Positive Inotropic Effect Of Phenylephrine Research Articles

α1-Adrenergic receptor regulates papillary muscle and aortic segment contractile function via modulation of store-operated Ca2+ entry in long-tailed ground squirrels Urocitellus undulatus.

The effect of phenylephrine (PE) on right ventricle papillary muscle (PM) and aortic segment (AS) contractile activity was studied in long-tailed ground squirrels Urocitellus undulatus during summer activity, torpor and interbout active (IBA) periods in comparison to rat. We found that PE (10μM) exerts positive inotropic effect on ground squirrel PM that was blocked by α1-AR inhibitor-prazosin. PE differently affected frequency dependence of PM contraction in ground squirrels and rats. PE significantly increased the force of PM contraction in summer and hibernating ground squirrels including both torpor and IBApredominantly at the range of low stimulation frequencies (0.003-0.1Hz), while in rat PM it was evident only at high stimulation frequency range (0.2-1.0Hz). Further, it was found that PE vasoconstrictor effect on AS contractility is significantly higher in ground squirrels of torpid state compared to IBA and summer periods. Overall vasoconstrictor effect of PE was significantly higher in AS of ground squirrels of all periods compared to rats. Positive inotropic effect of PE on PM along with its vasoconstrictor effect on AS of ground squirrels was not affected by pretreatment with inhibitors of L-type Ca2+ channels, or Na+/Ca2+ exchanger or Ca2+-ATPase but was completely blocked by an inhibitor of store-operated Ca2+ entry (SOCE)-2-APB, suggesting the involvement of SOCE in the mechanisms underlying PE action on ground squirrel cardiovascular system. Obtained results support an idea about the significant role of alpha1-AR in adaptive mechanisms critical for the maintaining of cardiovascular contractile function in long-tailed ground squirrel Urocitellus undulatus.

Pharmacokinetic–pharmacodynamic modeling of the effect of propofol on α 1-adrenoceptor-mediated positive inotropy in rat heart

Since it is unclear whether and how propofol affects the α 1-mediated inotropic response, we used a pharmacokinetic–pharmacodynamic modeling approach in isolated rat hearts to analyze the effect of propofol on receptor binding and signal transduction. In Langendorff rat hearts perfused with buffer containing 12.3 μM phenylephrine, 1.27 nmol doses of [ 3H]-prazosin were infused (over 1 min), in the absence and presence of propofol (28 μM). Simultaneous analysis of prazosin outflow concentration and inotropic response (left ventricular developed pressure) using an agonist–antagonist interaction model allowed to estimate receptor affinity, as well as the parameters of the operational model of agonism. Propofol significantly ( P < 0.05) increased the negative inotropic effect of prazosin. Modeling suggested that propofol increased the Hill coefficient, which determines the steepness of the stimulus–response curve for the positive inotropic effect of phenylephrine, from 1 to 2.6 ± 0.1 and decreased the maximum achievable inotropic effect from 121.2 ± 12 to 91.8 ± 6 mmHg. Thus, propofol may attenuate the positive inotropic effect of α 1-agonists by decreasing the transduction efficiency of α 1-adrenergic receptor signaling primarily at lower receptor occupancy.

Role of calmodulin kinase II in inotropic effect of α1‐adrenergic stimulation in the heart

The multifunctional Ca2+ and calmodulin‐dependent kinase II (CaMKII) may play a role in cardiac function by affecting Ca2+ release or uptake mechanisms through phosphorylation of its downstream targets phospholamban (PLB) and the ryanodine receptor. Here we examined the role of CaMKII in the positive inotropic effect of the α1‐adrenoceptor agonist phenylephrine (PE). In left atria isolated from a genetic mouse model of cardiac CaMKII inhibition (AC3‐I) PLB‐T17 phosphorylation was 20% of wild type (WT) or AC3‐C (scrambled peptide). Respectively, basal force (1 Hz) was 20% lower at 1.8 mM Ca2+, but normal at high Ca2+ (&gt;4.5 mM). The maximal positive inotropic effect of PE, which was more pronounced at low frequencies in WT and AC3‐C, lost its frequency dependence in the AC3‐I atria. Thus, the effect of PE was reduced by 50% at 1 Hz, but was normal at 8 Hz. Post‐rest (30 s, 1 Hz) potentiation of force was also absent. Subsequent studies were carried out examining response to PE in CaMKIIδ knockout mice. Basal and PE (100 μM)‐stimulated left ventricular pressures were not altered in Langendorff perfused hearts from these mice, despite significantly reduced PLB‐T17 phosphorylation after 10 min PE (20% of WT). Our data suggest a role of CaMKII in the positive inotropic effect of atrial α1‐adrenergic stimulation. Further studies are needed to determine the role of CaMKII in cardiac function under pathophysiological conditions.

