SPECIFIC INTRACELLULAR BINDING OF RUBIDIUM BY RAT DIAPHRAGM MUSCLE

Effect of inhibitors on D-xylose permeability in rat diaphragm muscle

The influence of metabolic inhibitors and low temperatures upon D-xylose transfer has been studied in rat diaphragm muscle preparations in vitro. Using intact fiber preparations, it has been confirmed that at body temperature metabolic inhibitors like DNP have an insulin-like action in that they permit D-xylose to distribute into previously unavailable intracellular aqueous regions; inhibitors, unlike insulin, disturb cation distribution in association with increased sugar penetration. Although the studies with inhibitors suggest an energy requirement for maintenance of D-xylose exclusion, the D-xylose exclusion mechanism is effectively maintained at 0° for many hours, and under these conditions, inhibitors have little or no effect on D-xylose distribution, though they do produce potassium loss. In cut muscle fiber preparations, in which insulin significantly increases the rate at which D-xylose equilibrates between cell water and external medium, DNP does not increase the rate of D-xylose entry, but does abolish the effect of insulin in this preparation. The results suggest that insulin action upon sugar permeability in muscle involves two barrier systems; some of the characteristics of these systems have been defined.

Incorporation of leucine into human skeletal muscle proteins

The in vitro incorporation of labelled leucine into human skeletal muscle proteins was studied with the aim to elucidate the relationship between the amino acid tissue pools and protein biosynthesis. The distribution volumes of leucine and cycloleucine in skeletal muscle tissue were similar but the equilibration time was shorter for leucine than for cycloleucine. The cellular uptake of leucine and cycloleucine was competitively inhibited by increased concentration of amino acids in the medium indicating an active transport. Optimal stimulation for incorporation of leucine into proteins was obtained at an amino acid concentration in the medium corresponding to 10 times that of normal human plasma. The incorporation of 14C-leucine into skeletal muscle proteins was linear before the total pool of free intracellular 14C-leucine and the incorporation rate of leucine calculated from the specific activity in the medium versus the amino acid concentration in the medium were different in the same experiment indicating a re-utilization of amino acids released at protein degradation. The results are compatible with the hypothesis that the proteolytically released amino acids have a competitive advantage for incorporation as compared with extra- and intracellular free amino acids. It is concluded that the amino acid pool which is in the immediate continuity with the protein biosynthesis sites equilibrates rapidly with the extracellular amino acid pool.