when fed a high-salt diet (HS). DS rats fed a low-salt diet were controls. Serial echocardiography disclosed compensated hypertrophy at 6 wk of HS, followed by progressive dilatation and impaired function. Mechanical properties of skinned left ventricular papillary muscle strips were analyzed at 6 wk of HS and then during failure (12 wk HS) by applying small amplitude (0.125%) length perturbations over a range of calcium concentrations. No differences in isometric tension-calcium relations or cross-bridge cycling kinetics or mechanical function were found at 6 wk. In contrast, 12 wk HS strips exhibited increased calcium sensitivity of isometric tension, decreased frequency of minimal dynamic stiffness, and a decreased range of frequencies over which cross bridges produce work and power. Thus the transition from hypertrophy to heart failure in DS rats is characterized by major changes in cross-bridge cycling kinetics and mechanical performance.

Crataegus special extract WS 1442 increases force of contraction in human myocardium cAMP-independently.

The mode of action of Crataegus extracts in the treatment of heart failure is still under examination. WS 1442, a standardized special extract from Crataegus leaves with flowers, exerts direct positive inotropic effects. This study was designed to investigate the mode of inotropic action of WS 1442 in human myocardium from patients with congestive heart failure (left ventricular myocardium from explanted hearts; NYHA IV, n = 8) as well as in nonfailing controls (right auricular trabeculae from patients with coronary heart disease, n = 8). WS 1442 effectively displaced specifically bound 3H-ouabain but did not influence the activity of adenylate cyclase [control, + Gpp(NH)p (10(-4) microM) 3,500 pmol cyclic adenosine monophosphate (cAMP)/20 min). In isolated left ventricular papillary muscle strips, WS 1442 significantly increased the force of contraction [basal, 1.8+/-0.2 mN; WS 1442 (50 microg/ml), 2.4+/-0.1 mN (130%)] and improved the frequency-dependent force generation (0.5 vs. 2.5 Hz: control, +0.1+/-0.01 mN; WS 1442, +0.9+/-0.3 mN) even in failing human myocardium. In fura-2-loaded muscle strips (right atrial trabeculae), WS 1442 increased both the Ca2+-transient and force generation. These effects also were observed in the lipophilic ethyl acetate-soluble fraction A, enriched in flavone derivatives. In conclusion, these findings suggest a pharmacologic mechanism of WS 1442 similar to the cAMP-independent positive inotropic action of cardiac glycosides. In addition, WS 1442 improves the force-frequency relation in failing human myocardium.

Altered crossbridge kinetics in the alphaMHC403/+ mouse model of familial hypertrophic cardiomyopathy.

A mutation in the cardiac beta-myosin heavy chain, Arg403Gln (R403Q), causes a severe form of familial hypertrophic cardiomyopathy (FHC) in humans. We used small-amplitude (0.25%) length-perturbation analysis to examine the mechanical properties of skinned left ventricular papillary muscle strips from mouse hearts bearing the R403Q mutation in the alpha-myosin heavy chain (alphaMHC403/+). Myofibrillar disarray with variable penetrance occurred in the left ventricular free wall of the alphaMHC403/+ hearts. In resting strips (pCa 8), dynamic stiffness was approximately 40% greater than in wild-type strips, consistent with elevated diastolic stiffness reported for murine hearts with FHC. At pCa 6 (submaximal activation), strip isometric tension was approximately 3 times higher than for wild-type strips, whereas at pCa 5 (maximal activation), tension was marginally lower. At submaximal calcium activation the characteristic frequencies of the work-producing (b) and work-absorbing (c) steps of the crossbridge were less in alphaMHC403/+ strips than in wild-type strips (b=11+/-1 versus 15+/-1 Hz; c= 58+/-3 versus 66+/-3 Hz; 27 degrees C). At maximal calcium activation, strip oscillatory power was reduced (0. 53+/-0.25 versus 1.03+/-0.18 mW/mm3; 27 degrees C), which is partly attributable to the reduced frequency b, at which crossbridge work is maximum. The results are consistent with the hypothesis that the R403Q mutation reduces the strong binding affinity of myosin for actin. Myosin heads may accumulate in a preforce state that promotes cooperative activation of the thin filament at submaximal calcium but blunts maximal tension and oscillatory power output at maximal calcium. The calcium-dependent effect of the mutation (whether facilitating or debilitating), together with a variable degree of fibrosis and myofibrillar disorder, may contribute to the diversity of clinical symptoms observed in murine FHC.

