An electron paramagnetic resonance study of skeletal muscle membrane fluidity in malignant hyperthermia

Skeletal muscle sarcolemma (SL), transverse tubule (TT) and heavy sarcoplasmic reticulum (HSR) membranes were isolated from malignant hyperthermia susceptible (MHS) and normal pigs, and the rotational dynamics of lipid hydrocarbon chain motion was examined by electron paramagnetic resonance (EPR) spectroscopy. The stearic acid spin probe 16-SASL was incorporated into MHS and normal membranes and both the order parameter (S) and effective correlation time (τ_r) of probe motion were calculated from spectra recorded over the temperature range of 2 to 40°C. At any given temperature, TT membranes exhibited significantly greater values for both the S and τ_r of probe motion than did SL, which exhibited significantly greater values than did HSR membranes. The order of decreasing S and τ_r values for 16-SASL mobility correlated with the decreasing cholesterol content of these membranes (TT>SL>HSR), however there was no difference in the S or τ_r values for a given membrane fraction isolated from both MHS and normal muscle. Arrhenius plots of 16-SASL mobility in SL, TT and HSR were linear from 2 to 40°C, indicating no abrupt thermotropic change in the lipid hydrocarbon phase of any of the membrane types studied. Apparent activation energies (E_a), calculated from the Arrhenius plots, were similar for MHS and normal membranes derived from a given cellular location. However, the E_a of probe motion for TT membranes (2.3 ± 0.1 and 2.4 ± 0.1 kcal/mol/degree for MHS and normal, respectively) was significantly less than for SL (3.4 ± 0.4 and 2.9 ± 0.1 kcal/mol/degree for MHS and normal, respectively) which, in turn, was significantly less than the E_a for HSR (3.7 ± 0.1 and 3.7 ± 0.1 kcal/mol/degree for MHS and normal, respectively). Since 16-SASL motion was similar in MHS and normal membranes, we conclude that there is no evidence for a generalized membrane defect affecting lipid mobility in these MHS muscle membranes.