Increase In Force Of Contraction Research Articles

Effect of <i>Aloe Barbadensis</i> on Rat's Uterine Contractility

The present study was undertaken to investigate the effect of the aqueous extract of Aloe barbadensis on the contractility of the uterine stip of a rat. Aqueous extract at final bath concentrations (FBC)1x10-4 mg/ml to 3x101 mg/ml produced a progressive increase in frequency of contraction of the uterine strip. The force of contraction of the uterine strip decreased gradually from FBC of 1x10-4mg/ml to 3x10-3mg/ml. However at higher FBC, Aloe bardadensis showed a progressive increase in force of contraction. We therefore suggest that Aloe barbadensis has some abortifacient effects at high does. KEY WORDS: Aloe barbadensis, Uterine contraction, Purgative. Nigerian Journal of Health and Biomedical Sciences Vol.3(2) 2004: 120-122

Calcium overload and cardiac function.

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

Ouabain increases myofibrillar Ca2+ sensitivity but does not influence the Ca2+ release in human skinned fibres

The present study investigated the impact of the Na+ pump inhibitor ouabain (g-strophanthin) on Ca2+ sensitivity and Ca2+ release in human right auricular trabeculae (coronary bypass) and in skinned muscle fibres from left ventricular myocardium (cardiac transplantation, dilated cardiomyopathy). A time-dependent increase in force of contraction was observed in right auricular trabeculae in response to ouabain (100 nM) before the intracellular Ca2+ transient (fura-2) increased (n=6). In triton X-skinned fibres (no sarcoplasmic reticulum) of human failing myocardium, ouabain (1–100 nM) concentration-dependently increased tension at a free extracellular Ca2+ concentration of 1 μM and the Hill coefficient of the Ca2+-dependent tension development. Ouabain (1–100 nM) did not directly induce a Ca2+ release out of the sarcoplasmic reticulum, nor did it alter the caffeine (10 mM) induced sarcoplasmic reticulum Ca2+ release in saponin-skinned fibre preparations in which the sarcoplasmic reticulum had been Ca2+-loaded.In conclusion, ouabain increases myofibrillar Ca2+ sensitivity possibly due to an increase in the cooperativity of the thick and thin myofilaments. This mechanism may additionally contribute to the positive inotropic effect of ouabain.

Functional roles of the alpha isoforms of the Na,K-ATPase.

The Na,K-ATPase is composed of two subunits, alpha and beta, and each subunit consists of multiple isoforms. In the case of alpha, four isoforms, alpha1, alpha2, alpha3, and alpha4 are present in mammalian cells. The distribution of these isoforms is tissue- and developmental-specific, suggesting that they may play specific roles, either during development or coupled to specific physiological processes. In order to understand the functional properties of each of these isoforms, we are using gene targeting, where animals are produced lacking either one copy or both copies of the corresponding gene or have a modified gene. To date, we have produced animals lacking the alpha1 and alpha2 isoform genes. Animals lacking both copies of the alpha1 isoform gene are not viable, while animals lacking both copies of the alpha2 isoform gene make it to birth, but are either born dead or die very soon after. In the case of animals lacking one copy of the alpha1 or alpha2 isoform gene, the animals survive and appear healthy. Heart and EDL muscle from animals lacking one copy of the alpha2 isoform exhibit an increase in force of contraction, while there is reduced force of contraction in both muscles from animals lacking one copy of the alpha1 isoform gene. These studies indicate that the alpha1 and alpha2 isoforms carry out different physiological roles. The alpha2 isoform appears to be involved in regulating Ca(2+) transients involved in muscle contraction, while the alpha1 isoform probably plays a more generalized role. While we have not yet knocked out the alpha3 or alpha4 isoform genes, studies to date indicate that the alpha4 isoform is necessary to maintain sperm motility. It is thus possible that the alpha2, alpha3, and alpha4 isoforms are involved in specialized functions of various tissues, helping to explain their tissue- and developmental-specific regulation.

The preferential beta3-adrenoceptor agonist BRL 37344 increases force via beta1-/beta2-adrenoceptors and induces endothelial nitric oxide synthase via beta3-adrenoceptors in human atrial myocardium.

