Negative Inotropy Research Articles

Influence of vasostatins, the chromogranin A-derived peptides, on the working heart of the eel ( Anguilla anguilla): negative inotropy and mechanism of action

We have studied the effects of exogenous human recombinant Vasostatin-1 (VS-1), Vasostatin-2 (VS-2) and the human Chromogranin A (CGA) 7–57 synthetic peptides on the mechanical performance of the isolated and perfused working eel ( Anguilla anguilla) heart. Under basal conditions, the three peptides decreased stroke volume (SV) and stroke work (SW), thus exerting negative inotropism. The VS-1-mediated negative inotropism was abolished by exposure to inhibitors of either Gi/o protein (pertussis toxin; PTx) or M1 muscarinic receptors (Pirenzepine) or calcium (Lantanum and Diltiazem) and potassium (Ba 2+, 4-aminopyridine, tetraethylammonium, glibenclamide) channels, while it required an intact endocardial endothelium (EE). Using N G-monomethyl- l-arginine ( l-NMMA) as an inhibitor of nitric oxide (NO) synthase (NOS), and hemoglobin as a NO scavenger, we demonstrated the obligatory role of NO signaling in mediating the vasostatin response. Pretreatment with either a specific inhibitor of soluble guanylate cyclase (GC) 1H-(1,2,4)oxadiazolo-(4,3- a)quinoxalin-1-one (ODQ), or the inhibitor of the cGMP-activated protein kinase (PKG) KT 5823, abolished the VS-1-mediated inotropism, indicating the cGMP-PKG component as a crucial target of NO signaling. Of note, VS-1 was effective in counteracting the adrenergic (Isoproterenol and Phenylephrine)-mediated positive inotropism. These findings provide the first evidence that vasostatins exert cardiotropic action in fish, thus suggesting their long evolutionary history as well as their species-specific mechanisms of action.

Mechanism of vasoselective action of mebudipine, a new calcium channel blocker

In previous studies, mebudipine, a newly synthesized dihydropyridine calcium channel blocker, showed a considerable vasoselective action. To investigate the mechanism of this property, the pattern of inhibitory action on the KCl-induced contraction in isolated rat aortic rings and the effect of changing resting membrane potential on the potency of this compound were considered. In addition, its chronotropic and inotropic actions were also studied. Mebudipine inhibited KCl-induced contractions. Its inhibitory effect was progressive and needed time to reach maximum. Incubation of the aortic rings in depolarizing physiological solution (high potassium, zero calcium) resulted in the augmentation of mebudipine effect. The potency of mebudipine in inhibiting aortic contractions and its time- and voltage-dependent action were significantly greater than those of nifedipine. In comparison with nifedipine, mebudipine showed a greater negative chronotropic effect, but in the case of negative inotropism, the reverse relation was observed. It is concluded that mebudipine has a greater time- and voltage-dependent inhibitory effect, as compared to nifedipine and this property could explain its prominent vasoselective action. It has also marked negative chronotropic effect and minor negative inotropic action. With regard to the above findings, mebudipine might have a selective and protective calcium channel blocking effect in ischemic regions (ischemia-selectivity), and the potential to be used in cardiovascular diseases without causing harmful effects such as reflex tachycardia and heart failure which have sometimes been seen with the older agents.

Cannabinoid antagonist SR-141716 inhibits endotoxic hypotension by a cardiac mechanism not involving CB1 or CB2 receptors.

