Biomedical subjects

Ariel L Escobar

Publications and source records attributed to Ariel L Escobar.

8 recordsLinked to original sources

PubMed2006

IgGs and Mabs against the beta2-adrenoreceptor block A-V conduction in mouse hearts: A possible role in the pathogenesis of ventricular arrhythmias.

Autoantibodies against beta-adrenoceptors might be involved in different cardiomyopathic diseases such as idopathic dilated cardiomyopathy, Chagas' disease and ventricular arrhythmias. To study the effects of such antibodies on the whole heart, we made use of a new technique allowing the measurement of Ca++ transients as well as action potentials in Langendorff preparations of mouse hearts. Mouse antibodies directed against the second extracellular loop of the beta2-adrenoceptor induced conduction blocks which could be washed away by the beta2-adrenoceptor inverse agonist ICI118,551, confirming the specificity and non-toxicity of these events. These results were confirmed by the use of a monoclonal antibody, monospecific for the beta2-adrenoceptor and the beta2-specific full agonist, clenbuterol. Both increased slightly, but significantly, the beating frequency but their main effect was the production of conduction blocks. In contrast, a monoclonal antibody, monospecific for the beta1-adrenoceptor, highly increased the beating frequency without interfering with the conduction. Our results suggest that stimulation of the beta2-adrenoceptor by anti-receptor antibodies in the conduction tissues leads to conduction disturbances, probably mediated by coupling to a different pathway than the classical Gs pathway. They confirm that anti-beta2 adrenoceptor antibodies could be responsible for ventricular arrhythmias.

Action Potentials↗

PubMed2006

Phospholamban phosphorylation sites enhance the recovery of intracellular Ca2+ after perfusion arrest in isolated, perfused mouse heart.

OBJECTIVE: To investigate the importance of the phosphorylation of Ser16 and Thr17 sites of phospholamban (PLN) on intracellular Ca2+ (Cai2+) handling and contractile recovery of the stunned myocardium. METHODS: Cai2+ (Rhod-2, pulsed local-field fluorescence microscopy) and contractility (isovolumic left ventricular developed pressure, LVDP) were simultaneously measured in Langendorff perfused hearts from transgenic mice expressing either intact PLN (PLN-WT) or PLN with both phosphorylation sites mutated to Ala (PLN-DM), subjected to 12 min of global ischemia followed by a reperfusion period of 30 min. RESULTS: Pre-ischemic values of Cai2+ and LVDP were similar in both groups. In PLN-WT, a transient increase in Thr17 phosphorylation at early reperfusion preceded a recovery of Ca2+ transient amplitude, virtually completed by the end of reperfusion. LVDP at 30 min reperfusion was 67.9+/-7.6% of pre-ischemic values, n=14. In contrast, in PLN-DM, there was a poor recovery of Cai2+ transient amplitude and LVDP was significantly lower (28.3+/-6.7%, n=11, 30 min reperfusion) than in PLN-WT hearts. Although myofilament Ca2+ responsiveness and troponin I (TnI) degradation did not differ between groups, the episodes of mechanical alternans, typical of Cai2+ overload, were significantly prolonged in PLN-DM vs. PLN-WT hearts. CONCLUSIONS: PLN phosphorylation appears to be crucial for the mechanical and Cai2+ recovery during stunning and protective against the mechanical abnormalities typical of Cai2+ overload. The importance of PLN phosphorylation would primarily reside in the Thr17 residue, which is phosphorylated during the critical early phase of reperfusion. Our results emphasize that, although ablation of PLN phosphorylation does not affect basal contractility, it does alter Ca2+ handling and mechanical performance under stress situations.

Animals↗

PubMed2005

Analysis of macroscopic ionic currents mediated by GABArho1 receptors during lanthanide modulation predicts novel states controlling channel gating.

