ROLE OF NA-H+ EXCHANGE, AND NA+-K+-2CL(-) SYMPORT IN THE REGULATORY VOLUME INCREASE OF RAT HEPATOCYTES( CONDUCTANCE, NA+)

1. In rat hepatocytes under hypertonic stress, the entry of Naf (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 contr ibutions of Na+ conductance, Na+ - H+ exchange and Na+-K+-2Cl(-) sympo rt to this process were quantified in confluent primary cultures by me ans of intracellular microelectrodes and cable analysis, microfluorome tric determinations of cell pH and buffer capacity, and measurements o f frusemide (furosemide)/bumetanide-sensitive Rb-86(+) uptake, respect ively. Osmolarity was increased from 300 to 400 mosmol l(-1) by additi on 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 stimula tion of Na+ extrusion via Na+-K+-ATPase plus the actual increase in ce ll Na+, namely 55 mmol l(-1) (10 min)(-1), as was determined on the ba sis of ouabain-sensitive Rb-86(+): uptake and by means of Na+-sensitiv e 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 e xpected 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 Naconductance in the RVI of rat hepatocytes could be confirmed on the ba sis of the pharmacological profile of this process, which was characte rized by means of confocal laser-scanning microscopy.