Dose-dependent Positive Inotropic Effect Research Articles

Inotropic and lusitropic, but not arrhythmogenic, effects of adipocytokine resistin on human atrial myocardium

The adipocytokine resistin is released from epicardial adipose tissue (EAT). Plasma resistin and EAT deposition are independently associated with atrial fibrillation. The EAT secretome enhances arrhythmia susceptibility and inotropy of human myocardium. Therefore, we aimed to determine the effect of resistin on the function of human myocardium and how resistin contributes to the proarrhythmic effect of EAT. EAT biopsies were obtained from 25 cardiac surgery patients. Resistin levels were measured by ELISA in 24-h EAT culture media (n = 8). The secretome resistin concentrations increased over the culture period to a maximal level of 5.9 ± 1.2 ng/mL. Coculture with β-adrenergic agonists isoproterenol (n = 4) and BRL37344 (n = 13) had no effect on EAT resistin release. Addition of resistin (7, 12, 20 ng/mL) did not significantly increase the spontaneous contraction propensity of human atrial trabeculae (n = 10) when given alone or in combination with isoproterenol. Resistin dose-dependently increased trabecula-developed force (maximal 2.9-fold increase, P < 0.0001), as well as the maximal rates of contraction (2.6-fold increase, P = 0.002) and relaxation (1.8-fold increase, P = 0.007). Additionally, the postrest potentiation capacity of human trabeculae was reduced at all resistin doses, suggesting that the inotropic effect induced by resistin might be due to altered sarcoplasmic reticulum Ca2+ handling. EAT resistin release is not modulated by common arrhythmia triggers. Furthermore, exogenous resistin does not promote arrhythmic behavior in human atrial trabeculae. Resistin does, however, induce an acute dose-dependent positive inotropic and lusitropic effect.

Istaroxime, a potential anticancer drug in prostate cancer, exerts beneficial functional effects in healthy and diseased human myocardium.

The current gold standard for prostate cancer treatment is androgen deprivation therapy and antiandrogenic agents. However, adverse cardiovascular events including heart failure can limit therapeutic use. Istaroxime, which combines Na+-K+-ATPase (NKA) inhibition with sarco/endoplasmic reticulum Ca2+-ATPase 2a (SERCA2a) stimulation, has recently shown promising anti-neoplastic effects in prostate cancer (PC) models and may also improve cardiac function. Considering the promising anticancer effects of istaroxime, we aimed to assess its functional effects on human myocardium.ResultsIstaroxime and strophanthidin elicited dose-dependent positive inotropic effects with a decline in developed force at supraphysiological concentrations in human atrial, nonfailing, and failing ventricular (ToF) myocardium. Diastolic force and RT50% did not change after exposure to both drugs. The maximal developed force in our in-vitro model of heart failure (ToF) was significantly higher after istaroxime administration. Such a difference did not occur in atrial or nonfailing ventricular trabeculae and was not applicable to the diastolic force.Materials and MethodsHuman atrial and ventricular trabeculae were isolated from nonfailing hearts and hearts of infants with tetralogy of Fallot (ToF), which were used as an in-vitro model of heart failure. The samples were electrically stimulated and treated with increasing concentrations of istaroxime and strophanthidin (10 nM–1 μM). Systolic and diastolic force development and relaxation parameters (RT50%) were analyzed.ConclusionsCombined NKA inhibition/SERCA2a stimulation increases contractility in atrial, nonfailing, and failing myocardium. Considering that heart failure is a potential side effect of current PC treatments, especially in elderly patients, istaroxime might combine beneficial cardiac and anti-cancer properties.

Abstract 334: An Inotropic S100A1-derived Peptide Enhances Contractile Performance and Survival of Post-ischemic Failing Hearts

