Laura Mantelli

Electrophysiological and antiarrhythmic properties of propafenon in isolated cardiac preparations.

Propafenon, a new antiarrhythmic drug, caused a 30% decrease in maximal driving frequency and a 65 ms net increase in functional refractory period of isolated guinea pig atria at a concentration as low as 0.5 μg/ml. The spontaneous rate of isolated atria and the contractility of electrically driven ventricular strips were reduced after treatment with propafenon 1μg/ml. Propafenon 0.5 μg/ml also altered action potentials of sheep Purkinje fiber. It reduced the action potential and overshoot amplitudes, decreased the maximum rate of depolarization of the action potential upstroke in a frequency-dependent fashion, shortened the action potential and effective refractory period, and depressed the membrane responsiveness. Moreover, propafenon antagonized the chronotropic and inotropic effects of isoprenaline in isolated guinea pig heart preparations; the pA2 value was about 6.4. Finally, propafenon possessed very weak calcium antagonist properties, being about 100 times less potent than verapamil in this respect. We conclude that propafenon is an antiarrhythmic drug with β-adrenoceptor blocking and “membrane stabilizing” activities in the same range of concentrations and that it has a calcium antagonistic activity only at much higher concentrations.

Roles of nitric oxide and endothelium-derived hyperpolarizing factor in vasorelaxant effect of acetylcholine as influenced by aging and hypertension.

We investigated vasodilator responses to acetylcholine (ACh) in isolated mesenteric vascular bed preparations (preconstricted with methoxamine) of young (2 months) and old (18 months) normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). ACh produced a similar dose-dependent vasorelaxant effect in preparations from both 2-month old normotensive and hypertensive rats. This vasodilator response to ACh decreased with age, especially in hypertensive animals. In preparations from young WKY, the vasorelaxant effect of ACh was not affected by 100 microM NG-nitro-L-arginine methyl ester (L-NAME), and was only slightly reduced by 500 microM L-NAME. The K+ channel blocker tetraethylammonium (TEA 2.5-10 mM) concentration-dependently antagonized the ACh-induced vasodilation in the same preparations. In preparations obtained from aged WKY animals, as well as in those from young and aged SHR animals, ACh-induced vasodilation was significantly and concentration-dependently reduced by 100 and 500 microM L-NAME. On the other hand, TEA induced a lesser antagonistic effect than that observed in young normotensive animals. In preparations preconstricted with 80 mM KCl, ACh caused vasodilation that was weaker in preparations from young WKY than in those from aged WKY; on the contrary, ACh was more effective in young than in aged SHR. These results confirm that the vasodilating response to ACh decreases with age and hypertension and suggest that the main mechanism responsible for the effect of ACh in vessels of young normotensive animals consists of activation of K+ channels. In preparations from old normotensive, as well as in those from young and old hypertensive animals, ACh induces vasorelaxation mainly through nitric oxide (NO) release.

Inhibition of the cardiac response to sympathetic nerve stimulation by opioid peptides and its potentiation by morphine and methadone

[D-Ala2,D-Leu5]enkephalin (1-10 microM) and [Met5]enkephalin-Arg-Phe (1-10 microM) produced concentration-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals in preparations pretreated with peptidase inhibitors (captopril 10 microM, bestatin 10 microM, thiorphan 0.3 microM and L-leucyl-L-leucine 2 mM). The inhibitory response to the opioid agonists was evident in preparations superfused with solutions containing 1.8 mM calcium, but not in those containing 3.6 mM calcium. Moreover the inhibition was antagonized by naloxone 10 microM. [D-Ala2,Met5]enkephalinamide (1-3 microM) and beta-endorphin (1-3 microM) did not significantly affect the sympathetic response. The cardiac response to sympathetic stimulation was not inhibited but, on the contrary, was potentiated by morphine (3-10 microM) and methadone (3-10 microM). It is suggested that the depressant effect of the opioid peptides was due to stimulation of presynaptic inhibitory opiate receptors on adrenergic nerve terminals of the heart, and that the potentiation of the sympathetic response by morphine and methadone was probably attributable to an unspecific inhibitory effect on the neuronal uptake of noradrenaline.

