Expression of Cystic Fibrosis Transmembrane Conductance Regulator Alters the Responses to Hypotonic Cell Swelling and ATP of Chinese Hamster Ovary Cells

Abstract

The aim of this study was to examine whether the stable expression of wild-type cystic fibrosis transmembrane conductance regulator (CFTR) in Chinese hamster ovary (CHO) cells alters the properties of these cells towards hypotonic cell swelling and ATP. According to many previous studies this was not expected a priori, since overexpression of CFTR should not affect the conductive pathways upregulated by the purinergic agonist or cell swelling. Three types of CHO cells were examined: a control group of normal CHO cells; a group of CFTR-CHO cells stably expressing wild-type CFTR at high levels (CHO-CFTR), and a group ΔF508-CFTR-CHO cells, stably expressing the frequent mutation ΔF508 CFTR (CHO-ΔF508). Whole cell patch-clamp studies were performed to measure the membrane voltage (V_m), the membrane conductance (G_m), and the membrane capacitance (C_m). Hypotonic cell swelling (Hypo, 150 mosm/l) was used, because it activates Cl^– and K⁺ channels and enables the cell to extrude KCl in many cells, and ATP because it is known to activate Ca²⁺-regulated channels in a large variety of cells. Hypo depolarized all three types of cells. This depolarization was accompanied by an increase in Cl^– conductance. The selectivity of the conductance was I^– ≥ Br^– ≥ Cl^– in CHO cells, but Cl^– = Br^– = I^– in the CFTR cells. Even more surprising: ATP (100 µmol/l) hyperpolarized CHO and ΔF508 cells and predominantly enhanced K⁺ conductance, whilst it depolarized and increased mostly a Cl^– conductance in CFTR cells. The selectivity of this anion conductance was atypical for ATP: Br^– > Cl^– > I^–. C_m was increased by ATP and Hypo in all three types of cells. ATP enhanced cytosolic Ca²⁺ ([Ca²⁺]_i) in all three types of cells but did not enhance cAMP. These data indicate that the expression of CFTR profoundly alters the properties of CHO cells. Agonists which stimulate characteristic Ca²⁺-regulated channels now enhance a Cl^– conductance resembling the properties of CFTR-Cl^– conductance.