Influence of Repeated Contractions of Muscle on its Lipid Content.

Whereas muscular activity is accompanied by striking changes in carbohydrate content, most attempts to demonstrate changes in the lipid content of muscle have failed. Leathes 1 found no difference in the amount of fat in corresponding muscles of the 2 legs of frogs, after one leg had been stimulated for several hours by a teta-nizing current. Winfield 2 obtained equally negative results on excised frog muscle. Since the respiratory quotient of isolated, glycogen-rich amphibian muscle is generally close to unity, indicating a predominant use of carbohydrate, there was the possibility that utilization of fat occurred only in a muscle with low glycogen content. In support of this idea may be cited the observation of Niernierko 3 that stimulation of the isolated gastrocnemius of frogs in winter had no effect on fat content, while in spring, when the glycogen content is known to be low, stimulation resulted in a decrease in fat content of 12 to 31%.

After a fasting period of 24 hours close to 90% of the heat production of rats is derived from fat oxidation, as indicated by a non-protein R. Q. of 0.71. Obviously the muscles of such animals are using chiefly fat (either directly or indirectly) and it therefore seemed worth while to investigate whether the marked increase in oxygen consumption brought about by activity, would result in a demonstrable change in lipid content.

The gastrocnemius of the rat (weighing 1 to 1.5 gm.) was thoroughly ground with sand, transferred to a 100 cc. volumetric flask and boiled for 15 minutes with 75 cc. of a 3 to 1 alcohol-ether mixture on a water bath. After making up to the mark, aliquot portions of the filtered extract were analyzed for free and total cholesterol, using Okey's 4 digitonide method and for phospholipids and total fatty acids, following Bloor's 5 procedure.