1 The present study investigated the effects of the preferential beta(3)-AR agonist BRL 37344 (BRL) on force of contraction (FOC), Ca(2+)-transient and eNOS-activity in human right atrial myocardium. 2 BRL concentration-dependently caused an increase in FOC that was paralleled by an increase in Ca(2+)-transient and a shortening of time to half peak relaxation (T0.5T). These effects were abolished in the presence of propranolol (0.3 micro M). 3 BRL acted as a competitive antagonist towards isoprenaline and in binding experiments it was shown to have a distinct affinity towards beta(1/2)-AR. 4 In immunohistochemical experiments BRL (10 micro M) increased detection of activated eNOS. This effect remained constant in the presence of propranolol (0.3 micro M). 5 BRL increased directly detected NO in DAF-staining experiments. This increase was significantly smaller in the presence of the NO-inhibitor L-NAME. 6 The inotropic effects of BRL were not changed in the presence of L-NMA. 7 These results suggest that the inotropic effects of BRL in human atrium are mediated via beta(1/2)-AR, whereas the increase of atrial eNOS-activity is due to beta(3)- adrenergic stimulation. This increase in eNOS-activity did not influence atrial myocardial contractility. In conclusion, this study shows that beta(3)-adrenergic stimulation is present in human atrium, but may not be functionally as significant as in the left ventricle.

Effects of the benzodiazepine tetrazepam on rat diaphragm tetanic contractions

AbstractA saturated solution of tetrazepam in Krebs‐Ringer did not affect the contraction force of the isolated diaphragm in vitro significantly during indirect stimulation at frequencies from 10 to 50 Hz. An overdose of crystalline tetrazepam, added to the bath (Krebs‐Ringer solution) induced an increase of contraction force of the tetanus at various frequencies, with a maximum at 30 Hz, up to 400% ± 30 (n=4) of the control value. It is suggested that the solubility of tetrazepam in Krebs‐Ringer solution is increased by substances released from the muscle. At a concentration of 10–4 M tetrazepam in Tween 80 (final concentration 0.25%), a comparable increase in contraction force occurred at indirect stimulation with 30 Hz. The maxima occurred at about one hour after exposure and were followed by a gradual decrease. At a stimulation frequency of 70 Hz, a gradual decrease in contraction force of 68% (± 3) (n=4) occurred after 2.5 hours, while the control force (in Tween 80, 0.25% ‐ in order to enhance the solubility of tetrazepam) decreased with 18% (± 6). At a concentration of 10–5 M, tetrazepam caused an increase in contraction force to about 150% (± 6) at 30 Hz (first stimulation of each cycle), and significant decrease in contraction force of about 20% at 70 Hz. At direct stimulation with 30 Hz the tetanic contraction force was increased as well by 10–4 M tetrazepam, whereas at 10–3 M of the benzodiazepine the force was depressed. These in vitro results fit with the muscle relaxation mentioned in a case report (two times 100 mg dose) in relation to 50 mg doses resulting in plasma level with a peak of about 2×10–6 M reported earlier (1). The results of the study tentatively suggest that the use of tetrazepam in the appropriate dose is more favourable than using diazepam in treating spasms associated with lumbago.

Effects of LY117018 and the Estrogen Analogue, 17α-Ethinylestradiol, on Vascular Reactivity, Platelet Aggregation, and Lipid Metabolism in the Insulin-Resistant JCR:LA-cp Male Rat: Role of Nitric Oxide

The JCR:LA-cp rat is obese and insulin resistant and develops a major vasculopathy, with associated ischemic damage to the heart. Male rats were treated with 17alpha-ethinylestradiol (EE), LY117018, and/or the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). LY117018 decreased plasma cholesterol esters, with a 40% reduction in total cholesterol. EE increased triglyceride levels and modestly decreased cholesterol esters. L-NAME increased blood pressure and aortic contractile sensitivity to phenylephrine and inhibited acetylcholine-induced relaxation. LY117018 decreased the force of contraction. The L-NAME-mediated increase in force of contraction and decrease in response to acetylcholine was inhibited by LY117018. L-NAME-induced hypertension was prevented by LY117018. Platelet aggregation was not different between obese and lean rats and was unaffected by L-NAME. LY117018, both in the absence and presence of L-NAME, inhibited platelet aggregation. The effects of LY117018 are apparently mediated through both NO-dependent and -independent mechanisms. The changes induced by EE and LY117018 may reflect the activation of multiple mechanisms, both estrogen receptor-dependent and -independent. The changes induced by LY117018 are significant and may prove to be cardioprotective in the presence of the insulin resistance syndrome.