S100A1 gene therapy was suggested as therapeutic for cardiomyopathies since myocardial levels of S100A1 are decreased in heart failure. S100A1 increases systolic and diastolic performance of cardiomyocytes through enhanced Ca2+-induced SR Ca2+ release and augmented SR Ca2+ re-uptake. These effects are due to enhanced systolic Type 2 Ryanodine Receptor (RyR2) and diastolic Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase 2a (SERCA2a) activity. The S100A1 C-terminus (S100A1ct) was recently shown to be the bioactive domain of the protein. The aim of this study was therefore to characterize the effects of an S100A1ct-derived peptide on cardiomyocyte function and to test its therapeutic action in a mouse model of heart failure. An S100A1ct-derived peptide encompassing aa75-85 of the human S100A1 protein and preceded by a hydrophilic motif (DKDDPP) was named S100A1ct6/11. S100A1ct6/11 was cell permeable and accumulated in the intracellular space of intact rat ventricular cardiomyocytes in a striated pattern, similar to endogenous S100A1. S100A1ct6/11 exerted a time- and dose-dependent positive inotropic effect in electrical field stimulated rat ventricular cardiomyocytes. This effect was associated with the regulation of the SR calcium content. Peptides encompassing aa75-85 derived from S100 paralogs A4 and B did not mimic S100A1ct6/11-mediated inotropy. S100A1ct6/11 protected cardiomyocytes from pro-arrythmic store overload-induced calcium leak as well as from chronic caffeine exposure-induced apoptotic cell death. Mice intravenously injected with S100A1ct6/11 exhibited a significant enhancement of LV contractile performance under basal and βAR-stimulated conditions. Daily intraperitoneal administration of S100A1ct6/11 for two weeks to mice with post-ischemic contractile dysfunction resulted in significantly improved contractile performance and survival due to reduced myocardial apoptosis. Moreover, this treatment protected failing hearts with caffeine-induced leaky RyR2 channels from βAR-triggered lethal ventricular tachyarrhythmias. In conclusion, administration of the S100A1ct6/11 peptide may represent a novel option for the treatment of cardiomyopathies without apparent side effects.

The novel chromogranin A‐derived serpinin and pyroglutaminated serpinin peptides are positive cardiac β‐adrenergic‐like inotropes

Three forms of serpinin peptides, serpinin (Ala26Leu), pyroglutaminated (pGlu)-serpinin (pGlu23Leu), and serpinin-Arg-Arg-Gly (Ala29Gly), are derived from cleavage at pairs of basic residues in the highly conserved C terminus of chromogranin A (CgA). Serpinin induces PN-1 expression in neuroendocrine cells to up-regulate granule biogenesis via a cAMP-protein kinase A-Sp1 pathway, while pGlu-serpinin inhibits cell death. The aim of this study was to test the hypothesis that serpinin peptides are produced in the heart and act as novel β-adrenergic-like cardiac modulators. We detected serpinin peptides in the rat heart by HPLC and ELISA methods. The peptides included predominantly Ala29Gly and pGlu-serpinin and a small amount of serpinin. Using the Langendorff perfused rat heart to evaluate the hemodynamic changes, we found that serpinin and pGlu-serpinin exert dose-dependent positive inotropic and lusitropic effects at 11-165 nM, within the first 5 min after administration. The pGlu-serpinin-induced contractility is more potent than that of serpinin, starting from 1 nM. Using the isolated rat papillary muscle preparation to measure contractility in terms of tension development and muscle length, we further corroborated the pGlu-serpinin-induced positive inotropism. Ala29Gly was unable to affect myocardial performance. Both pGlu-serpinin and serpinin act through a β1-adrenergic receptor/adenylate cyclase/cAMP/PKA pathway, indicating that, contrary to the β-blocking profile of the other CgA-derived cardiosuppressive peptides, vasostatin-1 and catestatin, these two C-terminal peptides act as β-adrenergic-like agonists. In cardiac tissue extracts, pGlu-serpinin increased intracellular cAMP levels and phosphorylation of phospholamban (PLN)Ser16, ERK1/2, and GSK-3β. Serpinin and pGlu-serpinin peptides emerge as novel β-adrenergic inotropic and lusitropic modulators, suggesting that CgA and the other derived cardioactive peptides can play a key role in how the myocardium orchestrates its complex response to sympathochromaffin stimulation.

