Dobutamine: positive inotropy by nonselective adrenoceptor agonism in isolated guinea pig and human myocardium

Summary Positive inotropic responses to dobutamine have been examined using isolated myocardium from guinea pigs und humans. The potency (EC₅₀) of dobutamine was 1.5 × 10^–6 mol/l on guinea pig papillary muscles, 1.8 × 10^–6 mol/l on guinea pig left atria and 2.5 × 10^–6 mol/l on human papillary muscle strips. In guinea pig cardiac muscles, Schild plots for the ₁-selective antagonist, 1-practolol, using dobutamine as agonist, had slopes of less than unity. This suggested the involvement of other receptors in the inotropic response to dobutamine. The ₂-selective antagonist, ICI 118,551, but not the ₁-selective antagonist, prazosin, attenuated the dobutamine response in guinea pig papillary muscles. Both ICI 118,551 and prazosin shifted the dobutamine concentration-response curve in guinea pig left atria. The positive inotropic response to dobutamine in human papillary muscles was antagonised by 1-practolol and ICI 118,551 but not by prazosin. The maximal inotropic response to dobutamine was 90% that of calcium measured in the same guinea pig papillary muscles but only 37% that of calcium in human papillary muscle strips. This reduced maximal effect of dobutamine in human myocardium is probably a disease-induced change but species variations cannot be excluded. Send offprint requests to E. Erdmann at the above addressThese studies were supported by the Deutsche Forschungsgemeinschaft (Er 65/4-4). Part of these studies was presented at the XII Congress of the International Society for Heart Research in Melbourne, Australia, February 1986 (Brown et al. 1986a)     

Antagonism of novel inotropic agents at A1 adenosine receptors and m-cholinoceptors in human myocardium

Summary The effects of the new inotropic agents saterinone, sulmazole, UD-CG 212.C1 and milrinone at A₁ adenosine receptors and m-cholinoceptors were evaluated in human myocardium from patients with heart failure. At A₁ adenosine receptors, all compounds inhibited ³H-DPCPX-binding to ventricular membrane preparations at micromolar concentrations. As judged from the K_i-values, the rank order of potency was saterinone > sulmazole > UD-CG 212.C1 > milrinone. The new inotropic agents also displaced the binding of ³H-QNB at m-cholinoceptors. Except for saterinone, the concentration ranges of mean K_i-values were considerably higher at m-cholinoceptors than at A₁ adenosine receptors. The rank order of potency was saterinone > sulmazole > UD-CG 212.Cl > milrinone. Competition of the A₁ adenosine receptor agonist R-PIA to ³H-DPCPX-binding showed a biphasic curve with a shallow slope (Hill coefficient n_H = 0.63) and revealed two affinity states of the A₁ adenosine receptor. In the presence of guanine nucleotides [Gpp(NH)p], the competition curve showed one low affinity class of binding sites and was shifted to the right. In contrast, the competition curves of the new inotropic agents were characterized by a monophasic, steeper slope (mean Hill coefficient n_H = 0.98). Guanine nucleotides had no effect. Similar results were obtained with saterinone and carbachol at m-cholinoceptors. Competition with carbachol revealed three affinity states of the m-cholinoceptor, the superhigh affinity binding was reversed by Gpp(NH)p. Competition with saterinone revealed one class of binding sites which was not influenced by Gpp(NH)p. Accordingly, in isolated, electrically driven human atrial trabeculae, the negative inotropic effect of adenosine was antagonized concentration-dependently by saterinone, sulmazole and UD-CG 212.Cl. Similarly the negative inotropic effect of carbachol was antagonized concentration-dependently by saterinone. It is concluded that the new inotropic agents bind to A₁ adenosine receptors and that their interaction is of antagonist nature. This mechanism might contribute to their capacity to enhance force of contraction by stimulation of cAMP-formation in addition to phosphodiesterase inhibition. The effects of saterinone may be partially due to antagonism at m-cholinoceptors. This is presumably not the case with the other inotropic agents studied given their low affinity for this receptor.Send offprint requests to M. Böhm at the above addressSupported by the Deutsche Forschungsgemeinschaft     