Age- and Sex-associated Changes in Cardiac β1-adrenoceptors from the Muscular Dystrophy (mdx) Mouse

S. Lu and A. Hoey. Age- and Sex-associated Changes in Cardiac β1-adrenoceptors from the Muscular Dystrophy (mdx) Mice. Journal of Molecular and Cellular Cardiology (2000) 32, 1661–1668. Variations in β1-adrenoceptor function due to age or sex were examined in the mouse model of Duchenne muscular dystrophy. Positive chronotropic and inotropic responses to (-)-isoprenaline and antagonist effects of CGP20712A were determined in isolated right and left atria from young (12 week) and old (12 month) male and old (12 month) female mdx mice and their age- and sex-matched C57BL/10ScSn (C57) mice. There was no difference in efficacy to (-)-isoprenaline when expressed as an increase in the rate of contraction or force of contraction as a percentage of Ca2+-induced increase respectively in right or left atria from age- and sex-matched mdx and C57. Old mdx males showed a decreased sensitivity to (-)-isoprenaline (P<0.05) and a reduced affinity to CGP 20712A (P<0.05) in both right and left atria compared with old C57 males. These same changes were also observed in left atria between old and young mdx males. A reduced efficacy to (-)-isoprenaline was also evident when young and old mdx males were compared. In contrast, in old females, mdx showed an increased affinity to CGP20712A in left and right atria (P<0.05), and an enhanced sensitivity to (-)-isoprenaline in right atria. Finally, in left atria, the maximum Ca2+-induced increase in force of contraction was lower in all mdx compared to their age- and sex-matched C57 (P<0.05). In conclusion, age- and sex-associated changes inβ1 -adrenoceptor function and responses to calcium were demonstrated in cardiac muscle from mdx mice, with a marked deterioration in β1-adrenoceptor function occurring with aging in male mdx being particularly evident.

CAMP- and cGMP-independent stretch-induced changes in the contraction of rat atrium.

The stretch-induced changes in contraction force, cAMP and cGMP in isolated rat left atrium were studied. Increasing the diastolic intra-atrial pressure from 1 cmH2O to 8 cmH2O caused an immediate (<500 ms) increase in contraction force, the magnitude of which was 2.24+/-0.29 (n=6) times the force elicited by 1 cmH2O. This was followed by a slower, gradual increase of the force, which was maximal 8 min after the stretch (4.33+/-0.31, n=6). These phenomena were not accompanied by changes in the cAMP (n=24) or cGMP (n=24) concentrations within the tissue at any duration of stretch tested (2, 8, 20 and 36 min, n=6 at each time point). Furthermore, it was estimated that if the beta-adrenergic receptor agonist isoprenaline (100 nM) was used to produce an increase of the contraction force of the same magnitude as that induced by stretch, the cAMP concentration was greater (4.20+/-0.29 pmol/mg, n=5, P<0.001) when compared to that produced after 20 min of stretch (2.69+/-0.12 pmol/mg, n=6). Even without significantly changing the cGMP concentration, isoprenaline significantly increased the [cAMP]/[cGMP] ratio (3.4+/-0.36, n=5, P<0.01) compared to stretch (1.95+/-0.14, n=6). This result shows that in the rat atrium stretch does not regulate the production or breakdown of cyclic nucleotides (cAMP or cGMP). Thus it seems very unlikely that the effects of stretch on rat atrium function are caused by cAMP or cGMP.

Means of therapeutic physical culture in the treatment of atypical pain (motor) patterns in myofascial syndromes

There were 68 patients with low back pain which developed 2-6 years after myofascial injury of the lumbar spine. All patients were examined neuro-orthopaedically and with EMG, rheography, manual muscular testing and ultrasonography. Course of treatment using pathogenetic method of therapeutic exercises which were differentiated depending on the type of atypical pain pattern and the stage of its development was applied. Treatment resulted in the increase of contraction force and bioelectric activity amplitude of muscle-antagonist, restoration of the pattern of muscles activation (according to EMG data), elimination of static and dynamic overload of the contracted and hyperexcitable muscular groups (according to manual muscular testing data), restoration of the optimum dynamic stereotype. In 92.0% of patients recovery with the elimination of subjective neurologic symptoms and restoration of spine function was achieved.

