Regulation of cell volume in a human biliary cell line: Activation of K⁺ and Cl^- currents

The mechanisms responsible for recovery from cell swelling were evaluated in M(z)-ChA-1 cells from human cholangiocarcinoma, a model biliary cell line. Exposure to hypotonic buffer (40% less NaCl) rapidly increased relative cell volume to 1.35 ± 0.10 as measured by a Coulter Multisizer, followed by regulatory volume decrease to 1.08 ± 0.03 by 30 min. The same maneuver increased ⁸⁶Rb (69 ± 17%) and ¹²⁵I (422 ± 58%) effiux in cell monolayers. ⁸⁶Rb effiux was selectively inhibited by Ba²⁺ [half-maximal inhibitory concentration (IC₅₀) ~1.5 mM], and ¹²⁵I by 5-nitro-2-(3- phenylpropylamino)-benzoic acid (NPPB) (IC₅₀ ≃ 50 μM). Inhibition of these conductive pathways partially inhibited recovery from swelling. Membrane conductance measured by whole cell patch-clamp analysis increased in 57 of 57 cells during swelling due to activation of both K⁺ and Cl^- conductances in most cells. K⁺ currents (75% of cells, 881 ± 150 pA at 0 mV) were nearly linear and Ba²⁺ sensitive; Cl^- currents (70% of cells, 2,696 ± 244 pA at + 60 mV) were outwardly rectified, showed time-dependent inactivation at depolarizing potentials, and were inhibited by NPPB. Chelation of cytosolic Ca²⁺ decreased swellinginduced isotope effiux, prevented activation of macroscopic K⁺ and Cl^- currents, and blocked volume recovery. These studies indicate that biliary cells are able to regulate cell volume during osmotic stress by activation of separate K⁺ and Cl^- conductances through a mechanism that depends in part on Ca²⁺-sensitive signaling pathways.