Studies on Insulin Action. 11. the Extracellular Space of Frog Muscle: Demonstration of D-mannitol and Sucrose Entry into Isolated Single Muscle Fibers and Intact Muscles

The distribution of labeled D-mamitol, sucrose, and other widely used extracellular space probe materials was studied in four types of muscles from leopard frogs and bullfrogs as well as in connective tissues and isolated single and multiple muscle fiber preparations from bullfrog semitendinosus muscle. In leopard frog muscle the extracellular space complex (consisting of the extracellular space proper along with connective tissues, small blood vessels, nerves, the sarcolemma and the transverse tubular system), accomrnodates D-mannitol and sucrose to a total concentration of about 9% of that of the external solution. Both D-mannitol and sucrose enter into muscle cells; an equilibrium concentration in the cell water of from 12.5% to as high as 34% of that in the external solution was observed.