Changes in Function of Cardiac Receptors Mediating the Effects of the Autonomic Nervous System in the Muscular Dystrophy (MDX) Mouse

S. Lu and A. Hoey. Changes in Function of Cardiac Receptors Mediating the Effects of the Autonomic Nervous System in the Muscular Dystrophy (MDX) Mouse. Journal of Molecular and Cellular Cardiology (2000) 32, 143–152. Adrenergic and muscarinic receptor mediated effects on the force of contraction and heart rate were studied in the isolated left atria and right atria from dystrophin-deficient mdx mice and age matched C57BL/10ScSn (C57) mice, respectively. The pD2and pA2values of (-)-isoprenaline and CGP 20712A, respectively, were not different in left atria and right atria from mdx and C57 mice. (-)-Phenylephrine produced a small positive inotropic effect on mdx left atria that could be antagonized by prazosin, whereas in C57 left atria no positive inotropic response was seen. In contrast, the positive chronotropic effect of (-)-phenylephrine was reduced in right atria from mdx compared to C57 right atria (P<0.05). The potency and efficacy to carbachol in the presence of (-)-isoprenaline were higher in right atria from mdx compared to C57 mice (P<0.05), although in left atria only a greater efficacy was evident in mdx mice. In left atria, basal force of contraction and maximum Ca2+-induced increases in force of contraction were lower from mdx compared to C57 mice (P<0.001 and P<0.05, respectively). In conclusion, marked changes were demonstrated in the function ofα1-adrenoceptors and muscarinic receptors, but not in β1-adrenoceptors in left and right atria from mdx mice.

Regulation of cardiac beta-adrenergic response by nitric oxide.

Time for primary review 12 days. The force and frequency of myocardial contraction are physiologically regulated by neurotransmitters and hormones. Norepinephrine released by the sympathetic nerves in the heart and epinephrine released into the circulation by adrenal glands increase myocardial contractility by acting on both α- and β-adrenergic receptors on heart muscle. Opposed to that is the action of acetylcholine released from parasympathetic nerves which reduces contractility by binding to muscarinic cholinergic receptors. The first demonstration that these two autonomic pathways are regulated by nitric oxide (NO) produced endogenously within cardiac muscle cells [1] both completes the understanding of the molecular mechanism of action of neurotransmitters in the heart and offers potential new therapeutic approaches for the correction of the altered responsiveness of cardiac muscle to autonomic regulation in circumstances such as heart failure. We will briefly review the molecular pathways leading to the intracellular actions of β-adrenergic and muscarinic cholinergic agonists in the cardiac myocyte, integrating the more recent evidence implicating the NO pathway from the single myocyte to clinical situations. ### 1.1 β-Adrenergic pathway β-Adrenergic agonists are well known to increase the force and frequency of contraction and increase the rate of relaxation of cardiac muscle. Despite some differences between regions of the heart, in general, the early increase in force of contraction is related directly to an increase in transmembrane flux of calcium, through phosphorylation of the L-type Ca channel, and the subsequent calcium-induced calcium release from the sarcoplasmic reticulum, whereas the sustained inotropic effect during the subsequent beats results from both increased influx and increased mobilization of internal calcium resulting from an increased replenishing of the reservoir of calcium in the sarcoplasmic reticulum [2,3]. A hallmark of the β-adrenergic effect on the heart is to increase the rate of relaxation which has been related to more rapid termination of … * Corresponding author. Tel.: +32-2-764-5349; fax: +32-2-764-9322 balligand{at}mint.ucl.ac.be

Effect of beta-blockers on free radical-induced cardiac contractile dysfunction.

