WATER TRANSPORT IN INVERTEBRATE PERIPHERAL NERVE FIBERS

Osmotic and diffusion permeabilities (P(f) and P(d)) of invertebrate nerve fibers to tritiated water were measured to determine what water flux studies could reveal about "the nerve membrane" and to directly test the possibility of active transport of water into or out of invertebrate nerve fibers. P(f)/P(d) ratios for lobster walking leg nerve fibers were found to be about 20 ± 7 at 14°C. P(d) measurements were made for squid giant axons at 25°C. and found to yield a value of 4 x 10(–4) cm.(–1) sec.(–1). When combined with the data of D. K. Hill for P(f), a P(f)/P(d) ratio of 21 ± 5 is obtained. These P(f)/P(d) ratios correspond to "effective pore radii" of about 16 ± 4 angstrom units, according to theories developed by Koefoed-Johnsen and Ussing and independently by Pappenheimer and his colleagues. Variations of water flux ratios with temperatures were studied and apparent activation energies calculated for both diffusion experiments and osmotic filtration experiments using the Arrhenius equation, and found to be close to 3 to 5 cal. per mole of water transferred. Cyanide (5 x 10(–3) molar) and iodoacetate (1 x 10(–3) molar) poisoned lobster leg nerve fibers showed no appreciable change in diffusion or osmotic filtration water effluxes. Caution in interpreting these proposed channels as simple pores was emphasized, but the possibility that such channels exist and are related to ionic flow is not incompatible with electrophysiological data.