Abstract P211: MRP4: A Novel Protein Involved in Camp Homeostasis in the Heart

Rationale: Transporters of the ABCC family, called MRP4 and MRP5, have been described to control cAMP homeostasis by extruding it from various cell types. Objective: The aim of our study was to determine the role of MRP4 in cAMP homeostasis and heart function and its impact on cardiac hypertrophy (CH). Methods and results: MRP4 KO mice and MRP4 KO with cardiac overexpression of the HCN-based cAMP sensor were used. Echocardiography and morphology studies showed that 3 month-old MRP4 KO mice displayed normal cardiac function (FS: 35.7% ± 1.02 in KO vs 36% ± 1.97 in WT) and cardiac phenotype (HW/TL: 7.59 ± 0.14 in WT mice vs 7.75 ± 0.15 in KO) in basal condition. Stimulation of WT and MRP4 KO mice by increasing concentrations of isoproterenol (ISO: 0.1μg/kg to 3 mg/kg) induced dose-dependent positive inotropic and chronotropic effects but there was no difference in the rate of contraction (Vmax: 11214 ± 682 in WT vs 10697 ± 691 in KO) or in the heart rate (724 ± 21 in WT vs 711 ± 21 in KO) between both groups. Cardiac myocytes were isolated from WT and MRP4 KO mice with cardiac expression of the HCN-based FRET sensor and intracellular cAMP concentrations were measured by FRET. Application of ISO (1nM) induced a similar increase in cAMP level in WT and MRP4 KO mice (YFP/CFP: 4.4 ± 0.15 in WT vs 4.1 ± 0.44 in KO cells), whereas cAMP signal was greater in MRP4 KO compared to WT mice after PDE inhibition by IBMX (ISO 1nM + IBMX 300µM: 8.1 ± 0.6 in WT vs 11.8 ± 0.62 in KO cells; p&lt;0.01). PDE3A and PDE4A mRNA level was 1.6 fold higher in the myocardium of MRP4 KO compared to WT mice. MRP4 KO mice displayed age-dependent CH (HW/TL in 9 month-old mice: 9.8 ± 0.57 in KO vs 7.82 ± 0.13 in WT mice; p&lt;0.01). 3 month-old MRP4 KO and WT mice were subjected to thoracic aortic clamping (TAC) or sham operated (Sh) and CH was assessed at 12 weeks after the surgery by echocardiography and morphometric studies. Heart function was also examined by hemodynamic technique in basal conditions and upon a β-adrenergic stimulation in WT (Sh and TAC) and MRP4 KO (Sh and TAC) mice. Alteration of left ventricular dP/dt upon β-adrenergic stimulation after TAC was more severe in MRP4 KO compared to WT mice. Conclusion: Taken together, our results show that both PDEs and MRP4 are important in cAMP homeostasis.

Polyphenols from Parabarium huaitingii and their positive inotropic and anti-myocardial infarction effects in rats

Eight phenolic compounds, including (-)-epicatechin (1) and seven proanthocyanidins (2-8), were obtained from the butanol extract of Parabarium huaitingii (PHB). Their chemical structures were identified based on analyses of mass spectra (MS), NMR, CD spectra, and partial acid catalyzed thiolytic degradation. The observation made by laser scanning confocal microscope found a significant increase of the concentration of intracellular Ca²+ ([Ca²+](i)) in single myocytes when the PHB was added, while compounds 1 and 3 had the same physiological effect. Further investigations showed PHB had a dose-dependent positive inotropic effect on isolated right atria and papillary muscle of left ventricle of the rat, while having no significant influence on the spontaneous beating rate of the isolated right atria. The inotropic effect of PHB could be greatly abolished by pretreating the myocardium in Ca²+-free solution. These findings indicated that PHB could significantly increase [Ca²+](i) in myocytes, which was greatly dependent on the influx of extracellular Ca²+. Compounds 1 and 3 might be the effective ingredients of the inotropic effect of PHB. In addition, PHB could also significantly decrease the infarct size of the heart on acute myocardial infarction (AMI) model rats, which suggested its myocardial protective effect on ischemic myocardium. The positive inotropic effect of PHB, together with its myocardial protective effect on AMI, suggested that PHB had a promising potential for the prevention and treatment of heart failure, especially the one that was caused by AMI.