Reduced contractile response to α 1-adrenergic stimulation in atria from mice with chronic cardiac calmodulin kinase II inhibition

The sustained positive inotropic effect of α-adrenoceptor agonists in the heart is associated with a small increase in intracellular Ca 2+ transients together with a larger sensitization of myofilaments to Ca 2+. The multifunctional Ca 2+ and calmodulin-dependent protein kinase II (CaMKII) could contribute to this effect, either by affecting the Ca 2+ release (ryanodine receptor) or by an uptake mechanism (via phospholamban [PLB] and SR Ca 2+ ATPase). Here we examined the role of CaMKII in the positive inotropic effect of the α-adrenoceptor agonist phenylephrine in left atria isolated from a genetic mouse model of cardiac CaMKII inhibition (AC3-I). Compared to atria from wild-type (WT) or AC3-C (scrambled peptide), AC3-I atria showed the following abnormalities. PLB phosphorylation at Thr17, a known CaMKII target, was significantly lower (∼ 20%). Post-rest (30 s, 1 Hz, 37 °C) potentiation of force was absent (AC3-C, 190% of pre-rest amplitude). Basal force was ∼ 20% lower at 1.8 mM Ca 2+, but normal at high Ca 2+ concentration (> 4.5 mM). The maximal positive inotropic effect of phenylephrine, which was more pronounced at low frequencies in WT and AC3-C atria, lost its frequency dependence (1 Hz to 8 Hz). Thus, the effect of phenylephrine was reduced by ∼ 50% at 1 Hz, but was normal at 8 Hz. All three groups showed a negative force–frequency relation, and did not differ in the frequency-dependent acceleration of relaxation. Our data indicate a role of CaMKII in post-rest potentiation and the positive inotropic effect of α-adrenergic stimulation at low frequencies.

Noradrenaline-induced desensitization in cultured heart cells as a model for the defects of the adenylate cyclase system in severe heart failure

Cultivation of rat heart muscle cells for up to 5 days in the presence of 10(-6) mol/l (-)noradrenaline leads to a desensitization of the cells to inotropic stimulation and cAMP formation at different levels of the cAMP-system: The decrease in the responsiveness to the beta-adrenoceptor agonist isoprenaline quantitatively parallels the down-regulation of beta 1-adrenoceptors. The impairment of the positive inotropic effect of the phosphodiesterase-inhibitor IBMX is associated with a diminished basal cAMP-formation but an unchanged phosphodiesterase-activity in noradrenaline-treated cells. This decrease in basal cAMP-accumulation as well as the attenuation of the forskolin-stimulated cAMP-formation indicate that a defect in the adenylate cyclase system beyond the receptor must be involved in noradrenaline-induced desensitization. In contrast to the diminished effectiveness of cAMP-increasing agents the positive inotropic effect of ouabain and the alpha-adrenoceptor-mediated positive inotropic effect of phenylephrine are unaltered in noradrenaline-treated cells. It is concluded from these results that in addition to the down-regulation of beta 1-adrenoceptors a defect on the post-receptor level of adenylate cyclase occurs in noradrenaline-induced desensitization not affecting mechanisms beyond the catalytic subunit of adenylate cyclase. As these findings are very similar to those observed in isolated preparations from severely failing human hearts, the presumption is confirmed that noradrenaline-induced desensitization might constitute an important etiological factor in the subsensitivity of failing human hearts to inotropic stimulation.

