Interaction of the inotropic effect of norepinephrine and acetylcholine on the guinea pig's myocardium]

The experiments on guinea pig myocardium slices have been carried out to study the interaction of inotropic effects of different doses of norepinephrine (NE, from 10(-7) to 10(-5) mol/l) and acetylcholine (AC, from 10(-8) to 10(-6) mol/l). With an increase of NE concentration the negative influence of AC on the inotropic action is replaced by positive one. It is shown that there are optimal concentrations of NE and AC to exert a negative influence of AC on adrenergic inotropic effect (in these experiments--3 x 10(-7) mol/l for both influences). A decrease in frequency of contractions of AC on NE effect and positive influence of adrenergic myocardium stimulation on inotropic effect of AC, respectively. Such a type of relation of cardial effects of choline- and adrenergic influences is suggested to be designated by term "negatively accentuative antagonism" unlike the opposite type of choline-adrenergic interaction--"positive accentuative antagonism", under which AC increases inotropic effect of adrenergic myocardium stimulation, while adrenergic positive inotropic influences decrease AC effect.     

The atropine-like action, not removed by naloxone, of the opioid dermorphin on the inotropic effects of acetylcholine

Opioid dermorphin induced a negative inotropic effect on the isolated perfused by Straube frog's heart, this effect was blocked by naloxone. On the background of dermorphin negative inotropic effect acetylcholine inhibited the ventricular contractile activity to the same degree as in the control experiments before dermorphin injection. But after the combined infusion of naloxone and dermorphin removed the opioid inotropic effect, the negative inotropic effect of acetylcholine became significantly weaker than in the control. It has been concluded that there are opiate receptors in the frog's ventricular myocardium, their activation leads to the negative inotropic effect. Dermorphin may act like atropine on the inotropic effects of acetylcholine, this action doesn't depend on the opiate receptors activation.     

Effect of myocardial infarct on the cholinoreactivity of the heart atria and on their acetylcholine content

The effect of left ventricular experimental infarction (caused by left coronary artery ligation) on the isolated right atrium contractile function and acetylcholine content in both atria was studied in male Wistar rats. It was shown that a 24-hour infarction induced an increase in atrial chronotropic response to acetylcholine, which proved an increase in the pacemaker cholinoreactivity. Atrial inotropic response to acetylcholine characterizing the contractile myocardium cholinoreactivity remained unchanged. At the same time atrial endogenous acetylcholine content decreased fourfold. An increase in pacemaker cholinoreactivity was not accompanied by changes in its adrenoreactivity; those changes increased the pacemaker sensitivity to cholinergic influences which could help elucidate the ectopic excitation foci, thus promoting the onset of arrhythmia.     

The effects of adenyl compounds on the heart of the axolotl (Ambystoma mexicanum)

In the spontaneously beating axolotl atrium, adenosine 5'-triphosphate (ATP) produced initial excitation followed by inhibition and then by a secondary excitation. This third phase of the ATP response was only seen in electrically driven preparations in the presence of 8-phenyltheophylline (8-PT), an adenosine receptor antagonist. alpha,beta-Methylene ATP (APCPP), a stable analogue of ATP, produced only excitatory effects, while adenosine and beta,gamma-methylene ATP (APPCP), a slowly degradable ATP analogue, produced inhibition or inhibition followed by excitation. 2-Chloroadenosine produced inhibition. The excitatory effects were not antagonized by 8-PT, indomethacin or propranolol and phentolamine. The negative inotropic responses of these compounds were antagonized by 8-PT and, with the exception of 2-chloroadenosine, potentiated by dipyridamole, an adenosine uptake blocker. In the ventricle, ATP, APCPP and APPCP produced positive inotropic effects, which were not affected by 8-PT, indomethacin or propranolol and phentolamine. Adenosine produced a negative inotropic effect which was not antagonized by 8-PT nor atropine nor potentiated by dipyridamole. The effects of adenyl compounds on the axolotl (urodele) heart suggest that, like the frog (anuran) heart, both P1- and P2-purinoceptors are present in the axolotl atrium and that only P2-purinoceptors are present in the axolotl ventricle, although adenosine does produce an inhibitory effect on the ventricle which is probably mediated via the release of a neurotransmitter other than acetylcholine.     