Endocannabinoids and CB1 receptors have been implicated in endotoxin (LPS)-induced hypotension: LPS stimulates the synthesis of anandamide in macrophages, and the CB1 antagonist SR-141716 inhibits the hypotension induced by treatment of rats with LPS or LPS-treated macrophages. Recent evidence indicates the existence of cannabinoid receptors distinct from CB1 or CB2 that are inhibited by SR-141716 but not by other CB1 antagonists such as AM251. In pentobarbital-anesthetized rats, intravenous injection of 10 mg/kg LPS elicited hypotension associated with profound decreases in cardiac contractility, moderate tachycardia, and an increase in lower body vascular resistance. Pretreatment with 3 mg/kg SR-141716 prevented the hypotension and decrease in cardiac contractility, slightly attenuated the increase in peripheral resistance, and had no effect on the tachycardia caused by LPS, whereas pretreatment with 3 mg/kg AM251 did not affect any of these responses. SR-141716 also elicited an acute reversal of the hypotension and decreased contractility when administered after the response to LPS had fully developed. The LPS-induced hypotension and its inhibition by SR-141716 were similar in pentobarbital-anesthetized wild-type, CB1(-/-), and CB1(-/-)/CB2(-/-) mice. We conclude that SR-141716 inhibits the acute hemodynamic effects of LPS by interacting with a cardiac receptor distinct from CB1 or CB2 that mediates negative inotropy and may be activated by anandamide or a related endocannabinoid released during endotoxemia.

Preconditioning: evolution of basic mechanisms to potential therapeutic strategies.

Preconditioning describes the phenomenon by which a traumatic or stressful stimulus confers protection against subsequent injury. Originally recognized in dog heart subjected to ischemic challenges, preconditioning has been demonstrated in multiple species, can be induced by various stimuli, and is applicable in different organ systems. Tremendous progress has been made elucidating the signal transduction cascade of preconditioning. Preconditioning represents a potent tissue-protective condition, and mechanistic understanding may allow safe clinical application. This review recalls the history of preconditioning and how it relates to the history of the investigation of endogenous adaptation; summarizes the current mechanistic understanding of acute preconditioning; outlines the signal transduction cascade leading to the development of delayed preconditioning; discusses preconditioning in noncardiac tissue; and explores the potential of using preconditioning clinically.

The Hippocampal Cholinergic Neurostimulating Peptide, the N-terminal Fragment of the Secreted Phosphatidylethanolamine-binding Protein, Possesses a New Biological Activity on Cardiac Physiology

Phosphatidylethanolamine-binding protein (PEBP), alternatively named Raf-1 kinase inhibitor protein, is the precursor of the hippocampal cholinergic neurostimulating peptide (HCNP) corresponding to its natural N-terminal fragment, previously described to be released by hippocampal neurons. PEBP is a soluble cytoplasmic protein, also associated with plasma and reticulum membranes of numerous cell types. In the present report, using biochemistry and cell biology techniques, we report for the first time the presence of PEBP in bovine chromaffin cell, a well described secretion model. We have examined its presence at the subcellular level and characterized this protein on both secretory granule membranes and intragranular matrix. In addition, its presence in bovine chromaffin cell and platelet exocytotic medium, as well as in serum, was reported showing that it is secreted. Like many other proteins that lack signal sequence, PEBP may be secreted through non-classic signal secretory mechanisms, which could be due to interactions with granule membrane lipids and lipid rafts. By two-dimensional liquid chromatography-tandem mass spectrometry, HCNP was detected among the intragranular matrix components. The observation that PEBP and HCNP were secreted with catecholamines into the circulation prompted us to investigate endocrine effects of this peptide on cardiovascular system. By using as bioassay an isolated and perfused frog (Rana esculenta) heart preparation, we show here that HCNP acts on the cardiac mechanical performance exerting a negative inotropism and counteracting the adrenergic stimulation of isoproterenol. All together, these data suggest that PEBP and HCNP might be considered as new endocrine factors involved in cardiac physiology.

Effects of endogenous adenosine and adenosine receptor agonists on hypoxia-induced myocardial stunning in Guinea-pig atria and papillary muscles.

