3,5-Di-t-butyl catechol is a potent human ryanodine receptor 1 activator, not suitable for the diagnosis of malignant hyperthermia susceptibility

3,5-Di-t-butyl catechol (DTCAT) releases Ca(2+) from rat skeletal muscle sarcoplasmic reticulum (SR) vesicles. Hence, it is a candidate for use as a substitute for halothane or caffeine in the in vitro contracture test for the diagnosis of susceptibility to malignant hyperthermia (MH). To characterize the effect of DTCAT at cell level, Ca(2+) release experiments were performed on cultured, human skeletal muscle myotubes using the fluorescent Ca(2+) indicator fura2-AM. DTCAT was also assayed in the in vitro contracture test on human skeletal muscle bundles obtained from individuals diagnosed susceptible (MHS), normal (MHN) or equivocal for halothane (MHEH) and compared to the standard test substances caffeine and halothane. DTCAT increased, in a concentration-dependent manner and with a higher efficacy as compared to caffeine, the free, intracellular Ca(2+) levels of cultured MHN and MHS skeletal muscle myotubes. This effect was similar in both types of myotubes and involved the release of Ca(2+) from SR stores as well as Ca(2+)-influx from the extracellular space. Inhibition of ryanodine receptors either with ryanodine or with ruthenium red markedly reduced DTCAT-induced increase in intracellular Ca(2+) concentration while abolishing that induced by caffeine. In MHN skeletal muscle bundles, DTCAT induced contractures with an EC(50) value of 160 ± 91 μM. However, the sensitivity of MHS or MHEH muscles to DTCAT was similar to that of MHN muscles. In conclusion, DTCAT is not suitable for the diagnosis of MH susceptibility due to its failure to discriminate between MHN and MHS muscles.

Lost in translation: regulation of skeletal muscle protein synthesis

Skeletal muscle accounts for about 50% of the body's mass in higher vertebrates. Besides its obvious role in motor activity, it also can serve as a reservoir for amino acids during times of starvation, or even as a metabolic water supply for migratory birds' during long flights. An imbalance between anabolic and catabolic processes can lead to the loss of muscle mass and life-threatening cachexia or sarcopenia. This review summarizes the current state of knowledge about the regulation of protein translation in skeletal muscle; it also discusses the role of the mTOR pathway, as well as new findings about the influence of specific miRNAs on protein expression in skeletal muscle.

4-Chloro-m-Cresol Cannot Detect Malignant Hyperthermia Equivocal Cells in an Alternative Minimally Invasive Diagnostic Test of Malignant Hyperthermia Susceptibility

Malignant hyperthermia (MH) is an inherited skeletal muscle disorder triggered by commonly used anesthetics. Mutated ryanodine receptors have been identified as molecular targets. The sensitivity of myotubes from individuals classified by the in vitro contracture test as MH susceptible (MHS), normal (MHN), and equivocal (MHEH) was assessed for the Ca2+-releasing activity of 4-chloro-m-cresol (4-CmC) and caffeine. In this study, we sought to determine whether 4-CmC can differentiate the MH status of an individual on the basis of the release of intracellular Ca2+, particularly in regard to MHEH diagnosis. Intracellular Ca2+ concentration was determined photometrically with Fura2. Regions of the ryanodine receptor 1 harboring most of the described MH mutations were sequenced from MHS and MHEH cells. One MH mutation (Gly2434Arg) was found in one MHS individual. Results of the caffeine-induced Ca2+ release in MHS and MHN cells correlated well with the in vitro contracture test results. MHS cells showed a higher sensitivity against caffeine and, to a lesser extent, against 4-CmC. Cells of MHEH individuals showed low sensitivities against both caffeine and 4-CmC, comparable to those of the MHN group. Therefore, with myotubes, caffeine was able to discriminate between MHS and MHN cells, but both caffeine and 4-CmC failed to detect MHEH cells.

