Further Studies on Ionic Regulation in the Muscle Fibres of Carcinus Maenas

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Abstract
1. The muscle fibres of Carcinus maenas contain 91 mM./kg. fibre water of acid soluble phosphate compounds, of which arginine phosphate is the most abundant. These compounds behave passively under conditions of blood dilution and the phosphate radicals do not exchange freely with inorganic phosphate in the blood. 2. Muscle potassium exchanges freely and completely with the blood potassium. The exchange constant is about 0·4 hr.-1. In the normal fibre potassium ions are approximately in equilibrium with those in the blood. Potassium is lost from the muscle when the blood potassium concentration is reduced, but both the muscle-to-blood potassium ratio and the membrane potential are increased. The behaviour of potassium appears to be largely passive but there may be some small active component in its retention. 3. Muscle sodium exchanges very rapidly and completely with blood sodium. The exchange is complete in 10 min. Sodium is also lost rapidly from the muscle to Ringer solutions with a reduced sodium concentration. It is suggested that the bulk of the muscle sodium is situated in a region distinct from the true fibre interior and that sodium ions are rigidly excluded from the latter. 4. These results are discussed in relation to the adaptation of the muscle fibre to reduced blood concentrations.