Developmental changes in dopamine modulation of the heart in the isopod crustacean Ligia exotica: reversal of chronotropic effect

Developmental changes in dopamine modulation of the heart were examined in the isopod crustacean Ligia exotica. The Ligia cardiac pacemaker is transferred from the myocardium to the cardiac ganglion during juvenile development and the heartbeat changes from myogenic to neurogenic. In the myogenic heart of early juveniles, dopamine affected the myocardium and caused a decrease in the frequency and an increase in the duration of the myocardial action potential, resulting in negative chronotropic (decrease in beat frequency) and positive inotropic (increase in contractile force) effects on the heart. Contrastingly, in the heart of immature adults just after juvenile development, dopamine caused effects of adult type, positive chronotropic and positive inotropic effects on the heart affecting the cardiac ganglion and myocardium. During the middle and late juvenile stages, dopamine caused individually a negative or a positive chronotropic effect on the heart. These results suggest that the chronotropic effect of dopamine on the Ligia heart is reversed from negative to positive in association with the cardiac pacemaker transfer from the myocardium to the cardiac ganglion during juvenile development.     

Positive and negative inotropic effects of muscarinic receptor stimulation in mouse left atria

In isolated mouse left atria, acetylcholine (ACh) produced a biphasic inotropic response; a transient decrease in developed tension was followed by an increase. Both negative and positive responses were concentration dependent and were inhibited by atropine. The negative and positive inotropic responses were also observed with a nonselective muscarinic stimulant, oxotremorine-M, but not with an M1-receptor selective stimulant, McN-A343. Pirenzepine, an M1-receptor antagonist, inhibited both negative and positive inotropic responses at high concentrations. Gallamine, an M2-receptor antagonist, inhibited the negative response. Hexahydro-siladifenidol hydrochloride, p-fluoro analog (p-F-HHSiD), an M3-receptor antagonist, inhibited the positive response with no effect on the negative phase. In pertussis toxin (PTX) treated preparations, negative inotropic response to ACh was not observed. These results suggest that the negative and positive inotropic responses to acetylcholine in mouse atria are mediated by M2 and M3 receptors, respectively. The negative phase, but not the positive phase, was mediated by a PTX-sensitive G protein.     

The topological characteristics of rat myocardial reactivity to noradrenaline, acetylcholine and a change in the electrostimulation frequency]

Dose-dependent effects of noradrenaline (10-7-10-6M), acetylcholine (10-8-3x10-6M) and stimulation rate (0.2-2.0 Hz) were obtained in experiments on myocardium preparations of the right and left atria and ventricles in rat. Three types of topological differences of the rat myocardium reactivity were observed: between the atria and ventricles (A/V), between the right and left atria and ventricles (R/L), between the right atrium (RA) and other cardiac chambers. A/V differences were most pronounced in the reactivity to acetylcholine (the atria were more reactive), the highest R/L differences were observed in the reactivity to noradrenaline (the myocardium of the right chambers was more reactive). RA reactivity greatly exceeded reactivity of other myocardial preparations to all three test influences. Topological peculiarities of chrono-inotropism permit supposing, that inotropic effects of rate changes in vivo are able to compensate, to some extent, the regional nonuniformity of cholin- and adrenergic regulatory inotropic effects.     

Fade of the responses of the isolated, blood-perfused dog atrium to cholinergic interventions

In the isolated, blood-perfused, canine right atrium, intramural parasympathetic nerve stimulation and intra-arterial infusions of acetylcholine induced substantial negative chronotropic and inotropic responses. The responses to parasympathetic stimulation reached their maximum values quickly, and then usually faded back toward control levels over the next 1 or 2 min of stimulation. The fade of the responses at high stimulation frequencies (greater than or equal to 30 Hz) was significantly greater than that at lower frequencies. The inotropic responses to acetylcholine infusion (1 microgram/min) faded slightly but significantly, whereas the chronotropic responses did not fade at all. These results suggest that the fade of the cardiac responses to parasympathetic stimulation is mainly ascribable to a progressive reduction in the rate of acetylcholine release from the nerve endings, especially at higher stimulation frequencies. The fade of the inotropic responses was more pronounced and had a longer time course than that of the chronotropic responses. Furthermore, the fade of the inotropic responses diminished significantly as the response magnitude was augmented by an increase in stimulation voltage. Conversely, the fade of chronotropic responses was not significantly affected by this intervention. These differences in the inotropic and chronotropic responses to neural stimulation, and the occurrence of a slight fade of the inotropic response to acetylcholine infusion, suggest that in addition to the predominant prejunctional mechanism, a postjunctional phenomenon may also be partly responsible for the fade of the inotropic response to cholinergic interventions.     

