A positive inotropic effect of adenosine in cardiac preparations of right atria from diseased human hearts

The actions of adenosine in the human atrium are of clinical relevance, for instance, to stop supraventricular arrhythmias. The purpose of the present study was to re-evaluate the inotropic effect of adenosine in the human atrium. We studied the effect of adenosine (cumulatively applied) on force of contraction in isolated electrically driven right atrial preparations from patients with coronary heart disease undergoing cardiac surgery. It has been known for some time that adenosine via A₁-adenosine receptors can elicit a negative inotropic effect in human atrial preparations. We report here that in 25% of the patients studied, a positive inotropic effect to adenosine was detectable. The A₁-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine attenuated this effect. Hence, we conclude that at least in some patients, adenosine can lead to a hitherto overlooked positive inotropic effect.     

Separate receptors mediating the positive inotropic and chronotropic effect of histamine in guinea-pig atria

The direct positive inotropic effect of histamine was studied on paced left atrial preparations from guinea pigs. Histamine (10^?8 to 10^?4 M) increased the maximum tension developed in left atria incubated at 35°C and driven at 2 Hz. The maximum increase in tension was 60% of that observed with norepinephrine. Metiamide (3 × 10^?5 M; a specific H₂-receptor antagonist) did not alter the inotropic response of left atria to histamine. However, tripelennamine (a typical H₁-receptor antagonist) competitively shifted the histamine inotropic dose—response curve to the right at concentrations from 10^?8 to 10^?7 M. Higher concentrations (3 × 10^?7 and 10^?6 M) caused little further additional shift to the right. The positive chronotropic effect of histamine on spontaneously beating atria was competitively antagonized by metiamide (10^?6 and 3 × 10^?6 M). These results demonstrate that in guinea-pig atria histamine increases myocardial contractility by an interaction with receptors closely related to classical H₁-receptors while its chronotropic effects is mediated by interaction with H₂-receptors.     

胺碘酮对离体心肌的负性肌力作用

对离体左,右心房肌,胺碘酮(Ami)1～100μmol·L~(-1)均有负性肌力作用,30μmol·L~1明显抑制心肌静息后收缩,正阶梯和成对刺激效应,1～10μmol·L~1使苯福林,异丙肾上腺素,组胺和CaCl_2正性肌力作用的量效曲线呈非竞争性拮抗作用,心肌动作电位和收缩力同步记录发现,Ami 30μmol·L~1减弱心肌收缩,延长APD,但对平台期和V_(max)无影响.故Ami的负性肌力作用可能是抑制心肌细胞外Ca~(2+)内流和细胞内Ca~(2+)释放所致,并非选择性阻滞心肌α,β和H_2受体以及电压依赖性钙通道引起 .     

Studies on the positive inotropic effect of dopamine in the guinea-pig heart

Summary The positive inotropic effect of dopamine has been studied in isolated ventricular strips of guinea-pig heart.The concentration-inotropic response curve for dopamine was significantly shifted to the right by pretreatment with reserpine.In preparations obtained from animals pretreated with reserpine (2.5 mg/kg, 24 h prior to the experiment) the dose-response curve was not significantly affected by haloperidol, a dopamine vascular receptor antagonist (10^–6–3×10^–6 M).The inotropic effect of dopamine was antagonized by practolol (3×10^–7–10^–6 M), but not by phentolamine (3×10^–6–10^–5 M); moreover the alpha-adrenoceptor blocking drug (10^–5 M) did not affect the curve for dopamine in the presence of practolol (3×10^–7 M).In preparations in which fast sodium channels were blocked by K⁺-rich medium, slow electrical responses (calcium-mediated action potentials) as well as contractions were induced by high concentrations of dopamine (10^–4–3×10^–4 M); again these responses were unaffected by phentolamine or haloperidol, but were blocked by practolol.It was concluded that in the guinea-pig ventricular muscle dopamine induced a positive inotropic effect through both indirect and direct action, and that the latter is due to the activation of beta-adrenoceptors.     

