Effects of nifedipine on smooth muscle cells of the rabbit mesenteric artery

The effects of nifedipine on electrical and mechanical responses of smooth muscle cells of the rabbit mesenteric artery were investigated using microelectrode and isometric tension recording methods for intact cells. The effects of nifedipine on the mechanical response on saponin-treated skinned muscles were also studied. Nifedipine inhibited the Ca spike evoked by outward current pulses in the presence of tetraethylammonium and that by perivascular nerve stimulation without affecting the amplitude of excitatory junction potentials. Nifedipine (less than 3 X 10(-7) M) modified neither the amplitude of excitatory junction potentials nor the facilitation process. This drug inhibited the contractions evoked by direct muscle stimulation under conditions of treatment with guanethidine and tetrodotoxin, excess concentrations of [ K ]O, exogenously applied norepinephrine (NE) and perivascular nerve stimulation. The K-induced contraction was markedly inhibited by nifedipine (greater than 3 X 10(-9) M) and the potency of the inhibitory action of nifedipine appeared in the following order: direct muscle stimulation greater than perivascular nerve stimulation greater than exogenously applied NE. Nifedipine inhibited the NE-induced oscillatory contractions more than the NE-induced tonic and phasic contractions. In Na-free solution, the tissue generated a small tonic contraction after 20 to 30 min superfusion. This contraction ceased with application of nifedipine. In the saponin-treated skinned muscles (50 micrograms/ml for 20 min), Ca accumulation into and Ca release from the store sites, as well as the contractile proteins including calmodulin, were not affected by nifedipine (1 X 10(-7) M). These results indicate that nifedipine only acts on the myoplasmic membrane of smooth muscles of the mesenteric artery. The nifedipine-induced relaxation appears to be due to inhibition of the voltage-dependent Ca channel.     

Differential effects of d- and l-pimobendan on cardiac myofilament calcium sensitivity

The effects is of the optical isomers of pimobendan (UD-CG 115 BS), an inotropic agent, were studied on the electrical and mechanical activity of intact and detergent-skinned preparations of cardiac muscle from guinea pig and dog. Racemic pimobendan has been shown to increase contractile force and to potentiate slow action potentials (AP) induced by stimulation of papillary muscle partially depolarized with 25 mM [K]o. These effects are shown in this study to be mainly due to the l-optical isomer of pimobendan. When slow APs were maximally stimulated by 1 microM isoproterenol, addition of either the d- or l-isomer of pimobendan did not affect the slow AP parameters. However, under these conditions, contractile force was significantly increased to 124% of control by the d-isomer and to 184% of control by the l-isomer. These results suggest that pimobendan may have direct effects on the myofilaments and that these effects are dependent on the optical isomer of the compound. To test this directly, the effects of d- and l-pimobendan were compared on Ca++-activated force developed by detergent-skinned heart muscle fibers. Submaximal force developed at constant Ca++ was increased by both optical isomers, but the l-isomer had a significantly greater Ca++-sensitizing effect. For example at pCa 6.75 force was 270% of control in the presence of the d-isomer and 400% of control in the presence of the l-isomer. At pCa 5, there was no effect of either isomer on force developed by the skinned fiber preparations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Negative inotropic effect of leukotrienes: leukotrienes C4 and D4 inhibit calcium-dependent contractile responses in potassium-depolarized guinea-pig myocardium

The effects of the sulfidopeptide leukotrienes (LTs) on the contractile response of electrically paced guinea-pig right ventricular papillary muscles in vitro were studied. LTs caused a concentration-dependent (1 nM-20 microM) negative inotropic effect; the order of relative potency was LTC4 greater than or equal to LTD4 greater than LTE4. A maximal 30% decrease in contractility occurred with 1 microM LTC4. The LT-induced decrease in contractile force was not mediated by cyclooxygenase products of the arachidonic acid cascade, as it was not influenced by indomethacin (14 microM). On the other hand, the slow-reacting substance-antagonist compound FPL 55712 (480 nM) caused a marked shift to the right of the LTC4 concentration-response curve. Because the negative inotropic effect of LTD4 was attenuated by increasing [Ca++]o, we next assessed the negative inotropic effect of LTs under conditions in which myocardial contractility depends solely on the slow inward Ca++ current. As a model, we used the isoproterenol- or histamine-induced restoration of contractile response in papillary muscles rendered inexcitable by 22 mMK+. LTC4 (16-480 nM) and LTD4 (20-600nM) inhibited isoproterenol- and histamine-induced restoration of contractility in a dose-dependent manner; a maximal 90% inhibition occurred with 0.48 microM LTC4. This effect of LTs was reversed by an elevation in [Ca++]o from 1.8 to 5.4 mM and prevented by FPL 55712 (480 nM). In muscles maintained at 5.4 mM [K+]o, LTC4 (160 and 480 nM) and LTD4 (1 microM) shifted the force-frequency curve (0.1-2 Hz) downwards in a parallel fashion; a similar alteration was obtained by lowering [Ca++]o to 1 mM.     