Comparison of the Electromechanical Responsiveness of Alpha-1-Adrenoceptor Stimulation in Ventricles of Normal and Cardiomyopathic Hamsters

Alterations in α₁-adrenoceptor (α₁AR) density and related signal transduction proteins were reported in cardiomyopathic hearts in the failing stage. The electromechanical modification of α₁-adrenergic stimulation in the failing heart is unclear. The present study compares the α₁AR-stimulated electromechanical response in failing ventricles of genetically cardiomyopathic BIO 14.6 hamsters (280–320 days old) with that in age-matched normal Syrian hamsters. The action potential was recorded with a conventional microelectrode technique, and twitch force was measured with a transducer. In the presence of propranolol, phenylephrine increased the contraction and prolonged the action potential duration (APD) to similar values in ventricles of both strains, despite a prolonged basal APD in cardiomyopathic ventricles. The positive inotropism stimulated by phenylephrine was inhibited by staurosporine, and was potentiated by 4β-phorbol-12,13-dibutyrate (PDBu) in both strains. The maximum positive inotropic effect of phenylephrine in PDBu-treated ventricles of normal hamsters was significantly greater than that in BIO 14.6 hamsters. The effects of phenylephrine on the ventricular force-frequency relationship and on the mechanical restitution in both normal and BIO 14.6 strain hamsters were examined. The uniform negative force-frequency relationship and the altered mechanical restitution reveal a defect of intracellular Ca²⁺ handling in cardiomyopathic BIO 14.6 hamsters. α₁-Adrenergic modulation cannot convert the defective properties in the model of the failing heart. Nevertheless, phenylephrine decreased postrest potentiation in short rest periods, and enhanced postrest decay after longer resting periods. The results indicate that α₁-adrenergic action enhances a gradual loss of Ca²⁺ from the sarcoplasmic reticulum, although its action in prolonging the APD can indirectly increase the influx of Ca²⁺.

Xestospongin C, a selective and membrane-permeable inhibitor of IP(3) receptor, attenuates the positive inotropic effect of alpha-adrenergic stimulation in guinea-pig papillary muscle.

We evaluated the role of the inositol 1,4,5-triphosphate (IP(3)) receptor-mediated Ca(2+) release on the positive inotropic effects of alpha-adrenergic stimulation using a novel, potent, selective membrane-permeable blocker of IP(3) receptor, xestospongin C. Guinea-pig papillary muscle permeabilized with saponin exhibited spontaneous oscillatory contractions in solution buffered with pCa(2+) 6.5 by a low concentration of EGTA. The oscillatory activity was increased by adding 100 microM IP(3) and abolished by 1 microM ryanodine or 30 microM cyclopiazonic acid. Xestospongin C (3 microM) inhibited the IP(3)-induced increase in the oscillatory contractions without affecting basal oscillations. In intact papillary muscle, xestospongin C (3 microM) inhibited the positive inotropic effects of phenylephrine, resulting in a rightward and downward shift of the concentration-response curve for phenylephrine. On the contrary, xestospongin C did not affect the concentration-response curve for phenylephrine obtained in the presence of ryanodine (1 microM). On the other hand, xestospongin C affected neither basal contractions nor the positive inotropic effects of a high extracellular Ca(2+) concentration (3.2 mM) or that of isoprenaline (1 and 10 nM). These results suggest that the IP(3)-mediated increase in Ca(2+) release is involved in the positive inotropic effects of alpha-adrenergic stimulation in the guinea-pig cardiac muscle.

Effects of α1‐ or β‐adrenoceptor stimulation on work‐loop and isometric contractions of isolated rat cardiac trabeculae

1. We studied the effects of alpha1- or beta-adrenoceptor stimulation on the contractility of isolated rat ventricular trabeculae at 24 degrees C using the work-loop technique, which simulates the cyclical changes in length and force that occur during the cardiac cycle. Some muscles were injected with fura-2 to monitor the intracellular Ca2+ transient. 2. Comparison of twitch records revealed that peak force was greater and was reached earlier in work-loop contractions than in corresponding isometric contractions. This was attributed to the changes in muscle length and velocity during work-loop contractions, since the Ca2+ transients were largely unaffected by the length changes. 3. Stimulation of alpha1-adrenoceptors (with 100 microM phenylephrine) increased net work, power production, the frequency for maximum work, and the frequency for maximum power production (fopt). The increase in net work was due to the positive inotropic effect of phenylephrine, which was similar at all frequencies investigated (0. 33-4.5 Hz). The increase in fopt was attributed to an abbreviation of twitch duration induced by alpha1-stimulation at higher frequencies (> 1 Hz), even though the twitch became longer at 0.33 Hz. 4. beta-Adrenoceptor stimulation (with 5 microM isoprenaline) produced marked increases in net work, power output, the frequency for net work, and fopt. These effects were attributed both to the positive inotropic effect of beta-stimulation, which was greater at higher frequencies, and to the reduction in twitch duration. beta-stimulation also abolished the frequency-dependent acceleration of twitch duration. 5. The increase in power output and fopt with alpha1- as well as beta-adrenoceptor stimulation suggested that both receptor types may contribute to the effects of catecholamines, released during stress or exercise, although the greater effects of beta-stimulation are likely to predominate.