Influence of temperature on the positive inotropic effect of levosimendan, dobutamine and milrinone

Patients in cardiac surgery and critically ill patients often demonstrate either hypothermia or fever. In addition, owing to heart failure, they frequently require inotropic support. The relative effectiveness of modern inotropic agents at various temperatures has not yet been evaluated. Therefore, we investigated the influence of levosimendan, dobutamine and milrinone on the contractile response of myocardial trabeculae at various temperatures. A total of 120 guinea pig ventricular trabeculae were placed in oxygenated 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer, stimulated at a frequency of 1.3 Hz and randomly assigned to a temperature of 31 degrees C, 34 degrees C, 37 degrees C or 40 degrees C. Concentrations of all substances were increased stepwise from 10(-9) to 10(-5) mol l(-1) (milrinone up to 10(-4) mol l(-1)). Maximum developed force, time to peak tension, Tsystolic(50%) and Tdiastolic(50%) were continuously recorded. All agents showed a dose-dependent positive inotropic effect (P < 0.0001 for all). Levosimendan acted at every temperature as a positive inotrope (P = 0.0643). Dobutamine-related inotropy showed a clear trend towards temperature dependence, although statistical evaluation did not prove this (P = 0.0624). Milrinone-related inotropy was abolished at 31 degrees C and 34 degrees C, and temperature dependence was significant (P < 0.0001). Hypothermia induced a positive inotropic effect. Our results suggest no modulation of levosimendan-induced inotropy under the experimental temperatures tested. This observation is possibly due to its Ca2+-sensitizing mechanism, which might not be influenced by temperature-related changes in intracellular Ca2+ levels. In contrast, the inotropic effect of cyclic AMP-coupled dobutamine and milrinone is suppressed under hypothermia-related interaction with intracellular Ca2+ homeostasis. Hence, levosimendan might prove to be the preferred inotropic drug in hypothermic patients.

Prolactin-releasing peptide regulates cardiac contractility

High levels of specific prolactin-releasing peptide (PrRP) binding sites have been found in the myocardium; however, the functional importance of PrRP in the regulation of cardiac function is unknown. In isolated perfused rat hearts, infusion of PrRP (1–100 nM) induced a dose-dependent positive inotropic effect. Inhibition of cAMP catabolism by IBMX, a phosphodiesterase inhibitor, failed to augment the contractile effect of PrRP. The protein phosphatase (PP1/PP2A) inhibitor calyculin A increased the inotropic response to PrRP, whereas the PP2A inhibitor okadaic acid had no effect. Ro32-0432, a protein kinase Cα (PKCα) inhibitor, significantly enhanced the inotropic effect of PrRP as well as the phosphorylation of phospholamban at Ser-16. In conclusion, the present data define a hitherto unrecognized role for PrRP in the regulation of cardiovascular system by showing that PrRP exerts a direct positive inotropic effect. Moreover, our results suggest that the cAMP-independent inotropic response to PrRP is suppressed by concurrent activation of PKCα and PP1.

Positive inotropic effect of the leaf extracts of parent and tissue culture plants of Coleus amboinicus on an isolated perfused frog heart preparation

The leaves of the indigenous plant Coleus amboinicus Lour (Lamiaceae) possess a characteristic aroma and are used in many food preparations to enhance the flavour. Apart from this, they also exhibit medicinal properties. Aqueous extracts of fresh leaves of C. amboinicus Lour. parent (CAP), as well as tissue culture-raised plants (CAT), showed a dose-dependent positive inotropic effect on isolated frog heart. The extracts of CAT showed higher activity. The responses to CAP and CAT (50–800 μg) were not affected in either way by propranolol, atropine, verapamil or imidazole. The responses to both CAP and CAT were inhibited after lowering sodium chloride concentration (from 111 to 55.6 mM) and potentiated after increasing sodium chloride concentration (from 111 to 222 mM). Lignocaine was found to inhibit the responses significantly. Thus, C. amboinicus induces a positive inotropic effect, possibly by increasing sodium influx, leading to greater intracellular availability of calcium.

Purinergic regulation of rat heart function in ontogeny

The effect of exogenous ATP and its analogs on heart function was studied in 14-100-day-old rats. Extracellular purines had a positive chronotropic effect on the heart. Intravenous administration of exogenous ATP and its stable analogs induced a dose-dependent increase in heart rate depending on animal age. The analysis of isometric contraction of myocardial strips demonstrated a dose-dependent positive inotropic effect of ATP. The family and subtype of the P2 receptors realizing the positive chronotropic and inotropic effects were identified using selective agonists and blockers. P2X receptors demonstrated the highest sensitivity during early postnatal ontogeny. The age-related pattern of the receptor response to exogenous purines indicated the heterochronic maturation of P2X and P2Y receptors in the myocardium.

