Guinea Pig Heart Muscle Research Articles

The influence of the neuropeptide galanin on the contractility and the effective refractory period of guinea-pig heart papillary muscle under normoxic and hypoxic conditions.

The aim of this study was to discover the effects of galanin, a neuropeptide comprising 29 amino acids capable of activating inward-rectifier K+ channels (IK1) in cardiomyocytes, on the force of contraction (Fc), velocity of contraction (+dF/dt), velocity of relaxation (-dF/dt) and effective refractory period (ERP) of guinea-pig heart. The influence of galanin on the time-course of hypoxia-induced disturbances in contractility and ERP was also examined. Experiments were performed on the isolated right ventricle papillary muscle of guinea-pig heart. In the concentration range 0.001-0.01 microM, galanin had no significant effect on the measured parameters. At 0.03 and 0.1 microM, galanin exerted a positive inotropic action and prolonged ERP. Further increasing the concentration led to a negative inotropic action and significant shortening of ERP. Although simulated hypoxia induced a significant drop in Fc, +dF/dt and -dF/dt, and a significant shortening of ERP, recovery of all the measured parameters was complete after 10 min reperfusion. In the presence of 0.03 and 0.1 microM galanin the effect of hypoxia on the contractility of papillary muscle was more profound and reperfusion did not result in complete recovery. In contrast, addition of 1 microM galanin to the hypoxic solution significantly protected the muscle and recovery of the tissues during reperfusion was rapid and complete (in 5 min). One can conclude that galanin at lower concentrations induced a positive inotropic action and a prolongation of ERP, but increased the sensitivity of heart muscle to hypoxia. At higher concentrations however, galanin exerted a negative inotropic action but protected the muscle against hypoxia-induced disturbances in contractility.

Cardiotonic effect of Apocynum venetum L. extracts on isolated guinea pig atrium

The effects on guinea-pig heart muscle of extracts of Apocynum venetum L. leaf, root, stem, old stem and Venetron--a polyphenol-rich extract of leaves--were studied by recording the mechanical activity and heart rate of isolated right atria. Cymarin--a cardiac glycoside--was also determined in A. venetum extracts by LC-MS/MS analysis. All extracts examined here showed a weak cardiotonic effect, i.e., induced a contractile response of the isolated atria and increased the pulse at a concentration of 1 mg/mL, which was not inhibited by propranolol (1 microM)-a beta-adrenoceptor blocker. The cymarin content in extracts of A. venetum was ranked as follows: old stem >> stem > root > leaf >> Venetron. Since the cardiotonic effects of A. venetum extracts did not reflect the cymarin content, a possible mechanism other than that of cardiac glycosides was investigated. The inhibitory effects on phosphodiesterase 3 (PDE3) were studied in a cell-free enzyme assay; all extracts of various parts of A. venetum inhibited PDE purified from human platelets. These results suggest that PDE3 inhibition may contribute to the cardiotonic effects of A. venetum extracts.

Bradykinin induced sustained anti–adrenergic actions in guinea pig heart muscle involves PKC

Objectives. Adenosine attenuates the effect of β-adrenergic stimulation upon contractile force, both when present during β-adrenergic stimulation and for a period after washout of adenosine. Protein kinase C (PKC) mediates the sustained effect. The present study investigated if bradykinin (500 nM) could induce anti-β-adrenergic actions. Design. Guinea pig papillary muscles (36.5°C, 1Hz) were subjected to 15 min exposure to and 20 min washout of bradykinin. Isoproterenol 100 nM was added for 3 min at the end of test exposure and at the end of washout. The PKC-inhibitor bisindolylmaleimide (20 nM) was given to investigate the importance of PKC. Results. Bradykinin significantly reduced the increase in contractile force in response to isoproterenol, both when present and after washout of test substance. Inhibition of PKC abolished the sustained effect, while inhibition of PI3-kinase with wortmannin did not seem to affect the results. Conclusions. Anti-β-adrenergic effects of bradykinin on contractile force and action potential duration were demonstrated. PKC was important for the sustained effect on contractile force, but not for the acute effect and the effect on action potential duration.