Effect of theophylline on calcium exchangeability in ventricular myocardium

Summary The effects of theophylline on contractile force and myocardial calcium exchangeability were studied in isolated, electrically driven Langendorff perfused guinea-pig hearts. Following a 30-min exposure to ⁴⁵Ca, total cellular calcium and ⁴⁵Ca activity were measured in right ventricular samples. Non-toxic theophylline concentrations (5×10^–5–10^–3 g/ml) which augmented contractile force without producing arrhythmias or contractures had no effect on total tissue calcium and did not alter the size of the fraction of cellular calcium exchangeable under steadystate conditions. A toxic concentration of theophylline (2×10^–3 g/ml) induced contractures and increased the amount of exchangeable cellular calcium. The latter effect was due to an increase in total calcium; the unlabelled cellular calcium fraction remained unchanged under the influence of all theophylline concentrations studied. The results suggest that theophylline increases the steady-state calcium exchangeability in ventricular myocardium only when the total calcium concentration is also increased.     

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.     

Influence of thyroid hormone on the positive inotropic effects mediated by alpha- and beta-adrenoceptors in isolated guinea pig atria and rabbit papillary muscles.

Effects of thyroxine treatment for 7--11 days on the positive inotropic effects mediated by alpha- and beta-adrenoceptors were studied in isolated guinea pig atria and rabbit papillary muscles. In guinea pig atria, the thyroxine treatment inhibited the positive inotropic effect of lower concentrations of phenylephrine (PHE), and attenuated the inhibitory effect of phentolamine on the PHE response. The effect of isoproterenol (ISO) was potentiated by the thyroxine treatment. In rabbit papillary muscles, the thyroxine treatment shifted the dose--response curve for PHE to the right and attenuated the inhibitory effect of phentolamine on the PHE response. Propranolol, in both guinea pig atria and rabbit papillary muscles, inhibited the PHE response more effectively in preparations from thyroxine-treated animals than in controls. In guinea peg atria, the attenuation of the PHE response mediated by alpha-adrenoceptors was observed after the thyroxine treatment for only 2 days, whereas the potentiation of the ISO response required the thyroxine treatment for a longer period. It was concluded that the thyroxine treatment attenuated the positive inotropic effect mediated by alpha-adrenoceptors and potentiated that mediated by beta-adrenoceptor-mediated positive inotropic effects due to the thyroxine treatment may be independent of each other.     

Electrophysiologic effects of saterinone and milrinone in the isolated guinea pig myocardium

The positive inotropic agent milrinone and the newly synthesized compound saterinone [+/-)-1,2-dihydro-5-[4-[2-hydroxy-3-[4-(2-methoxyphenyl)-1-piperazinyl] propoxy]phenyl]-6-methyl-2-oxo-3-pyridine-carbonitrile, BDF 8634) dose-dependently increased the contractile force of guinea pig left atria. During a 90-min exposure, 10(-4) mol/l saterinone caused a continuous increase of the functional refractory period (FRP), while the initial positive inotropic effect faded gradually. No change of the FRP was observed with milrinone. Saterinone (10(-4) mol/l) also increased the action potential duration and the FRP of guinea pig papillary muscles, while milrinone had no influence on either parameter. Both milrinone and saterinone increased the amplitude, depolarization velocity and duration of slow response action potentials in K+-depolarized muscles. These effects appeared in the presence of tetrodotoxin or propranolol and could be reversed by carbachol. It is concluded that saterinone increases the force of contraction and the slow inward current by inhibiting cardiac phosphodiesterase. The increase of the FRP may be attributed to a decrease of the membrane K+ conductance.     