The effects of endogenous adenosine and adenosine receptor agonists were examined on hypoxia-induced myocardial stunning of guinea-pig isolated paced left atria and papillary muscles. Hypoxia (30 minutes) reduced developed tension and increased diastolic tension (contracture) of left atria (41.8 +/- 11.5%) and papillary muscles (17.7 +/- 6.2%). Developed tension recovered to 80.8 +/- 3.15 and 77.2 +/- 5.3% 15 minutes after reoxygenation (stunning). Recovery of left atria was unaffected by adenosine deaminase (1 IU mL) but was depressed in papillary muscles (15 minutes, 48.6 +/- 4.3%) and contracture (46.1 +/- 7.5%) increased. Endogenous adenosine therefore protects from ventricular but not atrial stunning. Adenosine receptor agonists were introduced at 10 minutes into hypoxia. CPA (A1 selective, 3 x 10 M) impaired left atrial recovery (5 minutes, 38.1 +/- 5.0%), through direct negative inotropy, but did not affect papillary muscles. CGS21680 (A2A selective, 3 x 10 M) did not affect recovery. APNEA (A1/A3 receptor agonist, 10 M), increased recovery rate of left atria. Improved rate and extent of recovery of papillary muscles by APNEA (15 minutes, 94.8 +/- 3.1%) was prevented by the A3 receptor antagonist, MRS-1220 (10 M). IB-MECA (A3 selective, 3 x 10 M) increased atrial recovery rate but not the maximum developed tension reached in either tissue. However, when added at reoxygenation, IB-MECA caused complete recovery of both tissues, in the absence or presence of adenosine deaminase. Thus, A3 receptor stimulation reverses myocardial stunning of isolated atria and papillary muscles.

Chromogranin A N-terminal fragments vasostatin-1 and the synthetic CGA 7–57 peptide act as cardiostatins on the isolated working frog heart

Chromogranin A (CGA) N-terminal fragments corresponding to residues 1–76 and 1–113, named vasostatins for their inhibitory effects on vascular tension, have been postulated as important homeostatic regulators of the cardiovascular system. We have used an in vitro isolated working frog ( Rana esculenta) heart as a bioassay to study the effects of exogenous human recombinant CGA 1–76 (VS-1) and human CGA 7–57 synthetic peptide on cardiac performance. Under basal conditions, the concentration–response curves of the two peptides exhibited a significant negative inotropism. This vasostatin response was unaffected by pretreatment with either Triton X-100 or two nitric oxide synthase inhibitors, i.e., N G-monomethyl- l-arginine and l- N 5(1-iminoethyl) ornithine or the soluble guanylate cyclase inhibitor 1 H-(1,2,4) oxadiazolo-(4,3- a) quinoxalin-1-one, indicating an endocardial endothelium–nitric oxide–cGMP-independent mechanism. The negative inotropism was also unaffected by either adrenergic (i.e., phentolamine and propranolol) or muscarinic (atropine) receptor or G proteins (pertussis toxin) inhibition. On the contrary, it was dependent from both extracellular Ca 2+ and K + channels, since it was abolished by pretreatment to either the Ca 2+ channel inhibitors lanthanum and diltiazem or the K + channel inhibitors Ba 2+, 4-aminopyridine, tetraethylammonium chloride, and glibenclamide. In conclusion, the findings that vasostatins exert an inhibitory modulation on basal cardiac performance and counteract, as previously reported, the adrenergic-mediated positive inotropism, strongly support a cardio-regulatory role for these peptides.

Evolutionary Medicine

Nothing in biology makes sense except in the light of evolution. Evolutionary, or darwinian, medicine takes the view that contemporary diseases result from incompatibility between the conditions under which the evolutionary pressure had modified our genetic endowment and the lifestyle and dietary habits in which we are currently living, including the enhanced lifespan, the changes in dietary habits and the lack of physical activity. An evolutionary trait express a genetic polymorphism which finally improve fitness, it needs million years to become functional. A limited genetic diversity is a necessary prerequisite for evolutionary medicine. Nevertheless, search for a genetic endowment would become nearly impossible if the human races were genetically different. From a genetic point of view, homo sapiens, is homogeneous, and the so-called human races have only a socio-economic definition. Historically, Heart Failure, HF, had an infectious origin and resulted from mechanical overload which triggered mechanoconversion by using phylogenically ancient pleiotropic pathways. Adaptation was mainly caused by negative inotropism. Recently, HF was caused by a complex remodelling caused by the trophic effects of mechanics, ischemia, senescence, diabetes and, neurohormones. The generally admitted hypothesis is that cancers were largely caused by a combination of modern reproductive and dietary lifestyles mismatched with genotypic traits, plus the longer time available for a confrontation. Such a concept is illustrated for skin and breast cancers, and also for the link between cancer risk and dietary habits.