Na+-channel modulating effect of the inotropic compound S(-)BDF 9196 in human myocardium.

S(-)BDF 9196, the active enantiomer of racemic (+/-)BDF 9148, has been shown to increase force of contraction in myocardium from different species including humans. The present study aimed to investigate the mechanism of the positive inotropic action of the active enantiomer S(-)BDF 9196 in human myocardium. In electrically driven human left ventricular papillary muscle strips (dilated cardiomyopathy, NYHA IV, cardiac transplantation, n=9), S(-)BDF 9196 increased force of contraction concentration-dependently. The maximal positive inotropic effect remained unchanged after the addition of carbachol (1 mmol/l, indicating a cAMP-independent mode of action of S(-)BDF 9196. While [3H]ouabain binding in human myocardial membranes was not influenced by S(-)BDF 9196 up to 10 micromol/l, the inward Na(+)-current in isolated human left ventricular myocytes was increased significantly by S(-)BDF 9196 (1 micromol/l, n=5). These results provide evidence that S(-)BDF 9196 increases force of contraction in human myocardium primarily by enhancing Na(+)-influx, while cAMP-dependent or Na(+),K(+)-ATPase blocking effects do not seem to play a role.

Effect of inotropic interventions on the force-frequency relation in the human heart.

In severe human heart failure, an increase in frequency of stimulations is accompanied by a reduced force of contraction in vivo and in vitro. This contrasts the findings in nonfailing human hearts. To investigate influences of inotropic stimulation on the force-frequency relationship in human myocardium, the effects of the cAMP-independent positive inotropic agents ouabain (Na+/K(+)-ATPase inhibitor) and BDF 9148 (Na(+)-channel modulator) as well as of the beta-adrenoceptor agonist isoprenaline on the force-frequency relationship in electrically driven left ventricular papillary muscle strips from nonfailing and terminally failing human myocardium were studied. In nonfailing myocardium, force of contraction increased following an increase in stimulation frequency, whereas in failing human myocardium force of contraction gradually declined following an increase in stimulation frequency. Moderate stimulation of contractility by isoprenaline reversed the negative force-frequency relationship in failing myocardium and preserved the positive force-frequency relationship in nonfailing myocardium. In the presence of ouabain and BDF 9148 the positive force-frequency relationship was completely restored in failing myocardium. In contrast, in the presence of high concentrations of isoprenaline the former positive force-frequency relationship became negative even in nonfailing myocardium. The negative force-frequency relationship in failing human myocardium is accompanied by alterations in the intracellular Ca(2+)-homeostasis. The latter may be due to an impaired function of the sarcoplasmic reticulum (SR) in failing human myocardium. Therefore, the activity of the SR-Ca(2+)-ATPase (SERCA2) of crude membrane preparations was investigated and was significantly reduced in failing compared to nonfailing human myocardium. It is concluded that the negative force-frequency relationship may be due to alterations in the intracellular Ca(2+)-handling caused by an impaired function of the SERCA2 in failing human myocardium. The beneficial effects of cAMP-increasing agents on the force-frequency relationship in failing human hearts could result from an enhanced phosphorylation status of phospholamban in the presence of beta-adrenoceptor-stimulation. The effect of the [Na+]i-modulating agents BDF 9148 and ouabain demonstrates that the intracellular Na(+)-homeostasis influences intracellular Ca(2+)-handling as well. Differences observed in failing compared to nonfailing myocardium may be due to an altered expression or function of the Na+/Ca(2+)-exchanger, Na(+)-channels or the Na+/K(+)-ATPase in addition to the blunted activity of the SERCA2 in failing myocardium.

Positive inotropic effects of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium.

