Experimental hyperlipidemia prevents the protective effect of ischemic preconditioning on the contractility and responsiveness to phenylephrine of rat-isolated stunned papillary muscle.

This study was designed to establish a hyperlipidemic diet (significant increase in the cholesterol and triglycerides blood levels, but without atherogenic changes in heart muscle and coronary vessels) and to investigate the influence of experimental hyperlipidemia on the effects of ischemic preconditioning (PC) of rat-isolated papillary muscle on the time course of contractility during simulated ischemia and reperfusion and responsiveness to phenylephrine under such a condition. The animals were divided in four experimental groups: standard diet-fed control group (SD), SD underwent ischemic preconditioning (SD-PC), hyperlipidemic diet-fed group (HLD) and HLD underOFFt PC (HLD-PC). Force of contraction (Fc), velocity of contraction (+dF/dt), and velocity of relaxation (-dF/dt) were measured. HLD preparations were more sensitive to ischemia then SD ones. PC, performed by 5-min perfusion with no-substrate solution gassing with 95% N2/5% CO2 in the presence of fast electrical stimulation, and 10 min of reperfusion with normal solution and rate of stimulation, significantly increased the resistance of isolated cardiac tissues to simulated ischemia in SD-PC group, but not in HLD-PC group. Negative inotropic action of phenylephrine occured in SD group of preparations after simulated-ischemia/reperfusion period was also prevented by PC. Therefore, we conclude that experimental hyperlipidemia significantly influenced the function of rat heart muscle including the higher sensitivity to ischemia and different reaction to the same PC procedure.     

DMA对正常大鼠离体心脏和乳头肌的正性变力作用及其机制

目的研究二甲基氨氯吡咪(dimethylamiloride,DMA)对正常大鼠离体心脏和乳头肌的正性变力作用,并探讨其机制。方法采用Langendorff心脏灌流和离体乳头肌灌流方法观察DMA的变力性作用,并应用L-型钙通道阻滞药和Na+/Ca2+交换体(sodium calcium exchanger,NCX)抑制剂探讨其作用机制。结果①1~20μmol.L-1DMA对正常大鼠离体心脏产生正性变力作用,即增强左室主动收缩压(LVSP-LVDP)、左室压最大上升速率(+dp/dtmax)、左室压最大下降速率(-dp/dtmax);增强乳头肌收缩功能,即升高发展张力(TC),缩短收缩至最大速率的时间(T-dp/dtmax)。与用药前相比,差异有显著性(P<0.05或P<0.01)。②DMA的正性变力作用不能被L-型钙通道阻滞药尼卡地平(nicardipine)阻断。③NCX抑制剂氯化镍(NiCl2)可阻断DMA的正性变力作用。结论①(1~20)μmol.L-1DMA对正常大鼠离体心脏和乳头肌产生正性变力作用。②DMA正性变力作用与L-型钙通道无关。③DMA通过激动NCX产生正性变力作用。     

Age-dependent differences in the positive inotropic effect of phenylephrine on rat isolated atria

The age-dependent differences in the involvement of alpha-adrenoceptors in the positive inotropic effect of phenylephrine (Phe) were examined in isolated atria of male Wistar rats 6 weeks (6W), 10 weeks and 7 months (7M) of age. The maximal increase in tension development induced by Phe increased with age, whereas the EC50 values for the positive inotropic effect of Phe did not change with age. The inhibitory effect of phentolamine on the response to Phe increased with age. Propranolol caused only slight inhibition of the effect of Phe in both 6W and 7M rats, and the EC50 values for Phe in the presence of propranolol did not change significantly with age. The EC50 values for isoprenaline and 5-hydroxytryptamine in 7M rats were higher than those in 6W rats. In 7M rats, the duration of the tension development was only slightly affected by Phe in the presence or absence of propranolol, but it was markedly decreased by Phe in the presence of phentolamine. The dose-response curve for Phe was markedly shifted to the left by papaverine in 6W rats, but slightly in 7M rats. The dose-response curve for isoprenaline was markedly shifted to the left by papaverine in both groups. These results are consistent with effects of Phe being mediated by both alpha- and beta-adrenoceptors in both 6W and 7M rats, but there is a shift in the balance from rather more beta-receptors in the young animals to more alpha-receptors in the adults.     

No evidence for involvement of dopaminergic receptors in the positive inotropic action of dopamine on the isolated rabbit papillary muscle.