Dissociation of phosphoinositide hydrolysis and positive inotropic effect of histamine mediated by H1-receptors in guinea-pig left atria

Summary The present study was undertaken to determine whether the phosphoinositide hydrolysis is responsible for the positive inotropic effect of histamine in guinea-pig left atria. Histamine induced hydrolysis of phosphoinositides and a positive inotropic effect in a concentration-dependent manner. These effects were antagonized by chlorpheniramine (0.1 mol/l) but not by cimetidine (10 mol/l). At a concentration of 1 mol/l histamine produced a dual-component positive inotropic response composed of an initial increasing phase and a second and late developing, greater positive inotropic phase. Histamine (10 mol/l) caused a gradual increase in the formation of [³H]inositol trisphosphate (IP3) and a significant increase in the [³H]IP₃ level was detected 10 min after the stimulation. Thus, the increase in IP₃ did not precede the increase in force of contraction. The phospholipase C inhibitors 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (100 mol/l) and neomycin (100 mol/l) significantly reduced the histamine-induced [³H]inositol monophosphate accumulation. However, pretreatment with the phospholipase C inhibitors did not affect the positive inotropic effect of histamine, either in its extent or in its pattern. The phorbol esters 12-O-tetradecanoylphorbol-13-acetate (TPA) (100 nmol/l) and phorbol-12,13-dibutyrate (PDBu) (100 nmol/l) also significantly inhibited the phosphoinositide hydrolysis induced by histamine. The inhibitory effect of the phorbol esters on the phosphoinositide response was completely abolished in the presence of 10 mol/l 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor. TPA significantly attenuated the positive inotropic effect of histamine without changing the dual-component pattern, whereas PDBu merged two distinct components of the histamine inotropic response into one and potentiated the early part of the positive inotropic effect. However, neither of the changes which the phorbol esters produced in the positive inotropic response to histamine was blocked by H-7. In addition, H-7 itself failed to modify the positive inotropic effect of histamine. These results indicate that histamine induces hydrolysis of phosphoinositides in guinea-pig left atria that is mediated by H₁-receptors, but this biochemical event does not appear to contribute to the H₁-receptor-mediated positive inotropic action. Send offprint requests to Y. Hattori at the above address     

Influence of cyclooxygenase inhibitors and of lithium on the positive inotropic effect mediated by α1-adrenoceptors in guinea-pig left atrium

Summary In experiments on the isolated guinea-pig left atrium we tried to get more information about the intracellular signal transmission of the alpha₁-adrenoceptor. We were able to demonstrate that the cyclooxygenase inhibitors indometacin and acetylsalicylic acid enhance the positive inotropic effect of relatively low phenylephrine concentrations at an extracellular calcium concentration of 1.22 mmol/l. Preincubation with prazosin as well as an increased calcium concentration of 2.5 mmol/l abolished this effect. These observations led us to suppose that an elevated level of receptor-generated arachidonic acid, whose degradation is inhibited by the cyclooxygenasen inhibitors, caused the increased contractility by releasing more calcium from the sarcoplasmic reticulum. Under these conditions also lithium caused a distinct enhancement of the positive inotropic effect evoked by alpha, -adrenergic agonists, probably by inhibiting the degradation of the second messenger inositol trisphosphate. Send offprint requests to H.-J. Schümann     

Analysis of the actions of cimetidine and metiamide on gastric acid secretion in the isolated guinea pig gastric mucosa

Summary Cumulative dose-response curves for histamine were determined on acid secretion from the isolated guinea pig gastric mucosa. Two H₂-receptor antagonists—metiamide and cimetidine—behaved like competitive antagonists to histamine on gastric acid secretion in vitro. The isolated guinea pig gastric mucosa seems to be a suitable in vitro model for analysing the action of compounds on receptors involved in acid secretion.     