The aim of this study was to investigate the inotropic properties of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium. For comparison the Na+-channel modulator BDF 9148, the beta-adrenoceptor-agonist isoprenaline, and calcium were studied. Concentration-response curves for BDF 9198 (0.01-30 microM), BDF 9148 (0.01-30 microM), isoprenaline (0.001-1 microM), and calcium (1.8-15 mM) were obtained in electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilated cardiomyopathy, NYHA IV, heart transplantation; nonfailing, donor hearts). Whereas isoprenaline was significantly less effective and less potent in increasing the force of contraction in failing human myocardium than in nonfailing myocardium (p < 0.01), BDF 9198 and BDF 9148 were (in NYHA IV) as effective as in nonfailing human tissue. In both tissues, BDF 9198 and BDF 9148 exerted similar positive inotropic effects as calcium, with the novel Na+-channel modulator BDF 9198 being more potent in increasing force of contraction than was the preceding agent BDF 9148. The potencies of both Na+-channel modulators, BDF 9198 and BDF 9148, were enhanced in human failing myocardium when compared with nonfailing myocardium. In summary, the novel Na+-channel modulator BDF 9198 increases force of contraction to the same extent as calcium and with a higher potency than BDF 9148. The sensitivity of failing human myocardium to Na+-channel modulators is increased when compared with nonfailing myocardium, which might be the result of an altered Na+ homeostasis in human heart failure.

Oxygen Dose-Response Curve of Cardiac Papillary Muscle From Fetal and Nonpregnant Adult Sheep Exposed to Long-Term, High-Altitude Hypoxemia

We tested the hypothesis that hearts of fetal and nonpregnant adult sheep exposed to long-term hypoxemia would be able to sustain higher contractile function during exposure to acute hypoxia than hearts from normoxic animals. Pregnant and nonpregnant sheep were exposed to high altitude (3820 m) for 100 days. Right and left ventricular papillary muscle strips were obtained from fetuses and nonpregnant adults, mounted in an isolated bath system, stimulated electrically and subjected to acute hypoxia in a dose response manner. Measurements were made of maximum tension production (Tmax), maximum rate of tension development (+dT/dtmax), maximum rate of relaxation (-dT/dtmax), time to peak tension, and duration of contraction. Results were compared to papillary muscle from a normoxic group of animals. Baseline values (95% O2 + 5% CO2 bubbled in the bath) of Tmax and +/- dt/dtmax for each ventricle were greater in adults than fetuses in both normoxic and long-term hypoxemic groups. During hypoxia (at 40 and 20% O2) Tmax and +/- dT/dtmax, were all maintained at significantly higher values in papillary muscle from long-term hypoxemic fetuses than in papillary muscle from normoxic fetuses. Duration of contraction and time to peak tension did not differ between the normoxic and hypoxemic groups. In both ventricles of the long-term hypoxemic adult, Tmax and +/- dT/dtmax, as well as duration and time to peak tension, were significantly higher than in normoxic adults, but only at the lowest level of hypoxia (20% O2). Contrary to the original hypothesis, heart muscle from both fetal and adult sheep that had been exposed to long-term hypoxemia could maintain contractile function better during acute hypoxia. The responsible mechanisms are not clearly understood.

Increase in force of contraction by activation of the Na+/Ca(2+)-exchanger in human myocardium.

The aim of the present study was to investigate whether agents which enhance force of contraction via increasing intracellular Na+, i.e. cAMP-independently, remain effective in failing human myocardium. Cumulative concentration-response curves with (+/-)BDF 9148 (0.01-10 mumol l-1), a Na(+)-channel activator, and ouabain (0.01-0.1 mumol l-1), a Na+/K(+)-ATPase inhibitor, were performed on electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilative cardiomyopathy, NYHA IV, heart transplantation, n = 16; nonfailing, donor hearts, n = 5). The beta-adrenoceptor agonist isoprenaline (0.001-1 mumol l-1) and Ca2+ (1.8-15 mmol l-1) were studied for control. In addition, Ca2+ response curves were obtained on skinned fibre preparations from left ventricular myocardium (NYHA IV, n = 7) in the presence of BDF 9148 (1 mumol l-1) or a high Na+ concentration (50 mmol l-1) to investigate a possible direct or indirect interaction of (+/-)BDF 9148 with the myofilaments. While isoprenaline was significantly less effective in increasing force of contraction in failing human myocardium than in nonfailing myocardium (P < 0.01), in NYHA IV, (+/-)BDF 9148 and ouabain were as effective as in nonfailing human tissue. In failing and nonfailing myocardium, (+/-)BDF 9148 and ouabain exerted positive inotropic effects similar to those of Ca2+. However, the potency for (+/-)BDF 9148 to increase force of contraction was higher in NYHA IV than in nonfailing human myocardium (P < 0.05). Neither (+/-)BDF 9148 (1 mumol l-1) nor an increased concentration of Na+ (50 mmol l-1) altered the Ca2+ sensitivity or maximal developed tension of the contractile apparatus in experiments on chemically skinned left ventricular fibres. The enhanced sensitivity of the failing human myocardium towards Na(+)-channel modulation is not due to a direct or indirect interaction of (+/-)BDF 9148 with cardiac myofilaments but may be due to an altered Na(+)-homeostasis in human heart failure.