Pharmacological characterization of a new Ca(2+) sensitizer

The benzimidazole molecule was modified to synthesize a Ca(2+) sensitizer devoid of additional effects associated with Ca(2+) overload. Newly synthesized compounds, termed 1, 2, 3, 4, and 5, were evaluated in spontaneously beating and electrically driven atria from reserpine-treated guinea pigs. Compound 3 resulted as the most effective positive inotropic agent, and experiments were performed to study its mechanism of action. In spontaneously beating atria, the inotropic effect of 3 was concentration-dependent (3.0 microM-0.3 mM). Compound 3 was more potent and more active than the structurally related Ca(2+) sensitizers sulmazole and caffeine, but unlike them it did not increase the heart rate. In electrically driven atria, the inotropic activity of 3 was well preserved and it was not inhibited by propranolol, prazosin, ranitidine, pyrilamine, carbachol, adenosine deaminase, or ruthenium red. At high concentrations (0.1-1.0 mM) 3 inhibited phosphodiesterase-III, whereas it did not affect Na(+)/K(+)-ATPase, sarcolemmal Ca(2+)-ATPase, Na(+)/Ca(2+) exchange carrier, or sarcoplasmic reticulum Ca(2+) pump activities of guinea pig heart. In skinned fibers obtained from guinea pig papillary muscle and skeletal soleus muscle, compound 3 (0.1 mM, 1 mM) shifted the pCa/tension relation curve to the left, with no effect on maximal tension and no signs of toxicity. Compound 3 did not influence the basal or raised tone of guinea pig isolated aorta rings, whose cells do not contain the contractile protein troponin. The present results indicate that the inotropic effect of compound 3 seems to be primarily sustained by sensitization of the contractile proteins to Ca(2+).     

Frequency modulation of the inotropic action of isoproterenol in mouse heart

In mouse right ventricular strips, field-stimulated to contract isometrically in an oxygenated bicarbonate-buffered physiological salt solution at 22--24 degrees C, the EC50 for the inotropic action of isoproterenol decreased from 37 nM in muscles stimulated at 0.2 Hz to 5 nM in muscles stimulated at 3.3 Hz. At higher rates of contraction, there was also an increased sensitivity to the inotropic actions of norepinephrine and epinephrine but not to those of Ca++ and N6,O2'-dibutyryl cyclic AMP. Increasing the Ca++ concentration further decreased the EC50 for isoproterenol at 3.3 Hz but had no effect on the EC50 at 0.2 Hz. The leftward shift of the contractile response curve at 3.3 Hz was inhibited by verapamil (0.6 microM) and Mn++ (0.25 mM). The stimulation of cyclic AMP accumulation was approximately 6-fold more sensitive to isoproterenol at 3.3 than at 0.2 Hz, but isoproterenol increased contractile force at concentrations two orders of magnitude lower than those that significantly increased cyclic AMP accumulation. Inhibition of cyclic AMP phosphodiesterase activity further increased the sensitivity to the inotropic actions of isoproterenol but did not attenuate the frequency difference. The results indicate that isoproterenol-stimulated Ca++ influx through the slow channel plays an important role in the mechanism of the increased sensitivity to the inotropic action of isoproterenol found at higher frequencies of contraction. Although cyclic AMP accumulation was also frequency dependent, its role in the inotropic action of isoproterenol in mouse heart is not clear.     