Endothelin ET A receptors and endothelium partially mediate the positive inotropic and lusitropic effects of angiotensin II

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% d T/d t max and 86 ± 9% d T/d t 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% d T/d t max; and 39 ± 8%, 48 ± 19% and 59 ± 11% d T/d t min; and 40 ± 10%. EC 50 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 ET A receptor activation and intact endocardial endothelium.

Modulation of the positive inotropic effect of pyruvate by energetic substrate availability

Dependence of pyruvate’s positive inotropic effect on energetic substrate availability and potential role of its mitochondrial uptake were investigated. Pyruvate (3, 10 and 15 mM) was added to rabbit right ventricular papillary muscles (protocol I; n = 10) and human right auricular trabeculae (protocol II; n = 6), using glucose as energetic substrate. In protocols III & IV (rabbit papillary muscles; n = 8 and n = 10, respectively) pyruvate’s mitochondrial uptake was inhibited by α-cyano-4-hydroxycinnamate (0.5 mM) with octanoate as energetic substrate at 5 and 0.2 mM, respectively. In 8 additional rabbit papillary muscles, effects of L-alanine (10, 20 and 50 mM) were tested. In protocols I&II, pyruvate had a dose-dependent positive inotropic effect that was maximal at 10 mM, increasing in rabbit myocardium: 45.0±9.4% active tension, 20.5±7.4% peak rate of tension rise, 32.5±8.6% peak isotonic shortening, 31.2±11.7% peak rate of lengthening, 27.8±3.2% twitch duration. In protocol III (5 mM octanoate), pyruvate’s positive inotropic effect was still present and even enhanced, while in protocol IV (0.2 mM octanoate) it was decreased and not observed with 3 mM of pyruvate.We conclude that, in rabbit papillary muscles, the positive inotropic effect of pyruvate is modulated by the availability of metabolic substrates and presumably does not depend on its mitochondrial uptake.

Apelin, the novel endogenous ligand of the orphan receptor APJ, regulates cardiac contractility.

The orphan receptor APJ and its recently identified endogenous ligand, apelin, exhibit high levels of mRNA expression in the heart. However, the functional importance of apelin in the cardiovascular system is not known. In isolated perfused rat hearts, infusion of apelin (0.01 to 10 nmol/L) induced a dose-dependent positive inotropic effect (EC50: 33.1+/-1.5 pmol/L). Moreover, preload-induced increase in dP/dt(max) was significantly augmented (P<0.05) in the presence of apelin. Inhibition of phospholipase C (PLC) with U-73122 and suppression of protein kinase C (PKC) with staurosporine and GF-109203X markedly attenuated the apelin-induced inotropic effect (P<0.001). In addition, zoniporide, a selective inhibitor of Na+-H+ exchange (NHE) isoform-1, and KB-R7943, a potent inhibitor of the reverse mode Na+-Ca2+ exchange (NCX), significantly suppressed the response to apelin (P<0.001). Perforated patch-clamp recordings showed that apelin did not modulate L-type Ca2+ current or voltage-activated K+ currents in isolated adult rat ventricular myocytes. Apelin mRNA was markedly downregulated in cultured neonatal rat ventricular myocytes subjected to mechanical stretch and in vivo in two models of chronic ventricular pressure overload. The present study provides the first evidence for the physiological significance of apelin in the heart. Our results show that apelin is one of the most potent endogenous positive inotropic substances yet identified and that the inotropic response to apelin may involve activation of PLC, PKC, and sarcolemmal NHE and NCX.

Mechanical and electrophysiological effects of thiopental on rat cardiac left ventricular papillary muscle

Thiopental induces a negative inotropic effect on mammalian heart muscle, where it decreases Ca2+ current and Ca2+ release from the sarcoplasmic reticulum and reduces K+ currents. We analysed the effects of thiopental on the mechanical and electrical activities of rat myocardium, which differ markedly from those of other mammals. The effects of thiopental on mechanical parameters and on the transmembrane resting (RP) and action (AP) potentials of rat left ventricular papillary muscle were investigated. These effects were also studied in the presence of atenolol, a beta-blocking agent, and 4-aminopyridine (4-AP), a blocker of the transient outward K+ current. Thiopental (3.8 x 10(-6), 3.8 x 10(-5) and 1.1 x 10(-4) M) induced a dose-dependent positive inotropic effect. This positive inotropic effect persisted in the presence of atenolol (1 x 10(-6) M) but did not develop in the presence of 1 mM 4-AP; 4-AP had a positive inotropic effect but not in the presence of thiopental. Moreover, thiopental (3.8 x 10(-5) M) lengthened the plateau and the slow repolarizing phase of the AP, while 1 mM 4-AP only prolonged the plateau duration. In rat myocardium, the positive inotropic effect of thiopental in part mimics that of 4-AP, and in part may be explained by the lengthening of the slow repolarizing phase of the AP.