Frequency dependence of the α-adrenoceptor-mediated positive inotropic effect in guinea pig heart

Abstract Evidence for the role of α-adrenoceptors in the production of a positive inotropic effect has been obtained chiefly in heart preparations stimulated at a low rate (1 or 1.4 Hz). However, our previous results suggested that the α-receptor-mediated positive inotropic effect could be frequency dependent. In the present study cumulative concentration—inotropic effect curves were obtained in guinea pig ventricle strips treated with adrenaline and stimulated at 1 and 2.5 Hz. Phentolamine (3 × 10 −6 M) and practolol (10 −6 M) were employed as antagonists. Phentolamine antagonized the effect of low concentrations of adrenaline (10 −9 –10 −8 M) in preparations driven at 1 Hz, but did not modify the curve of the agonist at the higher stimulation rate. In preparations driven at 1 Hz, practolol antagonized the effect of adrenaline at concentrations above 3 × 10 −8 M without affecting the curve at lower concentrations; the β-blocker competitively antagonized the effect of the agonist at 2.5 Hz. It was concluded that a shortening of the interval between beats abolishes the α-mediated positive inotropic effect.

β1- and β2-adrenoceptors in sheep cardiac ventricular muscle

Abstract The presence of β 2 -adrenoceptors in the sheep ventricular myocardium was assessed by the radioligand binding technique and functional studies. In membrane preparations, the competition curve between [ 3 H]-dihydroalprenolol and the selective β 1 -antagonist CGP 20712A (0.1 n m –1 m m ) was clearly biphasic, and revealed the presence of two different binding sites showing an affinity (p K D ) for CGP 20712A of 9.5 ± 0.9 and 4.5 ± 0.4, respectively. The relative proportion of β 1 : β 2 adrenoceptors was about 70:30 in both the right and left ventricle. In ventricular trabeculae driven at 1Hz, isoprenaline (1–300 n m ) caused a dose-dependent increase in the force of contraction, the maximum effect being 298 ± 26 mg, associated with reduction of time to peak tension ( t 1 , clinotropic effect) and relaxation time ( t 2 , lusitropic effect). The inotropic dose-response curve for isoprenaline was significantly shifted to the right by pretreatment of the preparations with 0.1 μ m CGP 20712A or with the selective β 2 -antagonist ICI 118 551 (50 n m ). In the presence of CGP 20712A (0.1 μ m ), isoprenaline, up to a concentration of 10 μ m , did not affect either t 1 or t 2 ; on the other hand, pretreatment of the preparations with ICI 118 551 (50 n m ) fully antagonized the clinotropic but not the lusitropic effect of isoprenaline. In the presence of CGP 20712A procaterol (0.01–10 μ m ), a β 2 -adrenoceptor agonist, induced a positive intropic effect which was not associated with any significant modifications in t 1 or t 2 . This effect was completely abolished by ICI 118 551 (50 n m ). The positive inotropic action of isoprenaline (1 μ m ) was associated with a significant decrease in action potential duration measured at −60 mV (220 ± 8 and 193 ± 10 ms in the absence and presence of isoprenaline, respectively; P m ) alone, isoprenaline (1μ m ) still induced a significant increase in contractility but the action potential profile was only slightly affected. The effects of isoprenaline were fully antagonized by the simultaneous presence of CGP 20712A and ICI 118 551 (10 n m ). It is concluded that both β 1 - and β 2 -adrenoceptors appear to coexist in sheep ventricular myocardium where their stimulation mediates a positive inotropic effect. However, their functional role on the relaxation phase of the twitch may be different.

Blockade of adenosine receptors unmasks a stimulatory effect of ATP on cardiac contractility

1 The effects of ATP, α,β‐methylene ATP and β,γ‐methylene ATP on the contractile tension of guinea‐pig isolated left atria were evaluated. 2 ATP (1– 100 μm) produced a concentration‐dependent negative inotropic effect; this response was converted to a positive inotropic effect in the presence of the antagonist of adenosine A1 receptors, 1,3‐dipropyl‐8‐cyclopentylxanthine (DPCPX; 0.1 μm), and in the presence of 8‐phenyltheophylline (10 μm), an antagonist of A1 and A2 receptors. 3 The positive inotropic effect of ATP was antagonized by the P2 receptor antagonist, suramin (500 μm). Reactive blue 2 (30‐ 500 μm), a putative P2y receptor antagonist, concentration‐dependently reduced and finally abolished the effect of ATP. 4 In the presence of 8‐phenyltheophylline, the stable analogues of ATP, α,β‐methylene ATP and β,γ‐methylene ATP (1– 30 μm), produced a concentration‐dependent increase in atrial contractility of a lesser degree than that induced by ATP. 5 The results suggest that when inhibitory adenosine receptors are blocked, ATP produces a positive inotropic effect, probably mediated by P2y receptor stimulation.