Studies on the chemistry of thienoanellated O, N- and S, N-containing heterocycles. Part 30: Synthesis and pharmacological properties of thieno[2,3- b][1,4]thiazines with potential vasopressin receptor antagonistic activity

A series of new nonpeptide vasopressin antagonists with a 6-ethyl-thieno[2,3- b][1,4]thiazine or 6-benzyl-thieno[2,3- b][1,4]thiazine skeleton and structural modifications of the aryl side chain were synthesized in this study. The effects on guinea pig heart and smooth muscle preparations were investigated. In the presence of AVP the compounds showed an antagonistic effect. The compounds did not change spontaneous rate in right atria and exerted a slight but not significant negative inotropic effect in papillary muscles. The relaxing effect on vascular smooth muscle and terminal ileum was far more pronounced. Generally the relaxing effect on terminal ilea was more potent maybe due to difference in V 1a receptor density. Our results demonstrate that compounds with an ethyl group in position six on the thienothiazine ring ( 14, 16, 18 and 22) exerted the most potent relaxing activity in terminal ilea, whereas compounds with a phenyl ring in position six reduced this effect.

Effects of novel synthesized pyridothiazines on various guinea pig heart muscle preparations.

Calcium channel blockers have become important tools in the treatment of cardiovascular disorders and other diseases. Hybridization of well established calcium antagonist subclasses was an attempt to optimize their pharmacological profile. The intension of this study was to investigate the electrophysiological properties of MM 10 and MM 11 two newly synthesized compounds structurally closely related to KT-362 (5-[3-[[2-(3,4-dimethoxyphenyl)ethyl]amino]-1-oxopropyl]-2,3,4,5-tetrahydro-1,5-benzothiazepine fumarate) in various isolated guinea pig heart muscle preparations by means of the conventional intracellular microelectrode tech-nique. MM 10 (2,3-dihydro-1-[N-[2-(3,4-dimethoxyphenyl)ethyl]-N-methylaminoacetyl]-1H-pyrido[2,3-b][1,4]thiazine fumarate) and MM 11 (2,3-dihydro-1-[N-[2-(3,4-dimethoxyphenyl)ethyl]-N-methylaminopropionyl]-1H-pyrido[2,3-b][1,4]thiazine fumarate) exerted very similar effects though the action of MM 11 was more pronounced. Whereas action potential amplitude and maximum upstroke velocity (V(max)) in papillary muscle, left atria and spontaneously beating Purkinje fibers was not affected by the compounds in a concentration range from 3 to 30 micromol/l, action potential duration at 90% time to repolarization was significantly prolonged in a concentration-dependent manner. Action potential duration at 20% time to repolarization was decreased in spontaneously beating Purkinje fibers and remained unchanged in papillary muscles and left atria. In sinoatrial nodes both compounds reduced rate of activity, action potential amplitude, maximum upstroke velocity and slope of slow diastolic depolarization while time to repolarization was prolonged. In 3 out of 6 experiments with spontaneously beating Purkinje fibers, MM 11 (30 micromol/l) led to the occurrence of early afterdepolarizations with a take off potential between -50 and -60 mV. All observed effects were completely reversible during washout with drug-free physiological salt solution. From these results it was concluded that both compounds in addition to their calcium antagonistic properties might depress repolarizing potassium currents. In contrast to the mother compound KT-362 they do not seem to affect the fast sodium inward current. Replacement of the benzothiazepine nucleus by a pyridothiazine structure may weaken or even eliminate sodium channel blocking ability. Shortening of the side chain might result in a general loss in activity.

Fifty years of cardiovascular science together.