Gentamicin blockade of slow Ca++ channels in atrial myocardium of guinea pigs.

Cardiac dysfunction is occasionally detected in patients undergoing treatment with amino-glycoside antibiotics, however, the mechanism responsible for the negative inotropic effect of these agents has not been identified. In the present investigation electrically driven left atria of guinea pigs were used to study the effects of gentamicin on calcium ion (Ca++)-dependent contractile events in heart muscle isolated from in vivo influences. When atria were first inactivated by excess potassium ion (K+; 22mM) and contractions were then restored by isoproterenol (an experimental model that accentuates the contractile dependence of myocardial fibers on influx of Ca++ through specific "slow channels" of the sarcolemma), the cardiac depressant activity of gentamicin (0.1 mM) was profoundly augmented. Conversely, the negative inotropic effect of tetrodotoxin (23.5 micron) was abolished by the same experimental conditions. Also, gentamicin (1 mM) and La+++ (0.5 mM) markedly decreased the positive inotropic response to increased frequency of stimulation; whereas, D600 (1.05 micron) converted the positive frequency-force relationship to a negative relationship. Present data indicate a direct cardiac depressant action of gentamicin, and suggest that this antibiotic adversely affects either the transport system responsible for Ca++ movement through slow channels of the sarcolemma, the availability of Ca++ for translocation to these sites, or both.     

Mechanism of inotropic action of xestoquinone, a novel cardiotonic agent isolated from a sea sponge.

Xestoquinone (XQN) isolated from the sea sponge Xestospongia sapra produced dose-dependent cardiotonic effects on guinea pig left and right atria. A direct action of XQN (1-30 microM) on the contractile machinery of cardiac myofilaments was demonstrated in chemically skinned fiber preparations from guinea pig papillary muscles. In atrial preparations, the XQN-induced inotropic effect was markedly inhibited by verapamil or nifedipine, but was not affected by practolol, chlorpheniramine, cimetidine, tetrodotoxin or reserpine. The Ca++ dependence curve for the contractile response of the atria was substantially shifted to the left by XQN (10 microM), and this XQN-induced shift was reversed by verapamil. The time-to-peak tension and relaxation times of the atrial contractions were shortened by XQN, and the action potential duration was markedly prolonged. Whole-cell patch clamp recordings in left atrial strips confirmed that XQN (30 microM) increased the slow inward current. However, there was a temporal dissociation between altered tension development and prolongation of the action potential duration. Cyclic AMP phosphodiesterase activity was inhibited and tissue cyclic AMP content of guinea pig left atria was increased by XQN (0.3-10 microM) in a concentration-dependent manner, but increases in cyclic AMP content did not occur in parallel with increases in contractile response. These observations suggest that an enhancement of intracellular cyclic AMP content and Ca++ influx across the cell membrane contribute to the late phase of XQN-caused cardiotonic responses, whereas the early phase may largely be elicited through direct activation of contractile elements. XQN may provide a novel leading compound for valuable cardiotonic agents.     

Amrinone effects on electromechanical coupling and depolarization-induced automaticity in ventricular muscle of guinea pigs and ferrets

The effects of the cardiotonic agent, amrinone (0.05-4 mM), on electrical and mechanical activities of ferret and guinea-pig papillary muscles were studied using current and voltage clamp (single sucrose gap) techniques. In current clamp studies, amrinone increased, in a dose-dependent manner, contractile force elicited by action potential in both species. Depolarization-induced automaticity was facilitated in ferret muscles at all maximum diastolic potentials between -70 and -15 mV. Facilitation of automaticity in guinea-pig muscles occurred only at potentials more negative than -35 mV and was suppressed at more positive potentials. Cimetidine (10 microM) partially reversed the effects of amrinone on automaticity in both species. In voltage clamp studies, amrinone increased the slow inward current. Steady-state outward current was increased in guinea-pig but not in ferret muscles. A dual effect of amrinone on tension was observed. Amrinone was found to increase phasic tension of ferret papillary muscles only for depolarizations lasting less than 250 to 300 msec. For longer depolarizations, amrinone decreased the phasic tension (in a dose-dependent manner), whereas the tonic tension was not modified. The decrease as well as the increase in tension was associated with an increase of the slow inward current. The results suggest that amrinone may be arrhythmogenic and may have an intracellular action at the sarcoplasmic reticulum level (partial inhibition) in addition to its action on the calcium current.     

