EFFECTS OF EXTERNAL IONS ON MEMBRANE POTENTIALS OF A CRAYFISH GIANT AXON

Transmembrane potentials in the cray~h giant axon have been investigated as a function of the concentration of normally occurring external cations. Results have been compared with data already available for the lobster and squid giant axons. The magnitude of the action potential was shown to be a linear function of the log of the external sodium concentration, as would be predicted for an ideal sodium electrode. The resting potential is an inverse function of the external potassium concentration, but behaves as an ideal potassium electrode only at the higher external concentrations of potassium. Decrease in external calcium results in a decrease in both resting potential and action potential; an increase in external calcium above normal has no effect on magnitude of transmembrane potentials. Magnesium can partially substitute for calcium in the maintenance of normal action potential magnitude, but appears to have very little effect on resting potential. All ionic effects studied are completely reversible. The results are in generally good agreement with data presently available for the lobster giant axon and for the squid giant axon. In a previous paper (Dalton, 1958) intracellularly recorded resting and action potentials from a lobster giant axon were investigated as a function of the external concentration of normally occurring cations (sodium, potassium, calcium, and magnesium). Investigations of the lobster giant axon were undertaken in part to compare the resting and action potentials with data already available for the squid giant axon. A logical next step in a comparative study of transmembrane potentials in giant axons was an investigation of the crayfish giant axon, which makes an interesting comparison with the lobster axon for several reasons. Although the crayfish and lobster are, of course, rather closely related organisms, their habitats are quite different. Thus the opportunity is afforded for comparing axons from related animals * This investigation was supported by Research Grant B-1748 from the National Institute of Neurological Diseases and Blindness, National Institutes of Health, Public Health Service. 971 J. G ~ . P~,lsmI.., 1959, Vol. 42, No. 5 The Journal of General Physiology 972 EP~'ECTS OF EXTERNAl, IONS Olq MEMBRANE POTENTIALS which have quite different blood ion concentrations: the normal blood ion concentration of the American lobster is approximately twice that of the crayfish (van Harreveld, 1936; Cole, 1941). Another important advantage of such a comparative s tudy is concerned with the morphological similarity of the lobster and crayfish. The same giant axon preparation may be obtained from both animals, and experimental techniques found to be suitable for the lobster are adaptable to the crayfish with only minor modifications. These investigations were planned in an a t tempt to compare the responses of transmembrane potentials to variations in the external concentrations of normally occurring cations in the crayfish giant axon with the existing data for squid and lobster giant axons. Materials and Methods Most of the experiments were performed on Orconectes virilis obtained from the vicinity of Cambridge, Massachusetts, and from Oshkosh, Wisconsin. Some additional experiments utilized specimens of Procambarus clarkii supplied by Carolina Biological Supply Company, Elon College, North Carolina. No differences were distinguished in the responses of the axons from these two crayfish. The details of the dissection of the circumesophageal connectives from the ventral nerve cord of the crayfish are virtually identical with those already described for the lobster (Dalton, 1958). The minor differences in the details of dissection are related to size difference: the crayfish is somewhat more difficult to dissect, and it is only possible to obtain about 1 cm. of nerve from a crayfish in contrast to about 3 cm. which can be dissected from an average sized lobster. Each circumesophageal connective contains one large (about 100 microns in diameter) giant axon, and two or three more axons which are about one-half the diameter of the largest giant. Penetration of the largest giant is more easily accomplished and this axon was used for the majority of the studies. However, some experiments were run on some of the smaller axons, but no differences in their responses were noted. Before penetration with a glass capillary microelectrode the nerves were desheathed (except for the region of the nerve which was placed over the external stimulating electrodes). No attempt was made to isolate the giants from the rest of the nerve bundle, since they lie on the surface of the nerve bundle, and are easily accessible to the microelectrode. Experimental techniques were similar to those described for the lobster giant axon. The same nerve chamber was used, and the system for circulating experimental solutions was similar to that already described. In this system about 2 minutes are required for the experimental solution to reach the nerve chamber after switching, and another 2 to 4 minutes for the previous solution to be washed out of the chamber and for an equilibrium to be reached. I t seems reasonable to suppose that the presence of the entire nerve bundle in the chamber contributes to a delay in reaching an equilibrium with a change in the external ion concentration. For changes in external sodium and potassium, with the system and rate of circulation being used (about 2 to 3 cc./minute) a steady-state value was reached in 6 to 8 minutes after switching solutions. In such cases the nerve was immediately returned to the "norreal" or reference solution after a steady-state had been reached. In the case of