Circulation Research began 50 years ago, as I entered medical school in St Louis, and we have grown up together. I have luckily worked during a golden age for cardiovascular medicine, when biological and engineering sciences have prospered and greatly benefited our patients. This remarkable half-century testifies to the power of science to promote health. It is a vindication of the commitment my colleagues and I made to medical science so long ago and a tribute to the mission of Circulation Research . I will trace some of the scientific milestones, reflected by my Cardiology career. Having majored in history, I entered medical school in St Louis seriously deficient in science. I fortunately soon fell into the hands of Earl Sutherland, a physician who studied with Carl Cori and by 1952 was part of the Biochemistry faculty. He was trying to understand epinephrine action on glycogen metabolism, which subsequently led him to discover adenylyl cyclase and cAMP as the prototype “second messenger system,” winning the Nobel Prize in 1971. I worked half-time in his laboratory, purifying phosphorylase and absorbing his enthusiasm for science. During my second-year Pharmacology course, I was taught by Robert Furchgott, who received the Nobel Prize in 1998 for his work on nitric oxide. Medical school was exciting. During internship interviews with the prestigious Chairmen of Medicine, I was surprised that they grilled me on my research, not my clinical skills. So I returned to school and with two other students embarked on my first independent research effort to develop an antibody to lupus protein for immunolocalization. I collected plasma from a patient with severe systemic lupus erythematosus by withdrawing whole blood and returning packed red cells. Surprisingly, the patient immediately went into remission, possibly the first therapeutic plasmapheresis.1 Several supportive faculty provided the resources I …

Isometric force kinetics upon rapid activation and relaxation of mouse, guinea pig and human heart muscle studied on the subcellular myofibrillar level.

The kinetics of force development and relaxation upon rapid application and removal of Ca2+ was measured in bundles of few myofibrils isolated from triton X-100 skinned left ventricular trabeculae of mice (M), guinea pigs (G) and humans (H). Upon rapidly switching from relaxing solution (pCa 7.5) to activating solution (pCa 4.5) at 10 degrees C, force rose by a single exponential with a rate constant k(act) of 5.2 s(-1) (M), 1.7 s(-1) (G) and 0.3 s(-1) (H) to a plateau of 0.14 microN/microm2 (M), 0.16 microN/microm2 (G) and 0.15 microN/microm2 (H). A rapid release followed by a rapid restretch to the original length applied during steady-state Ca2+ activation at pCa 4.5 induced an exponential force redevelopment with a rate constant k(redev) that was indistinguishable from k(act), indicating that k(act) is limited by cross-bridge turnover kinetics rather than by the Ca2+-induced activation of the regulatory system. Upon rapidly switching from pCa 4.5 to pCa 7.5, force decayed in a pronounced biphasic manner. Thus a slow initial, almost linear decay with a rate constant k(lin) of 1.8 s(-1) (M), 0.6 s(-1) (G) and 0.15 s(-1) (H) and a duration t(lin) of 0.06 s (M), 0.11 s (G) and 0.3 s (H) was followed by a rapid exponential decay with a rate constant k(rel) of 18 s(-1) (M), 11 s(-1) (G) and 4.6 s(-1) (H). The pronounced biphasic shapes of the force decays determined here for the first time in cardiac myofibrils differs from the force decays that had been reported for multicellular skinned trabeculae in which relaxation was induced by rapid removal of Ca2+ by flash photolysis of caged Ca2+ chelators. In the skinned trabeculae, no pronounced initial slow phase was observed. The force decays shown here are much more similar to those reported for single skeletal myofibrils. The kinetics of isometric relaxation of skinned trabeculae (i.e., multicellular preparations), therefore, do not reflect the kinetics of force relaxation at the cardiac myofibrillar level.

DOCA-salts induce heart failure in the guinea pig.