Effect of MCI-154, a calcium sensitizer, on calcium sensitivity of myocardial fibers in endotoxic shock rats

This study was designed to investigate the effect of a calcium sensitizer on the Ca2+ sensitivity of myocardial fibers in endotoxic shock rats. Right ventricular papillary muscles from sham shock or endotoxic shock rats were skinned by incubation in saponin solution. Forces of the skinned muscles were recorded when they were activated sequentially by different pCa (-log[Ca2+]) activating solutions with or without positive inotropic agents. Tension-pCa relationship curve of skinned fibers delineated the affinity of troponin C(TnC) for Ca2+ and the medium value pCa50 (pCa required for producing 50% of maximal Ca2+-activated tension) was taken as the quantitative index of Ca2+ sensitivity of TNC. It was found that the maximal Ca2+ activated tension (Tmax) was lower, tension-pCa relationship curve was shifted rightward, and the pCa50 was reduced significantly in endotoxic shock group compared with that of sham shock group. Milrinone could not counteract the above abnormalities. However, when skinned right ventricular papillary fibers from endotoxic shock rats were dealt with activating solutions containing 1 x 10(-5) M MCI-154, the Tmax was significantly increased, the tension-pCa relationship curve was shifted leftward. The pCa50 in MCI-154 group was increased to an extent similar to that of sham shock group and markedly higher than the values of endotoxic shock group and milrinone group. Furthermore, such effects of MCI-154 were concentration dependent. It can been concluded that the sensitivity of cardiac contractile proteins to Ca2+ in endotoxic shock rats is decreased. MCI-154, a calcium sensitizer, can significantly reverse the decreased sensitivity and increase Tmax of myocardial muscles from endotoxic shock rats.     

Increase in Ca++ sensitivity of the contractile system by MCI-154, a novel cardiotonic agent, in chemically skinned fibers from the guinea pig papillary muscles

The effects of MCI-154, a novel cardiotonic agent, on the contractile protein system and the sarcoplasmic reticulum (SR) were investigated by using thin bundles of chemically skinned fibers from the guinea pig papillary muscles. In the skinned muscle fibers treated with 50 micrograms/ml of saponin, MCI-154 shifted the -log[Ca++]M-tension relation curve to the left and upward in the concentration-dependent manner (10(-7) to 10(-4) M). This was confirmed also in the skinned muscle fibers treated with 250 micrograms/ml of saponin which destroyed not only the surface membrane but also the function of SR. Sulmazole (10(-4) M) shifted the -log[Ca++]M-tension relation curve to the left but the effect was about 100 times less potent than that of MCI-154. Unlike MCI-154, sulmazole had little effect on the maximum tension development induced by -log[Ca++]M 4.4. Milrinone did not affect the Ca++-induced tension development in the skinned cardiac fibers. Higher concentration of MCI-154 (10(-4) M) also increased amplitude of -log[Mg-ATP]M-tension-curve in the absence of free Ca++ ion (bell-shaped curve) to the upward. Initial rate and plateau phase of Ca++ uptake by the SR in the skinned fibers treated with 50 micrograms/ml of saponin was increased slightly by MCI-154 at the concentrations of 10(-6) and 10(-4) M. MCI-154 had no effect on the Ca++-induced Ca++ release mechanism in the SR. These results suggest that an increase in Ca++ sensitivity of the contractile protein system is responsible for, at least in part, the mechanism of the positive inotropic action of MCI-154.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of pimobendan, a novel inotropic agent, on intracellular calcium and tension in isolated ferret ventricular muscle

