Role of Na+ conductance, Na(+)-H+ exchange, and Na(+)-K(+)-2Cl- symport in the regulatory volume increase of rat hepatocytes.

1. In rat hepatocytes under hypertonic stress, the entry of Na+ (which is thereafter exchanged for K+ via Na(+)-K(+)-ATPase) plays the key role in regulatory volume increase (RVI). 2. In the present study, the contributions of Na+ conductance, Na(+)-H+ exchange and Na(+)-K(+)-2Cl- symport to this process were quantified in confluent primary cultures by means of intracellular microelectrodes and cable analysis, microfluorometric determinations of cell pH and buffer capacity, and measurements of frusemide (furosemide)/bumetanide-sensitive 86Rb+ uptake, respectively. Osmolarity was increased from 300 to 400 mosmol l-1 by addition of sucrose. 3. The experiments indicate a relative contribution of approximately 4:1:1 to hypertonicity-induced Na+ entry for the above-mentioned transporters and the overall Na+ yield equalled 51 mmol l-1 (10 min)-1. 4. This Na+ gain is in good agreement with the stimulation of Na+ extrusion via Na(+)-K(+)-ATPase plus the actual increase in cell Na+, namely 55 mmol l-1 (10 min)-1, as we determined on the basis of ouabain-sensitive 86Rb+ uptake and by means of Na(+)-sensitive microelectrodes, respectively. 5. The overall increase in Na+ and K+ activity plus the expected concomitant increase in cell Cl- equalled 68 mmol l-1, which fits well with the increase in osmotic activity expected to occur from an initial cell shrinkage to 87.5% and a RVI to 92.6% of control, namely 53 mosmol l-1. 6. The prominent role of Na+ conductance in the RVI of rat hepatocytes could be confirmed on the basis of the pharmacological profile of this process, which was characterized by means of confocal laser-scanning microscopy.