Experiments were carried out on the isolated rabbit papillary muscle driven at 0.5 Hz in order to further elucidate the mechanism of the positive inotropic effect evoked by dopamine. The dose-response curve for dopamine was not affected by the antagonists pimozide (10(-6) M), yohimbine (10(-5) M) pindolol (3 x 10(-8) M) and phentolamine (10(-6) M) when these agents were given separately. Only the simultaneous administration of yohimbine plus pindolol and phentolamine plus pindolol, respectively, shifted the entire curve to the right. This shift was not further influenced by pimozide. Dopamine (10(-4) M) increased the cyclic AMP content of the papillary muscle by about 50%; this increase was not affected by pimozide, but was markedly elevated by yohimbine and completely depressed by pindolol. From the present results it is concluded, that dopamine produces its positive inotropic effect through stimulation of myocardial alpha-as well as beta-adrenoceptors to about the same degree; stimulation of specific dopaminergic receptors, however, is not involved. The stimulation of beta-adrenoceptors is accompanied by an increase of the cyclic AMP level, while that of alpha-adrenoceptors is not.     

Positive inotropic effects of adrenomedullin on rat papillary muscle

Adrenomedullin is a peptide recently isolated from pheochromocytoma that has vasorelaxant and long-lasting hypotensive activities. Plasma levels of adrenomedullin are elevated in patients with congestive heart failure, but the effects of adrenomedullin on the cardiac function are unclear. We, thus, investigated the effects of adrenomedullin on the contraction of rat papillary muscles. We measured the isometric tension and cAMP contents of isolated rat papillary muscles. Adrenomedullin exhibited concentration-dependent inotropic effects. Adrenomedullin also significantly increased intracellular contents of cAMP. Addition of the calcitonin gene-related peptide (CGRP) receptor antagonist inhibited both contractile force and cAMP generation of papillary muscles stimulated by adrenomedullin. The adrenomedullin-induced inotropic effect was further increased in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX), while the effect was significantly suppressed by KT5720 and Rp-8-bromoadenosine-3',5'-cyclic monophosphorothioate (Rp-8-Br-cAMPS), protein kinase A inhibitors. These results indicate that adrenomedullin has positive inotropic effects on the heart, at least partially through a cAMP-dependent pathway.     

Positive inotropic action of saponins on isolated atrial and papillary muscles from the guinea-pig.

The effects of several saponins of animal and plant origin on the contractile activity of atrial and papillary muscles of the guinea-pig were tested. In a concentration of 1 X 10(-5)M, holothurin-A (HL-A), holothurin-B, echinoside-A, echinoside-B and sakuraso-saponin (Saku) exhibited positive inotropic and chronotropic actions whereas desacyl-jego-saponin and ginsenoside-Rd did not. Saponins having a positive inotropic action caused haemolysis of rabbit erythrocytes whereas those without inotropic action did not cause haemolysis. The positive inotropic action of saponins was not affected by practolol, chlorpheniramine, cimetidine and indomethacin. Verapamil (10(-6)M) inhibited the inotropic actions due to HL-A and isoprenaline (10(-8)M) to the same extent but had a small effect on those due to ouabain (10(-7)M). In high K+ (30 mM K+) medium where the action potential and the contraction were depressed, HL-A, Saku and isoprenaline restored the action potential and the contraction of the 'slow response' type whereas ouabain failed to do so. In normal medium HL-A and Saku reduced the resting membrane potential by 15-20 mV. These results suggest that modification of the Ca channel is involved in the positive inotropic action of saponins.     