1. In this study we have investigated the effects of a novel inotropic agent, pimobendan (UDCG 115-BS), on skinned and intact ventricular muscle from ferrets. 2. Pimobendan (20 or 100 mumol/l) increased tension at a given free [Ca2+] when applied to skinned ventricular muscle, i.e. it increased the Ca2+ sensitivity of the myofibrils. 3. Tension and intracellular free Ca2+ [( Ca2+]i) were measured simultaneously in intact papillary muscles using the aequorin technique. When 25 mumol/l pimobendan was added to the superfusing solution, a slowly developing positive ionotropic effect was produced, which was accompanied by an increase in the size of the systolic rise in [Ca2+]i (Ca2+ transients) with a similar time course. 4. In order to determine whether pimobendan increased the Ca2+ sensitivity of myofibrils in an intact papillary muscle, we compared the increase in Ca2+ transients and tension observed in response to changes in extracellular [Ca2+] with those observed in response to pimobendan. The result of this comparison was that in intact muscle pimobendan caused no apparent increase in myofibrillar Ca2+ sensitivity. 5. Pimobendan caused an abbreviation of the time course of the Ca2+ transients, but the twitch was slightly prolonged. 6. When isoprenaline was added to the superfusing solution, a positive inotropic effect was produced, which was accompanied by a marked increase in the size of the Ca2+ transients. Isoprenaline caused an abbreviation of the time course of both the Ca2+ transients and the twitch. When the Ca2+ sensitivity of the intact myofibrils was determined as described above, isoprenaline caused a desensitization. Pimobendan produced a sensitization when compared with isoprenaline. 7. These results are consistent with the hypothesis that pimobendan produces an inotropic effect in isolated cardiac muscle which is mediated both by an increase in Ca2+ sensitivity and by an increase in adenosine 3':5'-cyclic monophosphate due to its phosphodiesterase-inhibiting activity. Such a combination of activities may be particularly advantageous for an inotropic agent.     

Vasodilator effects of LF 2.0254, a new 1,4-dihydropyridine. Comparison with nifedipine

LF 2.0254 is a new 1.4 dihydropyridine that relaxes vascular smooth muscles by blockade of calcium entry mediated by depolarisation. In rabbit aortas contracted by KCl (15 to 55 mM) or CaCl2 (2 mM), LF 2.0254 differs markedly from nifedipine and nicardipine because of its slow onset of action, the inhibitory effect increasing with the duration of tissular contact. LF 2.0254 has only slight negative chronotropic and inotropic effects on isolated guinea-pig atria as well as in anesthetized dogs after intravenous administration. Furthermore, in open-chest anesthetized dogs, LF 2.0254 decreases mean arterial blood pressure and since cardiac output is maintained total peripheral resistance is decreased. LF 2.0254 administered orally to perinephritic hypertensive dogs (0.1 and 0.3 mg/kg) and to spontaneously hypertensive rats (0.3 to 10 mg/kg) induces a more pronounced and long-lasting hypotensive action than nifedipine. In conclusion, these results suggest that LF 2.0254 could be useful in the treatment of hypertension associated or not with cardiac insufficiency.     

TMB-8 as a pharmacologic tool in guinea pig myocardial tissues. I. Effects of TMB-8 on force of contraction and on action potential parameters in atrial and papillary muscles.

The compound 8-)N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8) had been introduced as an intracellular Ca++ antagonist. We have studied the effects of TMB-8 on electrical and mechanical activity of isolated cardiac tissues in order to estimate its spectrum of action in heart muscle. In spontaneously beating right atria of the guinea pig, TMB-8 (1-100 microM) had a negative chronotropic effect. In left atria, TMB-8 (1-100 microM) induced a frequency-dependent biphasic inotropic effect: A transient increase in force of contraction was followed by a sustained decrease; the latter could be antagonized partially by an increase in [Ca++]o. TMB-8 prolonged the time-to-peak force. At high concentrations of TMB-8 (greater than 10 microM), the electrical stimulation threshold was elevated. TMB-8 (20 microM) competitively inhibited the positive inotropic effect of Bay K 8644 and reduced the magnitude of the positive inotropic and/or chronotropic effects of veratridine, (-)-isoproterenol, forskolin, histamine and (-)-phenylephrine. TMB-8 (30 microM) prolonged the action potential duration (APD) [in particular at 90% of repolarization (APD90)] and the refractory period, and decreased the AP amplitude and Vmax. In right ventricular papillary muscles, TMB-8 (30 microM) shortened the APD (APD20 = APD50 greater than APD90) and the refractory period but hardly affected the AP amplitude and Vmax. The resting membrane potential remained unchanged in both tissues. These findings suggest that in addition to interference with the Ca++ release from the sarcoplasmic reticulum, TMB-8 also affects the membrane conductances for cations.     

