The dependence of electrogenic sodium pump activity on changes in the cell volume of Helix pomatia neurons with different levels of intracellular sodium ion concentration was studied. Hypertonic solutions caused hyperpolarization of the membrane and increased membrane resistance in cells with a low sodium content (low-sodium cells; LSC). The activity of the electrogenic sodium pump in hypertonic solutions was increased compared to the activity in hypotonic solutions in LSC and decreased in cells with a high sodium content (high-sodium cells; HSC). The concentration of ouabain which led to maximal inhibition of active 22Na efflux from the neurons was 10(-4) M. Lower concentrations of ouabain (10(-8) M and lower) did not inhibit the sodium pump but stimulated it. The swelling of neurons in hypotonic solutions was accompanied by an increase in the number of binding sites for ouabain, while shrinking in hypertonic solutions led to the opposite effect--a decrease in binding sites. An increase in the number of binding sites also took place in normal isotonic potassium-free solutions compared with normal Ringer's solution. Two saturable components of ouabain binding were detectable in all solutions examined. gamma-Aminobutyric acid (GABA) and acetylcholine (ACh) increased the number of ouabain binding sites on the membrane. The results suggest that there are two opposite mechanisms by which cell volume changes can modulate the pump activity. One of them depends on the intracellular sodium ion concentration and causes pump activation in hypertonic solutions in LSC and saturation in HSC, while a second mechanism mediates the activating effect of cell swelling on the sodium pump in HSC. In addition, there may be a negative feedback between the pump activity and the number of functioning pump units in the membrane.

The effect of short-chain fatty acids on both ouabain-sensitive and ouabain-insensitive fractions of 22Na efflux from the neurons of Helix pomatia was studied. Fatty acids, having fewer than 10 carbon atoms in the hydrocarbon chain, increased the ouabain-sensitive 22Na efflux from the neurons, while fatty acids, having more than 9 carbon atoms, inhibited the 22Na efflux in comparison with that in normal physiological solution. All the fatty acids used had an inhibiting effect on the ouabain-insensitive 22Na efflux from the cells independent on the number of carbon atoms in the hydrocarbon chain. These studies indicate that these short-chain fatty acids can be effective modulators of both ouabain-sensitive and ouabain-insensitive fractions of Na efflux from the cells.