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Four mexiletine analogues have been tested for their antiarrhythmic, inotropic, and chronotropic effects on isolated guinea pig heart tissues and to assess calcium antagonist activity, in comparison with the parent compound mexiletine. All analogues showed from moderate to high antiarrhythmic activity. In particular, three of them ( 1b,c,e ) were more active and potent than the reference drug, while exhibiting only modest or no negative inotropic and chronotropic effects and vasorelaxant activity, thus showing high selectivity of action. All compounds showed no cytotoxicity and 1b,c,d did not impair motor coordination. All in, these new analogues exhibit an interesting cardiovascular profile and deserve further investigation. [ABSTRACT FROM AUTHOR]

Background Reactive oxygen species, such as superoxide, are being increasingly recognized as key components of a vast array of signaling pathways. Angiotensin II is a well-recognized stimulus for superoxide production through NADPH oxidase activation and opening of the mitochondrial ATP-sensitive potassium channels (mK ATP ). A role for this mechanism has been proposed to explain several physiological effects of the peptide. The aim of this study was to evaluate the involvement of this mechanism in the inotropic response to 100 nmol/L angiotensin II. Methods Sarcomere shortening and intracellular pH (BCECF-epifluorescence technique) were evaluated in isolated cat ventricular myocytes placed in a perfusion chamber on the stage of an inverted microscope. Myocardial superoxide production was evaluated by the lucigenin quimioluminiscence method. Results Angiotensin II (100 nmol/L) increased~70% sarcomere shortening, effect that was only partially prevented by NADPH oxidase inhibition, mK ATP channel blockade or inhibition of the cardiac Na + /H + exchanger (NHE-1). Moreover, angiotensin II stimulates NHE-1 activity by a NADPH oxidase-dependent mechanism. Myocardial superoxide production was also increased by angiotensin II, and this action was completely prevented either by NADPH oxidase inhibition or mK ATP channel blockade. Conclusions The positive inotropic response to 100 nmol/L angiotensin II is due to both ROS/NHE-1 dependent and independent pathways, this being a point of divergence with the signaling previously described to be triggered by lower concentrations of angiotensin II (i.e.: 1 nmol/L). [ABSTRACT FROM AUTHOR]

Abstract: Introduction: Dobutamine stress echocardiography (DSE) is performed to detect subtle ischemic regions in the myocardium in patients suffering from stenosed coronary artery disease. While in a clinical and veterinary context DSE is performed daily, in the preclinical context, a very limited application has been described in the literature. In the drug development process, the safety of new chemical entities on the cardiovascular system is a primary requirement for the development of new agents/drugs for administration in humans. There are limited tools to evaluate the impairment of myocardial performance and cardiac contractility in preclinical species and particularly in rodents. In this present paper a possible imaging application is presented. Methods: Anesthetised male Crl:CD(SD) rats (n =14) were given single intravenous doses of vehicle (5% w/v Dextrose solution) and dobutamine hydrochloride at 10μg/kg/min. Echocardiography was performed, starting immediately before the infusion and animals were monitored up to 5min after the end of infusion. Standard and non-standard echocardiography parameters were evaluated to detect test article related changes compared to vehicle infused animals. Results: Thickening of left ventricle walls and interventricular septum and increased velocity of circumferential stress were observed in dobutamine infused animals compared to those receiving vehicle. These changes were considered to be a direct effect on myocardial contractility, as also confirmed by increased functional parameters (i.e., Ejection Fraction, End Diastolic Volume and decrease of the ratio between pre-ejection period and ejection time of the aortic flow). In dobutamine infused rats the “Tei index” increased up to approximately 32% compared to vehicle animals, showing a clear relationship with increased contractility. Discussion: As expected dobutamine infusion in the anaesthetised rat is capable of inducing a positive inotropic effect and echocardiography results are a reliable tool for the determination of changes in contractility induced by pharmacological stress testing in the rat. [Copyright &y& Elsevier]

Abstract: Adrenomedullin (AM) effects were studied in rabbit papillary muscles by adding increasing concentrations (10−10 to 10−6 M) either alone or after pre-treatment with l-NNA, indomethacin, AM22–52 (AM receptor antagonist), CGRP(8–37) (CGRP receptors antagonist), KT5720 (PKA inhibitor), as well as after endocardial endothelium (EE) removal. Passive length–tension relations were constructed before and after a single concentration of AM (10−6 M). AM concentration-dependently induced negative inotropic and lusitropic effects, and increased resting muscle length (RL). At 10−6 M, AT, dT/dt max and dT/dt min decreased 20.9±4.9%, 18.3±7.3% and 16.7±7.8%, respectively, and RL increased to 1.010±0.004 L/L max. Correcting RL to its initial value resulted in a 26.6±6.4% decrease of resting tension, indicating decreased muscle stiffness, also patent in the down and rightward shift of the passive length–tension relation. The negative inotropic effect of AM was dependent on its receptor, CGRP receptor, PKA, the EE and NO, while the effects of AM on myocardial stiffness were abolished by EE damage and NO inhibition. This latter effect represents a novel mechanism of acute neurohumoral modulation of diastolic function, suggesting that AM is an important regulator of cardiac filling. [Copyright &y& Elsevier]

