CELL-VOLUME CHANGES UPON SODIUM-PUMP INHIBITION IN HELIX-ASPERSA NEURONS

1. Identified neurones of the suboesophageal ganglia of Helix aspersa were loaded with tetramethylammonium (TMA+). Experimentally induced changes in cell water volume and membrane potential were measured continuously by monitoring changes in intracellular [TMA+] using ion-sensitive double-barrelled microelectrodes. The technique allowed measurements of cell water volume changes of less than 5 %. 2. Exposure to hyperosmotic (up to + 24 %) or hyposmotic (up to about - 10 %) solutions caused reversible decreases and increases in cell water volume respectively, which agreed with near-ideal osmometric behaviour. Upon exposure to hyposmotic solutions whose osmolality was decreased by 30-40 %, the cell water volume increased to maximum values below those expected for ideal osmometric behaviour and exhibited partial regulatory volume decrease. 3. The sodium pump was inhibited in twenty identified neurones by sustained exposure to 1 mm ouabain. In every case ouabain caused cell membrane depolarization, as expected for inhibition of an electrogenic sodium pump. 4. Upon pump inhibition most cells (n = 14) shrank by up to 13 % of their initial water volume. In five of these cells, shrinkage was preceded by one or more short-lived swelling phases. In two other neurones short-lived swelling was followed by cell volume recovery without appreciable shrinkage. In four out of the twenty cells, there were no measurable volume changes. 5. The lack of an initial swelling phase in the cells that shrank, as well as the absence of detectable volume changes in some of the neurones, was not due to loss of ion-selective electrode sensitivity since predictable changes in cell volume elicited by osmotic challenges were monitored in the same cells. 6. It is concluded that neurones can be endowed with ouabain-insensitive mechanisms of volume control, whose activation following Na+ pump inhibition prevents them from short-term swelling and lysis.