Hepatocellular ATP-binding cassette protein expression enhances ATP release and autocrine regulation of cell volume

In a model liver cell line, recovery from swelling is mediated by a sensitive autocrine pathway involving conductive release of ATP, P₂ receptor stimulation, and opening of membrane Cl^- channels (Wang, Y., Roman, R. M., Lidofsky, S. D., and Fitz, J. G. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 12020-12025). However, the mechanisms coupling changes in cell volume to ATP release are not known. Based on evidence that certain ATPbinding cassette (ABC) proteins may function as ATP channels or channel regulators, we evaluated the potential role of ABC proteins by comparing ATP release and volume regulation in rat HTC and HTC-R hepatoma cells, the latter of which overexpress Mdr proteins. In both cell types, Cl^- current activation (I(Cl- swell)) and volume recovery following swelling were dependent on conductive ATP efflux. The rate of volume recovery was ~6-fold faster in HTC-R cells compared with HTC cells. This effect is likely due to enhanced ABC protein- dependent ATP release since (i) I(Cl-swell) and cell volume recovery were eliminated by inhibition of P-glycoprotein transport (20 μM verapamil and 15 μM cyclosporin A); (ii) swelling-induced Cl^- current density was similar in both cell types (approximately -50 pA/pF; not significant); and (iii) ATP conductance measured by whole-cell techniques was increased ~3-fold in HTC- R cells compared with HTC cells. Moreover, HTC-R cells exhibited enhanced survival during hypotonic stress. By modulating ATP release, hepatic ABC proteins may play a key role in the cellular pathways coupling changes in cell volume to ion permeability and secretion.