The electrical and mechanical response of adult guinea pig and rat ventricular myocytes to ω3 polyunsaturated fatty acids

Single adult guinea-pig and rat ventricular cardiac myocytes were used to study the effects of two members of the ω3 class of polyunsaturated fatty acids, docosahexaenoic acid and eicosapentaenoic acid, on the electrical and mechanical activity of cardiac muscle. Docosahexaenoic acid and eicosapentaenoic acid reduced the electrical excitability of both guinea-pig and rat cells in a dose-dependent manner. Both agents produced a dose-dependent negative inotropic response in guinea-pig cells but in the rat cells there was first a dose-dependent positive inotropic effect at low concentrations (<10 μM) followed by a negative inotropic effect at higher concentrations (>10 μM). Possible mechanisms by which these agents affect contraction were studied using conventional electrophysiological techniques. The polyunsaturated fatty acids reduced the action potential duration and the plateau potential of the guinea-pig cells in a simple, dose-dependent manner. In contrast, the effect on the rat action potential mirrored the inotropic effect. At low concentrations (<10 μM) there was a concentration-dependent increase in action potential duration followed by a concentration-dependent decrease at higher concentrations (>10 μM). Both polyunsaturated fatty acids decreased the fast Na + current and the L-type Ca 2+ current in a concentration-dependent but not use-dependent manner in cells from both species. In the rat cells these agents inhibited the transient outward current resulting in an increase in the duration of the rat action potential. The effects of polyunsaturated fatty acids on the Ca 2+, Na + and K + currents underlie these changes in the action potentials in guinea-pig and rat heart cells. The effects on the L-type Ca 2+ current and action potential duration can also explain both the simple negative inotropic effects of the agents on the guinea-pig cells and the more complex effects on the rat cells. These effects of polyunsaturated fatty acids on membrane currents may account for their anti-arrhythmic properties.

DIRECT EFFECTS OF REMIFENTANIL ON CARDIOMYOCYTE IN RATS

S476 Does-dependent hypotension and bradycardia have been reported hemodynamically with remifentamil. However, the direct effects of remifentanil on myocardium has not been characterized. This study was designed to investigate the effects of fentanyl on contractility and electric excitability of rat cardiomyocytes. METHODS: With institutional approval, 30 cardiomyocytes were enzymatically isolated from the hearts of 4 anesthetized male Sprague-Dawley rats, the cardiomyocyte was projected onto a monitor via an inverted video camera. Computer-controlled solenoids allowed instantaneous changeover of various supersantes containing remifentanil 0, 1, 5, 10, 50, 100 [micro sign]g/ml prepared with Tyrode solution, administered in that order. The range of contractile shortening was detected by a computerized phase-contrast edge detector. Contractility was quantified by myocyte shortening in response to supramaximal field stimulation at 0.6 Hz. Excitability was determined by stimulation threshold and excitability curve of cardiomyocytes. Data were analyzed by repeat-measure ANOVA and T-test for p<0.01 RESULTS: The results are summarized in Figure 1.Figure 1CONCLUSIONS: Our study shows that remifentanil has a dose-dependent positive inotropic effect up to 100 [micro sign]g/ml with no change of chronaxie on rat cardiomyocytes. The mechanism is unclear. Further study is needed.

Cardiovascular effects of the aqueous extract of Moringa pterygosperma

AbstractThe aqueous extract of stem bark of Moriga pterygosperma (Family Moraingaceae) was investigated for its effect on various pharmacological parameters. In cardiovascular profile at lower concentrations (1–10 ng) it produced a dose dependent positive inotropic effect (n = 3, 1.29 ± 0.021 for 10 ng) and at higher concentrations (0.1–1 μg) a dose dependent negative inotropic effect (n = 3, 0.53 ± 0.033 for 1 μg) on the isolated frog heart. It also produced a dose dependent hypotensive effect on dog blood pressure (n = 3, 82 ± 0.98 for 20 mg/kg). It failed to elicit any effect on isolated guinea‐pig ileum, rat stomach fundus or frog rectus abdominis muscle.

