Halothane and sevoflurane inhibit Na/Ca exchange current in rat ventricular myocytes

Experimental assessment of a myocyte-based multiscale model of cardiac contractile dysfunction

Cardiac contractile dysfunction (CD) is a multifactorial syndrome caused by different acute or progressive diseases which hamper assessing the role of the underlying mechanisms characterizing a defined pathological condition. Mathematical modeling can help to understand the processes involved in CD and analyze their relative impact in the overall response. The aim of this study was thus to use a myocyte-based multiscale model of the circulatory system to simulate the effects of halothane, a volatile anesthetic which at high doses elicits significant acute CD both in isolated myocytes and intact animals. Ventricular chambers built using a human myocyte model were incorporated into a whole circulatory system represented by resistances and capacitances. Halothane-induced decreased sarco(endo)plasmic reticulum Ca²⁺ (SERCA2a) reuptake pump, transient outward K⁺ (I_to), Na⁺-Ca²⁺ exchanger (I_NCX) and L-type Ca²⁺ channel (I_CaL) currents, together with ryanodine receptor (RyR2) increased open probability (P_o) and reduced myofilament Ca²⁺ sensitivity, reproduced equivalent decreased action potential duration at 90% repolarization and intracellular Ca²⁺ concentration at the myocyte level reported in the literature. In the whole circulatory system, model reduction in mean arterial pressure, cardiac output and regional wall thickening fraction was similar to experimental results in open-chest sheep subjected to acute halothane overdose. Effective model performance indicates that the model structure could be used to study other changes in myocyte targets eliciting CD.

Sevoflurane Protects Ventricular Myocytes against Oxidative Stress-induced Cellular Ca2+ Overload and Hypercontracture

Background:

Oxidative stress is implicated in pathogenesis of cardiac reperfusion injury, characterized by cellular Ca2+ overload and hypercontracture. Volatile anesthetics protect the heart against reperfusion injury primarily by attenuating Ca2+ overload. This study investigated electrophysiological mechanisms underlying cardioprotective effects of sevoflurane against oxidative stress-induced cellular injury.

Methods:

The cytosolic Ca2+ levels and cell morphology were assessed in mouse ventricular myocytes, using confocal fluo-3 fluorescence imaging, whereas membrane potentials and L-type Ca2+ current (ICa,L) were recorded using whole-cell patch-clamp techniques. Phosphorylation of Ca2+/calmodulin-dependent protein kinase II was examined by Western blotting.

Results:

Exposure to H2O2 (100 μm) for 15 min evoked cytosolic Ca2+ elevation and hypercontracture in 56.8% of ventricular myocytes in 11 experiments, which was partly but significantly reduced by nifedipine, tetracaine, or SEA0400. Sevoflurane prevented H2O2-induced cellular Ca2+ overload in a concentration-dependent way (IC50 = 1.35%). Isoflurane (2%) and desflurane (10%) also protected ventricular myocytes by a degree similar to sevoflurane (3%). Sevoflurane suppressed H2O2-induced electrophysiological disturbances, including early afterdepolarizations, voltage fluctuations in resting potential, and abnormal automaticities. H2O2 significantly enhanced ICa,L by activating Ca2+/calmodulin-dependent protein kinase II, and subsequent addition of sevoflurane, isoflurane, or desflurane similarly reduced ICa,L to below baseline levels. Phosphorylated Ca2+/calmodulin-dependent protein kinase II increased after 10-min incubation with H2O2, which was significantly prevented by concomitant administration of sevoflurane.

Conclusions:

Sevoflurane protected ventricular myocytes against H2O2-induced Ca2+ overload and hypercontracture, presumably by affecting multiple Ca2+ transport pathways, including ICa,L, Na+/Ca2+ exchanger and ryanodine receptor. These actions appear to mediate cardioprotection against reperfusion injury associated with oxidative stress.

Modulation of TRPC5 cation channels by halothane, chloroform and propofol

BACKGROUND AND PURPOSE

TRPC5 is a mammalian homologue of the Drosophila Transient Receptor Potential (TRP) channel and has expression and functions in the cardiovascular and nervous systems. It forms a calcium-permeable cation channel that can be activated by a variety of signals including carbachol (acting at muscarinic receptors), lanthanides (e.g. Gd3+) and phospholipids (e.g. lysophosphatidylcholine: LPC). Here we report the effects of inhalational (halothane and chloroform) and intravenous (propofol) general anaesthetics upon TRPC5.

EXPERIMENTAL APPROACH

Human TRPC5 channels were expressed in HEK 293 cells and studied using fura-2 and patch-clamp recording to measure intracellular calcium and membrane currents respectively at room temperature. Human TRPM2 channels were studied for comparison.

KEY RESULTS

TRPC5 activation by carbachol, Gd3+ or LPC was inhibited by halothane and chloroform at > or =0.1 and 0.2 mM respectively. Neither agent inhibited TRPM2. Propofol had an initial stimulatory effect on TRPC5 (evident in patch-clamp recordings only) and an inhibitory effect at > or =10 microM. TRPM2 was not affected by propofol. Propofol inhibited activation of TRPC5 by Gd3+ but not LPC, suggesting the effect was not directly on the channel. Propofol's anti-oxidant property was not necessary for its inhibitory effect because di-isopropyl benzene, a propofol analogue that lacks the hydroxyl group, also inhibited TRPC5.

CONCLUSIONS AND IMPLICATIONS

The data show the sensitivity of TRPC5 channel to general anaesthetics and suggest that some of the effects could have clinical relevance. The effects may be explained in part by the sensitivity of the channel to biophysical properties of the lipid bilayer.

Kinetics of anesthetic-induced conformational transitions in a four-alpha-helix bundle protein

Inhaled anesthetics are thought to alter the conformational states of Cys-loop ligand-gated ion channels (LGICs) by binding within discrete cavities that are lined by portions of four alpha-helical transmembrane domains. Because Cys-loop LGICs are complex molecules that are notoriously difficult to express and purify, scaled-down models have been used to better understand the basic molecular mechanisms of anesthetic action. In this study, stopped-flow fluorescence spectroscopy was used to define the kinetics with which inhaled anesthetics interact with (Aalpha(2)-L1M/L38M)(2), a four-alpha-helix bundle protein that was designed to model anesthetic binding sites on Cys-loop LGICs. Stopped-flow fluorescence traces obtained upon mixing (Aalpha(2)-L1M/L38M)(2) with halothane revealed immediate, fast, and slow components of quenching. The immediate component, which occurred within the mixing time of the spectrofluorimeter, was attributed to direct quenching of tryptophan fluorescence upon halothane binding to (Aalpha(2)-L1M/L38M)(2). This was followed by a biexponential fluorescence decay containing fast and slow components, reflecting anesthetic-induced conformational transitions. Fluorescence traces obtained in studies using sevoflurane, isoflurane, and desflurane, which poorly quench tryptophan fluorescence, did not contain the immediate component. However, these anesthetics did produce the fast and slow components, indicating that they also alter the conformation of (Aalpha(2)-L1M/L38M)(2). Cyclopropane, an anesthetic that acts with unusually low potency on Cys-loop LGICs, acted with low apparent potency on (Aalpha(2)-L1M/L38M)(2). These results suggest that four-alpha-helix bundle proteins may be useful models of in vivo sites of action that allow the use of a wide range of techniques to better understand how anesthetic binding leads to changes in protein structure and function.