甲基葡糖苷对豚鼠离体心房的正性肌力作用

目的　观察甲基葡糖苷对豚鼠离体心房的正性肌力作用 ,并探讨其作用机制。方法　采用豚鼠离体左右心房 ,测定药物对心房肌收缩力、右心房心率 ,以及对静息后收缩和正阶梯现象的影响 ,并测定大鼠心肌细胞膜Na+ K+ ATP酶活性。结果　甲基葡糖苷显著增强心房肌收缩力 ,减慢右心房心率 ,且呈剂量依赖性 ;能明显增强左心房静息后收缩和正阶梯现象 ,并能显著抑制大鼠心肌细胞膜Na+ K+ ATP酶活性。结论　甲基葡糖苷具有加强心房肌收缩力 ,降低右心房心率的作用 ,其正性变力作用可能与抑制心肌细胞膜Na+ K+ ATP酶活性 ,促进心肌细胞外钙内流和内钙释放有关     

山莨菪碱对豚鼠离体心肌生理特性的影响

盐酸山莨菪碱（Ani）290～713μmol／L对豚鼠离体左房肌呈现负性变力作用，能降低右心房的自动节律性，并能抑制肾上腺素诱发的异常节律性，还能延长心肌的功能不应期，但对兴奋性无明显影响。前两者可能是Ani抗心律失常的原因之一。     

Functional evidence against the existence of β2-adrenoceptors mediating positive inotropic effects in guinea-pig right ventricular myocardium

In guinea-pig isolated papillary muscles we studied the positive inotropic effects of the β-adrenoceptor agonists isoprenaline, fenoterol and noradrenaline in the presence and absence of the β₁-selective antagonist atenolol and the β₂-adrenoceptor antagonist ICI 118.551. In Schild regression analysis pA₂ values for either atenolol (7.14–7.16) and ICI 118.551 (6.79–6.84) were independent of the agonists used. These results support the view that the inotropic response in guinea-pig ventricular myocardium is mediated by β₁-adrenoceptors only. Received: 2 May 1996 / Accepted: 21 August 1996     

环维黄杨星D对豚鼠离体心肌的正性变力作用

本文研究了环维黄杨星D(CVB—D) 对豚鼠离体心肌的变力作用.在0.3～100μmol/L范围内,CVB—D对左、右心房肌均呈现剂量依赖性正性变力作用.CVB—D 30μmol/L增强左心房肌静息后收缩正性阶梯和成对刺激效应.乳头状肌动作电位和收缩力同步记录的结果表明,CVB—D 30μmol/L加强心肌收缩力,延长APD_(50)和APD_(90),但对V_(max)、APA和RP影响不大.因此,CVB—D的正性变力作用可能是由于促进心肌细胞外Ca~(2+)跨膜内流和增加细胞内Ca~(2+)释放所致.     

Mechanism underlying the reduced positive inotropic effects of the phosphodiesterase III inhibitors pimobendan,adibendan and saterinone in failing as compared to nonfailing human cardiac muscle preparations

