Muscle surface membranes: preparative methods affect apparent chemical properties and neurotoxin binding

Area of Science:

• Muscle physiology
• Membrane biochemistry

Background:

• Inconsistent findings exist regarding lipid composition and enzyme activity in purified muscle sarcolemma preparations.
• Previous studies have not adequately explained these discrepancies.

Purpose of the Study:

• To compare two methods for preparing muscle light surface membrane fractions.
• To investigate potential differences in sarcolemma and T-tubular membrane enrichment between fractions.

Main Methods:

• Preparation of muscle light surface membrane fractions using low-salt sucrose and LiBr-KCl extraction methods.
• Analysis of density profiles, lipid and cholesterol content, protein composition, and ATPase activity.
• Quantitation of sodium channels using [3H]saxitoxin binding and acetylcholine receptors using alpha-[125I]bungarotoxin.

Main Results:

• Both fractions predominantly contained surface membrane.
• The LiBr fraction showed higher p-nitrophenylphosphatase activity, free cholesterol, sodium channels, and acetylcholine receptors.
• Differences in protein composition were observed between the two fractions.

Conclusions:

• Light surface membrane fractions may be mixtures of sarcolemma and T-tubular membranes.
• The LiBr fraction is likely enriched in sarcolemma.
• The low-salt sucrose fraction may be enriched in T-tubular elements, explaining prior research variations.