Evidence against a role of a pertussis toxin-sensitive guanine nucleotide-binding protein in the alpha1-adrenoceptor-mediated positive inotropic effect in the heart

Summary Pertussis toxin, which specifically inactivates guanine nucleotide-binding proteins (N-proteins) involved in the signal transduction in various receptor systems, did not influence the positive inotropic effect of the alpha₁-adrenoceptor agonist phenylephrine in rat isolated left auricles. This indicates that the alpha₁-adrenoceptor-mediated positive inotropic effect does not involve a pertussis toxin-sensitive N-protein. Send offprint requests to W. Schmitz at the above addressSupported by the Deutsche Forschungsgemeinschaft     

Frequency dependence of the alpha-adrenoceptor-mediated positive inotropic effect in guinea pig heart.

Evidence for the role of alpha-adrenoceptors in the production of a positive inotropic effect has been obtained chiefly in heart preparations stimulated at a low rate (1 or 1.4 Hz). However, our previous results suggested that the alpha-receptor-mediated positive inotropic effect could be frequency dependent. In the present study cumulative concentration-inotropic effect curves were obtained in guinea pig ventricle strips treated with adrenaline and stimulated at 1 and at 2.5 Hz. Phentolamine (3 X 10(-6) M) and practolol (10(-6) M) were employed as antagonists. Phentolamine antagonized the effect of low concentrations of adrenaline (10(-9)-10(-8) M) in preparations driven at 1 Hz, but did not modify the curve of the agonist at the higher stimulation rate. In preparations driven at 1 Hz, practolol antagonized the effect of adrenaline at concentration above 3 X 10(-8) M without affecting the curve at lower concentrations, the beta-blocker competitively anatagonized the effect of the agonist at 2.5 Hz. It was concluded that a shortening of the interval between beats abolishes the alpha-mediated positive inotropic effect.     

Effects of centrally administered H2 antagonists in the behavioral despair test

Injection of the histamine-2 (H₂) receptor antagonist cimetidine into the lateral ventricles of mice produced a dose-related reduction in swimming in the behavioral despair test. This response can be attenuated by intracerebroventricular (ICV) injection of the histamine-1 (H₁) receptor antagonist chlorpheniramine, or the H₂ receptor agonist impromidine, given simultaneously with cimetidine. At doses which blocked cimetidine, neither chlorpheniramine nor impromidine alone had effects on swimming. A similar decrease in swimming behavior was also seen after ICV injections of the non-imidazole H₂ antagonist, BMY 25,368. This effect of BMY 25,368 was also attenuated by chlorpheniramine and impromidine. These results suggest that H₁ and H₂ receptors in the brain may mediate opposing behavioral effects.     

Relationship between structure,positive inotropic potency and lethal dose of grayanotoxins in guinea pig

Relationship between chemical structure, positive inotropic potency and lethal dose of grayanotoxins and the related compounds was studied using guinea pigs. The positive inotropic effect (PIE) was examined in their papillary muscle isolated from the heart.The potency of these compounds was expressed by pD₂ values, and was determined by depicting the concentration-PIE curve for each compound. The study has clarified the contribution of functional groups in the molecule; the presence of 3-hydroxyl, 6-hydroxyl and 10-methyl groups attached to the grayanane skeleton is established to be essential for the development of PIE. The inotropic potency of compounds carrying these essential groups is increased by a 10-hydroxyl group and the acylation of the 14-hydroxyl group. LD₅₀ value of 10 compounds with a high cardiotonic potency (pD₂>4) was determined by up and down method using male guinea pigs. The relation of LD₅₀ to pD₂ bore a significant correlation (r = 0.68, p<0.05). The most cardiotropic and toxic compound found in this study was asebotoxin III.     