Lanthanide-induced modulation of GABA(C) receptors expressed in Xenopus oocytes was studied. We obtained two-electrode voltage-clamp recordings of ionic currents mediated by recombinant homomeric GABArho(1) receptors and performed numerical simulations of kinetic models of the macroscopic ionic currents.GABA-evoked chloride currents were potentiated by La(3+), Lu(3+) and Gd(3+) in the micromolar range. Lanthanide effects were rapid, reversible and voltage independent. The degree of potentiation was reduced by increasing GABA concentration.Lu(3+) also induced receptor desensitization and decreased the deactivation rate of GABArho(1) currents. In the presence of 300 microM Lu(3+), dose-response curves for GABA-evoked currents showed a significant enhancement of the maximum amplitude and an increase of the apparent affinity. The rate of onset of TPMPA and picrotoxin antagonism of GABArho(1) receptors was modulated by Lu(3+). These results suggest that the potentiation of the anionic current was the result of a direct lanthanide-receptor interaction at a site capable of allosterically modulating channel properties. Based on kinetic schemes, which included a second open state and a nonconducting desensitized state that closely reproduced the experimental results, two nonexclusive probable models of GABArho(1) channels gating are proposed.

Chloride Channels↗

PubMed2004

Ryanodine receptor function in newborn rat heart.

The role of ryanodine receptor (RyR) in cardiac excitation-contraction (E-C) coupling in newborns (NB) is not completely understood. To determine whether RyR functional properties change during development, we evaluated cellular distribution and functionality of sarcoplasmic reticulum (SR) in NB rats. Sarcomeric arrangement of immunostained SR Ca(2+)-ATPase (SERCA2a) and the presence of sizeable caffeine-induced Ca2+ transients demonstrated that functional SR exists in NB. E-C coupling properties were then defined in NB and compared with those in adult rats (AD). Ca2+ transients in NB reflected predominantly sarcolemmal Ca2+ entry, whereas the RyR-mediated component was approximately 13%. Finally, the RyR density and functional properties at the single-channel level in NB were compared with those in AD. Ligand binding assays revealed that in NB, RyR density can be up to 36% of that found in AD, suggesting that some RyRs do not contribute to the Ca2+ transient. To test the hypothesis that RyR functional properties change during development, we incorporated single RyRs into lipid bilayers. Our results show that permeation and gating kinetics of NB RyRs are identical to those of AD. Also, endogenous ligands had similar effects on NB and AD RyRs: sigmoidal Ca2+ dependence, stronger Mg(2+)-induced inhibition at low cytoplasmic Ca2+ concentrations, comparable ATP-activating potency, and caffeine sensitivity. These observations indicate that NB rat heart contains fully functional RyRs and that the smaller contribution of RyR-mediated Ca2+ release to the intracellular Ca2+ transient in NB is not due to different single RyR channel properties or to the absence of functional intracellular Ca2+ stores.

Age Factors↗

PubMed2004

Calcium regulation of single ryanodine receptor channel gating analyzed using HMM/MCMC statistical methods.

Type-II ryanodine receptor channels (RYRs) play a fundamental role in intracellular Ca(2+) dynamics in heart. The processes of activation, inactivation, and regulation of these channels have been the subject of intensive research and the focus of recent debates. Typically, approaches to understand these processes involve statistical analysis of single RYRs, involving signal restoration, model estimation, and selection. These tasks are usually performed by following rather phenomenological criteria that turn models into self-fulfilling prophecies. Here, a thorough statistical treatment is applied by modeling single RYRs using aggregated hidden Markov models. Inferences are made using Bayesian statistics and stochastic search methods known as Markov chain Monte Carlo. These methods allow extension of the temporal resolution of the analysis far beyond the limits of previous approaches and provide a direct measure of the uncertainties associated with every estimation step, together with a direct assessment of why and where a particular model fails. Analyses of single RYRs at several Ca(2+) concentrations are made by considering 16 models, some of them previously reported in the literature. Results clearly show that single RYRs have Ca(2+)-dependent gating modes. Moreover, our results demonstrate that single RYRs responding to a sudden change in Ca(2+) display adaptation kinetics. Interestingly, best ranked models predict microscopic reversibility when monovalent cations are used as the main permeating species. Finally, the extended bandwidth revealed the existence of novel fast buzz-mode at low Ca(2+) concentrations.