We examined the effects of hydroxyl radicals (OH.) on human myocardial contractility and on sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) activity and the effects of the beta-receptor antagonists metoprolol, carvedilol, and its metabolite BM-910228. Isometric force of contraction was determined in isolated human myocardium. H(2)O(2) 1 mmol/L and Fe(3+)-nitrilotriacetic acid (Fe(3+)-NTA) 0.1 mmol/L used for generation of OH. induced a decrease in basal force of contraction and an increase in diastolic tension in atrial and left ventricular myocardial preparations. After challenge with OH., the maximum positive inotropic response to Ca(2+) 1.8 to 15 mmol/L was decreased by 60% and by 39%, respectively. The effects of OH. could be blocked by catalase. Carvedilol and its metabolite BM-910228 attenuated the OH.-induced impairment of the inotropic response to Ca(2+) in atrial myocardial preparations. Metoprolol had no significant effect. The stimulation frequency (0.5 to 3.0 Hz)-dependent increase in force of contraction and decrease in diastolic tension were abolished after exposure of atrial trabeculae to OH. In parallel, SERCA activity was decreased by OH. concentration-dependently, as determined in myocardial membrane preparations. BM-910228 partially restored the force-frequency relationship and preserved SERCA activity. OH. radicals induce an impairment of contraction and relaxation and an attenuation of the force-frequency relationship in human myocardium accompanied by an inhibition of SERCA. Carvedilol and BM-910228 partly prevented OH.-induced contractile dysfunction. These observations could explain the improvement of ejection fraction in heart failure trials with carvedilol without a restoration of beta-adrenergic receptor density.

Expression, activity and functional significance of inducible nitric oxide synthase in the failing human heart

Objectives. The study was designed to evaluate the functional impact of nitric oxide (NO) generation within the myocardium on cardiac contraction in the failing human heart.Background. Heart failure is associated with activation of cytokines and expression of inducible nitric oxide synthase (NOS II), which generates NO from L-arginine. Nitric oxide has been shown to modulate myocardial performance, raising the possibility that cardiac generation of NO by NOS II modulates cardiac contraction in the failing human heart.Methods. Left ventricular (LV) tissue of 24 patients with end-stage heart failure was obtained during cardiac transplantation. Gene expression of NOS II and endothelial NO-synthase (NOS III) was quantified by competitive reverse transcription-polymerase chain reaction and compared to tissues of five nonfailing donor hearts. Nitric oxide synthase II activity was determined by citrulline assay and related to changes in force of contraction induced by the β-adrenergic agonist isoproterenol, NO-donors and/or N-mono-methyl-L-arginine (L-NMMA), an inhibitor of NOS.Results. While NOS III mRNA was reduced in failing hearts, NOS II mRNA was increased in failing LV tissue and correlated with NOS II activity. High NOS II activity was associated with early relaxation and impaired responsiveness to β-adrenergic stimulation, that is, the inotropic response to isoproterenol in failing hearts was inversely related to NOS II activity (r = 0.61, p < 0.005). Nitric oxide donors or L-NMMA did not affect myocardial performance in failing hearts at baseline. However, L-NMMA enhanced the positive inotropic response to β-adrenergic stimulation in failing hearts with high NOS II activity. Nitric oxide donors attenuated the isoproterenol-induced increase in force of contraction of failing hearts.Conclusions. Cardiac production of NO by NOS II attenuates the positive inotropic effects of β-adrenergic stimulation and hastens relaxation in failing human hearts.

Unchanged sarcoplasmic reticulum Ca2+-ATPase activity, reduced Ca2+ sensitivity, and negative force-frequency relationship in transgenic rats overexpressing the mouse renin gene.

Transgenic rats overexpressing the mouse Ren-2 gene [TG(mREN2)27 rats, TGR] were used to characterize alterations in force generation and relaxation following cardiac hypertrophy. Age-matched Sprague-Dawley rats were used as the control group. The beta-adrenoceptor dependent increase in force of contraction was reduced in the transgenic animals but not the Ca2+-dependent increase in force generation. Additionally, force of contraction decreased after increasing stimulation frequencies (up to 7 Hz), but the frequency-dependent decrease in force of contraction was significantly more pronounced in the transgenic group. The Ca2+ sensitivity in chemically skinned fiber preparations of TGR was reduced than that in Sprague-Dawley rats while maximum effectiveness was the same. Unexpectedly, the sarcoplasmic reticulum Ca2+-ATPase activity measured in crude membrane preparations from TGR did not differ from that in Sprague-Dawley rats; however, the activity of the Na+/K+-ATPase was less while the Na+/Ca2+-exchanger activity was significantly greater. In the same preparations the protein expression of SERCA2 was reduced in TGR while expression of phospholamban and calsequestrin remained the same. Thus in the model of cardiac hypertrophy harboring the mouse Ren-2 gene the hypothesized correlation between SERCA2 function and force-frequency relationship was not observed. Possible reasons for the more negative force-frequency relationship in TGR included changes at the level of the myofilaments and altered intracellular Na+ homeostasis which may result from the reciprocal changes in the Na+/K+-ATPase and the Na+/Ca2+-exchanger activity.

