Autocrine signaling through ATP release represents a novel mechanism for cell volume regulation.

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Abstract
Recovery of cell volume in response to osmotic stress is mediated in part by increases in the Cl- permeability of the plasma membrane. These studies evaluate the hypothesis that ATP release and autocrine stimulation of purinergic (P2) receptors couple increases in cell volume to opening of Cl- channels. In HTC rat hepatoma cells, swelling induced by hypotonic exposure increased membrane Cl- current density to 44.8 +/- 7.1 pA/pF at -80 mV. Both the rate of volume recovery and the increase in Cl- permeability were inhibited in the presence of the ATP hydrolase apyrase (3 units/ml) or by exposure to the P2 receptor blockers suramin and Reactive Blue 2 (10-100 microM). Cell swelling also stimulated release of ATP. Hypotonic exposure increased the concentration of ATP in the effluent of perfused cells by 170 +/- 36 nM in the presence of a nucleotidase inhibitor (P < 0.01). In whole-cell recordings with ATP as the charge carrier, cell swelling increased membrane current density approximately 30-fold to 16.5 +/- 10.4 pA/pF. These findings indicate that increases in cell volume lead to efflux of ATP through opening of a conductive pathway consistent with a channel, and that extracellular ATP is required for recovery from swelling. ATP may function as an autocrine factor that couples increases in cell volume to opening of Cl- channels through stimulation of P2 receptors.