Negative inotropic effect of platelet-activating factor: association with a decrease in intracellular sodium activity

Platelet-activating factor (PAF) is an autacoid whose cardiovascular actions include a potent negative inotropic effect. The mechanism of this decrease in myocardial contractility is still at issue, as both a decrease and an increase in trans-sarcolemmal Ca++ influx have been reported. Because changes in intracellular sodium activity (aiNa) are known to influence myocardial contractility, we investigated whether PAF affects aiNa. Thus, we have measured contractile responses to PAF (1 nM-1 microM) in isolated guinea pig right ventricular papillary muscles paced at constant rate, and recorded transmembrane action potential and aiNa with conventional and sodium-selective microelectrodes, respectively. Our findings suggest that PAF does not affect slow inward Ca++ current, because PAF neither affected nor prevented histamine-induced restoration of contractile responses in K+-depolarized papillary muscles. On the other hand, we found the negative inotropic effect of PAF to be associated with a shortening of the action potential duration and with a decrease in aiNa. The specific PAF antagonist compound CV-3988 inhibited all three electro-mechanical responses. Our findings imply that the decrease in contractile force caused by PAF may depend on the reduction in aiNa; as aiNa falls, intracellular Ca++ may be lost via the Na+/Ca++ exchange and contractility decreases. The shortening of the action potential duration by PAF may reflect a decrease in Na+ influx and the consequent reduction in aiNa.     

Depression by isoflurane of the action potential and underlying voltage-gated ion currents in isolated rat neurohypophysial nerve terminals

We characterized the effects of the volatile anesthetic isoflurane on the ion currents that contribute to the action potential (AP) in isolated rat neurohypophysial (NHP) nerve terminals using patch-clamp electrophysiology. Mean resting membrane potential and AP amplitude were -62.3 +/- 4.1 and 69.2 +/- 2.9 mV, respectively, in NHP terminals. Two components of outward K(+) current (I(K)) were identified in voltage-clamp recordings: a transient I(K) and a sustained I(K) with minimal inactivation. Some terminals displayed a slowly activating I(K), probably the big Ca(2+)-activated K(+) current (BK). Isoflurane reversibly inhibited AP amplitude and increased AP half-width in normal extracellular Ca(2+) (2.2 mM). In high extracellular Ca(2+) (10 mM), isoflurane also reduced the afterhypolarization peak amplitude. A transient tetrodotoxin-sensitive Na(+) current (I(Na)) was the principal current mediating the depolarizing phase of the AP. A slowly inactivating Cd(2+)-sensitive current (probably a voltagegated Ca(2+) current; I(Ca)) followed the initial I(Na). Isoflurane reversibly inhibited both I(Na) and I(Ca) elicited by a voltage-stimulus based on an averaged AP waveform. The isoflurane IC(50) for AP waveform-evoked I(Na) was 0.36 mM. Isoflurane (0.84 +/- 0.04 mM) inhibited AP waveform-evoked I(Ca) by 37.5 +/- 0.16% (p < 0.05). The isoflurane IC(50) for peak I(K) was 0.83 mM and for sustained I(K) was 0.73 mM, with no effect on the voltage dependence of activation. The results indicate that multiple voltage-gated ion channels (Na(+) > K(+) > Ca(2+)) in NHP terminals, although not typical central nervous system terminals, are inhibited by the volatile general anesthetic isoflurane. The net inhibitory effects of volatile anesthetics on nerve terminal action potentials and excitability result from integrated actions on multiple voltage-gated currents.     