Heart failure (HF) is a common clinical problem confronting physicians and is often the final manifestation of many cardiovascular disorders. Despite recent advances in the pharmacological management of HF, it remains a highly lethal and disabling disorder. A number of animal models have been developed to study both the pathophysiology of HF and new therapeutic approaches to this complex syndrome. Only through an improved understanding of the basic biology of the early stages of the syndrome can HF be prevented or at least anticipated. With this in view, we have developed an easily realisable and inexpensive model in the guinea pig, which presents numerous structural, metabolic and biochemical similarities compared with the human heart. Thirty guinea pigs, aged 5 weeks and weighing 300 g were used. After anaesthesia, left nephrectomy was performed. After 1 week the guinea pigs were divided into: (a) control group (n=15), which received an injection of vehicle as well as tap water for 10 weeks; (b) DOCA-salts group (n=15), where the animals were treated with an IM injection of 10 mg DOCA 5 days a week for 10 weeks and with drinking water containing 9 g/l(-1) NaCl and 2 g/l(-1) KCl. Our results demonstrate that the administration of DOCA-salts to guinea pigs for 10 weeks caused a significant increase in blood pressure (BP+30%) associated with left ventricular hypertrophy (LVH), evaluated by LV weight (+37%), LV wall (+36%), by the ratio LV weight/Body weight (+23%) and by an increase in LV volume (+51%). Concerning HF, the latter was clinically evident through an increase in body weight, heart rate and dyspnoea. Indeed, guinea pigs presented pleural and/or pericardial effusion often associated with ascite. This model, which combines pressure and volume overload, results in a slow evolution towards HF. This allows a better understanding of the mechanisms in early LV remodelling which has the potential to develop into HF. Some recent studies have emphasised the value of using guinea pigs. The guinea pig heart muscle presents two major regulatory mechanisms of contractility that are closer to those found in humans, the isomyosin pattern which is predominantly V(3) and the phenomenon of Ca(2+)-induced Ca(2+)-release from the sarcoplasmic reticulum. The DOCA-salts model in the guinea pig is an easy surgical procedure with high post-operative survival, which causes an increase in arterial BP, LVH associated with HF. This model is a useful tool for studying some of the basic mechanisms of cardiovascular diseases.

Assessment of Affinities of Propranolol and Bopindolol to Membranes from COS-7 Cell Transiently Transfected with Beta-1- and Beta-2-Adrenoceptors Using a Radioligand-Binding Assay Method

This study was performed to assess the affinities of propranolol, bopindolol, its two metabolites (18-502, 20-785), pindolol, metoprolol, and atenolol to β₁- and β₂-adrenoceptor (β₁- and β₂-AR) subtypes using the membranes of COS-7 cells transiently expressing β₁- and β₂-AR subtypes. Radioligand-binding assays were performed and the results were compared with those (pKi or pA₂ values) obtained from the membrane-enriched fractions from the rat heart, cerebral cortex, bovine heart, tracheal smooth muscle or guinea-pig heart muscle. The pKi values of propranolol, bopindolol, its two metabolites, atenolol, pindolol and metoprolol to β₁-AR subtypes obtained from COS-7 cell membranes were 9.02 ± 0.04, 7.44 ± 0.12, 9.38 ± 0.31, 6.65 ± 0.16, 5.55 ± 0.14, 8.17 ± 0.15 and 5.99 ± 0.13, respectively. The rank order of pKi values for these agents to β-₂-ARs in COS-7 cell membranes was the same as that of β₁-ARs. In addition, good correlations were observed between pKi values of homogenates from various tissues and those of transfected COS-7 cell membranes to β₁- and β₂-ARs. Although good correlations were also observed between pA₂ values obtained from tracheal smooth muscle (β₂-ARs) and pKi values obtained from transfected COS-7 cell membranes to β₂-ARs, low correlation coefficient values to β₁-ARs were observed, however. In conclusion, these results suggested that binding characteristics of ³H-CGP-12177 to β-AR subtypes in these membranes from transfected COS-7 cells are similar to those from membrane fractions of various tissues.

Electrophysiological effects of newly synthesized 1,4-pyridothiazepines on various guinea pig heart muscle preparations.

