Sodium efflux in Myxicola giant axons.

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Abstract
Several properties of the Na pump in giant axons from the marine annelid M. infundibulum were determined in an attempt to characterize this preparation for membrane transport studies. Na0 and K0 [outside concentrations] activated the Na pump of normal microinjected Myxicola axons. In this preparation, the K0 activation was less and the Na0 activation much greater than that in the squid giant axon. When the intracellular ATP:ADP ratio of the Myxicola axon was elevated by injection of an extraneous phosphagen system, the K sensitivity of Na efflux increased to the magnitude characteristic of squid axons and the activating effect of Na0 disappeared. Several axons were injected with Na2SO4 in order to determine the effect of elevated Nai [inside] on the Na efflux. Increasing Nai enhanced a component of Na efflux which was insensitive to ouabain and dependent on [Ca] in Na-free (Li) seawater. After subtracting the Ca0-dependent fraction, Na efflux was related linearly to [Na]i in all solutions except in K-free (Li) seawater, where it appeared to reach saturation at high [Na]i.