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Journal of Experimental Biology 32,664-680 (1955)
Published by Company of Biologists 1955

Ionic Regulation in the Muscle Fibres of Carcinus Maenas : II. The Effect of Reduced Blood Concentration

J. SHAW ¹

¹ Department of Zoology, King's College, University of Durham Newcastle upon Tyne

1. The electrolyte composition of the carpopodite extensor and flexor muscles of the chelae of Carcinus maenas living in diluted sea waters has been studied.

2. The increase in water content of the muscles of crabs from 20% sea water corresponds to a maximum degree of swelling of 25% of the initial weight.

3. The resting membrane potential remains constant at 59 mV. over the whole range of blood concentrations studied.

4. Chloride is lost from the fibres and its concentration in the muscle is always about one-tenth of that in the blood.

5. Sodium ions are actively secreted from the muscles, and a steady state is reached when the concentration in the muscle is also about one-tenth that in the blood. The sodium and chloride concentrations in the muscle are approximately the same.

6. The potassium concentration of the muscle falls as the blood is diluted, but the ratio of muscle potassium to blood potassium increases from 10 to about 20. Potassium appears to be actively retained in the muscle.

7. The fall in concentration of muscle calcium and magnesium can be explained in terms of the swelling of the fibres alone and the fibre membrane is probably not permeable to these substances.

8. The fibre is also probably impermeable to the larger part of the anion fraction. It is only sodium and potassium ions which are actively regulated and the result of these processes is that the muscle sodium balances the muscle chloride and the muscle potassium balances the impermeable anion fraction. It is suggested that on the extent to which the muscle cell can withstand blood dilution depends the degree to which it can resist swelling.