Slow channel blockers play a major role in the treatment of cardiovascular disease. The intention of this study was to investigate the electrophysiological properties of MM 4 (1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminoacetyl]-1,2,3,4 -tetrahydropyrido[2,3-b][1,4]thiazepine fumarate) and MM 6 (1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminopropionyl]-1,2, 3,4-tetrahydropyrido[2,3-b][1,4]thiazepine fumarate), two newly synthesized compounds structurally related to KT-362 (5-[3-[[2-(3,4-dimethoxy-phenyl)ethyl]-amino]-1-oxopropyl]-2,3,4,5-tetra -hydro-1,5-benzothiazepine fumarate), by means of the conventional intracellular microelectrode technique. In various guinea pig heart muscle preparations, MM 4 and MM 6 exerted very similar effects though the action of MM 6 was more pronounced. In a concentration range from 3 to 100 micromol/l the compounds did not produce any significant change in transmembrane action potential parameters of papillary muscle and left atria, whereas the action potential duration at 20% and 50% time to repolarization in spontaneously beating Purkinje fibers was significantly shortened. In sinoatrial nodes action potential amplitude, Vmax, rate of activity and slope of slow diastolic depolarization were decreased, whereas the time to 50% and 90% repolarization was significantly prolonged. A decrease in the slow calcium inward current may account for the observed effects. In contrast to KT-362, MM 4 and MM 6 do not seem to affect the fast sodium inward current. It was concluded that replacement of the 1,5-benzothiazepine nucleus by a 1,4-pyridothiazepine structure and/or methylation of the side chain may weaken or even eliminate sodium channel blocking ability while calcium antagonistic characteristics are preserved. Shortening of the side chain might result in a general loss of activity.

Effects of novel pyridothiazepines and pyridothiazines on contractility of isolated guinea-pig heart muscle and vascular smooth muscle preparations.

The effects of newly synthesized pyridothiazepines MM 4 (1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminoacetyl]-1,2,3,4 -tetrahydro-pyrido[2,3-b][1,4]thiazepine fumarate), MM 6 (1-[N-[2-(3,4-dimethoxyphenyl)-ethyl]-N-methylaminopropionyl]-1,2, 3,4-tetrahydro-pyrido[2,3-b][1,4]thiazepine fumarate) and the novel pyridothiazines MM 10 (2,3-dihydro-1-[N-[2-(3,4-dimethoxyphenyl)ethyl]-N-methylaminoacetyl+ ++]-1H-pyrido[2,3-b][1,4]thiazine fumarate) and MM 11 (2,3-dihydro-1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminopropio nyl]-1H-pyrido[2,3-b][1,4]thiazine fumarate) on the contractility of isolated papillary muscles and aortic preparations of guinea pigs were studied using isometric contraction force measurements. The EC50 values for the negative inotropic effect were 27 micromol/l (MM 4), 19 micromol/l (MM 6), 32 micromol/l (MM 10) and 24 micromol/l (MM 11). In K+-precontracted aortic rings ([K+]o 60 mmol/l), the compounds induced relaxation with EC50 values of 27 micromol/l (MM 4), 24 micromol/l (MM 6), 84 micromol/l (MM 10) and 68 micromol/l (MM 11). Pyridothiazepines as well as pyridothiazines (100 micromol/l) were able to depress norepinephrine bitartrate (NE 10 micromol/l)-induced contraction of aortic rings in a calcium-free solution. It was concluded that the investigated compounds exert calcium antagonistic properties in both cardiac and smooth muscle. This antagonistic effect might be due to the inhibition of transmembrane calcium influx and/or intracellular calcium release.

Using Theoretical Descriptors in a Correlation Analysis of Adenosine Activity

A theoretical linear solvation energy relationship (TLSER) is used as a model for relating two guinea pig heart muscle activities to a set of computationally derived molecular descriptors for a set of 24 2-alkoxy and 25 2-aryloxy adenosines. The resulting equations are consistent with the structure activity relationship (SAR) study showing an increase in activity at one site with increase in substituent size and a hydrophobicity index.