Possible presynaptic inhibitory effect of etorphine on sympathetic nerve terminals of guinea-pig heart

Etorphine (1-4 microM) dose dependently reduced the sympathetic response induced by trains of field pulses in guinea-pig isolated atria stimulated at 4 Hz; this effect was antagonized by 10 microM naloxone. Since etorphine did not modify the dose-inotropic effect curve of exogenous noradrenaline in the same preparation, it is suggested that the depressant effect of the opioid agonist was due to stimulation of presynaptic inhibitory opiate receptors on adrenergic nerve terminals of the heart.

Sensitivity to dynorphin-(1-13) of the presynaptic inhibitory opiate receptors of the guinea-pig heart

Dynorphin-(1-13) at the concentrations of 1-10 microM, produced a dose-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals in guinea-pig atria pretreated with peptidase inhibitors. This inhibitory effect was competitively antagonized in a dose-dependent manner by 5 and 10 microM naloxone. Since dynorphin-(1-13) did not modify the dose-inotropic effect curve of exogenous noradrenaline, it was concluded that the depressant effect of the opioid agonist was due to the stimulation of the presynaptic inhibitory opiate receptors belonging to the kappa subtype.

Prejunctional prostanoid receptors on cardiac adrenergic terminals belong to the EP3 subtype

1 The effects of prostaglandin E2 (PGE2) and of several synthetic prostanoids on the cardiac response to sympathetic nerve stimulation in guinea‐pig atria have been evaluated. 2 PGE2 (0.01–100 nm), sulprostone (0.01–100 nm) and misoprostol (0.1–100 nm), but not butaprost (0.1–100 nm), dose‐dependently reduced the increase in cardiac contractility induced by electrical field stimulation of sympathetic terminals. 3 The EP1 antagonist AH6809 (1 μm) did not modify the inhibition of cardiac response induced by PGE2, sulprostone and misoprostol. 4 In preparations preloaded with [3H]‐noradrenaline, tritium overflow induced by electrical field stimulation was greatly and significantly reduced by 100 nm PGE2 and by 100 nm sulprostone. 5 These results indicate that PGE2 and other synthetic prostanoids reduce noradrenaline release from cardiac adrenergic nerve terminals acting on prejunctional inhibitory receptors belonging to the EP3 subtype.

α‐SYMPATHOMIMETIC AMINES AND CALCIUM‐MEDIATED ACTION POTENTIALS IN GUINEA‐PIG VENTRICULAR MUSCLE

1 The ability of amines, having α‐ or α‐ and β‐ adrenoceptor stimulating activity, to restore excitability and contractility in heart preparations partially depolarized by potassium, was investigated in guinea‐pig ventricular muscle in order to elucidate the mechanism of the positive inotropic effect mediated via α‐adrenoceptors. 2 In preparations in which fast sodium channels were inactivated by K+‐rich medium (22 mm) slow electrical responses as well as contractions were consistently induced by high concentrations of phenylephrine (10−4 to 3 × 10−4 m) and synephrine (3 × 10−4 m). 3 The restorative effects of both phenylephrine and synephrine were unaffected by phentolamine (10−5 m) but were readily abolished by practolol (10−5 m) or sotalol (10−5 m). 4 Methoxamine induced a dose‐dependent positive inotropic effect in ventricular strips paced at 0.5 Hz in normal Tyrode solution; the maximum increase in contractile tension was obtained with methoxamine 10−4 m. However, at the same concentration, the amine did not induce slow electrical responses in potassium‐depolarized preparations. 5 It is concluded that the induction of slow responses by phenylephrine and synephrine is due to β‐adrenoceptor stimulation, and that the increase in cardiac contractility caused by α‐adrenoceptor stimulation does not involve an increase in slow inward calcium current.