Purinergic-independent calcium signaling mediates recovery from hepatocellular swelling - Implications for volume regulation

Swelling of hepatocytes and other epithelia activates volume-sensitive ion channels that facilitate fluid and electrolyte efflux to restore cell volum e, but the responsible signaling pathways are incompletely defined. Previou s work in model HTC rat hepatoma cells has indicated that swelling elicits ATP release, which stimulates P2 receptors and activates Cl- channels, and that this mechanism is essential for hepatocellular volume recovery. Since P2 receptors are generally coupled to Ca2+ signaling pathways, we determine d whether hepatocellular swelling affected cytosolic [Ca2+], and if this in volved a purinergic mechanism. Exposure of HTC cells to hypotonic media evo ked an increase in cytosolic [Ca2+], which was followed by activation of K and Cl- currents. Maneuvers that interfered with swelling-induced increase s in cytosolic [Ca2+], including extracellular Ca2+ removal and intracellul ar Ca2+ store depletion with thapsigargin, inhibited activation of membrane currents and volume recovery. However, the swelling-induced increases in c ytosolic [Ca2+] were unaffected by either extracellular ATP depletion with apyrase or blockade of P2 receptors with suramin. These findings indicate t hat swelling elicits an increase in hepatocellular Ca2+, which is essential for ion channel activation and volume recovery, but that this increase doe s not stem from activation of volume-sensitive P2 receptors. Collectively, these observations imply that regulatory responses to hepatocellular swelli ng involve a dual requirement for a purinergic-independent Ca2+ signaling c ascade and a Ca2+-independent purinergic signaling pathway.