Ca 2+ overloading causes the negative inotropic effect of doxorubicin in myocytes isolated from guinea-pig hearts

We reported previously that conditions shown to increase Ca 2+ loading augment the negative inotropic effect of doxorubicin. To examine if the negative inotropic effect is caused by Ca 2+ overloading, doxorubicin-induced changes in diastolic and systolic Ca 2+ concentrations and twitch contractions were studied under altered Ca 2+ loading. Intracellular Ca 2+ concentrations ([Ca 2+] i) were monitored in fura-2-loaded myocytes isolated from the ventricular muscle of guinea-pig hearts. Twitch contractions were estimated from the shortening of myocytes. In myocytes incubated at 34°C in a medium containing 1.2 mM CaCl 2 (standard conditions), doxorubicin (100 μM) caused a significant decrease in diastolic length, and an increase in the amplitude of contraction. The positive inotropic effect of doxorubicin was followed by a negative effect. Concomitant with these changes in myocyte contractions, a monophasic increase in diastolic Ca 2+, and an increase and a subsequent decrease in the amplitude of Ca 2+ transients (peak [Ca 2+] i minus diastolic [Ca 2+] i) were observed. When the Ca 2+ load of myocytes was increased by an incubation at a low temperature (25°C) or in the presence of high Ca 2+ in the incubation medium (2.4 mM CaCl 2), diastolic [Ca 2+] i was elevated. Doxorubicin further increased diastolic [Ca 2+] i. Under these conditions, the doxorubicin-induced increase in the twitch contraction lasted a shorter period of time and the subsequent decrease in contraction was significantly enhanced. The peak [Ca 2+] i initially increased slightly and then decreased. Thus, the decrease in the amplitude of Ca 2+ transients, as well as myocyte contraction, was greater compared to the corresponding values observed under the standard conditions. These changes in the contraction and Ca 2+ transient developed with the same time course. The effects of the low-temperature incubation were antagonized by verapamil. These results indicate that the negative inotropic effect of doxorubicin results from a decrease in the amplitude of Ca 2+ transients caused by an increased diastolic [Ca 2+] i and a decreased peak [Ca 2+] i. These changes are likely to be caused by myocardial Ca 2+ overload. © 1997 Elsevier Science B.V. All rights reserved.

Xenoreactive natural antibodies and induced antibodies--their effects of beating cardiomyocytes as a model of a xenograft.

Xenotransplantation has been complicated by hyperacute rejection reactions, which are supposedly triggered by preformed natural antibodies (PNAb) of the recipient organism, whereas the role of antibodies specifically induced by previous antigen contact (IAb) is less clear. Primary cultures of spontaneously beating neonatal rat cardiomyocytes were used as a model of the heart to elaborate the effects of both PNAb and IAb from xenogeneic species and to investigate into their mechanisms of action. An experimental setup allowing for rapid medium exchange under continuous observation was employed. Sera containing PNAb reproducibly bring about a stereotype pattern of altered contractility including an initial increase in beating frequency followed by a temporary cessation of beating within the first minutes after administration. After recovery of spontaneous contractions, the cells within the monolayer exhibited a dissociation of the synchronicity of the beating persisting for several hours. The temporary pause in beating was prevented by a very high extracellular calcium concentration, but not by extracellular electrical stimulation sufficient to trigger contractions in control cells. Electrophysiological measurements carried out in adult ventricular guinea pig heart muscle cells under the same experimental conditions revealed an increase of the excitation threshold of the cells after application of sera containing PNAb due to an enhanced input resistance. These results indicate that the effect of PNAb is the consequence rather of a generally reduced excitability of the cell than of the inhibition of a singular ionic conductance. After specific absorption of PNAb directed against rat antigens beating of neonatal rate cardiomyocytes ensued without interruption. Sera specimens devoid of complement produced similar effects on contractility, although the duration of the standstill period was significantly shorter. The increase in input resistance visualized in guinea pig myocytes was absent after removing PNAb against guinea pig antigens but not after absorbing PNAb directed against rat epitopes. Signs of a permanent cytotoxicity after the administration of PNAb were lacking in all experiments. IAb against rat heart tissue raised in rabbits stopped the contractions of neonatal rat cardiomyocytes within 30 min after administration irreversibly and lead concentration-dependently to a destruction of the cells.