On the mechanism of action of phenylephrine in rat atrial heart muscle

Both in rat left atrial heart and in aortic smooth muscle preparations, phenylephrine (PE) caused a concentration-dependent increase in force of contraction (F_c) in the presence of atenolol (10 mol/l), which was antagonized by phentolamine, prazosin and WB 4101 in a competitive manner. The pA₂ values of the antagonists in the cardiac tissue were 10–20fold lower than those in the rat thoracic aorta. In the spontaneously beating right atrium, PE exerted a positive chronotropic action, which was not significantly antagonized by phentolamine or prazosin. It is therefore assumed that the effects of phenylephrine in the left atrium and in the aorta are mediated by different subtypes of ₁-adrenoceptors, whereas the effects in the sino-atrial node are probably unrelated to ₁-adrenoceptors. To further elucidate the mechanisms of the positive inotropic effect of PE, action potential configuration and ⁴⁵Ca²⁺ fluxes were monitored in the rat left atrium. The increase in F_c by PE was associated with an increase in action potential duration (APD) and a reduction in resting membrane potential (RP). In the presence of (–)-devapamil (13888), the effects of PE on APD and RP persisted, whereas the increase in F_c was antagonized in a non-competitive manner. Forskolin (300 nmol/l) enhanced the positive inotropic effect of PE. PE exerted a significant increase in ⁴⁵CA²⁺ uptake in beating preparations, which was abolished in the presence of (–)13888 (1 mol/l). In addition to the PE-induced increase in ⁴⁵Ca²⁺ uptake, a decrease in ⁴⁵Ca²⁺ efflux was observed. Similarly, depolarization of the membrane by raising [K⁺]_o to 85 mmol/l revealed an increase in ⁴⁵Ca²⁺ uptake and a decrease in ⁴⁵Ca²⁺ efflux. The latter observations support the view that the membrane potential strongly determines the movement of ⁴⁵Ca²⁺ across the membrane. It is assumed that the ₁-adrenoceptor-mediated changes in APD and RP may enhance F_c, first, by increasing net Ca²⁺ entry from the extracellular space through voltage-dependent Ca²⁺ channels and, second, by decreasing Ca²⁺ efflux possibly via the Na ⁺/Ca²⁺ exchange mechanism.     

Triphasic inotropic response of guinea-pig papillary muscle to murrayaquinone-A isolated from Rutaceae

Murrayaquinone-A, a carbazole alkaloid, was found to produce a triphasic inotropic response (first positive, second negative and third positive phases) of guinea-pig papillary muscle that normally paced at a slow rate of 0.2 Hz in Krebs-Henseleit solution at 30 degrees C. Murrayaquinone-A produced a concentration-dependent (10(-6) M-10(-4) M) positive inotropic effect (pD2 value 5.27 evaluated at the first phase). The triphasic pattern of inotropism of murrayaquinone-A was unaffected by reserpine, metoprolol or cimetidine treatment. Murrayaquinone-A increased the initial upstroke and the duration of the slow action potential in partially depolarized muscle (external K+ = 30 mEq). Murrayaquinone-A did not cause any positive inotropy under anoxic conditions and in the presence of 2,4-dinitrophenol and dicumarol. These results indicated that the triphasic inotropic effect of murrayaquinone-A is not mediated through a receptor mechanism but through a novel mechanism involving mitochondrial ATP production, thereby increasing the slow inward calcium current across the cardiac cell membrane via cyclic AMP converted from mitochondrial ATP.     

Negative inotropic action of corynanthine in the guinea pig papillary muscle is attenuated by glibenclamide

• 1.1. The change in force and rate of rise of force contraction in the guinea pig papillary muscle were measured.
• 2.2. Corynanthine, an alpha-1 adrenoceptor blocking agent, produced direct negative inotropic action and inhibited the isoprenaline positive inotropic action in a dose-dependent manner (3, 10 and 30 μmol/l).
• 3.3. Addition of 1 μmol/l glibenclamide, a potent inhibitor of ATP-sensitive potassium channels, significantly antagonized the direct negative inotropic action of corynanthine, but had no effect on the antagonism between isoprenaline and corynanthine.
• 4.4. The relationships between isoprenaline and corynanthine, and corynanthine and glibenclamide are discussed.

     

Phorbol ester potentiates alpha 1-adrenoceptor-mediated positive inotropic effect in rat papillary muscle

Tumor-promoting phorbol esters may alter alpha 1-adrenoceptor-mediated cardiac response by stimulating protein kinase C activity. We investigated the effect of phorbol-12,13-dibutyrate (PDBu) on the positive inotropic effect (PIE) in rat left ventricular papillary muscle. PDBu (1-100 nM) potentiated the phenylephrine (PE)-induced PIE in a dose- and time-dependent manner. The PIE induced by PE and PDBu was abolished by pretreatment with 3 x 10(-7) M prazosin. PDBu also enhanced PE-induced slow responses 2- to 3-fold. These results suggest that PDBu enhances alpha 1-adrenoceptor-mediated PIE by potentiating slow Ca2+ channels, presumably through the activation of protein kinase C.     