Summary The present study was performed to compare the effects of the new positive inotropic phosphodiesterase III inhibitors pimobendan, adibendan, and saterinone on the isometric force of contraction in electrically driven ventricular trabeculae carneae isolated from explanted failing (end-stage myocardial failure) with those from nonfailing (prospective organ donors) human hearts. In preparations from nonfailing hearts the phosphodiesterase inhibitors, as well as the a-adrenoceptor agonist isoprenaline, the cardiac glycoside dihydroouabain, and calcium, which were studied for comparison, revealed pronounced positive inotropic effects. The maximal effects of pimobendan, adibendan, and saterinone amounted to 56%, 36% and 45%, respectively, of the maximal effect of calcium. In contrast, in preparations from failing hearts the phosphodiesterase III inhibitors failed to significantly increase the force of contraction and the effect of isoprenaline was markedly reduced. The effects of dihydroouabain and calcium were almost unaltered. The diminished effects of isoprenaline were restored by the concomitant application of phosphodiesterase inhibitors.To elucidate the underlying mechanism of the lack of effect of the phosphodiesterase III inhibitors in the failing heart we also investigated the inhibitory effects of these compounds on the activities of the phosphodiesterase isoenzymes I–III separated by DEAE-cellulose chromatography from both kinds of myocardial tissue. Furthermore, the effects of pimobendan and isoprenaline on the content of cyclic adenosine monophosphate (determined by radioimmunoassays) of intact contracting trabeculae were studied. The lack of effect of the phosphodiesterase inhibitors in failing human hearts could not be explained by an altered phosphodiesterase inhibition, since the properties of the phosphodiesterase isoenzymes I–III and also the inhibitory effects of the phosphodiesterase inhibitors on these isoenzymes did not differ between failing and nonfailing human myocardial tissue. Instead, it may be due to a diminished formation of cyclic adenosine monophosphate in failing hearts, presumably caused mainly by a defect in receptor-adenylate cyclase coupling at least in idiopathic dilated cardiomyopathy. Both the basal and the pimobendan-stimulated or isoprenaline-stimulated contents of cyclic adenosine monophosphate of intact contracting trabeculae from failing hearts were decreased compared with the levels in nonfailing hearts. However, under the combined action of isoprenaline and pimobendan the cyclic adenosine monophosphate level reached values as high as with each compound alone in nonfailing preparations, and in addition the positive inotropic effect of isoprenaline was restored.These findings may have important clinical implications. Along with the elevated levels of circulating catecholamines the positive inotropic effects of the phosphodiesterase inhibitors may be maintained in patients with heart failure. Furthermore, the concomitant application of a -adrenoceptor agonist and a phosphodiesterase inhibitor might be beneficial in terminal heart failure refractory to conventional therapeutic regimens.Some of the results reported in this paper have already been presented in abstract form at the 61 st Session of the American Heart Association, Washington, DC, Nov. 1988 (von der Leyen et al., Circulation 78 (Suppl II): 11-360, 1988), at the Fall Meeting of the German Society of Pharmacology and Toxicology, Sept. 1988 (Schmitz et al., Naunyn-Schmiedeberg's Arch Pharmacol 338 (Suppl): R 16, 1988), at the 30th Spring Meeting of the German Society of Pharmacology and Toxicology, March 1989 (Meyer et al., Naunyn-Schmiedeberg's Arch Pharmacol 339 (Suppl): R 53, 1989), and at the XIII Congress of the International Society For Heart Research, Ann Arbor, MI, May 1989 (Meyer et al., J Mol Cell Cardiol 21 (Suppl 11): S. 50, 1989) mis|Send offprint requests to Wilfried Meyer at the above address     

Influence of temperature on the sensitivity of the adrenoceptors in the isolated atria of guinea pigs and rats.

The influence of the bath temperature on the responsiveness to sympathomimetic amines was studied with isolated guinea pig and rat atria. In electrically driven guinea pig left atria, the dose-response curve for the positive inotropic effect of isoproterenol (ISO) was shifted to the left by lowering the temperature from 36 to 24 degrees C. The positive inotropic effect of phenylephrine (PHE) in lower concentrations was attenuated by lowering the temperature. Phentolamine markedly inhibited the PHE response at 36 and 32 degrees C, whereas it produced no inhibition at 24 degrees C. Similar changes were observed with rat left atria. In guinea pig left atria, propranolol inhibited the response to PHE more effectively at 24 degrees C than 32 degrees C. With guinea pig and rat atria the dose--response curve for the positive inotropic effect of PHE in the presence of phentolamine was shifted to the left by lowering the temperature. The results suggest that lowering the temperature of the bath solution diminished the positive inotropic effect of PHE mediated by alpha-adrenoceptors and potentiated that mediated by beta-adrenoceptors.     