Trypsin and forskolin decrease the sensitivity of L-type calcium current to inhibition by cytoplasmic free calcium in guinea pig heart muscle cells

A key feature of trypsin action on ionic membrane currents including L-type Ca2+ current (ICa) is the removal of inactivation upon intracellular application. Here we report that trypsin also occludes the resting cytoplasmic free Ca2+ ([Ca2+]i)-induced inhibition of peak ICa in isolated guinea pig ventricular cardiomyocytes, using the whole-cell patch clamp in combination with the Fura-2 ratio-fluorescence technique. The effectiveness of trypsin to guard ICa against [Ca2+]i-induced inhibition was compared with that of forskolin, as cAMP-dependent phosphorylation had been suggested to confer protection against [Ca2+]i-induced inactivation. Intracellular dialysis of trypsin (1 mg/ml) augmented ICa by 7.2-fold, significantly larger than the threefold increase induced by forskolin (3 microM). Forskolin application after trypsin dialysis did not further enhance ICa. An increase in [Ca2+]i from resting levels (varied by 0.2, 10, and 40 mM EGTA dialysis) to submicromolar concentrations after replacement of external Na+ (Na(o)+) with tetraethylammonium (TEA+) resulted in monotonic inhibition of control ICa, elicited from a holding potential of -40 mV at 22 degrees C. AFter trypsin dialysis, however, ICa became less sensitive to submicromolar [Ca2+]i; the [Ca2+]i of half-maximal inhibition (K0.5, normally around 60 nM) increased by approximately 20-fold. Forskolin also increased the K0.5 by approximately threefold. These and accompanying kinetic data on ICa decay are compatible with a model in which it is assumed that Ca2+ channels can exist in two modes (a high open probability "willing" and a low open probability "reluctant" mode) that are in equilibrium with one another. An increase in [Ca2+]i places a larger fraction of channels in the reluctant mode. This interconversion is hindered by cAMP-dependent phosphorylation and becomes nearly impossible after tryptic digestion.

Synthesis and pharmacodynamic activity of 2-(3-(2-phenylethyl)benzofuran-2-yl)-N-propyl-ethanamine

The benzofuranethanamines 3a and 3b were synthesized and pharmacologically tested to investigate structure-activity relationships with antiarrhythmic compounds. The key-step in the synthesis was the chemoselective reduction of the chloroacetyl-dihydrobenzofurane 5 to chloroethylbenzofurane 9 using triethylsilane/BF3.Et2O. Results of a series of further attempts to reduce 5 are also described. Pharmacological investigations on isolated guinea pig heart muscle preparations showed that 3a exhibits similar negative inotropic and negative chronotropic action in comparison to propafenone and the conformationally restricted benzofurane 1a. In contrast to these substances, however, 3a shows no beta 1-adrenoreceptor blocking activity.

Synthesis of acetates of gomphogenin and gomphoside and evaluation of structure-activity relationships

Summary Acetates of gomphogenin and gomphoside have been synthesized and their structures established by NMR measurements, optical rotation, mass and infrared spectrometry. The kinetic and equilibrium parameters of the inhibitory interaction of the compounds with guinea-pig heart muscle Na + /K + -ATPase are presented and discussed. Acetylation of the 2α-OH group in gomphogenin or its 3β-acetate increases the binding affinity by 15- and 24-fold, respectively, whereas 3β- O -acetylation of gomphogenin and its 2α-acetate increases the affinity only two- and threefold. Acetylation of 4′-OH or 3′,4′-OH of gomphoside, instead, reduces the high affinity of gomphoside towards Na + /K + -ATPase.