Species differences in the negative inotropic effect of acetylcholine and soman in rat, guinea pig, and rabbit hearts.

1. Acetylcholine reduced atrial contractions by 82.5% in guinea pig, 50.8% in rat, and 41.5% in rabbit. 2. The EC50 values for the negative inotropic effect of acetylcholine were 3.3 x 10(-7) M in rat and guinea pig atria and 4.1 x 10(-6) M in rabbit atria. 3. There was no correlation between the species differences in the negative inotropic effect of acetylcholine in atria and the density or affinity of acetylcholinesterase or muscarinic receptors. 4. Inhibition of atrial acetylcholinesterase with soman reduced the EC50 of acetylcholine three-fold in all species, but did not change the maximal inotropic effect of acetylcholine. 5. Species differences in the negative inotropic effect of acetylcholine may be caused by differences in the coupling between myocardial muscarinic receptors and the ion channels that mediate negative inotropy.     

Age-related changes in inotropic effects of noradrenaline and acetylcholine on the myocardium of guinea pigs]

The inotropic effects of noradrenaline (10(-7)-10(-5) M) and acetylcholine (10(-8)-10(-6) M) were studied in experiments carried out on preparations of the right atria and on papillary muscles of the right ventricle in adult (4-5 months) and old (18-24 months) guinea pigs. An age-related decrease in inotropic noradrenaline effects and the displacement of dose-effect relationships to the right was revealed. Similar changes of the dose-related effects of acetylcholine superfused against the background of noradrenaline action were observed. The direct inotropic action of the acetylcholine did not change with ageing. A lack of the essential atrial-ventricular differences in age-related changes in myocardial reactivity is apparently very significant for support of effective functional coupling of cardiac chambers in ageing.     

Contrasting inotropic responses to alpha1-adrenergic receptor stimulation in left versus right ventricular myocardium

The left ventricle (LV) and right ventricle (RV) have differing hemodynamics and embryological origins, but it is unclear whether they are regulated differently. In particular, no previous studies have directly compared the LV versus RV myocardial inotropic responses to alpha(1)-adrenergic receptor (alpha(1)-AR) stimulation. We compared alpha(1)-AR inotropy of cardiac trabeculae from the LV versus RV of adult mouse hearts. As previously reported, for mouse RV trabeculae, alpha(1)-AR stimulation with phenylephrine (PE) caused a triphasic contractile response with overall negative inotropy. In marked contrast, LV trabeculae had an overall positive inotropic response to PE. Stimulation of a single subtype (alpha(1A)-AR) with A-61603 also mediated contrasting LV/RV inotropy, suggesting differential activation of multiple alpha(1)-AR-subtypes was not involved. Contrasting LV/RV alpha(1)-AR inotropy was not abolished by inhibiting protein kinase C, suggesting differential activation of PKC isoforms was not involved. However, contrasting LV/RV alpha(1)-AR inotropic responses did involve different effects on myofilament Ca(2+) sensitivity: submaximal force of skinned trabeculae was increased by PE pretreatment for LV but was decreased by PE for RV. For LV myocardium, alpha(1)-AR-induced net positive inotropy was abolished by the myosin light chain kinase inhibitor ML-9. This study suggests that LV and RV myocardium have fundamentally different inotropic responses to alpha(1)-AR stimulation, involving different effects on myofilament function and myosin light chain phosphorylation.     