Interactions between histamine H3 and dopamine D2 receptors and the implications for striatal function

The striatum contains a high density of histamine H(3) receptors, but their role in striatal function is poorly understood. Previous studies have demonstrated antagonistic interactions between striatal H(3) and dopamine D(1) receptors at the biochemical level, while contradictory results have been reported about interactions between striatal H(3) and dopamine D(2) receptors. In this study, by using reserpinized mice, we demonstrate the existence of behaviorally significant antagonistic postsynaptic interactions between H(3) and D(1) and also between H(3) and dopamine D(2) receptors. The selective H(3) receptor agonist imetit inhibited, while the H(3) receptor antagonist thioperamide potentiated locomotor activation induced by either the D(1) receptor agonist SKF 38393 or the D(2) receptor agonist quinpirole. High scores of locomotor activity were obtained with H(3) receptor blockade plus D(1) and D(2) receptor co-activation, i.e., when thioperamide was co-administered with both SKF 38393 and quinpirole. Radioligand binding experiments in striatal membrane preparations showed the existence of a strong and selective H(3)-D(2) receptor interaction at the membrane level. In agonist/antagonist competition experiments, stimulation of H(3) receptors with several H(3) receptor agonists significantly decreased the affinity of D(2) receptors for the agonist. This kind of intramembrane receptor-receptor interactions are a common biochemical property of receptor heteromers. In fact, by using Bioluminescence Resonance Energy Transfer techniques in co-transfected HEK-293 cells, H(3) (but not H(4)) receptors were found to form heteromers with D(2) receptors. This study demonstrates an important role of postsynaptic H(3) receptors in the modulation of dopaminergic transmission by means of a negative modulation of D(2) receptor function.     

Analysis of the histamine H2-receptor in human monocytes

Histamine receptors are G-protein-coupled receptors (GPCRs). Canonically, the histamine H₂-receptor (H₂R) couples to G_s-proteins and activates adenylyl cyclases (ACs) with subsequent adenosine-3′,5′-cyclic monophosphate (cAMP) generation. Recently, the classic two-state model describing GPCR activation has been extended to a multiple-state model implying that any ligand stabilizes a ligand-specific receptor conformation, also referred to as functional selectivity. In our present study we pharmacologically characterized the H₂R in human monocytes. We found dissociations in the effects of histamine (HA) and H₂R agonists on cAMP accumulation and inhibition of formyl peptide-induced production of reactive oxygen species (ROS). In addition, we excluded participation of protein kinase A (PKA) in HA-induced ROS inhibition. Comparison of the potencies and efficacies of H₂R agonists in monocytes, neutrophils and eosinophils unmasked cell type-specific pharmacological profiles of individual ligands. Taken together, our data extend the concept of functional selectivity to the H₂R in human monocytes and demonstrate striking cell-type specificity in the pharmacological profile of the H₂R.     

Inotropic effect of prostacyclin (PGI2) on isolated rat atria at different contraction frequencies

Summary The effects of a wide range of Prostacyclin (PGI₂) concentrations on the Isometric Developed Tension (IDT) of isolated left auricles driven at different frequencies (0.8, 1.6 or 3.3 Hz) were explored. A comparison of the positive inotropism of PGI₂, norepinephrine (NE), tyramine and phenylephrine (Phenyl) indicated that PGI₂, NE and tyramine enhanced peak tension significantly less at slow (0.8 and 1.6 Hz) than at higher rates (3.3 Hz); whereas Phenyl augmented equally the IDT at all of these three driving frequencies. The positive inotropism evoked by PGI₂ was inhibited by (–)-propranolol and also by a treatment with 6-OHDA. Cocaine, normetanephrine and U-0521, augmented the positive inotropic influence of PGI₂ on atria paced at a slow rate (0.8 Hz) but not at a faster one (3.3 Hz). These results suggest that the action of PGI₂ on atrial contractions is apparently indirect and mediated by the release of tissue catecholamines. In addition the effect of PGI₂ and NE at slow and fast rates appears associated with a different relative relevance of the processes which terminate the action of added sympathomimetic agonists, namely, neuronal or extraneuronal uptake as well as metabolization via COMT. These mechanisms seen to be more prominent at slower driving frequencies and could explain the diminished effect of PGI₂ and NE on slowly paced atria.This work was supported by Grant 6638 from the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET)     