Phosphoinositide 3-kinase gamma-deficient hearts are protected from the PAF-dependent depression of cardiac contractility.

Following an ischemic insult, cardiac contractile recovery might be perturbed by the release of autacoids, like platelet-activating factor (PAF), that depress heart function by acting through G protein-coupled receptors (GPCRs). The signaling events downstream the PAF receptor that lead to the negative inotropic effect are still obscure. We thus investigated whether the GPCR-activated phosphoisositide 3-kinase gamma (PI3Kgamma) could play a role in the cardiac response to PAF. The negative inotropic effect of PAF was studied ex vivo, in isolated electrically driven atria and in Langendorff-perfused whole hearts derived from wild-type and PI3Kgamma-null mice. Postischemic recovery of contractility was analyzed in normal and mutant whole hearts subjected to 30 min of ischemia and 40 min of reperfusion in the presence or absence of a PAF receptor antagonist. While wild-type hearts stimulated with PAF showed increased nitric oxide (NO) production and a consequent decreased cardiac contractility, PI3Kgamma-null hearts displayed reduced phosphorylation of nitric oxide synthase 3 (NOS3), blunted nitric oxide production and a complete protection from the PAF-induced negative inotropism. In addition, Langendorff-perfused PI3Kgamma-null hearts showed a better contractile recovery after ischemia/reperfusion, a condition where PAF is known to be an important player in depressing contractility. In agreement with a role of PI3Kgamma in this PAF-mediated signaling, postischemic contractile recovery in PI3Kgamma-null mice appeared overlapping with that of normal hearts treated with the PAF receptor antagonist WEB 2170. These data indicate a novel PAF-dependent signaling pathway that, involving PI3Kgamma and NOS3, contributes to postischemic contractile depression.

Blockers of the Kv1.5 channel for the treatment of atrial arrhythmias.

Atrial arrhythmias are a common problem in cardiological practice. Despite the availability of several antiarrhythmic drugs, there is a medical need for safer and more efficient antiarrhythmic treatment. Compounds that act atrial selectively without prolonging the QTc-time and without negative inotropy to terminate and/or prevent atrial arrhythmias would be of high interest. In this context, the voltage-gated potassium channel Kv1.5 is regarded as a promising target to achieve atrial selectivity, which in turn would be associated with fewer side effects than classical antiarrhythmics. This review summarizes patents and other publications on compounds which show this novel mode of action. The chemistry, selectivity and structure-activity data disclosed in the literature are discussed in light of recent work demonstrating the antiarrhythmic efficacy of Kv1.5 blockers in vivo. Several studies in pig, dog or goat models have confirmed their proposed atrial selective antiarrhythmic effect in vivo. Most of the more intensively characterized Kv1.5 blockers have turned out not to be selective but also block other ion channels. Based on the currently available data it seems that additional inhibition of Kv4.3 and KACh is beneficial for the desired antiarrhythmic effect or at least does not hamper the atrial selectivity of a Kv1.5 blocker. Significant block of IK1, HERG or sodium channels, however, clearly leads to loss of atrial selectivity and increases the risk of lethal ventricular proarrhythmia.

Mechanism of the negative inotropic effects of alpha 1-adrenoceptor agonists on mouse myocardium.

