Guinea Pig Atrial Myocardium Research Articles

A New Class III Antiarrhythmic Drug Niferidil Prolongs Action Potentials in Guinea Pig Atrial Myocardium via Inhibition of Rapid Delayed Rectifier.

A new class III antiarrhythmic drug niferidil (RG-2) has been introduced as a highly effective therapy for cases of persistent atrial fibrillation, but ionic mechanisms of its action are poorly understood. In the present study, the effects of niferidil on action potential (AP) waveform and potassium currents responsible for AP repolarization were investigated in guinea pig atrial myocardium. APs were recorded with sharp glass microelectrodes in multicellular atrial preparations. Whole-cell patch-clamp technique was used to measure K+ currents in isolated myocytes. In multicellular atrial preparations, 10-8M niferidil effectively prolonged APs by 15.2±2.8% at 90% repolarization level. However, even the highest tested concentrations, 10-6M and 10-5M failed to prolong APs more than 32.5% of control duration. The estimated concentration of niferedil for half-maximal AP prolongation was 1.13×10-8M. Among the potassium currents responsible for AP repolarization phase, I K1 was found to be almost insensitive to niferidil. However, another inward rectifier, I KACh, was effectively suppressed by micromolar concentrations of niferidil with IC50=9.2×10-6M. I KATP was much less sensitive to the drug with IC50=2.26×10-4M. The slow component of delayed rectifier, I Ks, also demonstrated low sensitivity to niferidil-the highest used concentration, 10-4M, decreased peak I Ks density to 46.2±5.5% of control. Unlike I Ks, the rapid component of delayed rectifier, I Kr, appeared to be extremely sensitive to niferidil. The IC50 was 1.26×10-9M. I Kr measured in ventricular myocytes was found to be less sensitive to niferidil with IC50=3.82×10-8M. Niferidil prolongs APs in guinea pig atrial myocardium via inhibition of I Kr.

P512A new antiarrhythmic drug niferidil prolongs action potentials in guinea pig atrial myocardium via inhibition of potassium currents

P512A new antiarrhythmic drug niferidil prolongs action potentials in guinea pig atrial myocardium via inhibition of potassium currents

Concentration estimation via curve fitting: quantification of negative inotropic agents by using a simple mathematical method in guinea pig atria

We created a simple method based on curve fitting in order to assess the concentration of pharmacological agonists or antagonists in the microenvironment of the receptors. We tested our method in electrically driven guinea pig left atria by estimating the concentration of N 6-cyclopentyladenosine (CPA; A 1 adenosine receptor agonist), acetyl- β-methylcholine (muscarinic receptor agonist) and verapamil (L-type Ca 2+ channel inhibitor) added previously to the atria in known amounts. Our results validated the fitness of the model under specified conditions. In addition, our data suggest a relatively slow elimination of CPA in isolated, practically bloodless guinea pig atrial myocardium.

Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro.

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.

Effects of levcromakalim and glibenclamide on paced guinea-pig atrial strips exposed to hypoxia

Isolated strips of guinea-pig atrial myocardium were mounted in isometric myographs and electrically paced for measurements of myocardial contractile function. Levcromakalim, a K + channel opener, completely inhibited the contractile force in a concentration-dependent way (EC 50 = 15 μM). Glibenclamide (3 μM), a blocker of ATP-regulated K + channels (K ATP), caused a 5-fold rightward shift of the concentration-effect curve. Exposure of the atrial strips to hypoxia caused a time-dependent loss of contractility from 100% to a minimum level of 60% within 12 min. Levcromakalim (1 μM, 3 μM and 10 μM) concentration-dependently enhanced the hypoxia-induced inhibition of contractile function whereas levcromakalim (0.01 μM and 0.1 μM) had no significant effect. In the presence of levcromakalim (10 μM) hypoxia reduced the contractile force to 25%. Glibenclamide (3 μM) totally antagonized the enhancing effect of levcromakalim. When hypoxia was induced in glucose-free Krebs solution with 2-deoxyglucose, the myocardial contractility was completely suppressed within 12 min. Glibenclamide by itself (3 μM) failed to influence the myocardial response to hypoxia both in normal Krebs solution and under conditions of impaired glycolysis. The results indicate that levcromakalim by activation of myocardial ATP-regulated K + channels accelerates and enhances the hypoxia-induced inhibition of myocardial contractile function. This effect may possibly contribute to the mechanism by which K + channel openers exert cardioprotection. The results further suggest that mechanisms different from activation of K ATP take a major part in the depressant mechanical response to hypoxia and glycolytic blockade in the guinea-pig atrial myocardium.

Adenosine receptors mediate both contractile and relaxant effects of adenosine in main pulmonary artery of guinea pigs