Nitric oxide inhibits isoprenaline-induced positive inotropic effects in normal, but not in hypertrophied rat heart.

Evidence has accumulated that, in the rat heart, nitric oxide (NO) inhibits beta-adrenoceptor-mediated positive inotropic effects. The aim of this study was to investigate whether this effect of NO may be altered in cardiac hypertrophy. For this purpose we studied the effects of the NO-donor SNAP (S-nitroso-N-acetyl-D,L-penicillamine) on isoprenaline-induced positive inotropic effects in left ventricular strips from three models of cardiac hypertrophy: a) 12-16 weeks old male spontaneously hypertensive rats (SHR) vs. age-matched normotensive Wistar-Kyoto (WKY) rats, b) six weeks old male Wistar WKY-rats sub-totally nephrectomized (SNX) 7 weeks after SNX vs. sham-operated rats (SOP) and c) four weeks old male Wistar WKY-rats supra-renal aortic-banded (AOB, band diameter 1.0 mm) 8 weeks after AOB vs. SOP. In all three models of cardiac hypertrophy the heart weight/body weight ratio was significantly higher than in their respective controls. On isolated electrically driven ventricular strips isoprenaline (10(-10)-10(-5) M) caused concentration-dependent increases in force of contraction. Maximal increases (Emax) were similar in SHR vs. WKY-rats, but reduced in SNX- (2.9+/-0.29 vs. 5.1+/-0.34 mN, p<0.01) and AOB-rats (2.3+/-0.37 vs. 4.2+/-0.33 mN, p<0.01). In control rats (WKY and the respective SOP) the NO-donor SNAP (10(-5) M) caused a significant rightward-shift of the concentration-response curve for isoprenalinel; this rightward-shift could be inhibited by methylene blue (10(-5) M). In ventricular strips of SHR, SNX- and AOB-rats, however, 10(-5) M SNAP failed to significantly affect isoprenaline-induced positive inotropic effect. We conclude that in cardiac hypertrophy effects of NO are attenuated. Such an impairement of the NO-system could contribute to the development and/or maintenance of cardiac hypertrophy.

Modification of cardiac actions of Ro 05-4864 by PK 11195 and flumazenil in the perfused rat heart

The benzodiazepine analogue Ro 05-4864 [chlorodiazepam] (2.10 −5 to 4.10 −4 M) induced a concentration-dependent increase of coronary flow rate (Emax 82.4% [± 2.2 SEM]) and an increase of contraction force (Emax 68.3% [± 4.7 SEM]) of the retrograde perfused, isolated Langendorff rat heart. The influence of PK 11195, antagonist of the peripheral type benzodiazepine receptor, and flumazenil (Anexate), antagonist of the central type benzodiazepine receptor, on these responses to Ro 5-4864 was studied. In concentrations of 10 −7 to 5.10 −5 M, PK 11195 significantly reduced both the increase of coronary flow rate and of contraction force, without affecting these functions by itself; the positive inotropic response produced by Ro 05-4864 was even abolished in the presence of 5.10 −5 M PK 11195. The Emax values of Ro 05-4864 on both coronary flow and inotropy were reduced significantly by PK 11195. In the presence of flumazenil, 10 −7 to 10 −5 M, both the vasodilatory and the positive inotropic response induced by Ro 05-4864 were significantly counteracted as well. The Emax values of Ro 05-4864 were reduced significantly. In conclusion, the results support earlier suggestions that it is tempting to involve peripheral type benzodiazepine receptors in cardiac actions of benzodiazepines. The finding that the centrally acting benzodiazepine antagonist flumazenil reduced the cardiac actions of Ro 05-4864 is as yet difficult to explain. On the other hand concentrations of both agonist and antagonist employed are so-much high that interference of other receptors than benzodiazepine receptors must be considered as well.

