1. Analysis of sodium loss from Mytilus cerebro-visceral connectives indicated a slow efflux component attributable to movement across the cellular membranes. This component was significantly slowed at 4 °C, or by treatment with DNP, ouabain or K-free media; this implies that a conventional Na/K exchange pump contributes to cellular efflux.
2. Dose-response curves for the effects of ouabain on cellular Na efflux gave preliminary indications of a significant increase in the sodium pump frequency in connectives from mussels adapted to 25% salinity relative to those from 100% salinity.
3. Assays of ATPase activity in the connectives showed a 63% increment in the Na/K-activated enzyme in 25%-adapted tissues, with no significant change in the magnesium-activated enzyme. The Na/K-ATPase also exhibited a marked shift in its optimal sodium requirement, from 100 mm at 100% salinity to 50 mm at 25% salinity; these figures are close to the respective values of [Na+]i determined for the axons under such conditions.
4. [3H]ouabain binding studies confirmed that there was an increment in active sodium pumping sites (in this case of 76%) in dilute-adapted nerves. Pump frequencies were in the range of 1000-4000 sites/μm2, in agreement with previous studies of invertebrate nerve membranes.
5. The significance of extra pumping capacity for sodium (and potassium) ions in the dilute adapted tissues is considered in relation to the electrophysiological functioning, volume regulation and solute balance of the nerves of an osmoconformer.