Modifications of the inotropic responses to alpha- and beta-adrenoceptor stimulation by propofol in rat myocardium.

Propofol induces cardiovascular depression but without significant effect on intrinsic myocardial contractility in many species. However, its interactions with adrenoceptor stimulation are unknown. We studied the effects of propofol (1 and 10 microg/mL) and its solvent on the inotropic response induced by phenylephrine (10(-8)-10(-4) M) or isoproterenol (10(-8)-10(-4) M) in rat left ventricular papillary muscles in vitro (Krebs-Henseleit solution, 29 degrees C, pH 7.40, calcium 0.5 mM, stimulation frequency 12 pulses/min). We also studied the lusitropic effects in isotonic and isometric conditions. In control groups, phenylephrine (127% +/- 3% of baseline; P < 0.05) and isoproterenol (169% +/- 11% of baseline; P < 0.05) induced a positive inotropic effect. Propofol (10 microg/mL) completely abolished the positive inotropic effect of phenylephrine (100% +/- 3% of baseline; P = not significant). In contrast, at the lowest concentration (1 microg/mL), propofol did not modify the positive inotropic effect of phenylephrine. Propofol did not modify the inotropic effect of isoproterenol. Propofol (10 microg/mL) enhanced the positive lusitropic effect of isoproterenol under low-load (P < 0.05) but not under high-load conditions. A high concentration of propofol abolished the positive inotropic effect of alpha- but not beta-adrenoceptor stimulation and enhanced the positive lusitropic effect of beta-adrenoceptor stimulation.

Modifications of the Inotropic Responses to alpha- and beta-Adrenoceptor Stimulation by Propofol in Rat Myocardium

Propofol induces cardiovascular depression but without significant effect on intrinsic myocardial contractility in many species. However, its interactions with adrenoceptor stimulation are unknown. We studied the effects of propofol (1 and 10 microg/mL) and its solvent on the inotropic response induced by phenylephrine (10(-8)-10(-4) M) or isoproterenol (10(-8)-10(-4) M) in rat left ventricular papillary muscles in vitro (Krebs-Henseleit solution, 29 degrees C, pH 7.40, calcium 0.5 mM, stimulation frequency 12 pulses/min). We also studied the lusitropic effects in isotonic and isometric conditions. In control groups, phenylephrine (127% +/- 3% of baseline; P < 0.05) and isoproterenol (169% +/- 11% of baseline; P < 0.05) induced a positive inotropic effect. Propofol (10 microg/mL) completely abolished the positive inotropic effect of phenylephrine (100% +/- 3% of baseline; P = not significant). In contrast, at the lowest concentration (1 microg/mL), propofol did not modify the positive inotropic effect of phenylephrine. Propofol did not modify the inotropic effect of isoproterenol. Propofol (10 microg/mL) enhanced the positive lusitropic effect of isoproterenol under low-load (P < 0.05) but not under high-load conditions.A high concentration of propofol abolished the positive inotropic effect of alpha- but not beta-adrenoceptor stimulation and enhanced the positive lusitropic effect of beta-adrenoceptor stimulation.

Role of Ca(2+)-independent PKC in alpha 1-adrenoceptor-mediated inotropic responses of neonatal rat hearts.