普罗帕酮对豚鼠心肌力学的影响

目的研究普罗帕酮心肌变力性作用并从组织水平探讨其作用机制。方法采用离体乳头肌灌流的方法观察普罗帕酮对豚鼠乳头肌主动张力(DT)、主动张力上升最大速度(+dT/dtmax)、主动张力下降最大速度(-dT/dtmax)的影响。经钙通道阻滞剂尼卡地平及Na+/Ca2+交换体阻滞剂KBR7943预处理后,分别观察普罗帕酮的上述作用。结果①在场刺激引起收缩的乳头肌,0.1、1、10、30μmol·L-1普罗帕酮分别使DT由对照(0.18±0.05)g降至(0.14±0.03)、(0.12±0.03)、(0.08±0.02)、(0.05±0.02)g(P<0.01),IC50为10μmol·L-1;+dT/dtmax由对照(1.79±0.45)mg·s-1降至(1.58±0.37)、(1.46±0.29)、(1.26±0.19)、(0.97±0.15)mg·s-1(P<0.01);-dT/dtmax由对照(1.61±0.29)mg·s-1降至(1.45±0.28)、(1.26±0.19)、(0.92±0.26)、(0.78±0.22)mg·s-1。②尼卡地平(2.0μmol·L-1)阻断L型钙通道后,普罗帕酮(10μmol·L-1)使DT、+dT/dtmax、-dT/dtmax由(0.10±0.02)g、(1.32±0.24)mg·s-1、(1.24±0.17)mg·s-1分别降低至(0.06±0.01)g、(1.11±0.23)mg·s-1、(0.89±0.23)mg·s-1(P<0.01)。③KBR7943(1.0μmol·L-1)阻断Na+/Ca2+交换体后,普罗帕酮(10μmol·L-1)使DT、+dT/dtmax、-dT/dtmax由(0.18±0.02)g、(1.48±0.28)mg·s-1、(1.63±0.23)mg·s-1分别降低至(0.09±0.01)g、(1.16±0.01)mg·s-1、(1.05±0.23)mg·s-1(P<0.01)。结论①普罗帕酮剂量依赖性抑制豚鼠乳头肌力学各项指标,显示有负性肌力作用。②普罗帕酮对L型钙通道及反向Na+/Ca2+交换的抑制作用参与其负性肌力的作用,且前者作用较大。     

Involvement of cyclic AMP in the direct inotropic action of amrinone

Summary Using the isolated guinea-pig papillary muscle and conventional methods for recording isometric force and transmembrane electrical activity we analysed the positive inotropic effect of the bipyridine derivative amrinone at a contraction frequency of 0.2 Hz. The drug produced a concentration-dependent (0.06–8.0 mmol/l), reversible increase in force of contraction associated with an abbreviation of relaxation time at low concentrations and an increase of this parameter at high concentrations. Part of the inotropic effect was manifested by the rested-state contraction. In muscles depolarized by 24 mmol/l [K]₀, amrinone increased the maximum rate of depolarization and overshoot of the slow responses. Carbachol reduced the inotropic effect of amrinone, and this antagonism was removed by the additional application of atropine. The inotropic effect of amrinone was not affected by propranolol or phentolamine and only slightly inhibited by cimetidine. Amrinone potentiated the positive inotropic effects of isoprenaline or histamine but interacted additively with dihydroouabain; the –log EC₅₀ of isoprenaline was increased by 0.803±0.077 and that of histamine by 1.14±0.054 logarithmic units in the presence of 0.2 mmol/l amrinone. Abbreviation of relaxation time, increase in force of the rested-state contraction, atropine-sensitive antagonism by carbachol, and the effects on the slow response are characteristic of the class of cardiotonic drugs thought to act by increasing the cellular concentration of cyclic AMP. Direct support for this hypothesis was provided by the demonstration that inotropically effective concentrations of amrinone produced an up to 3.5-fold increase in cyclic AMP content of guinea-pig papillary muscles. In addition, amrinone was found to inhibit phosphodiesterase in a crude enzyme preparation from guinea-pig ventricular strips. Lack of specific antagonism by propranolol or cimetidine and potentiation of the effects of isoprenaline or histamine are consistent with an inhibitory effect on cyclic nucleotide phosphodiesterase. At high concentrations, amrinone was shown to exert an additional theophylline-like effect on the contour of the isometric contraction that cannot be attributed to cyclic AMP accumulation.     

米利酮和哇巴因的强心效应、安全范围和心脏毒性的比较

在猫和豚鼠戊巴比妥钠心衰模型上比较了米利酮和哇巴因的强心作用、安全范围和心脏毒性。米利酮作用于衰竭心脏的最大有效量为１２３８±２０３μｇ·ｋｇ－１（猫）、２１０２±２１０μｇ·ｋｇ－１（豚鼠）米利酮安全范围远大于哇巴因。毒性剂量的米利酮发生主要以室性心律失常为主的心电图变化。结果表明，米利酮的强心作用可能无种族差异性，其安全性好．但也会诱发室性心律失常。     

