EFFECTS OF ESCAPABLE AND INESCAPABLE FOOT-SHOCK ON RAT ATRIAL BETA-ADRENOCEPTORS

The chronotropic responsiveness to norepinephrine (NE) and isoproterenol (ISO) was determined in right atria isolated from rats submitted to repeated escapable or inescapable foot-shock. Significant postjunctional supersensitivity to ISO, but not to NE, was observed in both groups. No significant change in the pA2 value of metoprolol (a selective beta1-adrenoceptor antagonist) was detected. However, a decrease of the maximum response to soterenol, a partial agonist at beta1-adrenoceptors, occurred only after inescapable foot-shock. The enhanced sensitivity to ISO was abolished by butoxamine (a selective beta2-adrenoceptor antagonist) and accompanied by a marked increase in the pA2 Value of this antagonist. We conclude that the ability to control the shock prevented the down-regulation of the pacemaker beta1-adrenoceptors but not the increased participation of beta2-adrenoceptors in the response of the rat sinoatrial node to catecholamines after repeated foot-shock.44486987

REDUCED RESPONSIVENESS TO NORADRENALINE IN ISOLATED RAT ATRIA EXPOSED TO HYPEROSMOTIC SOLUTIONS

1. The effects of hyperosmotic NaCl and sucrose solutions on the responsiveness to noradrenaline (NA) were studied in isolated rat right and left atria. 2. Sucrose caused subsensitivity to NA and isoprenaline in right atria, which was abolished by atropine. 3. NaCl caused subsensitivity only to NA in both atria, which was reversed by imipramine. 4. Both solutions decreased the maximum tension of left atria after maximal NA and Ca2+ concentrations. 5. The results suggest that hyperosmolality reduces the atrial responsiveness to NA by directly depressing contractility and increasing acetylcholine release. However, if the solute is NaCl, the main mechanism seems to be an increase of the catecholamine neuronal uptake.22115115

Determination of the vectorelectrogram in isolated rat atria: application to the study of arrhythmias

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Atrial tachyarrhythmias, the most frequent type of cardiac arrhythmia, are associated with increased stroke risk. Reentry and focal activity are considered as the main mechanisms underlying this dysfunction. In this study, we describe determination of the vectorelectrogram in isolated rat atria as a means to distinguish different patterns of electrical propagation. In all studied right atria beating at sinus rhythm, the mean electric vector (MEV) trajectory was clockwise, and each cycle was preceded by electric diastole (null MEV), either in the absence or presence of muscarinic cholinergic or beta-adrenergic receptor stimulation. During cholinergic tachyarrhythmia (induced by high-rate electric stimulation in both atria, plus exposure to carbachol in left atria), vector loops were ellipsoidal and stable, with variable direction, and did not cross the origin, which is consistent with reentrant activation and with findings obtained in vivo by other authors. In contrast, during spontaneous activity induced by rapid pacing in isoproterenol-exposed left atria, vector loops were similar to those in right atria at sinus rhythm, thus suggestive of focal activity. It is concluded that the vectorelectrogram approach allows discrimination of different patterns of propagation during arrhythmia in isolated atria and may be useful for high-output tests of pro- and anti-arrhythmic compounds.301112811291Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [300632/2005-3, 141175/2002-8

A method to estimate mitochondrial Ca2+ uptake in intact cardiac myocytes

In the present paper we describe a method to estimate mitochondrial Ca2+ uptake during the declining phase of Ca2+ transients (cell relaxation) in intact isolated myocardial cells. This method is based on inhibition of sarcoplasmic reticulum (SR) Ca2+ accumulation by caffeine, blockade of Ca2+ transport via sarcolemmal Ca2+-ATPase by treatment with carboxyeosin and inhibition of sarcolemmal Na+/Ca2+ exchange by removal of extracellular Na+ and Ca2+. Ca2+ transients were evoked in rabbit ventricular myocytes by quick and sustained caffeine application (10 mM) after a 5-min period of electrical stimulation to load the SR with Ca2+. Mitochondrial Ca2+ transport was estimated using a model described by Sipido and Wier (Journal of Physiology (1991), 435:605-630), which was originally proposed to describe Ca2+ fluxes during excitation-contraction coupling in cardiac cells. Our results indicate that, in intact rabbit myocytes, the Ca2+ flux due to net mitochondrial Ca2+ uptake may attain a value close to 1 mu M/sec.29121699170

Rest-dependence of twitch amplitude and sarcoplasmic reticulum calcium content in the developing rat myocardium

Post-rest contractile response was studied in isolated ventricular muscle from rats aged 1 to 90 days. Amplitude of rapid cooling contractures (RCC) was taken as an index of the sarcoplasmic reticulum (SR) Ca2+ content. We observed that: (a) developed tension (per cross-section area) increased with age: (b) time to peak twitch force and relaxation half-time decreased from 87 +/- 6 to 56 +/- 2 ms and from 68 +/- 6 to 36 +/- 1 ms, respectively, from the neonatal period to adulthood; (c) post-rest twitch potentiation was observed at all ages, with greater relative potentiation in younger preparations, although relative potentiation of [Ca2+](i) transient amplitude was similar in young and adult isolated ventricular myocytes; (d rest did not significantly affect the amplitude of RCC in muscle or caffeine-evoked [Ca2+]. transients in myocytes at any studied age: (e) favoring Ca2+ efflux via Na+-Ca2+ exchange (NCX) during rest reversed twitch potentiation and caused a similar decrease in RCC amplitude ( similar to 40%) at all ages; (f) stimulation of Ca2+ influx via NCX during rest increased RCC amplitude (similar to 40%) only in immature preparations. However, when this procedure was repeated after partial SR Ca2+ depletion, increase in RCC amplitude was not significantly age-dependent. We conclude that post-rest twitch potentiation is already present early after birth and does not require rest-dependent changes in SR Ca2+ content at any studied age. Our results suggest that NCX is close to equilibrium during rest in both adult and developing rat myocardium, and does not seem to mediate diastolic net Ca2+ fluxes which may affect the SR Ca2+ content. (C) 2001 Academic Press.33471172