This study was done to identify the mechanism of the alpha1-adrenoceptor (AR) mediated negative inotropic effects of phenylephrine (PE) on adult mouse myocardium. As reported by others, we also found that the nonselective alpha1AR agonist PE produced a negative inotropic effect on ventricular strips from adult mice that was inhibited by the alpha1AAR antagonist 5-methylurapidil (5MU) but not by the alpha1BAR antagonist chloroethylclonidine (CEC) or the alpha1DAR antagonist BMY 7378. The selective alpha1AAR agonist A61603 also produced a negative inotropic effect, which was antagonized by 5MU. Phorbol 12,13-dibutyrate (activator of all PKC isoforms) mimicked the negative inotropic responses to PE and A61603. The negative inotropic effects of PE were inhibited by bisindolylmaleimide (inhibitor of all PKC isoforms) but not by Gö 6976 (inhibitor of Ca2+-dependent PKC). Rottlerin, an inhibitor of Ca2+-independent PKCdelta, antagonized the negative inotropic effects of PE and A61603. PE and A61603 increased the translocation of PKCdelta, which was prevented by rottlerin. These data suggest that the alpha1AR-mediated negative inotropy on adult mouse myocardium is signaled by Ca2+-independent PKCdelta.

Peptides from the N-terminal domain of chromogranin A (vasostatins) exert negative inotropic effects in the isolated frog heart

The negative inotropic effects of synthetic peptides derived from the N-terminus of chromogranin A (CgA) were studied in an avascular model of the vertebrate myocardium, the isolated working frog heart ( Rana esculenta). The peptides were frog and bovine CgA 4–16 and CgA 47–66, and bovine CgA 1–40 with (CgA 1–40SS) and without an intact disulfide bridge (CgA 1–40SH). Under basal cardiac conditions, four of the peptides caused a concentration-dependent negative inotropism that was comparable to the negative inotropy reported for human recombinant vasostatin I (CgA 1–78) and bovine CgA 7–57. By comparison of the structural characteristics of the bovine and frog sequences with their minimally effective concentrations ranging from 68 to 125 nM of peptide, the results were consistent with the natural structure (CgA 17–38SS) being essential for the negative inotropism. In addition, the partial sequences of the frog and bovine vasostatin I were effective in counteracting the characteristic positive inotropism exerted by isoproterenol (1 nM) at minimally effective concentrations ranging from 45 to 272 nM. Taken together, these results extend the first evidence for a cardiosuppressive role of the N-terminal domain of chromogranin A known for its co-storage with catecholamines in the sympathoadrenal system of vertebrates.

Cardiac role of frog ANF: negative inotropism and binding sites in Rana esculenta

To elucidate the role of atrial natriuretic peptides (NPs) in the amphibian heart, the myotropic effects and the cardiac distribution of frog atrial natriuretic factor (fANF) have been studied in Rana esculenta. Spontaneously, beating in vitro isolated working heart preparations were treated with increased concentrations (10 −11–10 −8 M) of fANF-(1–24). The peptide at 10 −9 and 10 −8 M significantly reduced heart rate (HR) and, on the electrically paced preparations, decreased cardiac output (CO), stroke volume (SV) and work. Such negative inotropism was abolished by pretreatment with the pertussis toxin or by blocking the particulate guanylate cyclase (GC) with anantin while it was independent both from the functional impairment of the endocardium–endothelium by Triton X-100 and the inhibition of the soluble guanylate cyclase by 1 H-(1,2,4,) oxadiazolo-(4,3-a) quinoxalin-1-one (ODQ). By autoradiography, two classes of high and low affinity NPs binding sites were detected in the ventricular endocardium and myocardium and in the bulbus arteriosus. The analysis of displacement binding data using the radioligand [ 125I]-rat atrial natriuretic peptide [ 125I-rANP-(1–28)], its cold counterpart and the fANF-(1–24) showed that in the ventricular myocardium, the low affinity NPs sites bound both the heterologous and the homologous ligands at a concentration close to that responsible for the negative inotropism and chronotropism.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle.