In guinea pig main pulmonary artery precontracted with noradrenaline, adenosine exerted an initial phasic contraction followed by a tonic contraction and a slow relaxation. After selective blockade by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX: 10 nM) of A1 receptors, adenosine only elicited a rapid relaxation. This initial response was characterized by use of adenosine (AR) and its analogues N6-cyclopentyl-adenosine (CPA), R-N6-phenylisopropyladenosine (R-PIA), 2-chloroadenosine (CADO), 5'-N-ethyl-carboxamidoadenosine(NECA), N6-2-(4-aminophenyl) ethyl adenosine (APNEA) and 2-p-((carboxyethyl)-phenethylamino)-5'-carboxamidoadenosine (CGS 21 680). The order of potency of the adenosine analogues for purine-induced phasic contraction was CPA > R-PIA > NECA = APNEA > AR > CGS 21 680 suggesting the involvement of activation of A1 type adenosine receptors in the contraction phase. DPCPX antagonized the CPA-induced contraction with a pA2 = 9.27 +/- 0.26, but the Schild plot slope parameter was significantly lower than unity (0.58 +/- 0.09). In contrast, in electrically driven guinea pig atrial myocardium (a tissue reported to possess A1 receptors), the DPCPX-CPA antagonism was purely competitive (pA2 = 8.95 +/- 0.06; slope = 0.93 +/- 0.06). In the presence of 300 nM DPCPX, the rank order of potency for the purine-induced fast relaxation was NECA > CADO = AR > CGS 21 680 = R-PIA > CPA. The NECA- and adenosine-induced relaxation was influenced neither by 300 nM CP 66713 (an antagonist at A2a receptors), nor by endothelial removal and inhibition of nitric oxide synthase (100 microM NG-nitro-L-arginine: L-NOARG).(ABSTRACT TRUNCATED AT 250 WORDS)

ELectrophysiologic properties of bretylium tosylate on atrial myocardium.

The antifibrillatory property of bretylium tosylate was first observed in experimental atrial fibrillation, yet the cellular basis for this phenomenon has not been explored. The purpose of this study was to determine the electrophysiologic properties of bretylium tosylate on guinea pig atrial myocardium in the presence and absence of cholinergic influence. Bretylium (10(-6) M - 10(-4) M) produced a concentration-dependent prolongation of atrial action potential duration with a threshold concentration of 10(-5) M. This direct effect of bretylium was unaltered by blockade of beta-adrenergic receptors with propranolol (10(-6) M) or blockade of alpha-adrenergic receptors with phentolamine (10(-6) M). In a second series of experiments the muscarinic receptor blocking properties of bretylium were determined. Acetylcholine produced a concentration-dependent shortening of action potential duration in paced (200 ms) left atrial muscle strips. This well-recognized muscarinic effect was unaltered in the presence of bretylium (10(-6) M - 10(-3) M). These data indicate that bretylium tosylate physiologically exerts direct effects on the atrial myocardium to prolong action potential duration. This compound does not appear to physiologically antagonize the effects of acetylcholine and therefore its reported atrial antiarrhythmic properties cannot be explained by muscarinic receptor blockade.

Effect of dantrolene sodium on the transmembrane potentials and contractility of guinea pig atrial myocardium

Dantrolene sodium (3 × 10 −4 M) exerted a biphasic effect on the mechanical and electrical activity of the isolated guinea pig left atrium. In the first phase, the drug increased the contractile force by about 200%, and prolonged the action potential duration. This transient positive inotropic effect was antagonized by D-600, a slow Ca 2+ channel inhibitor. In the later phase, the drug gradually decreased the amplitude of contractions, shortened the action potential duration and induced contracture. Dantrolene sodium increased the rate of depolarization and overshoot of Ca 2+ -mediated slow action potentials. The results suggest that dantrolene sodium increases the slow inward Ca 2+ current, causing a positive inotropic effect in the atrial myocardium. The late negative inotropic effect of the drug could be due to a secondary inhibition of the slow inward Ca 2+ current due to an increased intracellular free Ca 2+ concentration as a consequence of other actions of the drug.

THE EXTRANEURONAL ACCUMULATION OF ISOPRENALINE IN TRACHEA AND ATRIA OF GUINEA-PIG AND CAT: A FLUORESCENCE HISTOCHEMICAL STUDY

1 The Falck-Hillarp histochemical technique was used to locate extraneuronal sites of accumulation of isoprenaline in trachea and atria from guinea-pig and cat. With a tissue exposure time to formaldehyde gas of 3 h, isoprenaline was located as green fluorescence.2 Quantitative microphotometry was used to measure fluorescence intensity within cells in the trachealis smooth muscle and the atrial myocardium of both species.3 After incubation of tissues in 50 muM isoprenaline, specific fluorescence was seen in trachealis smooth muscle of both species and in the atrial myocardium of cat but not guinea-pig. In both species, fluorescence was also seen in the chondroblasts of the tracheal cartilage and in blood vessels in all tissues.4 In trachealis smooth muscle of both species and in cat atrial myocardium, fluorescence brightness, resulting from incubation of tissues in 50 muM isoprenaline was significantly increased by 200 muM beta-thujaplicin, an inhibitor of catechol-O-methyl transferase (COMT). In the presence of beta-thujaplicin, fluorescence was not visible in guinea-pig atrial myocardium with 50 muM isoprenaline, although fluorescence brightness measured in myocardial cells was now greater than that in corresponding controls.5 The fluorescence intensity seen in cat and guinea-pig trachealis smooth muscle cells and in cat atrial myocardial cells after incubation in 50 muM isoprenaline was decreased significantly in the presence of phenoxybenzamine (100 muM). In guinea-pig atria, phenoxybenzamine had no effect on myocardial fluorescence. Fluorescence intensity was also decreased if the incubation with isoprenaline was carried out at 0 degrees C or if the post-incubation washing temperature was 37 degrees C instead of 0 to 2 degrees C.6 The results demonstrate that the fluorescence histochemical technique can be used to locate isoprenaline in tissues. They also indicate that guinea-pig and cat trachealis smooth muscle cells and cat atrial myocardial cells can accumulate isoprenaline (a) by a mechanism sensitive to phenoxybenzamine and (b) into sites in which COMT plays a functional role in inactivating isoprenaline at the concentration used in these histochemical experiments (50 muM). In contrast, the guinea-pig atrial myocardial cells may have a minimal capacity to accumulate isoprenaline by a phenoxybenzamine-sensitive uptake mechanism.