ISOVOLUMETRIC REGULATION IN A DISTAL NEPHRON CELL-LINE (A6)

The effects of gradually reducing the osmolality of the basolateral so lution (pi(b)) were examined in a renal epithelial cell line (A6). pi( b) was linearly decreased with time from 260 to 140 mosmol/kgH(2)O. Ce ll volume did not change when pi(b) was gradually decreased at dilutio n rates (D-r) of 1-1.5 mosmol . kgH(2)O(-1) . min(-1). Increasing D-r to 3 or 6 mosmol . kgH(2)O(-1) . min(-1) abolished this isovolumetric regulation (IVR). Replacing Cl- by NO3- or SCN- inhibited IVR markedly , whereas Br- substituted perfectly for Cl-. On the other hand, with a ll these anions, the regulatory volume decrease (RVD) was completely d eveloped. Ba2+ (30 mM) markedly slowed down RVD but improved IVR. The discrepancies between RVD and NR suggest that different mechanisms are used to control cell volume during gradual and shockwise hyposmotic p erturbations. During gradual and shockwise reductions of pi(b), cellul ar K+ content was reduced to the same extent; Rb-86 efflux was only pa rtially inhibited by Ba2+. The amount of intracellular K+ depletion co uld account for 70% of the cationic osmolyte loss, which suggests that K+ is the major cation excreted during both types of perturbations.