Abstract: Endogenous polyamines mediate acute metabolic effects and cardiac hypertrophy associated to β-adrenoceptor stimulation. The aim of this study is to characterize the role of polyamines on β-adrenoceptor system mediated responses. To this end, the functional interaction of polyamine modifying drugs on isoproterenol-elicited cardiotonic effect, in isolated left atria of male Wistar rats, and their effects on [3H]dihydroalprenolol (DHA) binding on β-adrenoceptors and on adenylyl cyclase activity of membrane heart were studied. Polyamines interact with β-adrenoceptors in rat heart, as shown by the displacement of [3H]DHA binding. Furthermore, putrescine (but not spermidine or spermine) increased adenylyl cyclase activity, elicited a positive inotropism and increased intracellular cAMP. The putrescine effect on adenylyl cyclase was not antagonized by the β-adrenoceptors blockers, alprenolol and ICI-118,551, and facilitated the isoproterenol effect. Neither alprenolol, atenolol nor ICI-118,551 antagonized putrescine-elicited positive inotropism. However, the effect was abolished in preparations with desensitized β-adrenoceptors. α-Difluoromethylornithine, an inhibitor of ornithine decarboxylase, antagonized the effect of isoproterenol on inotropism and cAMP increase. In addition, putrescine might elicit effects by mechanisms independent of β-adrenoceptor system, since in left atria with functional desensitized receptors an interaction with ouabain-elicited cardiotonic effect was observed. These results suggest that putrescine may act as a low affinity agonist on β-adrenoceptors and modulate acute responses mediated by β-adrenoceptors. These findings may be of importance in the physiology and in diseases involving cardiac β-adrenoceptors. [Copyright &y& Elsevier]

Abstract: Angiotensin II is an octapeptide whose effects are mediated by two types of receptors. AT1 receptors are responsible for the vasoconstrictor, positive inotropic and growth promoting properties, while AT2 receptors have been linked to vasodilator and anti-mitogenic properties. In this study we investigated the effects of selective AT2 receptor stimulation on myocardial contractility and lusitropy. Effects of selective AT2 receptor activation were evaluated in rabbit right papillary muscles (n =96) by adding increasing concentrations of H-9395, an AT2 receptor agonist, alone or in presence of a selective AT1 receptor antagonist (ZD-7155), or alternatively, by adding increasing concentrations of angiotensin II in presence of ZD-7155. In the latter conditions, selective AT2 receptor activation was also performed in presence of NG-nitro-l-Arginine, indomethacin, proadifen, hydroxocobalamin, apamin plus charybdotoxin, Hoe-140 or PD-123,319, as well as, after endocardial endothelium removal. Selective AT2 stimulation induced a negative inotropic and lusitropic effect in the first three protocols. This effect was completely abolished after selective removal of the endocardial endothelium and blunted in presence of Hoe-140, hydroxocobalamin, apamin plus charybdotoxin and PD-123,319, but maintained in presence of NG-nitro-l-Arginine, indomethacin or proadifen. Selective AT2 receptor stimulation induces a negative inotropic and lusitropic effect, which is modulated by endocardial endothelium and mediated by bradykinin B2 receptors through NO release and calcium dependent potassium channels activation. Such findings may help to better understand the therapeutic effects of selective AT1 antagonists, which are increasingly used for treating cardiovascular diseases. [Copyright &y& Elsevier]

Abstract: We analyzed the influence of endothelin-1 and endocardial endothelium on the myocardial effects of angiotensin-II. Angiotensin-II (10−9–10−5 M) was tested in rabbit right papillary muscles in absence (Protocol-A) or presence of PD-145065 (10−7 M; Protocol-B), BQ-123 (10−7 M; Protocol-C) or losartan (10−6 M; Protocol-E), as well as, after removing the endocardial endothelium with Triton X-100 0.5% (Protocol-D). In Protocol-F increasing concentrations of endothelin-1 (10−10–10−8 M) were added in presence of angiotensin-II (10−7 M) after selective removal of the endocardial endothelium. In Protocol-A, angiotensin-II had dose-dependent positive inotropic and lusitropic effects, maximal at 10−6 M increasing 122±13% active tension, 117±16% dT/dt max and 86±9% dT/dt min. In Protocols B, C and D the inotropic and lusitropic effects of angiotensin-II were significantly attenuated. The same concentration (10−6 M) of angiotensin-II increased respectively 48±11%, 59±27% and 72±16% active tension; 54±14%, 54±20% and 32±9% dT/dt max; and 39±8%, 48±19% and 59±11% dT/dt min; and 40±10%. EC50 for active tension significantly increased from −7.8±0.1 logM in Protocol A to −7.1±0.3, −6.7±0.4 and −6.8±0.3 logM in Protocols B, C and D respectively, while E max decreased from 106±14% in Protocol A to 50±14 and 51±19% in Protocols B and C respectively, but did not significantly change in Protocol D (114±25%). Losartan completely blocked the inotropic and lusitropic effects of angiotensin-II, while the attenuation of these effects after the selective removal of the endocardial endothelium was blunted by concomitant administration of endothelin-1 (Protocol F). In conclusion, angiotensin-II has a dose-dependent positive inotropic effect that depends, to a great extent, on endothelin ETA receptor activation and intact endocardial endothelium. [Copyright &y& Elsevier]