We investigated the role of protein kinase C (PKC) in alpha 1-adrenoceptor (alpha 1-AR)-mediated positive inotropic responses of neonatal rat myocardium. Bisindolylmaleimide (BIM), an inhibitor of all PKC isoforms, reduced the positive inotropic responses to phenylephrine and methoxamine but not to isoproterenol. The positive inotropic effect of phenylephrine was not attenuated by Gö-6976, a selective inhibitor of Ca(2+)-dependent isoforms; it was potentiated by the 1,2-diacylglycerol kinase inhibitor R-59949. Phorbol 12,13-dibutyrate, an activator of both Ca(2+)-dependent and-independent PKC isoforms, as well as thymeleatoxin, a selective activator of Ca(2+)-dependent PKC isoforms, inhibited myocardial contractions, which were prevented by BIM and Gö-6976. BIM inhibited the phenylephrine-induced increase in particulate PKC activity but not the increase in phosphatidylinositide turnover. Phenylephrine induced translocation of only Ca(2+)-independent PKC-epsilon and -delta. These results suggest that activation of Ca(2+)-independent PKC isoforms by alpha 1-AR agonists plays a key role in alpha 1-AR-mediated positive inotropic effect on neonatal myocardium.

Electrical and mechanical effects of a novel antihypertensive quinazoline derivative, 3-[[4-(2-methoxyphenyl) piperazin-1-ly]methyl]-5-(methylthio)-2,3-dihydroimidazo [1,2-c]quinazoline, on guinea pig ventricular muscles.

Electrical and mechanical effects of 3-[[4-(2-methoxyphenyl)piperazin-1-ly]methyl]-5-(methylthio)-2,3-+ ++dihydroimidazo[1,2-c]quinazoline (DL-017), a new synthesized antihypertensive agent, were studied in guinea-pig ventricular papillary muscles. In muscle fibers driven at 1 Hz, DL-017 decreased the twitch force in a concentration-dependent manner and significantly increased the action-potential duration and decreased intracellular Na+ activity (ai(Na)) and maximal rate of upstroke of action-potential (Vmax) when concentrations were greater than 1 microM. Phenylephrine in the presence of 1 microM propranolol produced a concentration-dependent positive inotropy. DL-017 (0.01 microM) antagonized the positive inotropic effect of phenylephrine and shifted the concentration-response curve to the right. In K+-depolarized muscle fibers, 0.1 microM DL-017 significantly decreased the contractile force without changing the slow action potential. In low-[K+]o and high-[Ca2+]o solutions, a train of stimuli triggered a spontaneous rhythm that could be abolished by 3 microM DL-017. Our results suggest that DL-017 not only exhibits an alpha1-antagonistic effect but also induces negative inotropy by a decrease in myofibrillar calcium sensitivity and inhibits Na+ channels at higher concentrations, contributing to the drug's negative inotropic and type-I antiarrhythmic effects.

Pretreatment with rimalkalim changes the adrenergic responsiveness of isolated guinea pig papillary muscle

It has been proposed that the positive inotropic action of different drugs is influenced by the activation of ATP-sensitive K + channels (K ATP). This hypothesis was tested by investigating the effects of rimalkalim ((3 S,4 R)-3-hydroxy-2,2-dimethyl-4-(oxo-1 pyrrolidinyl)-6-phenyl-sulfonylchroman hemihydrate (formerly HOE-234)) as activator of K ATP channels on the positive inotropic action of forskolin, l-phenylephrine hydrochloride and dibutyryl cyclic AMP ( N 6,2′- O-dibutyryladenosine 3′:5′-cyclic monophosphate). Experiments were performed with the isolated guinea-pig heart papillary muscle. The force of contraction ( F c), velocity of contraction (+d F/d t) and relaxation (−d F/d t), time to peak contraction ( t tp) and duration of contractions at the level of 10% or more of their total amplitude ( t t 10 ) were measured. Pretreatment with 1 μM rimalkalim had no significant influence on the positive inotropic effects of dibutyryl cyclic AMP and l-phenylephrine in the presence of 1 μM metoprolol. However, the positive inotropic effects of l-phenylephrine in the absence of metoprolol were significantly attenuated after pretreatment with rimalkalim. The increase in force and velocity of contraction induced by forskolin was strongly enhanced under these conditions. Addition of 1 μM glibenclamide, an inhibitor of K ATP channels, to rimalkalim, prevented the above-mentioned changes in the positive inotropic effects of phenylephrine and forskolin obtained after pretreatment with rimalkalim. What is more, addition of 0.2 μM thapsigargin, a selective blocker of Ca 2+-adenosinetriphosphatase of sarcoplasmic reticulum (Ca 2+ ATP-ase), abolished the potentiation of the positive inotropic action of forskolin induced by pretreatment with rimalkalim. These results demonstrate that activation of K ATP channels by rimalkalim alters β-adrenoceptors, but has no effect on α-adrenoceptor signalling pathways and enhances forskolin inotropic effects by a mechanism which probably involves Ca 2+ ATP-ase.