An improved model for assessment of positive inotropic activity in vitro

Responses of guinea pig paced left atria to positive inotropic agents that probably act by different mechanisms were compared at two different calcium concentrations in vitro (1.25 and 2.50 mM). Consistently good results were obtained with isoproterenol, ouabain, amrinone, and 3-isobutyl-1-methylxanthine only at the lower calcium concentration, which resulted in a greater percentage of responsive preparations, greater response discrimination, lower incidence of spontaneous and drug-induced arrhythmias, and therefore a better quantitative assessment of concentration-response relationships. Satisfactory responses were also obtained with the indirect-acting agent tyramine and with the new agent, vardax. An excellent correlation between the magnitude of tension developed and the maximum rate of tension development was obtained for various inotropic stimuli. The partial beta agonists, prenalterol and dichloroisoproterenol, gave relatively poor responses under both sets of experimental conditions. It is concluded that the use of a low calcium concentration in the bathing medium enhances responsiveness to a variety of inotropic agents acting by different mechanisms, providing a useful and more reliable model for testing in vitro. It is suggested that a low external calcium concentration results in a smaller predrug sarcolemmal calcium flux, which may be markedly enhanced by inotropic agents.     

Potent inhibitory action of chlorethylclonidine on the positive inotropic effect and phosphoinositide hydrolysis mediated via myocardial alpha1-adrenoceptors in the rabbit ventricular myocardium

Summary The influence of the alpha_lb-adrenoceptor-selective antagonist chlorethylclonidine on the alpha₁-adrenergic positive inotropic effect and the phosphoinositide hydrolysis induced by phenylephrine was investigated in the rabbit ventricular myocardium. Pretreatment of membrane fractions derived from the rabbit ventricular muscle with 10^–5 mol/l chlorethylclonidine decreased the specific binding of [³H]prazosin (at a saturating concentration of 10^–9 mol/l) from the control value of 11.27±0.48 to 4.18±1.87 fmol/mg protein. The inhibition by adrenaline of the binding of [³H]prazosin (slope factor and affinity) was not affected by chlorethylclonidine. The positive inotropic effect of phenylephrine (in the presence of 3 × 10^–7 mol/l bupranolol) was inhibited by chlorethylclonidine in a concentration-dependent manner (10^–7–10^–5 mol/l) and abolished by 10^–5 mol/l chlorethylclonidine. The concentration of chlorethylclonidine to inhibit the phenylephrine-induced maximum response to 50% was 2.4 × 10^–6 mol/l. The accumulation of [³H]inositol monophosphate and [³H]inositol trisphosphate induced by 10^–5 mol/l phenylephrine was inhibited by chlorethylclonidine in the same concentration range. These findings indicate that the myocardial alpha₁-adrenoceptors mediating a positive inotropic effect in the rabbit ventricular myocardium may belong to the chlorethylclonidine-sensitive alpha_1b-subtype, and that the subcellular mechanism of action involve phosphoinositide hydrolysis. Send offprint requests to M. Endoh at the above address     

Correlation between inhibition of (Na+, K+)-membrane-ATPase and positive inotropic activity of cardenolides in isolated papillary muscles of guinea pig

Summary Concentrations of 17 cardenolides, cardenolide glucuronides and sulfates producing halfmaximal inhibition of (Na⁺, K⁺)-membrane-ATPase from different organs and animal species were determined in vitro. In addition the concentrations that increased the contractility of guinea pig isolated papillary muscles to a particular level were investigated. Comparisons between ATPase-inhibiting and positive inotropic cardiac activities showed extensive parallelism: the correlation coefficients after log/log transformation were between 0.92 and 0.97. The same close correlations are found if dissociation constants of cardenolide receptor complexes and concentrations causing ⁸⁶Rb-uptake inhibition in human erythrocytes are examined.The concentrations necessary for inhibition of (Na⁺, K⁺)-membrane-ATPase of the guinea pig heart and the concentrations required to achieve a defined positive inotropic effect in guinea pig papillary muscle showed a log/log correlation coefficient of 0.97 (P<0.001). In both tests the potencies covered more than three orders of magnitude. The results support Repke's hypothesis on the digitalis receptor.     