Abstract: A mechanism underlying the increase in rat heart contractility modulated by BmK I, an α-like scorpion neurotoxin, was investigated using whole-cell patch-clamp and fluorescence digital imaging techniques. Results showed that (a) L-type Ca2+ current could not be modified by 500nM BmK I; (b) The inactivation process of Na+ current was significantly delayed with no change of its amplitude; (c) The overall intracellular Na+ and Ca2+ concentration could be augmented in the presence of BmK I; (p<0.05); (d) The increase of free intracellular Ca2+ concentration induced by BmK I was inhibited completely by 5mM NiCl2 (p<0.05), an inhibitor of Na+–Ca2+ exchanger; (e) The spontaneous Ca2+ release induced by 10mM caffeine from sarcoplasmic reticulum could not be modulated by 500nM BmK I in the absence of external Ca2+. These results indicate that cardiac voltage-gated Na+ channels are also targets of BmK I. Na+ accumulation through Na+ channels can trigger sarcoplasmic reticulum Ca2+ release in rat cardiac myocytes via reverse-mode Na+–Ca2+ exchanger. Furthermore, Ca2+ release from sarcoplasmic reticulum induced by BmK I most likely involves a Ca2+-induced release mechanism. [Copyright &y& Elsevier]

Putrescine, spermidine and spermine are natural compounds found in up to millimolar concentrations in eukaryotic and prokaryotic cells. At physiologic pH, the polyamines are protonated (+2, +3 and +4 charges), their polycationic properties lead to the assumption that they could affect physiological systems by binding to anionic sites of the cellular membrane and/or by modulating ion channels. At the cardiovascular level, their effects are not completely understood. However, these compounds may be able to exert the induction of synthesis and release of cellular mediators. In an attempt to explore this possibility, we used the isolated and perfused rat heart, Langendorff, model in order to evaluate the inotropic effects of these polyamines, putrescine, spermidine and spermine. Dose–response curves (0.1–0.6 mM) for putrescine, spermidine and spermine were constructed; with the finding that spermine had the largest negative effect. The obtained effects were not blocked by nitric oxide synthesis inhibitors (l-NAME), H1 and H2 receptor antagonists (Brompheniramine and Cimetidine) or by Glibenclamide, an antagonist of ATP-sensitive K+ channels.We found that spermine-induced and increased ATP concentration in cardiac effluents. Reactive Blue, a P2y purinoreceptor antagonist and Aminophylline, an unspecific adenosine receptor antagonist, blocked the spermine-induced effects. These results showed that ATP, at least in part, is responsible of the spermine cardiovascular effects. Adenosine was shown to also play an important role on those effects. [Copyright &y& Elsevier]

The effect of aqueous crude extract (ACE) of Eugenia uniflora L. (Myrtaceae) was studied on rat''s perfused ventricles. This plant is used in South American traditional medicine as an antihypertensive and we already demonstrated previously its hypotensive properties. In this paper, maximal left intraventriclular pressure (P) of rat''s hearts beating at 0.2 Hz firstly increased to 162.1±11.1% of basal value during 1–3 min of perfusing ACE 0.6%. Maximum rate of contraction (+P) also increased to duplicating +P/P ratio. Both types of effect were significantly decreased by either propranolol 0.35 μM, and pre-treatment with reserpine (5 mg/kg), suggesting that they were caused by a compound that releases cathecolamines with β-adrenergic action. Nevertheless, after 20 min of perfusing ACE, ventricles decreased P to about 50% of their basal value, suggesting a negative-inotropic compound present in the extract. The perfusion of 1.2% ACE decreased P in a pressure–[Ca]o curve (0.5–2 mM) in a non-competitive manner, suggesting that an irreversible Ca-blocking compound is also present in the extract. In summary, E. uniflora ACE has a dual effect on the heart related to its hypotensive action and is probably responsible for the therapeutic or adverse effects in patients under cardiac risk. [Copyright &y& Elsevier]