Phosphatidylinositol 3-kinase contributes to cell volume regulation through effects on ATP release

Regulated changes in cell volume represent a signal that modulates a broad range of cell and organ functions. In HTC hepatoma cells, increases in volume are coupled to membrane ion permeability through a pathway involving (i) ATP efflux, (ii) autocrine stimulation of P2 receptors, and (iii) increases in anion permeability and Cl- efflux, contributing to recovery of volume toward basal values. Based on recent evidence that cell volume increases also stimulate phosphoinositide kinases, the purpose of these studies was to determine if phosphatidylinositol 3-kinase (PI 3-kinase) modulates these pathways. Exposure of cells to hypotonic buffer (20 or 40% less NaCl) caused an initial increase in cell volume and stimulated a rapid increase in ATP release. Subsequent opening of Cl- channels was followed by recovery of cell volume toward basal values, despite the continuous presence of hypotonic buffer. Inhibition of PI 3-kinase with wortmannin (Ki = 3 nM) significantly inhibited both the rate of volume recovery and activation of Cl- currents; similar results were obtained with LY294002 (10 microM). Additionally, current activation was inhibited by intracellular dialysis with antibodies specific for the 110-kDa catalytic subunit of PI 3-kinase. Since release of ATP is a critical element in the volume-regulatory pathway, the role of PI 3-kinase on volume-stimulated ATP release was assessed. Both wortmannin and LY294002 decreased basal and volume-stimulated ATP permeability but had no effect on the current response to exogenous ATP (10 microM). These findings indicate that PI 3-kinase plays a significant role in regulation of cell volume and suggest that the effects are mediated in part through modulation of cellular ATP release.