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.

Pharmacological Evidence for Involvement of Phospholipase D, Protein Kinase C, and Sodium-Calcium Exchanger in -Adrenoceptor-Mediated Negative Inotropy in Adult Mouse Ventricle

Pharmacological Evidence for Involvement of Phospholipase D, Protein Kinase C, and Sodium-Calcium Exchanger in -Adrenoceptor-Mediated Negative Inotropy in Adult Mouse Ventricle

Cardiovascular effects of natriuretic peptides and their interrelation with endothelin-1.

The natriuretic peptides are a group of structurally related but genetically distinct peptides. Four types of natriuretic peptides have been found thus far: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP) and Dendroaspis natriuretic peptide (DNP). ANP and BNP are secreted mainly from the heart and function as hormones with vasodilatory and natriuretic effects. CNP originates mainly from endothelial cells with a paracrine effect to induce vasodilation. Other effects of natriuretic peptides including negative inotropy, antimitogenic and anticoagulation have been described. Three types of natriuretic peptide receptors mediate their functions, and among them two are cGMP-coupled. Clearance of natriuretic peptides is via its clearance receptor through the action of neutral endopeptidases. Natriuretic peptides interact with other vasoactive peptides including endothelin. The putative role of natriuretic peptides in the pathophysiology of various cardiovascular diseases including congestive heart failure, hypertension, ischemic heart disease, and cardiomyopathy are discussed. Natriuretic peptide plasma levels are used for the diagnosis and therapeutic follow-up of congestive heart failure patients. Increasing the levels of natriuretic peptides by natriuretic peptide mimetics and neutral endopeptidase inhibitors may provide a new therapeutic strategy for the treatment of cardiovascular diseases such as congestive heart failure and hypertension.

Alpha 1-adrenergic receptor responses in alpha 1AB-AR knockout mouse hearts suggest the presence of alpha 1D-AR.

Two functional alpha(1)-adrenergic receptor (AR) subtypes (alpha(1A) and alpha(1B)) have been identified in the mouse heart. However, it is unclear whether the third known subtype, alpha(1D)-AR, is also present. To investigate this, we determined whether there were alpha(1)-AR responses in hearts from a novel mouse model lacking alpha(1A)- and alpha(1B)-ARs (double knockout) (ABKO). In Langendorff-perfused hearts, alpha(1)-ARs were stimulated with phenylephrine. For ABKO hearts, phenylephrine reduced left ventricular pressure and coronary flow (to 87 +/- 2% and 86 +/- 4% of initial, respectively, n = 11, P < 0.01). These effects were blocked by prazosin and 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspirol[4,5]decane-7,9-dione] dihydrochloride, suggesting that alpha(1D)-AR-mediated responses were present. In contrast, right ventricular trabeculae from ABKO hearts did not respond to phenylephrine, suggesting that in ABKO perfused hearts, the effects of phenylephrine were not mediated by direct actions on cardiomyocytes. A novel finding was that alpha(1)-AR stimulation caused positive inotropy in the wild-type mouse heart, in contrast to negative inotropy observed in mouse cardiac muscle strips. We conclude that mouse hearts lacking alpha(1A)- and alpha(1B)-ARs retain functional alpha(1)-AR responses involving decreases of coronary flow and ventricular pressure that reflect alpha(1D)-AR-mediated vasoconstriction. Furthermore, alpha(1)-AR inotropic responses depend critically on the experimental conditions.

Contributions of heart rate and contractility to myocardial oxygen balance during exercise.