Regulation of gastric acid secretion by histamine H3 receptors in the dog: an investigation into the site of action

The involvement of histamine H₃ receptors in the regulation of gastric acid secretion was investigated in the conscious dog with gastric fistula, by the use of the selective agonist (R)-methylhistamine and the selective antagonist thioperamide. (R)-methylhistamine (0.3–1.2 mol/kg/h) induced a dose-related inhibition of the acid secretion induced by pentagastrin and by bombesin, maximum inhibition not exceeding 60–65%. The inhibitory effect of the H₃ agonist (0.6 mol/kg/h) was inhibitited by thioperamide (0.1 mol/kg/h), suggesting that the effect was entirely mediated by H₃ receptors. Thioperamide was also able to enhance the acid response to submaximal doses of pentagastrin and bombesin. The acid secretion induced by histamine was not modified by (R)a-methylhistamine (0.3–1.2 mol/kg/h) but it was significantly enhanced by thioperamide (0.1 mol/kg/h). Neither (R)-methylhistamine nor thioperamide significantly modified the increase in plasma gastrin levels induced by bombesin. In conclusion these data demonstrate that histamine H₃ receptors may represent an effective mechanism for the negative control of stimulated gastric acid secretion in the dog; however, since the inhibition was mainly evident against stimuli which involve the release of histamine, a location of H₃ receptors in paracrine cells of the gastric mucosa rather than in gastrin producing cells or parietal cells seems more likely. Correspondence to: G. Bertaccini at the above address     

Quabain: Temporal relationship between the inotropic effect and the in vitro binding to,and dissociation from, (Na++K+)-activated ATPase

Summary The time course of the inotropic response to ouabain in Langendorff preparations was compared with that of the in vitro ATP-dependent (³H)-ouabain binding to cardiac (Na⁺+K⁺)-activated ATPase preparations, and subsequent dissociation, to determine the temporal relationship between the inotropic response and (Na⁺+K⁺)-activated ATPase inhibition.Species differences were minimal either in the onset of inotropic response or the (³H)-ouabain binding. The rates of both loss of the inotropic response to ouabain during washout and the dissociation of the ouabain-enzyme complex, however, were rapid in guinea pig and rabbit (relatively ouabain-insensitive species) and slow in cat and dog (ouabain-sensitive species). The half-time of the loss of the inotropic response was similar to the half-time of the dissociation of the ouabain-enzyme complex in each species.Since ATP-dependent binding of cardiac glycosides has been related to enzyme inhibition, it was concluded that the time course of the inotropic response to ouabain parallels the time course of (Na⁺+K⁺)-activated ATPase inhibition, and that the dissociation of ouabain from the enzyme may terminate the inotropic response.A part of this study was presented at the Fifth Annual Meeting of the International Study Group for Research in Cardiac Metabolism, Winnipeg, Manitoba, June, 1972.     

Histamine receptors in the guinea pig ileum

Summary Histamine and some related compounds acting selectively on H₂-or H₁-receptors were tested for their ability to contract the guinea pig ileum, in the usual whole ileum preparation and in the longitudinal muscle preparation. The concentrations elicited by histamine in both kinds of preparations were not potentiated by cimetidine or metiamide and were not inhibited by administration of H₂ receptor selective agonists in doses which were subthreshold for contracting the guinea pig ileum; higher doses of the H₂ agonists could actually potentiate the effect of histamine. The results obtained suggest that H₂ receptors with relaxing effect do not occur in the guinea pig ileum or at least that they are not involved in the contraction of the longitudinal muscle layers. The possibility that a sub-type of H₂ receptors with properties different from those of the classical H₂ receptors so far known, exists in the guinea pig ileum, cannot be excluded.