The sensitivity of G protein-activated K+ channels toward halothane is essentially determined by the C terminus

G protein-activated K(+) channels (GIRKs or Kir3.x) are targets for the volatile anesthetic, halothane. When coexpressed with the m(2) acetylcholine (ACh) receptor in Xenopus oocytes, agonist-activated GIRK1(F137S)- and GIRK2-mediated currents are inhibited by halothane, whereas in the absence of ACh, high concentrations of halothane induce GIRK1(F137S)-mediated currents. To elucidate the molecular mechanism of halothane action on GIRK currents of different subunit compositions, we constructed deletion mutants of GIRK1(F137S) (GIRK1(Delta363*)) and GIRK2 (GIRK2(Delta356)) lacking the C-terminal ends, as well as chimeric GIRK channels. Mutated GIRK channels showed normal currents when activated by ACh but exhibited different pharmacological properties toward halothane. GIRK2(Delta356) showed no sensitivity against the inhibitory action of halothane but was activated by halothane in the absence of an agonist. GIRK1(Delta363*) was activated by halothane more efficiently. Currents mediated by chimeric channels were inhibited by anesthetic concentrations that were at least 30-fold lower than those necessary to decrease GIRK2 wild type currents. Glutathione S-transferase pulldown experiments did not show displacement of bound Gbetagamma by halothane, indicating that halothane does not interfere with Gbetagamma binding. Single channel experiments revealed an influence of halothane on the gating of the channels: The agonist-induced currents of GIRK1 and GIRK2, carried mainly by brief openings, were inhibited, whereas higher concentrations of the anesthetic promoted long openings of GIRK1 channels. Because the C terminus is crucial for these effects, an interaction of halothane with the channel seems to be involved in the mechanism of current modulation.

G protein-gated inwardly rectifying potassium channels are targets for volatile anesthetics

G protein-gated inwardly rectifying potassium channels (GIRKs) are a family of homo- and hetero-oligomeric K(+) channels composed of different subunits (GIRK1 to 4 in mammals). GIRK4 and GIRK1 are found mainly in the atrium, whereas neuronal cells predominantly express the GIRK1, GIRK2, and GIRK3 isoforms. When activated, GIRK channels slow the firing rate of atrial myocytes and neuronal cells. Because of their key role in controlling excitability, we investigated the influence of a prototypic anesthetic, halothane, on GIRK channels of different subunit composition expressed in Xenopus laevis oocytes. Halothane enhanced background currents through hetero-oligomeric GIRK1/GIRK4 and homo-oligomeric GIRK1(F137S) channels but not through homo-oligomeric GIRK2 channels. This activation of basal current did not depend on the presence of coexpressed G protein-coupled receptors but instead required the presence of G(beta/gamma). In contrast to basal GIRK currents, the agonist-induced GIRK current (via coexpressed m2 muscarinic receptors) was inhibited by halothane. For GIRK1/GIRK4 and GIRK1(F137S) channels this inhibition was most pronounced at low concentrations of the anesthetic (0.1-0.3 mM) and occurred also when channels had been activated by guanosine-5'-O-(3-thio)triphosphate. This inhibition, however, was overridden by high concentrations of halothane (0.9 mM) and augmentation of the agonist-induced current was observed. This increase in agonist-induced current was never seen with GIRK2 homo-oligomeric channels. Agonist-induced currents mediated by GIRK2 channels were always inhibited by halothane with an IC(50) value of approximately 60 microM. These data suggest a direct interaction of halothane with GIRK channels.

Dihydropyridine-induced Ca2+ release from ryanodine-sensitive Ca2+ pools in human skeletal muscle cells

Dihydropyridines (DHPs) are widely used antihypertensive drugs and inhibit excitation-contraction (E-C) coupling in vascular smooth muscle and in myocardial cells by antagonizing L-type Ca2+ channels (DHP receptors). However, contradictory reports exist about the interaction of the DHP with the skeletal muscle isoform of the DHP receptor and E-C coupling in skeletal muscle cells. Using the intracellular fluorescent Ca2+ indicator fura-2, an increase in [Ca2+]i was observed after extracellular application of nifedipine to cultured human skeletal muscle cells. The rise in [Ca2+]i was dose dependent with a calculated EC50 of 614 +/- 96 nM nifedipine and a maximum increment in [Ca2+]i of 80 +/- 3.2 nM. Similar values were obtained with nitrendipine. This effect of DHPs was restricted to differentiated skeletal muscle cells and was not seen in non-differentiated cells or in PC12 cells. In spite of the observed increase in [Ca2+]i, whole-cell patch clamp experiments revealed that 10 microM nifedipine abolished inward Ba2+ currents through L-type Ca2+ channels completely. Similar to nifedipine, (+/-)Bay K 8644, an agonist of the L-type Ca2+ channel, also increased [Ca2+]i. This effect could not be enhanced by further addition of nifedipine, suggesting that both DHPs act via a common signalling pathway. Based on the specific mechanism of the skeletal muscle E-C coupling, we propose the stabilization of a conformational state of the DHP receptor by DHPs, which is sufficient to activate the ryanodine receptor.