Algorithms↗

PubMed2003

Developmental changes of intracellular Ca2+ transients in beating rat hearts.

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

Age Factors↗

PubMed2003

Pulsed local-field fluorescence microscopy: a new approach for measuring cellular signals in the beating heart.

In cardiac research, single-cell experimental models have been extensively used to study the molecular mechanisms of intracellular Ca(2+) homeostasis. The results of these studies are usually extrapolated to the tissue level assuming that the phenomena studied at the cellular level are either similar in the intact organ, or only slightly modified by variables that exist at the whole-heart level. The validity of these assumptions has rarely been confirmed experimentally. Common obstacles associated with the study of intracellular Ca(2+) signals in beating hearts include motion artifacts and spatio-temporal limitations of the recording system. In this work, action potentials and intracellular Ca(2+) signals were measured in beating hearts from young rats, with spatio-temporal resolutions similar to cellular studies using a novel pulsed local-field fluorescence technique. This method was based on maximizing emitted fluorescence to increase the signal-to-noise ratio (S/N). The fluorescence emission of the indicator molecules was synchronized with brief (<1 ns), high-power (400 W) laser pulses, and the common mode noise of the fluorescence signal was differentially cancelled. To follow rapidly evolving signals, a highly sensitive and fast detection system was used (10 kHz). The spatial resolution was improved using a small (50-200 microm diameter) multimode fiberoptic. Mechanical artifacts were effectively reduced by inserting the fiberoptic into a "floating" glass micropipette sealed to the heart wall with negative pressure. Our results demonstrate that local-field fluorescence microscopy offers an outstanding experimental approach for studying physiological signals at the whole-organ level with the high spatio-temporal resolution common to normal cellular approaches.

Action Potentials↗

PubMed2002

Luminal Ca2+ controls termination and refractory behavior of Ca2+-induced Ca2+ release in cardiac myocytes.

Despite extensive research, the mechanisms responsible for the graded nature and early termination of Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) in cardiac muscle remain poorly understood. Suggested mechanisms include cytosolic Ca2+-dependent inactivation/adaptation and luminal Ca2+-dependent deactivation of the SR Ca2+ release channels/ryanodine receptors (RyRs). To explore the importance of cytosolic versus luminal Ca2+ regulatory mechanisms in controlling CICR, we assessed the impact of intra-SR Ca2+ buffering on global and local Ca2+ release properties of patch-clamped or permeabilized rat ventricular myocytes. Exogenous, low-affinity Ca2+ buffers (5 to 20 mmol/L ADA, citrate or maleate) were introduced into the SR by exposing the cells to "internal" solutions containing the buffers. Enhanced Ca2+ buffering in the SR was confirmed by an increase in the total SR Ca2+ content, as revealed by application of caffeine. At the whole-cell level, intra-SR [Ca2+] buffering dramatically increased the magnitude of Ca2+ transients induced by I(Ca) and deranged the smoothly graded I(Ca)-SR Ca2+ release relationship. The amplitude and time-to-peak of local Ca2+ release events, Ca2+ sparks, as well as the duration of local Ca2+ release fluxes underlying sparks were increased up to 2- to 3-fold. The exogenous Ca2+ buffers in the SR also reduced the frequency of repetitive activity observed at individual release sites in the presence of the RyR activator Imperatoxin A. We conclude that regulation of RyR openings by local intra-SR [Ca2+] is responsible for termination of CICR and for the subsequent restitution behavior of Ca2+ release sites in cardiac muscle.

Animals↗