Studies on the mechanism of the positive inotropic action evoked by epinine on the rabbit isolated papillary muscle at different rates of beating

1. Effects of epinine on cyclic AMP and contractility were investigated in rabbit papillary muscles driven at a rate of 0.5 or 2.0 Hz. 2. When the frequency of stimulation was increased from 0.5 to 2.0 Hz, the log dose-response curve for the positive inotropic effect of epinine was displaced to the left, whereas the maximum of the developed tension was not changed. 3. At both frequencies phentolamine (1 μmol/1) shifted the lower part of the log dose-response curve for epinine to the right, whereas pindolol (30 nmol/1) affected mainly the upper part. In the presence of both a- and β-adrenoceptor antagonists, the whole curve was shifted to the right in a parallel manner. However, cocaine (30 μmol/1) did not significantly influence the log dose-response curve of epinine. 4. At 0.5 Hz a submaximal effective concentration of epinine (100 μmol/1) led to an approximately 100% increase of the cyclic AMP level after 60 s; the same increase of the cyclic AMP level was induced at 2.0 Hz by one-third the concentration of epinine (30 μmol/1). 5. Phentolamine (1 μmol/1) did not affect the increase of the cyclic AMP level evoked by epinine, whereas pindolol (30 μmol/1) completely depressed it. 6. The present results indicate that epinine produces its positive inotropic effect through direct stimulation of myocardial α-adrenoceptors as well as β-adrenoceptors, depending upon the concentration: in lower concentrations it acts mainly on α-adrenoceptors, whereas in higher concentrations it acts predominantly on β-adrenoceptors. The positive inotropic effect through α-adrenoceptor stimulation is mediated by cyclic AMP, while that through a-adrenoceptors is not.     

Interaction between negative inotropic effects of halothane and nifedipine in the isolated rat heart

Isobolographic analysis was used to assess quantitatively the combined direct negative inotropic effect of halothane and nifedipine as well as to define the type of interaction between these two agents. Experiments were performed on an isolated rat heart preparation. Developed left ventricular pressure and dP/dtmax were used as indices of inotropic action. It was found that the combination of halothane and nifedipine gives mostly additive direct negative inotropic effect. However, with a small level of myocardial depression (ED25), there was some deviation from the additive interaction toward an infra-additive effect (p less than 0.01), and with a high level of depression (ED75), some deviation toward a supra-additive effect (p less than 0.05).     

A neurally mediated inotropic effect of veratridine and cevadine on isolated guinea-pig papillary muscle

Summary The positive inotropic effect of veratridine and cevadine was investigated in the isolated, isometrically contracting guinea-pig papillary muscle. The increase of the force of the rested-state contraction, elicited after incubation of the resting muscle with veratridine or cevadine, served as a measure of the neurally mediated, sympathomimetic effect of these alkaloids. Concentrations exceeding 5 mol/l veratridine or 15 mol/l cevadine produced concentration-dependent increases of the force of the rested-state contraction. These concentrations are larger than those causing the maximum positive inotropic effect on muscles contracting continually at a rate of 1 Hz. The positive inotropic effect of 30 mol/l veratridine as manifested by the rested-state contraction was absent in the presence of 100 nmol/l tetrodotoxin and in muscles from reserpine-pretreated animals. It was significantly inhibited by 50 nmol/l (–)-propranolol, but not by the same concentration of (+)-propranolol. The effect of 30 or 60 mol/l cevadine was likewise absent in catecholamine-depleted preparations. It is concluded that the indirect inotropic effect of veratridine or cevadine, which is attributed to their noradrenaline-releasing effect on intracardiac nerves, requires higher concentrations than the direct positive inotropic effect, which is a consequence of the increased transsarcolemmal influx of Na ions into the myocardial cell.These results were communicated to the Deutsche Pharmakologische Gesellschaft (Honerjäger 1977)     

The mechanism of the positive inotropic action of ketamine on isolated atria of the rat.

