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J. Biol. Chem., Vol. 276, Issue 33, 30871-30877, August 17, 2001

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Purinergic-independent Calcium Signaling Mediates Recovery from Hepatocellular Swelling
IMPLICATIONS FOR VOLUME REGULATION^*

Michael W. Roe, Ann L. Moore, and Steven D. Lidofsky^§¶

From the Departments of  Medicine and ^§ Pharmacology, University of Vermont College of Medicine, Burlington, Vermont 05401

Swelling of hepatocytes and other epithelia activates volume-sensitive ion channels that facilitate fluid and electrolyte efflux to restore cell volume, but the responsible signaling pathways are incompletely defined. Previous 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 determined whether hepatocellular swelling affected cytosolic [Ca²⁺], and if this involved a purinergic mechanism. Exposure of HTC cells to hypotonic media evoked an increase in cytosolic [Ca²⁺], which was followed by activation of K⁺ and Cl currents. Maneuvers that interfered with swelling-induced increases in cytosolic [Ca²⁺], including extracellular Ca²⁺ removal and intracellular Ca²⁺ store depletion with thapsigargin, inhibited activation of membrane currents and volume recovery. However, the swelling-induced increases in cytosolic [Ca²⁺] were unaffected by either extracellular ATP depletion with apyrase or blockade of P2 receptors with suramin. These findings indicate that swelling elicits an increase in hepatocellular Ca²⁺, which is essential for ion channel activation and volume recovery, but that this increase does not stem from activation of volume-sensitive P2 receptors. Collectively, these observations imply that regulatory responses to hepatocellular swelling involve a dual requirement for a purinergic-independent Ca²⁺ signaling cascade and a Ca²⁺-independent purinergic signaling pathway.

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