Lipid composition of transverse tubular membanes from normal and dystrophic skeletal muscle

Abstract

Vesiculated fractions of transverse tubular (TT) membranes were isolated from normal and genetically dystrophic breast muscle of the chicken. The lipid composition of the TT was compared to the lipids of highly purified sarcoplasmic reticulum (SR) membranes and enriched fractions of sarcolemma (SL) membranes. Normal TT membranes contained a significantly higher molar ratio of cholesterol to phospholipid phosphorous (0.86) when compared to both SR (0.17) and SL (0.38) membranes. The phospholipid phosphorous-to-protein ratio was highest for SL (1.55 μmol phosphorous/mg), intermediate for TT (1.09), and lowest for the SR (0.79) membranes. TT membranes contained lower amounts of phosphatidylcholine and higher amounts of sphingomyelin when compared to SR while the TT phospholipid distribution was not significantly different from that of the SL. These data indicate that our putative TT fraction displays a lipid composition distinctly different from that of SR and SL and confirms other biochemical and morphological data which indicate that they are of TT origin. All three membrane classes isolated from dystrophic muscle displayed significant alterations in the composition of various lipids. Cholesterol levels were elevated most notably in SL (2.4-fold) followed by the SR (1.3-fold) but not TT membranes. Dystrophic TT and SL but not SR displayed elevated levels of phospholipid phosphorous. Few differences in the distribution of the individual phospholipid classes were found among the dystrophic membranes except for a 2.2-fold increase in the sphingomyelin content of the SR. The observed lipid abnormalities may be the underlying cause of reported alterations in excitation-contraction coupling and suggest that avian dystrophy may result from a generalized membrane defect.