Receptors involved in the positive inotropic action induced by dopamine on the ventricle of a 7-day-old chick embryo heart

Summary— Earlier experiments only revealed involvement of sympathetic pre-synaptic dopaminergic receptors in dopamine induced inotropism in myocardium. We therefore used electrically stimulated (1 Hz) isolated 7-day-old chick embryo heart ventricles, thought to be devoid of functional sympathetic nerves, to re-investigate post-synaptic receptors involvement and particularly that of dopaminergic receptors in the positive inotropic effect of dopamine. The results snowed that noradrenaline, isoprenaline and dopamine produced a positive inotropic effect with a similar efficacy and with an order of potency as follows: Isoprenaline = Noradrenaline > Dopamine. Tyramine induced no significant modification of the “initial tension” indicating that functional sympathetic innervation and/or releasable endogenous catecholamines were not demonstrable in the 7-day-old chick embryo heart ventricle. Propranolol (1 μM) competitively antagonized the positive inotropic response to isoprenaline, noradrenaline and dopamine, meanwhile phentolamine (3 μM) failed to significantly modify the effects of both noradrenaline and dopamine, indicating that these catecholamines induced their positive inotropic effects via stimulation of β-adrenoceptors; involvement of α-adrenergic receptors stimulation was not demonstrable in these effects. Moreover, haloperidol (2 μM) antagonized the positive inotropic response to dopamine but had not any significant effect on the response to isoprenaline. The combined application of both propranolol and haloperidol antagonized the positive inotropic response to dopamine to a greater extent than when these two antagonists were given alone. Consequently, post-synaptic dopaminergic receptors were also involved in the positive inotropic effect of dopamine. Furthermore, in preparations in which sodium channels were inactivated by high potassium physiological salt solution, high concentrations of dopamine (0.1 mM to 1 mM) induced a slow developing electrical and positive inotropic responses which were also inhibited by propranolol and haloperidol, but not by phentolamine. These latter results indicated that like β-adrenergic stimulation, the slow inward calcium current activated by stimulation of adenylate cyclase, was at least in part involved in the positive inotropic response to dopamine. In conclusion, dopamine induced its positive inotropism via stimulation of post-synaptic β-adrenergic and dopaminergic receptors. The contribution of dopaminergic receptors in this positive inotropic effect might be of the DA-2 receptors since haloperidol used had been reported to be more DA-2 than DA-1 antagonist. These DA-2 receptors subtypes would mediate activation of adenylate cyclase.     

Interaction of lidocaine and calcium on the electrical characteristics of rabbit atria.

The interactions of lidocaine (1-5 X 10(-5) M) and calcium ions (1.25-5.0 mM) on electrical characteristics of atrial potentials were determined with standard microelectrode techniques with major reference to the maximum rate of rise of the action potential (Vmax of AP), the time constant of recovery of the rapid sodium carrier (gamma) and repetitive firing due to early extra stimuli (arrhythmia). Lowering Ca caused depolarization and decreased Vmax and gamma; high Ca caused changes in the opposite direction. The relation of gamma to membrane potential was downward concave when membrane potential was changed by Ca but upward concave when equivalent changes in membrane potential were induced by changing the external potassium concentration. Lidocaine (1 X 10(-5) M) had no significant effect at 2.5 mM Ca but significantly decreased the overshoot and Vmax of AP, increased gamma and effective refractory period and was antiarrhythmic at 1.25 mM Ca. These changes were closely similar to the effects of lidocaine (5 X 10(-5) M) at 2.5 mM Ca. The effects of this high concentration were decreased when Ca was changed to 5.0 mM. The effect of lidocaine most clearly predictive of efficacy for the type of arrhythmia was that on Vmax, with changes in gamma in particular not being related to antiarrhythmic activity.     

Electrophysiological actions of the pimobendan metabolite, UD-CG 212 Cl, in guinea pig myocardium.