Na+ channel modulation and force-frequency relationship in human myocardium.

The enhancement of force of contraction (FOC) following increasing frequencies of stimulation is an important mechanism of positive inotropy in human myocardium. The present study aimed to investigate the influence of alterations in Na+ influx on FFR in human myocardium. Isometric FOC of electrically stimulated right auricular trabeculae (AUT, n = 12) from human non-failing hearts (n = 8) was measured at different stimulation rates (0.5-3 Hz) under control conditions, after increasing Na+ influx by the addition of (+/-)BDF 9148 (BDF, 3 mumol l-1) and after decreasing Na+ influx by the addition of lidocaine (LIDO, 10 mumol l-1). Additionally, the rate dependent changes in diastolic tension (DT) were measured in all experiments. Under control conditions FOC increased with increasing frequencies of stimulation. The rate at which maximal FOC was observed (SFmax) was 2.0 +/- 0.2 Hz and maximal increase in FOC (PIEmax) by increasing frequency of stimulation was +1.5 +/- 0.5 mN. After increase of Na+ influx by BDF (3 mumol l-1) SFmax was decreased to 0.8 +/- 0.1 Hz (p < 0.05 versus control) and PIEmax was +0.1 +/- 0.3 mN (p < 0.05). When Na+ influx was diminished by LIDO (10 mumol l-1) SFmax and PIEmax were increased compared to control (2.4 +/- 0.1 Hz and +4.1 +/- 0.9 mN, p < 0.05 versus control). The diastolic tension (DT) of AUT at 3 Hz was not changed at higher rates in the control group and after application of LIDO (10 mumol l-1), whereas after enhancement of Na+ influx by BDF there was an increase in DT of +0.7 +/- 0.2 at 3 Hz (p < 0.05 versus control and LIDO). An enhanced Na+ influx leads to a decrease in the optimal frequency and to a smaller force potentiation by higher stimulation rates which could be at least partly due to incomplete relaxation at higher frequencies, whereas a reduced Na+ influx is followed by opposite alterations. It is concluded that besides Ca2+ handling also Na+ influx and Na+ homeostasis might determine the frequency-induced force potentiation in human myocardium. Thus, the negative FFR in diseased human myocardium might result from changes in cellular Ca2+ or Na+ regulatory sites.

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.

Ca(2+)-dependent and Ca(2+)-independent regulation of contractility in isolated human myocardium.

Changes in contractile force of the myocardium may depend on changes in the intracellular Ca2+ concentration, changes in the responsiveness of the myofibrils for Ca2+, or a combination of both. We investigated in isolated muscle strip preparations from human nonfailing and endstage failing hearts the influence of physical (changes in preload, stimulation rate, or rhythm), and pharmacological interventions (alpha- or beta-adrenoceptor-stimulation, endothelin) on developed force of contraction and the corresponding intracellular Ca2+ transients. Isometric contraction, electrical stimulation, 37 degrees C. Simultaneous registration of force of contraction and intracellular Ca2+ transients (aequorin method). Increases in preload, alpha- and endothelin-receptor stimulation resulted in increases in force of contraction without increasing aequorin light emission. Increasing stimulation rate or increasing rest intervals resulted in parallel increases (nonfailing myocardium) or decreases (failing myocardium) of force of contraction and aequorin light emission. beta-Adrenoceptor-stimulation exerted inotropic and lusitropic effects in human failing myocardium associated with a large, overproportional increase in aequorin light emission. The human heart regulates intrinsic contractility via several subcellular mechanisms. Increases in preload (Frank-Starling-mechanism) and alpha- or endothelin-receptor-stimulation enhance myocardial contractility by increasing the Ca2+ responsiveness of the myofilaments; rate- and rhythm-dependent modulation of the contractile state directly depend on changes in the intracellular Ca(2+)-transients; beta-adrenoceptor stimulation results in an overproportional large increase in intracellular Ca2+ transients, probably due to additional cAMP-dependent Ca(2+)-desensitizing effects on the level of the myofibrils.