The effects of exogenous histamine in isolated rat hearts.

The role of histamine in cardiac physiology and pathophysiology is not clarified, but is dependent on species. The effects of exogenous histamine in Langendorff-perfused rat hearts were investigated. 1 mM, 100, 10, 1 and 0.1 microM of histamine (n = 7 each) as 15 min infusions were employed in a dose-response study, and compared to control perfused hearts (n = 7). In another experimental series, 100 microM histamine (n = 15) was added during reperfusion after 25 min global ischemia, and compared to control ischemia-reperfusion (n = 15). The maximal response to histamine in the dose-response study (100 microM) was an increase of left ventricular developed pressure to 126 +/- 8% of initial value (mean +/- SEM, p < 0.04), and increase of coronary flow to 152+6% (p < 0.02) after 5 min infusion. 100 microM histamine did not significantly influence heart rate or rhythm. The lowest concentration (0.1 microM) did not have effects cardiac performance. Reperfusion with histamine for 2 min after ischemia reduced left ventricular developed pressure to 68 +/- 10% of initial value versus 116+17% in ischemic controls (p < 0.05), and increased left ventricular end-diastolic pressure to 24 +/- 8 mmHg compared to 6 +/- 2 mmHg in controls (p < 0.04). Left ventricular pressures were similar in hearts reperfused with histamine and in ischemic controls for the rest of the observation. Coronary flow increased during reperfusion in hearts given histamine. Histamine had a dose-dependent positive inotropic and vasodilatory effect in isolated rat hearts. Exogenous histamine had only minor effects on post-ischemic cardiac function.

Effects of extracellular Cl − on the inotropic response to α-adrenoceptor stimulation

This study was designed to determine if the sustained positive inotropic action of α-adrenergic stimulation is affected by the absence of extracellular chloride ion (Cl o −). Atrial and papillary muscle were isolated from adult male rats, bathed in Krebs-Henseleit solution (30°C) with and without Cl − (methane-sulfonate substitution), and stimulated at 0.5 Hz. Isometric developed tension was monitored during cumulative addition of phenylephrine, isoproterenol and Ca 2+. The dose-dependent positive inotropic effects of isoproterenol and Ca 2+ were not altered by the absence of Cl o −. However, the magnitude of the response to phenylephrine was diminished in both tissues. In atrial muscle, the maximum positive inotropic effect of phenylephrine was reduced from 2.05±0.17 g in the presence of Cl o − to 0.39±0.06 g in the absence of Cl o −; control developed tension was 0.60±0.08 and 0.47±0.10 g in these two groups before exposure to the α-adrenoceptor agonist. In papillary muscle, control developed tension was 1.40±0.11 and 1.17±0.18 g in the presence and absence of Cl o −, respectively; and the maximum inotropic responses to phenylephrine were 0.71±0.12 and 0.27±0.13 g. EC 50 values for phenylephrine were not significantly affected by substitution for Cl −. Similar results were observed in a Hepes-buffered bathing solution without bicarbonate (HCO 3 −). These results indicate that the positive inotropic action of α-adrenergic stimulation is mediated in part by a mechanism requiring Cl −. Furthermore, data suggest that the antagonistic effect of Cl o − removal is not mediated via Cl −/HCO 3 − exchange.

Influence of formamide on the electrical and mechanical properties of the guinea pig ventricle

1. 1. Guinea pig papillary muscle in vitro upon exposure of Formamide (FMD) exerts a dose dependent positive inotropic effect. 2. 2. The increase in tension developed by papillary muscle is associated with a marked decrease in the duration of the action potential. 3. 3. The increase in extracellular calcium concentration does not modify the positive inotropic effect, but increases the duration of the action potential. 4. 4. FMD markedly potentiate the increase in tension induced by caffeine 3 M suggesting a possible synergetic effect upon the release of Ca 2+ ions from the S.R. 5. 5. These experiments suggest that FMD increases the intracellular calcium concentration by exerting a direct action on intracellular calcium stores.