Involvement of α1B-adrenoceptors in the positive inotropic effect of endogenous noradrenaline in rabbit myocardium

We studied the α 1-adrenoceptor subtypes mediating the positive inotropic effects of phenylephrine and noradrenaline as well as endogenous noradrenaline released by tyramine in rabbit papillary muscle. In the presence of propranolol, both phenylephrine and tyramine produced a positive inotropic effect in a concentration-dependent manner. WB4101 ( N-[2-(2,6-dimethoxyphenoxy]ethyl]-2,3-dihydro-1,4-benzodioxin-2-methanamine) and chlorethylclonidine each antagonized the positive inotropic effect of phenylephrine. On the other hand, only chlorethylclonidine significantly blocked the positive inotropic effect of tyramine. However, the presence of both antagonists was needed to block the positive inotropic effect elicited by the exogenous addition of the low concentration of noradrenaline. These data suggest that after extensive blockade of β-adrenoceptors the positive inotropic effects of phenylephrine and exogenous noradrenaline result from stimulation of the α 1A and α 1B-adrenoceptor subtypes, whereas that of endogenous noradrenaline is mediated via the α 1B-adrenoceptor subtype. This could be explained by assuming that the α 1B-adrenoceptor subtype population may be located on a space confronting more closely to the sympathetic nerve endings than the α 1A-adrenoceptor subtype population.

Alpha 1D-adrenoceptors do not contribute to inotropic responses of neonatal rat myocardium.

This study investigated the role of alpha 1D-adrenoceptors (ARs) in the inotropic responses of neonatal (7-day-old) rat myocardium by using the relatively selective antagonist BMY 7378. The positive inotropic effects of phenylephrine on right ventricular strips of neonatal rats were not inhibited by BMY 7378 up to a concentration of 300 nM but potently antagonized by the alpha 1AAR antagonist 5-methylurapidil (5-MU) (pKB, 8.73 +/- 0.12; n = 6). BMY 7378 was approximately 1,000-fold less potent (pKi, 6.63 +/- 0.7; n = 5) in inhibiting the binding of [3H]prazosin to right ventricular membrane preparations of neonatal rats than was 5-MU (pKi, 9.04 +/- 0.54; n = 5). Neonatal rat cerebral cortex and adult rat aorta were used as additional controls. BMY 7378 was a weak inhibitor of [3H]prazosin binding to neonatal cortex (pKi, 6.8 +/- 0.04; n = 3) but highly potent in inhibiting the binding to adult rat aortic membrane preparations (pKi for high-affinity binding, 9.70 +/- 0.30; n = 4); BMY 7378 potently antagonized the effects of phenyleprine on adult rat aortic strips (pKB, 9.05 +/- 0.11; n = 5). It is concluded that alpha 1DARs do not play a significant role in alpha 1AR-mediated inotropic responses of neonatal rat myocardium.

Interaction of halothane with alpha- and beta-adrenoceptor stimulations in rat myocardium.

Halothane induces negative inotropic and lusitropic effects in myocardium. It has been suggested that halothane potentiates beta-adrenoceptor stimulation. However, its effects on the inotropic response to alpha-adrenoceptor stimulation and its effects on the lusitropic effects of alpha- and beta-adrenoceptor stimulation are unknown. The effects of halothane (0.5 and 1 minimum alveolar concentration [MAC]) on the inotropic responses induced by phenylephrine (10(-8) to 10(-4) M) and isoproterenol (10(-8) to 10(-4) M) were studied in rat left ventricular papillary muscles in vitro (in Krebs-Henseleit solution at 29 degrees C, pH 7.40, with 0.5 mM calcium and stimulation frequency at 12 pulses/min). The lusitropic effects were studied in isotonic (R1) and isometric (R2) conditions. One MAC halothane induced a negative inotropic effect (54 +/- 3%, P < 0.05), increased R1 (109 +/- 3%, P < 0.05), and decreased R2 (88 +/- 2%, P < 0.05). In control groups, phenylephrine (137 +/- 7%, P > 0.05) and isoproterenol (162 +/- 6%, P < 0.05) induced a positive inotropic effect. Halothane did not significantly modify the positive inotropic effect of calcium, suggesting that it did not modify the inotropic reserve of papillary muscles. In contrast, 1 MAC halothane enhanced the positive inotropic effects of phenylephrine (237 +/- 19%, P < 0.05) and isoproterenol (205 +/- 11%, P < 0.05). Halothane did not modify the lusitropic effect of phenylephrine under high or low load. In contrast, 1 MAC halothane impaired the positive lusitropic effect of isoproterenol under low load (P < 0.05), whereas it did not modify the positive lusitropic effect of isoproterenol under high load. At clinically relevant concentrations, halothane potentiated the positive inotropic effects of both alpha- and beta-adrenoceptor stimulation. Furthermore, halothane alters the positive lusitropic-effect of beta-adrenoceptor stimulation under low load.

