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

Rate-dependent Ca2+ signalling underlying the force-frequency response in rat ventricular myocytes: A coupled electromechanical modeling study

Rate-dependent effects on the Ca2+ sub-system in a rat ventricular myocyte are investigated. Here, we employ a deterministic mathematical model describing various Ca2+ signalling pathways under voltage clamp (VC) conditions, to better understand the important role of calmodulin (CaM) in modulating the key control variables Ca2+/calmodulin-dependent protein kinase-II (CaMKII), calcineurin (CaN), and cyclic adenosine monophosphate (cAMP) as they affect various intracellular targets. In particular, we study the frequency dependence of the peak force generated by the myofilaments, the force-frequency response (FFR). Our cell model incorporates frequency-dependent CaM-mediated spatially heterogenous interaction of CaMKII and CaN with their principal targets (dihydropyridine (DHPR) and ryanodine (RyR) receptors and the SERCA pump). It also accounts for the rate-dependent effects of phospholamban (PLB) on the SERCA pump; the rate-dependent role of cAMP in up-regulation of the L-type Ca2+ channel (ICa;L); and the enhancement in SERCA pump activity via phosphorylation of PLB.Our model reproduces positive peak FFR observed in rat ventricular myocytes during voltage-clamp studies both in the presence/absence of cAMP mediated -adrenergic stimulation. This study provides quantitative insight into the rate-dependence of Ca2+-induced Ca2+-release (CICR) by investigating the frequency-dependence of the trigger current (ICa;L) and RyR-release. It also highlights the relative role of the sodium-calcium exchanger (NCX) and the SERCA pump at higher frequencies, as well as the rate-dependence of sarcoplasmic reticulum (SR) Ca2+ content. A rigorous Ca2+ balance imposed on our investigation of these Ca2+ signalling pathways clarifies their individual roles. Here, we present a coupled electromechanical study emphasizing the rate-dependence of isometric force developed and also investigate the temperature-dependence of FFR. Our model provides mechanistic biophysically based explanations for the rate-dependence of CICR, generating useful and testable hypotheses. Although rat ventricular myocytes exhibit a positive peak FFR in the presence/absence of beta-adrenergic stimulation, they show a characteristic increase in the positive slope in FFR due to the presence of Norepinephrine or Isoproterenol. Our study identifies cAMP-mediated stimulation, and rate-dependent CaMKII-mediated up-regulation of ICa;L as the key mechanisms underlying the aforementioned positive FFR

Modeling CICR in rat ventricular myocytes: voltage clamp studies

<p>Abstract</p> <p>Background</p> <p>The past thirty-five years have seen an intense search for the molecular mechanisms underlying calcium-induced calcium-release (CICR) in cardiac myocytes, with voltage clamp (VC) studies being the leading tool employed. Several VC protocols including lowering of extracellular calcium to affect <it>Ca</it>²⁺loading of the sarcoplasmic reticulum (SR), and administration of blockers caffeine and thapsigargin have been utilized to probe the phenomena surrounding SR <it>Ca</it>²⁺release. Here, we develop a deterministic mathematical model of a rat ventricular myocyte under VC conditions, to better understand mechanisms underlying the response of an isolated cell to calcium perturbation. Motivation for the study was to pinpoint key control variables influencing CICR and examine the role of CICR in the context of a physiological control system regulating cytosolic <it>Ca</it>²⁺concentration ([<it>Ca</it>²⁺]<it>_myo</it>).</p> <p>Methods</p> <p>The cell model consists of an electrical-equivalent model for the cell membrane and a fluid-compartment model describing the flux of ionic species between the extracellular and several intracellular compartments (cell cytosol, SR and the dyadic coupling unit (DCU), in which resides the mechanistic basis of CICR). The DCU is described as a controller-actuator mechanism, internally stabilized by negative feedback control of the unit's two diametrically-opposed <it>Ca</it>²⁺channels (trigger-channel and release-channel). It releases <it>Ca</it>²⁺flux into the cyto-plasm and is in turn enclosed within a negative feedback loop involving the SERCA pump, regulating[<it>Ca</it>²⁺]<it>_myo</it>.</p> <p>Results</p> <p>Our model reproduces measured VC data published by several laboratories, and generates graded <it>Ca</it>²⁺release at high <it>Ca</it>²⁺gain in a homeostatically-controlled environment where [<it>Ca</it>²⁺]<it>_myo</it>is precisely regulated. We elucidate the importance of the DCU elements in this process, particularly the role of the ryanodine receptor in controlling SR <it>Ca</it>²⁺release, its activation by trigger <it>Ca</it>²⁺, and its refractory characteristics mediated by the luminal SR <it>Ca</it>²⁺sensor. Proper functioning of the DCU, sodium-calcium exchangers and SERCA pump are important in achieving negative feedback control and hence <it>Ca</it>²⁺homeostasis.</p> <p>Conclusions</p> <p>We examine the role of the above <it>Ca</it>²⁺regulating mechanisms in handling various types of induced disturbances in <it>Ca</it>²⁺levels by quantifying cellular <it>Ca</it>²⁺balance. Our model provides biophysically-based explanations of phenomena associated with CICR generating useful and testable hypotheses.</p

Osmolality- and Na+-dependent effects of hyperosmotic NaCl solution on contractile activity and Ca2+ cycling in rat ventricular myocytes

Hypertonic NaCl solutions have been used for small-volume resuscitation from hypovolemic shock. We sought to identify osmolality- and Na+-dependent components of the effects of the hyperosmotic NaCl solution (85 mOsm/kg increment) on contraction and cytosolic Ca2+ concentration ([Ca2+](i)) in isolated rat ventricular myocytes. The biphasic change in contraction and Ca2+ transient amplitude (decrease followed by recovery) was accompanied by qualitatively similar changes in sarcoplasmic reticulum (SR) Ca2+ content and fractional release and was mimicked by isosmotic, equimolar increase in extracellular [Na+] ([Na+](o)). Raising osmolality with sucrose, however, augmented systolic [Ca2+](i) monotonically without change in SR parameters and markedly decreased contraction amplitude and diastolic cell length. Functional SR inhibition with thapsigargin abolished hyperosmolality effects on [Ca2+](i). After 15-min perfusion, both hyperosmotic solutions slowed mechanical relaxation during twitches and [Ca2+](i) decline during caffeine-evoked transients, raised diastolic and systolic [Ca2+](i), and depressed systolic contractile activity. These effects were greater with sucrose solution, and were not observed after isosmotic [Na+](o) increase. We conclude that under the present experimental conditions, transmembrane Na+ redistribution apparently plays an important role in determining changes in SR Ca2+ mobilization, which markedly affect contractile response to hyperosmotic NaCl solutions and attenuate the osmotically induced depression of contractile activity.455461762

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

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