Extracellular ATP is a potent signaling factor that modulates a variety of cellular functions through the activation of P 2 purinergic receptors in the plasma membrane. These receptors are widely distributed among different liver cell types, including hepatocytes, cholangiocytes, macrophages, and endothelial cells, but the physiologic roles have not been fully defined. Cells release ATP in response to both osmotic and mechanical stimuli, and one mechanism may involve opening of a channel-like pathway (1, 2). In respiratory epithelia, ATP release stimulated by cytosolic cAMP activates outwardly rectified Cl Ϫ channels coupled to P 2U receptors and enhances Cl Ϫ secretion (3).Recent studies in a model liver cell line support an alternative pathway where increases in cell volume induce conductive ATP efflux. In these cells, removal of extracellular ATP or P 2 receptor blockade prevents both Cl Ϫ channel activation and volume recovery (1). These findings suggest functional interactions between ATP release, P 2 receptor stimulation, and Cl Ϫ channel opening in epithelial secretion and volume regulation.Members of the ATP-binding cassette (ABC) 1 protein family are likely to be relevant to this volume regulatory pathway for two reasons. First, while the molecular basis for the transmembrane ATP conductance has not been established, heterologous expression or up-regulation of ABC family members in some cell models is associated with enhanced electrodiffusional ATP release. In cystic fibrosis respiratory epithelia, cAMP fails to stimulate channel-mediated ATP efflux, a response that is present in native epithelia; CFTR gene transfer restores the ATP conductance (3, 4). In other cell lines, ATP release is proportional to the expression of mammalian and Drosophila Mdr1 P-glycoproteins (5, 6). Second, in some but not all cell types, Mdr1 P-glycoproteins regulate swelling-activated Cl Ϫ currents (I Cl-swell ). Effects include enhancement of I Cl-swell and endowment of Cl Ϫ channel sensitivity to protein kinase C (mdr1 gene transfer) and increase in I Cl-swell for a given hypotonic stress (P-glycoprotein overexpression) (7,8). The cellular mechanisms involved in these responses and the implications for other cell types have yet to be clarified.In hepatocytes, P-glycoproteins transport both amphipathic compounds and phospholipids across canalicular membranes into bile (9, 10). However, the functions of multiple other ABC members present in liver cells are unknown. In light of the putative association of certain ABC proteins with channelmediated ATP and Cl Ϫ transport, we sought to investigate the role of hepatocellular ABC proteins in these processes. Findings in rat HTC hepatoma cells were compared with those in a selected population of HTC cells (HTC-R) that overexpress both endogenous and novel Mdr proteins (11). These studies demonstrate that inhibition of P-glycoprotein transport prevents recovery from swelling and that overexpression of Mdr proteins is associated with enhanced ATP release, volume recovery, and cell surv...