Enhanced sensitivity of the failing human myocardium to cardiac glycosides and Na +-channel activators

Cardiac glycosides and Na +-channel activators increase intracellular Na + and thereby enhance the transport rate of the sarcolemmal Na + Ca 2+ exchanger. We tested the hypothesis of whether increased expression of the Na + Ca 2+ exchanger in failing human myocardium is accompanied by enhanced sensitivity of the failing human myocardium toward cardiac glycosides and Na +-channel activators. We studied the positive inotropic effects of the new Na +-channel activator BDF and the cardiac glycoside ouabain in human failing (New York Heart Association [NYHA] functional class IV, heart transplants for dilated cardiomyopathy, n = 11) and nonfailing (donor hearts, n = 5) myocardium on electrically driven left ventricular papillary muscle strips (1 Hz, 37° C). The effectiveness of ouabain and BDF to increase force of contraction was similar in human nonfailing and failing myocardium. BDF was more potent to increase force of contraction in failing than in nonfailing tissue ( p < 0.05). The time until maximal inotropic effect developed after ouabain was significantly shorter in NYHA IV (mean 150 ± 16 min) than in nonfailing myocardium (mean 240 ± 20 min). These results suggest that human failing myocardium exerts an enhanced sensitivity to cardiac glycosides and Na +-channel activators, possibly because of enhanced expression of the Na + Ca 2+ exchanger or because of an altered intracellular Na +-homeostasis.

1014-98 Ca2+ -Uptake of Sarcoplasmic Reticulum Plays a Functional Role for the Force-Frequency Relationship in Man

Cyclopiazonic acid (CPA), a mycotoxin from Aspergillus and Penicillium, is a highly selective inhibitor of the Ca 2+ -ATPase in sarcoplasmic reticulum (SR). The present study aimed to investigate the functional role of the Ca 2+ -ATPase for the force-frequency-relationship (FFR) in human myocardium. The effect of CPA (0.01–30 μM) on Ca 2+ -uptake ( 45 Ca 2+ ) and Ca 2+ -ATPase ([free Ca 2+ ] 1.5 nM-1.5 μM) on SR-vesicles (left ventricle) as well as on the FFR (0.5–3 Hz, left ventricular papillary muscle strips) in the very same human nonfailing hearts (NF, donor hearts, n = 10) was examined. CPA concentration-dependently inhibited SR Ca 2+ -uptake and Ca 2+ -ATPase (EC 50 : 0.3 μ M) (p &lt; 0.05). In the presence of CPA (100 μ M) the former positive FFR in NF becomes negative (p &lt; 0.01). CPA reduced isometric force of contraction and peak rate of tension decay, and prolonged time to half relaxation (p &lt; 0.05). In conclusion: (1) the SR-Ca 2+ -ATPase plays a functional role for the positive FFR in man; (2) the altered FFR in failing human myocardium may be due to an altered SR-Ca 2+ -ATPase-function.

The failing human heart is unable to use the Frank-Starling mechanism.