Mechanism of the negative inotropic effects of alpha 1-adrenoceptor agonists on mouse myocardium

This study was done to identify the mechanism of the alpha1-adrenoceptor (AR) mediated negative inotropic effects of phenylephrine (PE) on adult mouse myocardium. As reported by others, we also found that the nonselective alpha1AR agonist PE produced a negative inotropic effect on ventricular strips from adult mice that was inhibited by the alpha1AAR antagonist 5-methylurapidil (5MU) but not by the alpha1BAR antagonist chloroethylclonidine (CEC) or the alpha1DAR antagonist BMY 7378. The selective alpha1AAR agonist A61603 also produced a negative inotropic effect, which was antagonized by 5MU. Phorbol 12,13-dibutyrate (activator of all PKC isoforms) mimicked the negative inotropic responses to PE and A61603. The negative inotropic effects of PE were inhibited by bisindolylmaleimide (inhibitor of all PKC isoforms) but not by G? 6976 (inhibitor of Ca2+-dependent PKC). Rottlerin, an inhibitor of Ca2+-independent PKCdelta, antagonized the negative inotropic effects of PE and A61603. PE and A61603 increased the translocation of PKCdelta, which was prevented by rottlerin. These data suggest that the alpha1AR-mediated negative inotropy on adult mouse myocardium is signaled by Ca2+-independent PKCdelta.     

Polyphenols from Parabarium huaitingii and their positive inotropic and anti-myocardial infarction effects in rats

Eight phenolic compounds, including (−)-epicatechin (1) and seven proanthocyanidins (2-8), were obtained from the butanol extract of Parabarium huaitingii (PHB). Their chemical structures were identified based on analyses of mass spectra (MS), NMR, CD spectra, and partial acid catalyzed thiolytic degradation. The observation made by laser scanning confocal microscope found a significant increase of the concentration of intracellular Ca²⁺ ([Ca²⁺]_i) in single myocytes when the PHB was added, while compounds 1 and 3 had the same physiological effect. Further investigations showed PHB had a dose-dependent positive inotropic effect on isolated right atria and papillary muscle of left ventricle of the rat, while having no significant influence on the spontaneous beating rate of the isolated right atria. The inotropic effect of PHB could be greatly abolished by pretreating the myocardium in Ca²⁺-free solution. These findings indicated that PHB could significantly increase [Ca²⁺]_i in myocytes, which was greatly dependent on the influx of extracellular Ca²⁺. Compounds 1 and 3 might be the effective ingredients of the inotropic effect of PHB. In addition, PHB could also significantly decrease the infarct size of the heart on acute myocardial infarction (AMI) model rats, which suggested its myocardial protective effect on ischemic myocardium. The positive inotropic effect of PHB, together with its myocardial protective effect on AMI, suggested that PHB had a promising potential for the prevention and treatment of heart failure, especially the one that was caused by AMI.     

The role of intra- and extracellular calcium sources in the rhythmic inotropic regulation of isometric contractions in the chick embryo myocardium

Isometric contractions and force-frequency relationships (FFR) were assessed in isolated ventricular preparations of 3- and 4-day chick embryos (EM) and posthatched (PH) chicks. Pacing protocols for the FFR assessment were applied in normal buffer, buffer with 50% and 25% of normal Na+ concentration, and in buffer with ryanodine. PH myocardium showed greater peak force and more prominent FFR than EM. 50% low Na+ superfusion induced positive inotropic effect and increase in the FFR in PH and EM. 25% low Na+ superfusion induced negative inotropic effect and suppressed FFR more prominent in EM. Ryanodine more suppressed FFR in PH myocardium. The observed age-dependent differences substantiate the fact that Na-Ca exchange plays a leading role in the FFR regulation in early embryonic myocardium and sarcoplasmic reticulum plays a leading role in PH myocardium.     

Oppositional effects of acetylcholine and isoproterenol on isometric tension and cyclic nucleotide concentrations in rabbit atria.