Pimobendan (UD-CG 115 BS), an inotropic agent and inhibitor of type III phosphodiesterase activity, is demethylated in vivo to form UD-CG 212 Cl, which is a more potent type III phosphodiesterase inhibitor. This study examined cyclic AMP (cAMP)-mediated actions of UD-CG 212 Cl. In guinea pig papillary muscles, UD-CG 212 Cl increased cAMP and stimulated Ca(++)-dependent slow action potentials (APs) in a dose-dependent manner. When compared to previous studies using pimobendan, UD-CG 212 Cl was approximately 100-fold more potent. UD-CG 212 Cl had no additional effects on slow APs in the presence of a maximal dose of isoproterenol (1 microM). Propranolol had little effect on UD-CG 212 Cl-induced slow APs. These results, along with previous studies, indicate that slow AP induction by UD-CG 212 Cl was cAMP-dependent, and the increase in cAMP levels was most likely due to phosphodiesterase inhibition and not beta receptor stimulation. Experiments with tetraethylammonium.Cl suggested that UD-CG 212 Cl probably did not induce slow APs by blocking K+ channels. In voltage-clamped ventricular myocytes UD-CG 212 Cl (100 microM) could stimulate Ca++ current (+21 +/- 5%) when basal cAMP levels were enhanced with a submaximal dose of isoproterenol (10(-9)-10(-8) M). Isoproterenol was not required to observe the stimulating effect of UD-CG 212 Cl on Ca++ current in intact, nondialyzed cells prepared using the nystatin-perforated patch method. Studies with the stereoisomers of UD-CG 212 Cl showed that the D-isomer was more potent than the L-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiac sarcolemma as a possible site of action of caffeine in rat heart

Caffeine (0.1-10 mM) produced a biphasic effect on Na(+)-K+ ATPase activity in the rat heart sarcolemmal preparations. The Na(+)-K+ ATPase activity was stimulated by about 25% at low concentrations (0.1-1 mM), whereas the enzyme was inhibited by about 25% at higher concentrations (10 mM) of caffeine. The stimulatory effect of 1 mM caffeine was associated with about 30% increase in the Vmax value for Na(+)-K+ ATPase, whereas the depressant action of 10 mM caffeine was associated with an increase of the Km value from 1.4 to 2.1 mM ATP. The Na(+)-induced Ca++ release from the sarcolemmal vesicles was stimulated with caffeine in a concentration-dependent manner; about 80% increase in the activity was observed at 0.1 mM caffeine. The apparent Ka (millimolar Na+) values for the Na(+)-induced Ca++ release were about 17 and 6 in the absence and presence of 1 mM caffeine, respectively. However, the sarcolemmal Na(+)-dependent Ca++ uptake and ATP-independent Ca++ binding were not affected, whereas the ATP-dependent Ca++ accumulation and Ca+(+)-stimulated ATPase activities were depressed by 1 to 10 mM caffeine. This agent at concentrations of 0.1 to 10 mM produced a biphasic effect on the contractile activity of the isolated perfused rat heart. The initial transient positive inotropic (30-60%) effect was followed by a sustained negative inotropic (50-80%) response of the drug; the delayed decrease in contractile force was associated with a significant increase (35-50%) in the resting tension. The initial positive inotropic effect of caffeine was dependent on the concentration of Ca++ (0.2-3 mM) in the perfusion medium; however, this response was attenuated either by lowering the concentration of Na+ from 140 to 35 mM or by different concentrations (0.5-1 mM) of amiloride in the medium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Specific actions of gallium on norepinephrine-induced tension and associated 45Ca movements in rabbit aortic smooth muscle

Gallium ion (Ga) dose-dependently (60-360 microM) inhibited contractions induced by norepinephrine (NE, 1 microM) in rabbit aortic (and media intimal) strips, but did not affect contractions elicited with high K+ (80 mM) solution. The initial phasic portion of the NE-induced response was either unaffected or only slightly (less than 10%) reduced, but the tonic portion of the response was inhibited completely by higher concentrations (greater than or equal to 300 microM) of Ga . In resting muscles, the equilibrated (90 min) 45Ca uptake was not altered by Ga (360 microM). Also, 45Ca efflux from either high- or low-affinity Ca++ binding sites was unaltered by Ga . The effects of Ga (360 microM) on 45Ca retained after a subsequent 60-min washout at 0.5 degrees C in an isosmotic (80.8 mM) La solution were also examined. High affinity La -resistant 45Ca released by NE (1 microM) was not altered by Ga . Under conditions favoring low affinity Ca++ uptake, 45Ca retention in control and K+-treated muscles was not changed by Ga , but the additional incremental 45Ca uptake associated with NE (in the presence of high K+) was blocked. Thus, Ga appears to have a selective inhibitory action on NE-associated 45Ca uptake without affecting either resting and high K+-induced 45Ca uptake or that 45Ca fraction released by NE. This action may result from a selective blockade by Ga of receptor-linked Ca++ channels in rabbit aortic smooth muscle.