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Journal of Experimental Biology 72,127-140 (1978)
Published by Company of Biologists 1978

Transmembrane Activity Gradients of K and cl in the Muscle Fibres of Osmoconforming Marine Decapod Crustaceans

ROBERT W. FREEL ¹

¹ Department of Biology, University of California, Los Angeles, California 90024

1. The resting membrane potentials (E_m) and the transmembrane activity gradients for K and Cl were measured in the muscle fibres of osmoconforming (Callianassa and Cancer) and weakly osmoregulating (Pachygrapsus) marine crustaceans acclimated to various osmotic conditions.

2. The muscle membranes of sea water acclimated crabs behave as good K electrodes. However, a slight contribution of Na to the resting potential was demonstrated in all species. The ratio P_Na/P_K was about 0.01. Equilibrium potentials (measured with ion-selective microelectrodes) for Cl were equal to E_m, while E_K was always more negative than E_m as a result of the slight Na contribution.

3. Acclimation to dilute or concentrated sea water had little effect on the K electrode properties or Na permeabilities of these fibres. The muscle fibres were depolarized in crabs acclimated to concentrated sea water and were hyperpolarized in crabs adapted to dilute sea water. These changes resulted solely from alterations in (a_K)_i/ (a_K)_O which were in turn brought about by changes in cellular and haemolymph hydration.

4. Since the Na contribution to E_m was so small in all conditions, it was concluded that the distributions of K and Cl are best considered in terms of Donnan equilibria, and that the cellular K and Cl adjustments observed during salinity adaptation reflect the passive re-establishment of new equilibrium distributions for these ions.