Role of acetylcholine in electrical stimulation-induced arrhythmia in rat isolated atria

In this study, we used the spontaneously beating, isolated rat right atrium as an in vitro model to study arrhythmogenic effects of electrical stimulation. A tetrapolar platinum electrode was used for stimulation and recording of atrial electrical activity at 36.5 degrees C (spontaneous rate, 4.9 +/- 0.3 Hz). A flutter-like pattern of arrhythmia was reproducibly induced by application of stimulus trains (250 pulses, 66.7 Hz), Arrhythmia was characterized by regular and very short cycle length (40-70 ms), each episode lasting from 3 s to >5 min. In control conditions, application of one to five pulse trains was sufficient to induce arrhythmia. However, atropine (but not propranolol) completely blocked arrhythmia induction (10-15 consecutive trains were ineffective). The ability of electrical stimulation to evoke arrhythmia was restored after atropine washout. A milder stimulation protocol(30 pulses, 50 Hz), which was unable to evoke arrhythmia in control conditions, was fully effective in the presence of 1 mu M acetylcholine (ACh). Furthermore, a similar flutter-like pattern could be induced in isolated left atria in the presence of ACh, Our results point out an arrhythmogenic effect of neurally released ACh in the isolated right atrium on atrial electrical stimulation.34447547

Estimation of the fractional sarcoplasmic reticulum Ca2+ release in intact cardiomyocytes using integrated Ca2+ fluxes

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The sarcoplasmic reticulum (SR) is the main source of contraction-activating Ca2+ in the adult mammalian myocardium. The fraction of the SR Ca2+ content released at a twitch (fractional SR Ca2+ release, FR) is an important parameter for assessing the efficiency of excitation-contraction coupling under physiological and pathophysiological conditions, as well as for identification of modulators of this process. We here describe and propose an approach for FR quantitation based on the estimation of integrated Ca2+ fluxes mediated by different transporters that remove the ion from the cytosol. These fluxes may be calculated solely from the measurement of cytosolic free Ca2+ concentration ([Ca2+](i)) during Ca2+ transients evoked under selective inhibition of the transporters, and from the cell Ca2+ buffering parameters available in the literature. The FR values obtained with this approach in intact rat ventricular myocytes (0.63 +/- 0.04; n = 12) were comparable to those estimated in the same cell type with an already established method, based on electrophysiological measurements with the patch-clamp technique, in addition to [Ca2+](i); measurement (0.69 +/- 0.05; n = 6; p > 0.40). We conclude that the proposed method might be a suitable and a technically simpler alternative to the electrophysiological method for FR estimation.314401408Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)CNPq [Proc N. 300632/2005-3, 141175/2002-8]FAPESP [Proc. N 2006/05717-7, 2008/54795-6

Electric field stimulation of cardiac myocytes during postnatal development

Studies on cardiac cell response to electric field stimulation are important for understanding basic phenomena underlying cardiac defibrillation, In this work, we used a model of a prolate spheroidal cell in a uniform external field (Klee and Plonsey, 1976) to predict the threshold electric field (E-T) for stimulation of isolated ventricular myocytes of rats at different ages. The model assumes that E-T is primarily determined by cell shape and dimensions, which markedly change during postnatal development, Neonatal cells showed very high E-T, which progressively decreased with maturation (experimental mean values were 29, 21, 13, and 5.9 and 6.3 V/cm for 3-6, 13-16, 20-21, 28-35, and 120-180 day-old rats, respectively, P V-T approximate to 35 mV, under our experimental conditions) was reasonably constant during development, except for cells from 1-mo-old animals, in which DeltaV(T) was lower than at other ages. We conclude that the model reasonably correlates E-T with cell geometry and size in most cases, Our results might be relevant for the development of efficient procedures for defibrillation of pediatric patients.48663063

INOTROPIC EFFECT OF HYPEROSMOTIC NACL SOLUTIONS ON THE ISOLATED RAT CARDIAC TISSUE

The inotropic effect of Krebs-Henseleit solution rendered hyperosmotic by addition of NaCl or sucrose (increments of 50, 100, 150 and 200 mOsm/l) on myocardial contractile activity was studied in rat isolated left atria paced ar 4, 16 and 64 stim/min. The solutions did not affect the peak tension (T(p)) at 4 stim/min, whereas sucrose caused a dose-dependent increase in T(p) at 16 stim/min and NaCl decreased T(p) at 64 stim/min. The total time duration of the contraction was increased in a dose-dependent fashion by both solutes, but the effect of NaCl was attenuated at 64 stim/min. The results showed that, in the isolated rat atrial tissue exposed to hyperosmotic NaCl solutions, the negative inotropic effect of increased Na+ concentration overcomes the positive influence of hyperosmolality only at higher pacing rates (about 1 Hz).98639740