1 The effect of ketamine (10-7 M-5 X10-4 M) on electrical and mechanical properties of isolated atria of the rat was investigated. 2. On spontaneously beating right atria, ketamine produced a dose-dependent positive inotropic effect which was accompanied by a progressive reduction in atrial rate. 3 In electrically driven left atria, ketamine produced a dose-dependent positive inotropic effect which was accompanied by a parallel increase in df/dtmax and by a prolongation in the time to peak tension and in the time for total concentration. The positive inotropic effect occurred concomitantly with an increase in the height and duration of the plateau phase of the action potential of atrial fibres. 4 The positive inotropic effect of ketamine varied with the concentration of Ca and Na in the bathing media and the rate of stimulation. 5 Ketamine decreased post-extrasystolic potentiation and the amplitude-interval relationship. 6 The positive inotropic effect of ketamine was inhibited by verapamil (10-6 M) and by caffeine (4 X10-3 M). 7 Ketamine, 5 X10-5M and 10-4M, increased 45Ca uptake in electrically driven left atria. At 10-4M and 5 X10-4M, ketamine also increased 45Ca efflux. 8 These results suggest that ketamine produces its positive inotropic effect by two possible mechanisms. One of these is presumed to be an effect on the cell membrane with leads to an increased Ca influx into atrial fibres; the other is probably related to the inhibition of Ca sequestration by the sarcoplasmic reticulum.     

Mechanism of the histamine-induced positive inotropic action in cardiac muscle.

Histamine, a positive inotropic agent which elevates cyclic AMP, was tested for ability to induce Ca2+ channels in 3 preparations of embryonic (16-day-old) chick ventricular myocardial cells whose fast Na+ channels were blocked by tetrodotoxin or voltage inactivated in 25 mM K+. In such inexcitable cells, histamine (10(-6)M) rapidly (1-3 min) induced slowly rising, overshooting, plateau-like responses accompanied by contractions. Mn2+, D600, or H2-receptor blocking agents abolished the slow responses. These results suggest that the positive inotropic action of histamine, like that of catecholamines, is mediated by an increased availability of slow Ca2+ channels.     

Positive inotropic and negative chronotropic effects of (-)-cis-diltiazem in rat isolated atria.

1. The cardiovascular effects of (-)-cis-diltiazem, an optical isomer of diltiazem, were studied in the isolated atrium and aortic strip. (-)-cis-Diltiazem (30 microM or more) increased the developed tension of the rat left atrium, while (+)-cis-diltiazem (1 microM or more) decreased it. 2. (-)-cis-Diltiazem (1 to 100 microM) decreased the rate of spontaneous beating in the right atrium as did (+)-cis-diltiazem. 3. The potency of the positive inotropic action of (-)-cis-diltiazem was almost the same as that of ouabain in the rat left atrium, but in the guinea-pig left atrium it was considerably weaker than that of ouabain. 4. In both endothelium-intact and endothelium-denuded aortic strips, (-)-cis-diltiazem relaxed the Ca(2+)-induced contraction. In the endothelium-intact rat aortic strip depolarized by 15 mM KCl, Bay K 8644, a calcium channel agonist, increased the contractile force, whereas (-)-cis-diltiazem did not. 5. These results indicate that (-)-cis-diltiazem has a positive inotropic action in isolated atria in rats and guinea-pigs, but the mode of positive inotropic action of (-)-cis-diltiazem is different from that of ouabain or Bay K 8644.     

Implications of altered inotropic effects of phenylephrine in pressure-overloaded cat ventricular muscle

The positive inotropic action of phenylephrine in cardiac muscle is mediated by alpha- and beta-adrenoceptors. Data suggest the responsiveness of myocardium to inotropic agents is altered in cardiac disease. We evaluated the actions of phenylephrine on isometric contraction and K+-induced contracture in isolated cat right ventricular muscle from normal hearts and hearts with partial pulmonary artery ligation-induced pressure overload of 5-11 days in duration. Peak contractile force (Po) and rate of force development (dP/dt) were lower (p less than or equal to 0.005) in pressure-overloaded (0.59 +/- 0.2 g/mm2 and 4.6 +/- 1.7 g/s/mm2, respectively) than in normal (1.33 +/- 0.2 g/mm2 and 10.5 +/- 1.6 g/s/mm2, respectively) muscle. Phenylephrine (10(-6), 5 X 10(-6), and 10(-5) M) significantly (p less than or equal to 0.01) increased Po and dP/dt in normal but not in pressure-overloaded muscle. Phenylephrine (5 X 10(-6) M) reduced peak K+-induced contracture force (Pc) similarly in normal (-30 +/- 8%) and pressure-overloaded (-36 +/- 11%) muscles. beta-Adrenergic blockade (nadolol, 10(-4) M) reduced, but did not abolish, the "relaxant" action of the drug on Pc in both muscle groups. The lack of a positive inotropic effect of phenylephrine in pressure-overloaded muscle suggests a derangement in both alpha- and beta-adrenoceptor function in this model of cardiac disease.