No abstract

Interdomain interactions between N-terminal and central domains serving as a "domain switch" are believed to be essential to the functional regulation of the skeletal muscle ryanodine receptor-1 Ca 2؉ channel. Mutational destabilization of the domain switch in malignant hyperthermia (MH), a genetic sensitivity to volatile anesthetics, causes functional instability of the channel. Dantrolene, a drug used to treat MH, binds to a region within this proposed domain switch. To explore its mechanism of action, the effect of dantrolene on MH-like channel activation by the synthetic domain peptide DP4 or anti-DP4 antibody was examined. A fluorescence probe, methylcoumarin acetate, was covalently attached to the domain switch using DP4 as a delivery vehicle. The magnitude of domain unzipping was determined from the accessibility of methylcoumarin acetate to a macromolecular fluorescence quencher. The SternVolmer quenching constant (K Q ) increased with the addition of DP4 or anti-DP4 antibody. This increase was reversed by dantrolene at both 37 and 22°C and was unaffected by calmodulin. [ 3 H]Ryanodine binding to the sarcoplasmic reticulum and activation of sarcoplasmic reticulum Ca 2؉ release, both measures of channel activation, were enhanced by DP4. These activities were inhibited by dantrolene at 37°C, yet required the presence of calmodulin at 22°C. These results suggest that the mechanism of action of dantrolene involves stabilization of domain-domain interactions within the domain switch, preventing domain unzipping-induced channel dysfunction. We suggest that temperature and calmodulin primarily affect the coupling between the domain switch and the downstream mechanism of regulation of Ca 2؉ channel opening rather than the domain switch itself.Dantrolene (hydrated 1-(((5-(4-nitrophenyl)-2-furanyl)methylene)amino)-2,4-imidazolidinedione sodium salt) is an intracellularly acting skeletal muscle relaxant used for the treatment of malignant hyperthermia (MH). 1 MH is a potentially deadly, pharmacogenetically mediated, hypermetabolic response to volatile anesthetics that results from unregulated intramyoplasmic Ca 2ϩ release (1). The drug is known to inhibit excitation-contraction coupling of skeletal muscle (2) by suppressing depolarization-induced sarcoplasmic reticulum (SR) Ca 2ϩ release in normal and MH-susceptible skeletal muscle without affecting voltage sensor activation (3). In MH, the voltage dependence of contractile activation is shifted to lower voltages (4), whereas in the presence of clinical concentrations of dantrolene, i.e. 10 M (5), the voltage dependence of contractile activation is shifted to higher voltages (6, 7). Normalization of the voltage dependence of contractile activation may therefore be one of the important components of the therapeutic action of dantrolene. Dantrolene also confers a normal Mg 2ϩ sensitivity to MH-susceptible muscle fibers, which would otherwise show a considerably reduced sensitivity to the normal inhibitory action of myoplasmic Mg 2ϩ on the SR Ca 2ϩ release mechanism (...

No abstract

Measurements were made of the average energy 8' per ion pair formed in liquid xenon by internal-conversion electrons from~Bi. We observed voltage pulses resulting from electron collection either in liquid xenon or in a gaseous mixture of argon (95%I and methane (5%). The relative pulse heights for the two material. s determine the ratio of the W values. Using the known W for the gaseous mixture, we obtained a liquid-xenon 8' of 15.6+ 0.3 eV. This value is considerably smaller than the gas-phase values, 21.5 or 21.9 eV. For interpretation, we adapted Platzman's energy-balance equation to liquids, assuming a conduction-band picture. Theoretical values thus calculated agree well with experiment.

No abstract