Pharmacological differentiation of synergistic contribution of L-type Ca2+ channels and Na+/H+ exchange to the positive inotropic effect of phenylephrine, endothelin-3 and angiotensin II in rabbit ventricular myocardium.

The present study was designed to delineate pharmacologically the role of sarcolemmal L-type Ca2+ channels and Na+/H+ exchange in the positive inotropic effect (PIE) of phenylephrine mediated by alpha-1 adrenoceptors, endothelin (ET) and angiotensin II (Ang II) that stimulate phosphoinositide (PI) hydrolysis in the rabbit ventricular muscle. The PIE of these receptor agonists was compared with the PIE of isoprenaline that accumulates cyclic AMP. For this purpose, we investigated the influence of a Ca2+ antagonist, verapamil, and of an inhibitor of Na+/H+ exchange, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), alone or in combination, on the cumulative concentration-response curve (CRC) for phenylephrine (with 0.3 microM bupranolol). ET-3 and Ang II in isolated right ventricular papillary muscles of the rabbit, which were electrically stimulated at 1 Hz in Krebs-Henseleit solution at 37 degrees C. Verapamil at 0.3 and 1 microM decreased the basal force of contraction to 37.0 +/- 4.0% and 13.2 +/- 1.1% of the control, respectively, while EIPA event at 10 microM affected the basal force to much less extent and decreased it to 87.0 +/- 1.4%. Verapamil (0.3 and 1 microM) and EIPA (1 and 10 microM), when used alone, each significantly attenuated but did not abolish the PIEs induced by phenylephrine, ET-3 and Ang II, while the simultaneous administration of verapamil (1 microM) and EIPA (10 microM) consistently and almost completely inhibited the PIE induced by these receptor agonists. By contrast, the PIE of isoprenaline was retained even in the presence of verapamil and EIPA. These results indicate that both the influx of Ca2+ ions through L-type Ca2+ channels and activation of Na+/ H+ exchange contribute synergistically to the PIE that is mediated by alpha-1 adrenergic, ET and Ang II receptor agonists, while these mechanisms are not essential for the beta-adrenoceptor-mediated PIE.

(±)-Tamsulosin, an α1A-adrenoceptor antagonist, inhibits the positive inotropic effect but not the accumulation of inositol phosphates in rabbit heart

The influence of (±)-tamsulosin, a selective α 1A-adrenoceptor antagonist, on the positive inotropic effect and the accumulation of inositol phosphates that are induced via α 1-adrenoceptors was studied in comparison with that of another α 1A-adrenoceptor ligand oxymetazoline in the rabbit ventricular myocardium. Phenylephrine elicited a concentration-dependent positive inotropic effect via α 1-adrenoceptors in the presence of either (±)-bupranolol or S(−)-timolol. The mode of antagonism induced by (±)-tamsulosin on the effect of phenylephrine was dependent on the concentration applied: (±)-tamsulosin at 1 and 3 nM acted in a competitive manner, the slope of the regression line of the Schild plot being unity and the p A 2 value being 9.12; at 10 nM, it shifted further the concentration-response curve to the right without affecting the maximal response but the slope became less than unity. At 100 nM and higher, it suppressed the maximal response to phenylephrine. (±)-Tamsulosin effectively antagonized the positive inotropic effect of phenylephrine even after inactivation of α 1B-adrenoceptors by treatment with chlorethylclonidine, which is an indication that the (±)-tamsulosin-sensitive subtype belongs to a class resistant to chlorethylclonidine. (±)-Tamsulosin, over the range of concentrations at which it antagonized the positive inotropic effect mediated by α 1-adrenoceptors, did not affect the accumulation of [ 3H]inositol phosphates that was induced by 10 μM phenylephrine. Oxymetazoline antagonized the positive inotropic effect of phenylephrine in a competitive manner without affecting the accumulation of inositol monophosphate induced by phenylephrine. These results indicate that the positive inotropic effect, mediated via (±)-tamsulosin- and oxymetazoline-sensitive subtype of α 1-adrenoceptors, is exerted by a subcellular mechanism that is independent of the accumulation of inositol phosphates.

