| Abstract: | A method has been developed for the isolation of sealed plasma membrane vesicles from rabbit white skeletal muscle. The final preparation was highly purified as indicated by enrichment of plasma membrane marker enzymes (i.e. ouabain-sensitive (Na+,K+)-ATPase, adenylate cyclase, and acetylcholinesterase). The absence of sarcoplasmic reticulum and mitochondria as contaminants was indicated by the low specific activity of marker enzymes, i.e. Ca2+-ATPase, succinate-cytochrome c reductase, and monoamine oxidase. Thin section and negative staining electron microscopy confirmed the absence of sarcoplasmic reticulum and mitochondrial contamination. The plasma membrane preparation consisted largely of sealed vesicles as observed by electron microscopy and as also demonstrated by latency of enzymic activities, which were unmasked by preincubation with detergent (sodium dodecyl sulfate). Membrane sidedness was estimated from latency of ouabain-sensitive (Na+,K+)-ATPase activity and acetylcholinesterase activity. The latency studies suggest that most of the vesicles are oriented inside out with respect to the orientation of the sarcolemma membrane in the muscle fiber. The inside-out plasma membrane vesicles actively accumulated sodium ions upon addition of ATP. The sodium ions were concentrated greater than 8-fold inside the vesicles and were released upon addition of the ionophore monensin. The sodium ions were taken up in the presence of K+ or NH4+ but not of choline. Uptake was inhibited by low concentrations of vanadate or digitoxin. The Na+ uptake was concomitant with Rb+ efflux. Therefore, the sodium ion transport and the resulting gradients formed appear to have been generated by the ouabain-sensitive (Na+,K+)-ATPase. Batrachotoxin, which opens Na+ channels in excitable tissues, prevents most of the Na+ uptake, suggesting the presence of toxin-activated Na+ channels in these plasma membrane vesicles. |