There is evidence that the failing human left ventricle in vivo subjected to additional preload is unable to use the Frank-Starling mechanism. The present study compared the force-tension relation in human nonfailing and terminally failing (heart transplants required because of dilated cardiomyopathy) myocardium. Isometric force of contraction of electrically driven left ventricular papillary muscle strips was studied under various preload conditions (2 to 20 mN). To investigate the influence of inotropic stimulation, the force-tension relation was studied in the presence of the cardiac glycoside ouabain. In skinned-fiber preparations of the left ventricle, developed tension was measured after stretching the preparations to 150% of the resting length. To evaluate the length-dependent activation of cardiac myofibrils by Ca2+ in failing and nonfailing myocardium, the tension-Ca2+ relations were also measured. After an increase of preload, the force of contraction gradually increased in nonfailing myocardium but was unchanged in failing myocardium. There were no differences in resting tension, muscle length, or cross-sectional area of the muscles between both groups. Pretreatment with ouabain (0.02 mumol/L) restored the force-tension relation in failing myocardium and preserved the force-tension relation in nonfailing tissue. In skinned-fiber preparations of the same hearts, developed tension increased significantly after stretching only in preparations from nonfailing but not from failing myocardium. The Ca2+ sensitivity of skinned fibers was significantly higher in failing myocardium (EC50, 1.0; 95% confidence limit, 0.88 to 1.21 mumol/L) compared with nonfailing myocardium (EC50, 1.7; 95% confidence limit, 1.55 to 1.86 mumol/L). After increasing the fiber length by stretching, a significant increase in the sensitivity of the myofibrils to Ca2+ was observed in nonfailing but not in failing myocardium. These experiments provide evidence for an impaired force-tension relation in failing human myocardium. On the subcellular level, this phenomenon might be explained by a failure of the myofibrils to increase the Ca2+ sensitivity after an increase of the sarcomere length.

Existence of PAF receptors in human platelets and human lung tissue but not in the human myocardium

The aim of this study was to investigate whether platelet-activating factor (PAF), PAF receptors, and PAF receptor-mediated effects in the human myocardium play a role in cardiac depression during anaphylaxis or septic shock. The effects of PAF, the biologically inactive derivative lyso-PAF, and the specific PAF antagonist WEB 2086 were studied in human myocardial tissue, in human coronary arteries, in human platelets, and in human lung tissue. PAF (C 16-PAF, C 18-PAF; 0.000001 to 1 μmol/L) had no effect on isometric force of contraction of electrically driven right atrial trabeculae (patients undergoing aortocoronary bypass surgery) and left ventricular papillary muscle strips (mitral valve replacement). PAF (0.2 μmol/L) did not influence the concentration-response curve of either the β-adrenoceptor agonist isoprenaline (ISO, 0.0001 to 1 μmol/L) or the m-cholinoceptor agonist carbachol (CARB, 0.0001 to 10 μmol/L). The effectiveness (ISO +4.7 ± 0.7 mN, PAF + ISO + 4.3 ± 0.44 mN, CARB −2.7 ± 1.06 mN; PAF + CARB −2.6 ± 0.52 mN) and the potency—as indicated by the EC 50 values—of both isoprenaline and carbachol were identical with and without pretreatment with PAF (0.2 μmol/L). PAF at concentrations of 0.000001 to 10 μmol/L exerted no effect on force of contraction either precontracted (prostaglandin F 2α, 0.3 μmol/L) or unprecontracted in human coronary artery rings. Histamine (0.01 to 100 μmol/L) and noradrenalin (0.001 to 30 μmol/L) initiated concentration-dependent contraction in human coronary artery rings (EC 50: histamine, 1.86 μmol/L; noradrenalin, 0.69 μmol/L). At lower concentrations (PAF, 0.01 μmol/L) PAF produced complete aggregation of human platelets. In human platelet membranes and lung membranes, 3H-WEB 2086 exhibited saturable high-affinity binding (K D 14.4 nmol/L and 14.3 nmol/L). The maximal binding capacity was 292 fmol/mg protein and 268 fmol/mg protein, respectively. In displacement experiments PAF (0.01 to 10000 nmol/L) and WEB 2086 (0.01 to 10000 nmol/L), but not lyso-PAF, completely displaced 3H-WEB 2086 from its binding sites on human platelet and lung membranes. In contrast, neither in left ventricular membranes nor in right atrial membranes was specific binding of 3H-WEB 2086 observed. These results suggest that there are neither specific PAF receptors nor direct PAF-mediated actions in human myocardial tissue or human coronary artery rings. The effects of PAF on myocardial function may be due to the activation of mediators (e.g., histamine).