The respective contributions of heart rate (HR) reduction and left ventricular (LV) negative inotropy to the effects of antianginal drugs are debated. Accordingly, eight instrumented dogs were investigated during exercise at spontaneous and paced HR (250 beats/min) after administration of either saline, atenolol, or ivabradine (selective pacemaker current channel blocker). During exercise, atenolol and ivabradine (both 1 mg/kg iv) similarly reduced HR (-30% from 222 +/- 5 beats/min), and LV mean ejection wall stress was not altered. LV dP/dt(max) was reduced by atenolol but not ivabradine. Diastolic time (DT) was increased by atenolol versus saline (195 +/- 6 vs. 123 +/- 4 ms, respectively) and to a greater extent by ivabradine (233 +/- 11 ms). Myocardial oxygen consumption (MVo(2)) was lower under ivabradine and atenolol versus saline (6.7 +/- 0.6 and 4.7 +/- 0.4 vs. 8.1 +/- 0.6 ml/min, respectively, P < 0.05). Under pacing, DT and MVo(2) were similar between ivabradine and saline but significantly reduced with atenolol. Thus HR reduction and negative inotropy equally contribute to the reduction in MVo(2) during exercise in the normal heart. The negative inotropy limits the increase in DT afforded by HR reduction.

Vasostatins exert negative inotropism in the working heart of the frog.

An in vitro isolated working frog heart (Rana esculenta) was used to study the effects of exogenous CGA(1-76) (vasostatin 1), CGA(1-113) (vasostatin 2), and the synthetic CGA(7-57) on cardiac performance. Under basal cardiac conditions, the dose-response curves of the three peptides from 10(-8) to 10(-7) M showed a significant calcium-dependent negative inotropism that involved neither the endocardial endothelium nor the adrenergic and muscarinic receptors. In addition, the CgA fragments clearly counteracted the typical positive inotropism of isoprenaline (10(-<9) M). Taken together, these results provide the first evidence for a cardio-suppressive role for the vasostatins.

α1-Adrenoceptor Subtypes Mediate Negative Inotropy in Myocardium from α1A/C-Knockout and Wild Type Mice

D. T. McCloskey, D. G. Rokosh, T. D. O'Connell, E. C. Keung, P. C. Simpson and A. J. Baker. α1-Adrenoceptor Subtypes Mediate Negative Inotropy in Myocardium from α1A/C-Knockout and Wild Type Mice. Journal of Molecular and Cellular Cardiology (2002) 34, 1007–1017. Cardiac α1-adrenoceptors (AR) have two predominant subtypes (α1A-AR and α1B-AR) however, their roles in regulating contraction are unclear. We determined the effects of stimulating α1A-AR (using the subtype-selective agonist A61603) and α1B-AR (using a gene knockout mouse lacking α1A-AR) separately, and together (using phenylephrine) on Ca2+ transients, intracellular pH, and contraction of mouse cardiac trabeculae. Stimulation of α1-AR subtypes separately or together caused a triphasic contractile response. After a transient (≈3%) force rise (phase 1), force declined markedly (phase 2), then partially recovered (phase 3). In phase 2, the force decline (% of initial) with combined α1A-AR plus α1B-AR stimulation (50±3%) was more than with separate subtype stimulation (P<0.01), suggesting α1A-AR and α1B-AR mediate additive effects during phase 2. Force decline in phase 2 paralleled decreases of Ca2+ transients that were reduced more with combined vs. separate subtype stimulation. During phase 3 the final force reduction was similar with stimulation of α1A-AR (20±5%), or α1B-AR (20±3%), or both (26±4%) suggesting α1A-AR and α1B-AR mediate non-additive effects during phase 3. In contrast, Ca2+ transients recovered fully in phase 3 suggesting reduced force in phase 3 involved decreased myofilament Ca2+-sensitivity. Decreased Ca2+-sensitivity was not mediated by changes of intracellular pH since this was not affected by α1-AR stimulation. In contrast to mouse trabeculae, rat trabeculae demonstrated a positive inotropic response to α1-AR stimulation. In conclusion, for mouse myocardium in vitro both α1-adrenoceptor subtypes mediate negative inotropy involving decreased Ca2+ transients and a decreased Ca2+ sensitivity that does not involve altered intracellular pH.