Cellular Ca 2+ loading and inotropic effects of doxorubicin in atrial muscle preparations isolated from rat or guinea-pig hearts

The inotropic effects of doxorubicin were examined under conditions that alter the Ca 2+ loading of myocardial cells. Atrial muscle preparations isolated from rat or guinea-pig hearts were used. Cellular Ca 2+ loading was altered by changing the temperature, extracellular Ca 2+ concentrations or the frequency of electrical stimulation. In guinea-pig heart muscle preparations stimulated at 2 Hz at 30°C in the presence of 1.2 mM CaCl 2, 30 μM doxorubicin caused biphasic (early and late phase) positive inotropic effects. A higher concentration (200 μM) of doxorubicin caused a transient positive inotropic effect followed by a gradual decrease in developed tension. An increase in CaCl 2 concentration from 1.2 to 2.4 mM decreased the positive inotropic effect of 30 μM doxorubicin and changed the inotropic effect of 200 μM doxorubicin from positive to negative after a transient increase in developed tension. At 0.5-Hz stimulation or 36°C incubation, 30 or 200 μM doxorubicin produced remarkable late phase positive inotropic effects. In rat heart muscle preparations, the patterns of the inotropic effects of doxorubicin were similar to those observed with guinea-pig hearts; however, the negative inotropic effect observed at 200 μM doxorubicin was greater. These results indicate that the early and the late phase positive inotropic effects of doxorubicin have different mechanisms. The pattern of inotropic effects of doxorubicin is affected by conditions that alter cellular Ca 2+ loading or the concentration of doxorubicin.

Hyperresponsiveness of cardiac muscles to histamine in reserpine-treated guinea pigs.

The hyperresponsiveness of cardiac tissue to histamine following treatment with reserpine was pharmacologically characterized. Guinea pig hearts were isolated 24 hours after intraperitoneal administration of 5 mg/kg reserpine. This treatment resulted in a complete depletion of tissue norepinephrine. Reserpine treatment potentiated the positive inotropic response of isolated perfused hearts to histamine at doses ranging from 0.3 to 3 micrograms (23 to 57% increase for reserpine-treated animals vs. 14 to 32% increase for control animals). Isolated left ventricular papillary muscle of reserpine-treated guinea pig hearts also showed hyperresponsiveness to histamine at concentrations of 0.1 microM or greater. The hyperresponsiveness of the papillary muscle contraction of the reserpine-treated animals to histamine was abolished in the presence of 1 microM cimetidine, but not attenuated in the presence of 1 microM diphenhydramine. This hyperresponsiveness was not modified by 1 microM bunazosin or 1 microM propranolol. The results suggest that H2-receptor mediated action plays a role in the hyperresponsiveness of cardiac muscles to histamine.

Improved synthesis and pharmacologic activity of the enantiomers of a new benzofurane type antiarrhythmic compound.

An improved synthesis of the enantiomers of the new benzofurane-type antiarrhythmic compound 1 is described, which makes use of the enantiomerically pure mbe-lactol. Thus, acylation of the benzofurane 4 with acetic anhydride and subsequent bromination gave the bromoacetyl-derivative 6, which, after reduction with LiAlH4, was protected with mbe-lactol to give a mixture of the diastereomers 8a and 8b. After separation via column chromatography assignment of absolute configuration was carried out using a well-established NMR-method. Reaction with propylamine and cleavage of the protective group gave (R)-1 and (S)-1, respectively. Enantiomeric purity was confirmed using a direct HPLC method with rsp-cyclodextrine as stationary phase. Pharmacological investigations on isolated guinea pig heart muscle preparations showed that GE 68 and its two enantiomers did not significantly differ from each other with regard to their negative inotropic, negative chronotropic, and lack of beta-adrenoceptor blocking action. In contrast, the reference drug propafenone was equally potent in its negative inotropic and chronotropic activity as GE 68, but additionally showed a weak beta-adrenoceptor blocking activity.