Modulation of hepatocellular swelling-activated K+ currents by phosphoinositide pathway-dependent protein kinase C.

K+ channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+ current (I-KVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+ flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to I-KVol activation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased I-KVol, whereas conditions that decrease cellular PIP2 decreased I-KVol. GTP and AlF4- stimulated I-KVol development, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased I-KVol and inhibited the stimulatory response to PIP2 or GTP. Protein kinase C (PKC) is involved, because K+ current was enhanced by 1-oleoyl-2-acetyl-sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both I-KVol and the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated I-KVol activation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+ current similar to I-KVol was activated by PIP2, lipid phosphatase inhibitors to counter PIP2 depletion, a PLC-coupled alpha(1)-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate I-KVol through a PIP2/PKC-dependent pathway. The results indicate that PIP2 indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.