Alpha 1-adrenoceptor subtypes mediating inotropic and electrophysiological effects in mammalian myocardium.

Stimulation of alpha 1-adrenoceptors produces a positive inotropic effect in rat and rabbit ventricular myocardium via different mechanisms, the prolongation of action potential duration (APD) exclusively in the former and an increase in myofibrillar Ca2+ sensitivity in large part in the latter. This study was designed to determine whether the two inotropic mechanisms are mediated by different alpha 1-adrenoceptor subtypes. In rat papillary muscles, the positive inotropic effect and APD prolongation induced by phenylephrine (in the presence of propranolol) were inhibited by WB-4101, but not affected by chlorethylclonidine (CEC). WB-4101, but not CEC, blocked the phenylephrine-induced inhibition of the transient outward current (Ito) in rat ventricular cells. On the other hand, WB-4101 and CEC each antagonized the positive inotropic effect of phenylephrine in rabbit papillary muscles. However, the phenylephrine-induced APD prolongation observed in rabbit papillary muscles was blocked only by WB-4101. These results indicate that the WB-4101 sensitive alpha 1-adrenoceptor subtype mediates the positive inotropism that is correlated with the APD prolongation resulting from Ito reduction, whereas the CEC-sensitive subtype mediates the positive inotropism that is probably associated with increased myofibrillar Ca2+ sensitivity. Radioligand binding studies with [3H] prazosin showed a similar ratio of alpha 1A-to alpha 1B-adrenoceptor subtypes in rat and rabbit ventricular myocardium, implying that the different degree of contribution of each action mechanism to the overall inotropic effect in the two species cannot be explained by distribution of the alpha 1-adrenoceptor subtypes.

Characterization of alpha 1 D-adrenoceptor subtype in rat myocardium, aorta and other tissues.

1. This study was done to characterize the functional role of alpha 1D-adrenoceptors in rat myocardium, aorta, spleen, vas deferens and prostate by use of the selective antagonist BMY 7378. 2. BMY 7378 inhibited [3H]-prazosin binding to aortic membranes with a potency (pKi 9.8 +/- 0.40) approximately 100 fold higher than in right ventricular membranes (pKi 7.47 +/- 0.11) and approximately 1,000 fold higher than that in plasma membranes of the prostate (pKi 6.62 +/- 0.39), vas deferens (pKi 6.67 +/- 0.15), salivary gland (pKi 6.46 +/- 0.38) and liver (6.58 +/- 0.06). 3. BMY 7378 antagonized the positive inotropic effects of phenylephrine (in the presence of 1 microM propranolol) on right ventricles (pA2 7.0 +/- 0.11), left atria (pKB 7.04 +/- 0.18) and papillary muscles (pKB 6.9 +/- 0.1) and inhibited phenylephrine-induced increase in inositol phosphates. 4. BMY 7378 was approximately 100 fold more potent as an antagonist of phenylephrine on aortic strips (pA2 9.0 +/- 0.13) than on vas deferens (pKB 7.17 +/- 0.08) and spleen (pKB 7.16 +/- 0.21); it was ineffective on the prostate. 5. Chloroethylclonidine suppressed the maximal effects of phenylephrine on spleen; 5-methylurapidil antagonized the effects of phenylephrine on aortic strips (pA2 7.98 +/- 0.08), vas deferens (pKB 8.89 +/- 0.07) and prostate (pKB 8.85 +/- 0.21). 6. BMY 7378 caused a dose (0.1-100 nmol kg-1)-dependent decrease in mean blood pressure of urethane-anaesthetized rats and its hypotensive efficacy was equal to that of hexamethonium. 7. The data suggest that alpha 1D-adrenoceptors play a significant role in rat aorta, a minor role in the heart, vas deferens and spleen and virtually no role in the prostate.