The effects of acetylcholine chloride and isoproterenol on myocardiial cyclic GMP, cyclic AMP and on isometric tension were studied in isolated electrically driven rabbit atria. Acetylcholine (0.5 muM) produced a significant decrease in isometric force that was associated with a significant elevation in atrial cyclic GMP. Cyclic AMP was significantly lowered at 15 seconds after the addition of acetylcholine, but was only slightly decreased at earlier time periods. Both the negative inotropic action and increase in cyclic GMP after addition of acetylcholine were blocked by atropine. Isoproterenol (0.1 muM) produced a significant increase in isometric tension that was associated with a significant elevation in atrial cyclic AMP levels, whereas cyclic GMP levels were not changed. These effects were blocked by practolol. The increases in atrial cyclic GMP and cyclic AMP following addition of acetylcholine and isoproterenol, respectively, preceded the changes in isometric tension in response to these agents. These data support the hypothesis that changes in intracellular levels of cyclic AMP and cyclic GMP may mediate the positive and negative inotropic effects of adrenergic and cholinergic agents.     

Atrial myocardium is the predominant inotropic target of adrenomedullin in the human heart

Adrenomedullin (ADM) is an endogenous peptide with favorable hemodynamic effects in vivo. In this study, we characterized the direct functional effects of ADM in isolated preparations from human atria and ventricles. In electrically stimulated human nonfailing right atrial trabeculae, ADM (0.0001-1 micromol/l) increased force of contraction in a concentration-dependent manner, with a maximal increase by 35 +/- 8% (at 1 micromol/l; P < 0.05). The positive inotropic effect was accompanied by a disproportionate increase in calcium transients assessed by aequorin light emission [by 76 +/- 20%; force/light ratio (DeltaF/DeltaL) 0.58 +/- 0.15]. In contrast, elevation of extracellular calcium (from 2.5 to 3.2 mmol/l) proportionally increased force and aequorin light emission (DeltaF/DeltaL 1.0 +/- 0.1; P < 0.05 vs. ADM). Consistent with a cAMP-dependent mechanism, ADM (1 micromol/l) increased atrial cAMP levels by 90 +/- 12%, and its inotropic effects could be blocked by the protein kinase A (PKA) inhibitor H-89. ADM also exerted positive inotropic effects in failing atrial myocardium and in nonfailing and failing ventricular myocardium. The inotropic response was significantly weaker in ventricular vs. atrial myocardium and in failing vs. nonfailing myocardium. In conclusion, ADM exerts Ca(2+)-dependent positive inotropic effects in human atrial and less-pronounced effects in ventricular myocardium. The inotropic effects are related to increased cAMP levels and stimulation of PKA. In heart failure, the responsiveness to ADM is reduced in atria and ventricles.     

Absence of purinoceptors in the heart of the turtle (Emys orbicularis)

The effects of adenosine, adenosine 5'-triphosphate (ATP), a slowly degradable ATP analogue beta,gamma-methylene ATP (APPCP) and a degradation resistant ATP analogue alpha,beta-methylene ATP (APCPP) were examined on the turtle heart. Adenosine, ATP, APPCP and APCPP had no effect on the rate or force of contraction of either the atrium or ventricle. The effects of acetylcholine and noradrenaline were also examined on the turtle heart. Acetylcholine decreased the force and rate of contraction of turtle atria in a concentration-dependent manner. Noradrenaline increased the rate of contraction but caused a slight decrease in the force of contraction of the atrium. Neither acetylcholine nor noradrenaline produced an inotropic effect on the ventricle.     

Cardiovascular effects of matrine isolated from the Chinese herb Shan-dou-gen

Matrine, a pure compound isolated from the Chinese herb Shan-don-Gen (Sophora subprostrata), was studied for its effects on the cardiovascular system of the rat. Intravenous injections of matrine at doses from 5 mg to 20 mg/kg body weight exhibited dose-dependent hypotensive and bradycardiac effects. These effects lasted only 1 to 3 min. In the isolated atria and ventricle preparations, matrine at doses from 50 micrograms to 200 micrograms/ml increased the amplitudes of spontaneous contraction of the atria and electrically induced contraction of the ventricle, whereas the frequency of the spontaneous beating of the atria was reduced. The dose-dependent effects of matrine on the isolated preparations persisted as long as the compound was present. Tachyphylaxis was not observed with repeated applications of this compound to the isolated preparations. The positive inotropic effects on both atria and ventricle and the negative chronotropic effect on spontaneous beating of the atria by matrine were not blocked by diphenhydramine, atropine, phenoxybenzamine, propranolol, trifluoperazine, or methysergide. In contrast, verapamil significantly reduced the positive inotropic effect of matrine on the ventricle. In the isolated aortic strip preparation, matrine at a dose of 200 micrograms/ml led to a slight increase in muscle tone. The same dose of matrine induced a 35% increase of perfusion pressure in the hindleg perfusion model. These results suggest that the in vivo transient hypotensive effect of matrine is likely associated with a decrease in heart rate itself, since positive inotropic effects on both the atria and the ventricle, and vasoconstriction of some vascular beds could not be the cause of hypotension.(ABSTRACT TRUNCATED AT 250 WORDS)     