Positive inotropic effects of the calcium channel activator Bay K 8644 on guinea-pig and human isolated myocardium

Summary 1. The positive inotropic effects of the dihydropyridine calcium activator Bay K 8644 were studied in guinea-pig isolated contracting myocardium and human papillary muscle strips obtained from patients undergoing mitral valve replacement or cardiac transplantation. 2. Bay K 8644 produced a slowly developing, concentration-dependent positive inotropic response in all cardiac tissues studied. In guinea-pig papillary muscle, the increase in force of contraction was half-maximal at 3.9 × 10^–8 mol/l and the maximal inotropic effect was comparable to that obtained with ouabain, dobutamine or calcium. The guinea-pig left atrium (EC₅₀, 2.1 × 10^–7 mol/l) was fivefold less sensitive than the papillary muscle. 3. The maximal inotropic response to dihydroouabain was significantly increased after preincubation with Bay K 8644 (1 × 10^–6 mol/l) in papillary muscles from both guinea-pig and human. In guinea-pig papillary muscles, the maximal inotropic response to dobutamine was not changed by preincubation with Bay K 8644 whereas in human papillary muscle strips, Bay K 8644 increased the inotropic response to dobutamine. 4. Bay K 8644 increased force of contraction (EC₅₀, 4 × 10^–8 mol/l) in human papillary muscle strips from patients undergoing mitral valve replacement. However, the maximal inotropic response to Bay K 8644 was reduced to 32 ± 4.4% that of calcium (15 mmol/l) measured in the same muscle strips. 5. A further reduction in maximal inotropic response to Bay K 8644 to 13 ± 1.2% that of calcium (15 mmol/l) with no change in potency was measured in human papillary muscle strips taken from terminally failing hearts of cardiac transplant recipients. 6. There was a significant correlation between the preoperative left ventricular ejection fraction and the maximal inotropic response to Bay K 8644 in isolated human papillary muscle strips. 7. These results suggest that Bay K 8644 affects excitation-contraction coupling of cardiac muscle so as to increase the maximal inotropic effect of the digitalis glycosides. Further, the inotropic response of human myocardial tissue to calcium channel activator Bay K 8644 may be reduced in states of pathological heart function.The human heart papillary muscles were provided by Prof. E. Kreuzer, Prof. B. Kemkes, Dr. C. Weinhold and their colleagues, Herzchirurgische Klinik der Universität, Klinikum Grosshadern, D-8000 München 70, Federal Republic of Germany Send offprint requests to E. Erdmann     

莲心碱对心肌电活动及机械活动的影响

莲心碱（Ｌｉｅ）１－３００ｕｍｏｌ·Ｌ１可浓度依赖性地降低离体豚鼠乳头状肌收缩力（Ｆｅ）和快反应劝作电位（ＡＰ）的零相上升幅度（ＡＰＡ）及最大除极速率（Ｖｍａｘ），延长动作电位时程（ＡＰＤ），且对Ｖｍａｘ的抑制作用有频率依赖性；同浓度下，其作用强于奎尼丁。对高Ｋ＋诱发的慢ＡＰ，Ｌｉｅ也能剂量依赖性地降低ＡＰＡ，Ｖｍａｘ，Ｆｅ，其作用弱于维拉帕米。Ｌｉｅ还可对抗乙酰胆碱缩短ＡＰＤ的作用提示Ｌｉｅ对Ｎａ＋，Ｋ＋Ｃａ２＋的跨膜转运均有非特异性阻滞作用。     

α-adrenoceptor-mediated positive inotropic effect of phenylephrine in isolated human ventricular myocardium

In isolated human ventricular myocardium the α-adrenoceptor agonist phenylephrine had a positive inotropic effect in preparations from 9 of 14 patients. This effect was seen in the presence of the β-adrenoceptor-bloking agent propronolol but was nearly abolished by the α-adrenoceptor blocking agent prazosin. In contrast to the β-adrenoceptor-mediated positive inotropic effect, the effect of phenylephrine was accompanied by a prolongation of the isometric contraction. The results suggest that α-adrenoceptors exist in human ventricular myocardium.