The kinetics of heparin inhibition of the esterase and basal lipase activities of lipoprotein lipase

The kinetics of inhibition of the esterase and lipase activities of bovine milk lipoprotein lipase (LPL) were compared. The esterase LPL activity against emulsified tributyrylglycerol was not affected by the enzyme activator apolipoprotein C-II (C-II) and amounted to about 15% of the "plus activator" lipase enzyme activity. Heparin at concentrations of 20 micrograms/ml inhibited 25% of the esterase activity. The reaction followed Henri-Michaelis-Menten kinetics and the inhibition by heparin followed a linear, intersecting, noncompetitive kinetic model. On the other hand, the basal lipase activity of LPL against emulsified trioleoylglycerol (TG) was very sensitive to inhibition by heparin: 1 microgram/ml inhibited about 80% of the reaction and 3 micrograms/ml drove the reaction to zero. The velocity curve for the uninhibited basal LPL activity was sigmoidal with an apparent nH(TG) of 2.94. Heparin inhibited the lipase activity competitively: heparin decreased nH(TG) and increased[TG]0.5 6.4-fold, while TG decreased the nH(Heparin) from 2.14 to 0.95 and caused a 3-fold increase in [Heparin]0.5. C-II, at concentrations lower than 2.5 X 10(-8) M (i.e., lower than KA), countered the inhibitory effects of heparin: at constant inhibitor concentrations, C-II increased nH(TG) from 1.78 to 2.52 and decreased [TG]0.5 about 10-fold; it also increased the apparent Vmax. At the lower C-II concentrations, nH(C-II) was approximately equal to 1.0 and increasing the TG concentrations decreased [C-II]0.5 from 3.8 X 10(-8) to 8.5 X 10(-9) M, with no effect on the nH(C-II). At the higher C-II concentrations, nH(C-II) was 2.5 and TG decreased [C-II]0.5 about 2-fold with no effect on the nH(C-II). In the absence of heparin, C-II had no effect on nH(TG) nor on [TG]0.5, but it increased the apparent Vmax. On the other hand, TG had no effect on nH(C-II) nor on [C-II]0.5, but at any given C-II concentration, the reaction velocity increased with increasing TG concentrations. It is concluded that TG and heparin as well as C-II and heparin are mutually exclusive and that lipoprotein lipase is a multisite enzyme, possibly a tetramer, with three high-affinity catalytic sites, and an equal number of sites for C-II and heparin per oligomer. However, LPL differs from classical allosteric enzymes in that its activator has no effect on substrate cooperativity nor on [S]0.5; its only effect is to increase Vmax by increasing the catalytic rate constant kp by inducing conformational changes in the enzyme.     

Hybridization of Euglena RNA to the heavy and the light chloroplast DNA components separated in alkaline CsC1 gradients

It has been suggested that increases in cyclic GMP levels are responsible for the negative inotropic effects of acetylcholine in the heart. This hypothesis was tested by monitoring the effects of acetylcholine and sodium nitroprusside on tension and cyclic nucleotide levels in strips of cat atrial appendage. Sodium nitroprusside markedly increased atrial cyclic GMP levels but did not decrease the twitch tension developed by the atrial strips. Low concentrations of acetylcholine, on the other hand, decreased twitch tension without increasing myocardial cyclic GMP levels. No significant change in cyclic AMP levels was observed in any of these experiments. These results are not consistent with the proposed role for cyclic GMP as the mediator of the negative inotropic effects of acetylcholine.